Introduction to Geothermal Power

First Edition, 2011 ISBN 978-93-81157-59-6

© All rights reserved. Published by: The English Press 4735/22 Prakashdeep Bldg, Ansari Road, Darya Ganj, Delhi - 110002 Email: [email protected]

Table of Contents

Chapter 1- Geothermal Electricity

Chapter 2 - Geothermal Energy

Chapter 3 - Enhanced Geothermal System

Chapter 4 - Ground-coupled Heat Exchanger

Chapter 5 - Geothermal Heating

Chapter 6 - Geothermal Heat Pump

Chapter 7 - Geothermal Power in Different Countries

Chapter-1

Geothermal Electricity

Geothermal electricity is electricity generated from geothermal energy. Technologies in use include dry steam power plants, flash steam power plants and binary cycle power plants. As a more recent technology, geothermal electricity generation is currently used only in 24 countries while geothermal heating is in use in 70 countries.

Estimates of the electricity generating potential of geothermal energy vary from 35 to 2000 GW. Current worldwide installed capacity is 10,715 megawatts (MW), with the largest capacity in the United States (3,086 MW), Philippines, and Indonesia.

Geothermal power is considered to be sustainable because the heat extraction is small compared to the Earth's heat content. The emission intensity of existing geothermal electric plants is on average 122 kg of CO2 per megawatt-hour (MW·h) of electricity, a small fraction of that of conventional fossil fuel plants.

History and development

Global geothermal electric capacity. Upper red line is installed capacity; lower green line is realized production.

In the 20th century, demand for electricity led to the consideration of geothermal power as a generating source. Prince Piero Ginori Conti tested the first geothermal power generator on 4 July 1904 in Larderello, Italy. It successfully lit four light bulbs. Later, in 1911, the world's first commercial geothermal power plant was built there. Experimental generators were built in Beppu, Japan and the Geysers, California, in the 1920s, but Italy was the world's only industrial producer of geothermal electricity until New Zealand built a plant in 1958.

In 1960, Pacific Gas and Electric began operation of the first successful geothermal electric power plant in the United States at The Geysers in California. The original turbine lasted for more than 30 years and produced 11 MW net power.

The binary cycle power plant was first demonstrated in 1967 in Russia and later introduced to the USA in 1981. This technology allows the use of much lower temperature resources than were previously recoverable. In 2006, a binary cycle plant in Chena Hot Springs, Alaska, came on-line, producing electricity from a record low fluid temperature of 57°C.

Geothermal electric plants have until recently been built exclusively where high temperature geothermal resources are available near the surface. The development of binary cycle power plants and improvements in drilling and extraction technology may

enable enhanced geothermal systems over a much greater geographical range. Demonstration projects are operational in Landau-Pfalz, Germany, and Soultz-sous-Forêts, France, while an earlier effort in Basel, Switzerland was shut down after it triggered earthquakes. Other demonstration projects are under construction in Australia, the United Kingdom, and the United States of America.

The thermal efficiency of geothermal electric plants is low, around 10-23%, because geothermal fluids are at a low temperature compared to steam from boilers. By the laws of thermodynamics this low temperature limits the efficiency of heat engines in extracting useful energy during the generation of electricity. Exhaust heat is wasted, unless it can be used directly and locally, for example in greenhouses, timber mills, and district heating. The efficiency of the system does not affect operational costs as it would for a coal or other fossil fuel plant, but it does factor into the viability of the plant. In order to produce more energy than the pumps consume, electricity generation requires high temperature geothermal fields and specialized heat cycles. Because geothermal power does not rely on variable sources of energy, unlike, for example, wind or solar, its capacity factor can be quite large – up to 96% has been demonstrated. The global average was 73% in 2005.

Resources

Enhanced geothermal system 1:Reservoir 2:Pump house 3:Heat exchanger 4:Turbine hall 5:Production well 6:Injection well 7:Hot water to district heating 8:Porous sediments 9:Observation well 10:Crystalline bedrock

The earth’s heat content is 1031 joules. This heat naturally flows to the surface by conduction at a rate of 44.2 terawatts, (TW,) and is replenished by radioactive decay at a rate of 30 TW. These power rates are more than double humanity’s current energy consumption from primary sources, but most of this power is too diffuse (approximately 0.1 W/m2 on average) to be recoverable. The Earth's crust effectively acts as a thick insulating blanket which must be pierced by fluid conduits (of magma, water or other) to release the heat underneath.

Electricity generation requires high temperature resources that can only come from deep underground. The heat must be carried to the surface by fluid circulation, either through magma conduits, hot springs, hydrothermal circulation, oil wells, drilled water wells, or a combination of these. This circulation sometimes exists naturally where the crust is thin: magma conduits bring heat close to the surface, and hot springs bring the heat to the surface. If no hot spring is available, a well must be drilled into a hot aquifer. Away from tectonic plate boundaries the geothermal gradient is 25-30°C per kilometre (km) of depth in most of the world, and wells would have to be several kilometres deep to permit electricity generation. The quantity and quality of recoverable resources improves with drilling depth and proximity to tectonic plate boundaries.

In ground that is hot but dry, or where water pressure is inadequate, injected fluid can stimulate production. Developers bore two holes into a candidate site, and fracture the rock between them with explosives or high pressure water. Then they pump water or liquefied carbon dioxide down one borehole, and it comes up the other borehole as a gas. This approach is called hot dry rock geothermal energy in Europe, or enhanced geothermal systems in North America. Much greater potential may be available from this approach than from conventional tapping of natural aquifers.

Estimates of the electricity generating potential of geothermal energy vary from 35 to 2000 GW depending on the scale of investments. This does not include non-electric heat recovered by co-generation, geothermal heat pumps and other direct use. A 2006 report by the Massachusetts Institute of Technology (MIT), that included the potential of enhanced geothermal systems, estimated that investing 1 billion US dollars in research and development over 15 years would allow the creation of 100 GW of electrical generating capacity by 2050 in the United States alone. The MIT report estimated that over 200 zettajoules (ZJ) would be extractable, with the potential to increase this to over 2,000 ZJ with technology improvements - sufficient to provide all the world's present energy needs for several millennia.

At present, geothermal wells are rarely more than 3 kilometres (2 mi) deep. Upper estimates of geothermal resources assume wells as deep as 10 kilometres (6 mi). Drilling at this depth is now possible in the petroleum industry, although it is an expensive process. The deepest research well in the world, the Kola superdeep borehole, is 12 kilometres (7 mi) deep. This record has recently been imitated by commercial oil wells, such as Exxon's Z-12 well in the Chayvo field, Sakhalin. Wells drilled to depths greater than 4 kilometres (2 mi) generally incur drilling costs in the tens of millions of dollars. The technological challenges are to drill wide bores at low cost and to break larger volumes of rock.

Geothermal power is considered to be sustainable because the heat extraction is small compared to the Earth's heat content, but extraction must still be monitored to avoid local depletion. Although geothermal sites are capable of providing heat for many decades, individual wells may cool down or run out of water. The three oldest sites, at Larderello, Wairakei, and the Geysers have all reduced production from their peaks. It is not clear whether these plants extracted energy faster than it was replenished from greater depths,

or whether the aquifers supplying them are being depleted. If production is reduced, and water is reinjected, these wells could theoretically recover their full potential. Such mitigation strategies have already been implemented at some sites. The long-term sustainability of geothermal energy has been demonstrated at the Lardarello field in Italy since 1913, at the Wairakei field in New Zealand since 1958, and at The Geysers field in California since 1960.

Power station types

Dry steam plant

Flash steam plant

Dry steam power plants

Dry steam plants are the simplest and oldest design. They directly use geothermal steam of 150°C or more to turn turbines.

Flash steam power plants

Flash steam plants pull deep, high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines. They require fluid temperatures of at least 180°C, usually more. This is the most common type of plant in operation today.

Binary cycle power plants

Binary cycle power plants are the most recent development, and can accept fluid temperatures as low as 57°C. The moderately hot geothermal water is passed by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to flash to vapor, which then drives the turbines. This is the most common type of geothermal electricity plant being built today. Both Organic Rankine and Kalina cycles are used. The thermal efficiency is typically about 10%.

Worldwide production

The International Geothermal Association (IGA) has reported that 10,715 megawatts (MW) of geothermal power in 24 countries is online, which is expected to generate 67,246 GWh of electricity in 2010. This represents a 20% increase in geothermal power online capacity since 2005. IGA projects this will grow to 18,500 MW by 2015, due to the large number of projects presently under consideration, often in areas previously assumed to have little exploitable resource.

In 2010, the United States led the world in geothermal electricity production with 3,086 MW of installed capacity from 77 power plants; the largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California. The Philippines follows the US as the second highest producer of geothermal power in the world, with 1,904 MW of capacity online; geothermal power makes up approximately 18% of the country's electricity generation.

Utility-grade plants

The largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California, United States. As of 2004, five countries (El Salvador, Kenya, the Philippines, Iceland, and Costa Rica) generate more than 15% of their electricity from geothermal sources.

Naknek Electric Association (NEA) is going to make an exploration well near King Salmon, in Southwest Alaska. It could cut the cost of electricity production by 71 percent and the planned power is 25 megawatts.

Geothermal electricity is generated in the 24 countries listed in the table below. During 2005, contracts were placed for an additional 500 MW of electrical capacity in the United States, while there were also plants under construction in 11 other countries. Enhanced geothermal systems that are several kilometres in depth are operational in France and Germany and are being developed or evaluated in at least four other countries.

Installed geothermal electric capacity

Country Capacity (MW)2007

Capacity (MW)2010

percentageof nationalproduction

USA 2687 3086 0.3% Philippines 1969.7 1904 27% Indonesia 992 1197 3.7% Mexico 953 958 3% Italy 810.5 843 New Zealand 471.6 628 10% Iceland 421.2 575 30% Japan 535.2 536 0.1% El Salvador 204.2 204 14% Kenya 128.8 167 11.2% Costa Rica 162.5 166 14% Nicaragua 87.4 88 10% Russia 79 82 Turkey 38 82 Papua-New Guinea 56 56 Guatemala 53 52 Portugal 23 29 China 27.8 24 France 14.7 16 Ethiopia 7.3 7.3 Germany 8.4 6.6 Austria 1.1 1.4 Australia 0.2 1.1 Thailand 0.3 0.3

TOTAL 9,731.9 10,709.7

Environmental impact

Krafla Geothermal Station in northeast Iceland

Fluids drawn from the deep earth carry a mixture of gases, notably carbon dioxide (CO2), hydrogen sulfide (H2S), methane (CH4) and ammonia (NH3). These pollutants contribute to global warming, acid rain, and noxious smells if released. Existing geothermal electric plants emit an average of 122 kg of CO2 per megawatt-hour (MW·h) of electricity, a small fraction of the emission intensity of conventional fossil fuel plants. Plants that experience high levels of acids and volatile chemicals are usually equipped with emission-control systems to reduce the exhaust. Geothermal plants could theoretically inject these gases back into the earth, as a form of carbon capture and storage.

In addition to dissolved gases, hot water from geothermal sources may hold in solution trace amounts of toxic chemicals such as mercury, arsenic, boron, antimony, and salt. These chemicals come out of solution as the water cools, and can cause environmental damage if released. The modern practice of injecting geothermal fluids back into the Earth to stimulate production has the side benefit of reducing this environmental risk.

Plant construction can adversely affect land stability. Subsidence has occurred in the Wairakei field in New Zealand. Enhanced geothermal systems can trigger earthquakes as part of hydraulic fracturing. The project in Basel, Switzerland was suspended because

more than 10,000 seismic events measuring up to 3.4 on the Richter Scale occurred over the first 6 days of water injection.

Geothermal has minimal land and freshwater requirements. Geothermal plants use 3.5 square kilometres per gigawatt of electrical production (not capacity) versus 32 and 12 square kilometres for coal facilities and wind farms respectively. They use 20 litres of freshwater per MW·h versus over 1000 litres per MW·h for nuclear, coal, or oil.

Economics

Geothermal power requires no fuel, and is therefore immune to fuel cost fluctuations, but capital costs tend to be high. Drilling accounts for over half the costs, and exploration of deep resources entails significant risks. A typical well doublet in Nevada can support 4.5 megawatt (MW) of electricity generation and costs about $10 million to drill, with a 20% failure rate. In total, electrical plant construction and well drilling cost about 2-5 million € per MW of electrical capacity, while the levelised energy cost is 0.04-0.10 € per kW·h. Enhanced geothermal systems tend to be on the high side of these ranges, with capital costs above $4 million per MW and levelized costs above $0.054 per kW·h in 2007.

Geothermal power is highly scalable: a large geothermal plant can power entire cities while a smaller power plant can supply a rural village.

Chevron Corporation is the world's largest private producer of geothermal electricity. The most developed geothermal field is the Geysers in California. In 2008, this field supported 15 plants, all owned by Calpine, with a total generating capacity of 725 MW.

Chapter-2

Geothermal Energy

Steam rising from the Nesjavellir Geothermal Power Station in Iceland.

Geothermal energy is thermal energy stored in the Earth. Thermal energy is energy that determines the temperature of matter. Earth's geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, from volcanic activity, and from solar energy absorbed at the surface. The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface.

From hot springs, geothermal energy has been used for bathing since Paleolithic times and for space heating since ancient Roman times, but it is now better known for electricity generation. Worldwide, about 10,715 megawatts (MW) of geothermal power is

online in 24 countries. An additional 28 gigawatts of direct geothermal heating capacity is installed for district heating, space heating, spas, industrial processes, desalination and agricultural applications.

Geothermal power is cost effective, reliable, sustainable, and environmentally friendly, but has historically been limited to areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the range and size of viable resources, especially for applications such as home heating, opening a potential for widespread exploitation. Geothermal wells release greenhouse gases trapped deep within the earth, but these emissions are much lower per energy unit than those of fossil fuels. As a result, geothermal power has the potential to help mitigate global warming if widely deployed in place of fossil fuels.

The Earth's geothermal resources are theoretically more than adequate to supply humanity's energy needs, but only a very small fraction may be profitably exploited. Drilling and exploration for deep resources is very expensive. Forecasts for the future of geothermal power depend on assumptions about technology, energy prices, subsidies, and interest rates.

Electricity

The International Geothermal Association (IGA) has reported that 10,715 megawatts (MW) of geothermal power in 24 countries is online, which is expected to generate 67,246 GWh of electricity in 2010. This represents a 20% increase in online capacity since 2005. IGA projects growth to 18,500 MW by 2015, due to the projects presently under consideration, often in areas previously assumed to have little exploitable resource.

In 2010, the United States led the world in geothermal electricity production with 3,086 MW of installed capacity from 77 power plants. The largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California. The Philippines is the second highest producer, with 1,904 MW of capacity online. Geothermal power makes up approximately 18% of the country's electricity generation.

Geothermal electric plants were traditionally built exclusively on the edges of tectonic plates where high temperature geothermal resources are available near the surface. The development of binary cycle power plants and improvements in drilling and extraction technology enable enhanced geothermal systems over a much greater geographical range. Demonstration projects are operational in Landau-Pfalz, Germany, and Soultz-sous-Forêts, France, while an earlier effort in Basel, Switzerland was shut down after it triggered earthquakes. Other demonstration projects are under construction in Australia, the United Kingdom, and the United States of America.

The thermal efficiency of geothermal electric plants is low, around 10-23%, because geothermal fluids do not reach the high temperatures of steam from boilers. The laws of thermodynamics limits the efficiency of heat engines in extracting useful energy. Exhaust heat is wasted, unless it can be used directly and locally, for example in greenhouses,

timber mills, and district heating. System efficiency does not materially affect operational costs as it would for plants that use fuel, but it does affect return on the capital used to build the plant. In order to produce more energy than the pumps consume, electricity generation requires relatively hot fields and specialized heat cycles. Because geothermal power does not rely on variable sources of energy, unlike, for example, wind or solar, its capacity factor can be quite large – up to 96% has been demonstrated. The global average was 73% in 2005.

Direct application

In the geothermal industry, low temperature means temperatures of 300 °F (149 °C) or less. Low-temperature geothermal resources are typically used in direct-use applications, such as district heating, greenhouses, fisheries, mineral recovery, and industrial process heating. However, some low-temperature resources can generate electricity using binary cycle electricity generating technology.

Approximately 70 countries made direct use of 270 petajoules (PJ) of geothermal heating in 2004. More than half went for space heating, and another third for heated pools. The remainder supported industrial and agricultural applications. Global installed capacity was 28 GW, but capacity factors tend to be low (30% on average) since heat is mostly needed in winter. The above figures are dominated by 88 PJ of space heating extracted by an estimated 1.3 million geothermal heat pumps with a total capacity of 15 GW. Heat pumps for home heating are the fastest-growing means of exploiting geothermal energy, with a global annual growth rate of 30% in energy production.

Direct heating is far more efficient than electricity generation and places less demanding temperature requirements on the heat resource. Heat may come from co-generation via a geothermal electrical plant or from smaller wells or heat exchangers buried in shallow ground. As a result, geothermal heating is economic at many more sites than geothermal electricity generation. Where natural hot springs are available, the heated water can be piped directly into radiators. If the ground is hot but dry, earth tubes or downhole heat exchangers can collect the heat. But even in areas where the ground is colder than room temperature, heat can still be extracted with a geothermal heat pump more cost-effectively and cleanly than by conventional furnaces. These devices draw on much shallower and colder resources than traditional geothermal techniques, and they frequently combine a variety of functions, including air conditioning, seasonal energy storage, solar energy collection, and electric heating. Geothermal heat pumps can be used for space heating essentially anywhere.

Geothermal heat supports many applications. District heating applications use networks of piped hot water to heat many buildings across entire communities. In Reykjavík, Iceland, spent water from the district heating system is piped below pavement and sidewalks to melt snow. Geothermal desalination has been demonstrated.

Environmental effects

Geothermal power station in the Philippines

Krafla Geothermal Station in northeast Iceland

Fluids drawn from the deep earth carry a mixture of gases, notably carbon dioxide (CO2), hydrogen sulfide (H2S), methane (CH4) and ammonia (NH3). These pollutants contribute to global warming, acid rain, and noxious smells if released. Existing geothermal electric plants emit an average of 122 kilograms (269 lb) of CO2 per megawatt-hour (MW·h) of electricity, a small fraction of the emission intensity of conventional fossil fuel plants. Plants that experience high levels of acids and volatile chemicals are usually equipped with emission-control systems to reduce the exhaust.

In addition to dissolved gases, hot water from geothermal sources may hold in solution trace amounts of toxic chemicals such as mercury, arsenic, boron, and antimony. These chemicals precipitate as the water cools, and can cause environmental damage if released. The modern practice of injecting cooled geothermal fluids back into the Earth to stimulate production has the side benefit of reducing this environmental risk.

Direct geothermal heating systems contain pumps and compressors, which may consume energy from a polluting source. This parasitic load is normally a fraction of the heat output, so it is always less polluting than electric heating. However, if the electricity is produced by burning fossil fuels, then the net emissions of geothermal heating may be comparable to directly burning the fuel for heat. For example, a geothermal heat pump

powered by electricity from a combined cycle natural gas plant would produce about as much pollution as a natural gas condensing furnace of the same size. Therefore the environmental value of direct geothermal heating applications is highly dependent on the emissions intensity of the neighboring electric grid.

Plant construction can adversely affect land stability. Subsidence has occurred in the Wairakei field in New Zealand and in Staufen im Breisgau, Germany. Enhanced geothermal systems can trigger earthquakes as part of hydraulic fracturing. The project in Basel, Switzerland was suspended because more than 10,000 seismic events measuring up to 3.4 on the Richter Scale occurred over the first 6 days of water injection.

Geothermal has minimal land and freshwater requirements. Geothermal plants use 3.5 square kilometres (1.4 sq mi) per gigawatt of electrical production (not capacity) versus 32 and 12 square kilometres (4.6 sq mi) for coal facilities and wind farms respectively. They use 20 litres (5.3 US gal) of freshwater per MW·h versus over 1,000 litres (260 US gal) per MW·h for nuclear, coal, or oil.

Economics

Geothermal power requires no fuel (except for pumps), and is therefore immune to fuel cost fluctuations, but capital costs are significant. Drilling accounts for over half the costs, and exploration of deep resources entails significant risks. A typical well doublet (extraction and injection wells) in Nevada can support 4.5 megawatts (MW) and costs about $10 million to drill, with a 20% failure rate.

In total, electrical plant construction and well drilling cost about 2-5 million € per MW of electrical capacity, while the break–even price is 0.04-0.10 € per kW·h. Enhanced geothermal systems tend to be on the high side of these ranges, with capital costs above $4 million per MW and break–even above $0.054 per kW·h in 2007. Direct heating applications can use much shallower wells with lower temperatures, so smaller systems with lower costs and risks are feasible. Residential geothermal heat pumps with a capacity of 10 kilowatt (kW) are routinely installed for around $1–3,000 per kilowatt. District heating systems may benefit from economies of scale if demand is geographically dense, as in cities, but otherwise piping installation dominates capital costs. The capital cost of one such district heating system in Bavaria was estimated at somewhat over 1 million € per MW. Direct systems of any size are much simpler than electric generators and have lower maintenance costs per kW·h, but they must consume electricity to run pumps and compressors. Some governments subsidize geothermal projects.

Geothermal power is highly scalable: from a rural village to an entire city.

Chevron Corporation is the world's largest private geothermal electricity producer. The most developed geothermal field is the Geysers in California.

Resources


The Earth's internal heat naturally flows to the surface by conduction at a rate of 44.2 terawatts (TW), and is replenished by radioactive decay of minerals at a rate of 30 TW. These power rates are more than double humanity’s current energy consumption from all primary sources, but most of it is not recoverable. In addition to heat emanating from deep within the Earth, the top 10 meters (33 ft) of the ground accumulates solar energy (warms up) during the summer, and releases that energy (cools down) during the winter.

Beneath the seasonal variations, the geothermal gradient of temperatures through the crust is 25–30 °C (77–86 °F) per kilometer of depth in most of the world. The conductive heat flux is approximately 0.1 MW/km2 on average. These values are much higher near tectonic plate boundaries where the crust is thinner. They may be further augmented by fluid circulation, either through magma conduits, hot springs, hydrothermal circulation or a combination of these.

A geothermal heat pump can extract enough heat from shallow ground anywhere in the world to provide home heating, but industrial applications need the higher temperatures of deep resources. The thermal efficiency and profitability of electricity generation is particularly sensitive to temperature. The more demanding applications receive the greatest benefit from a high natural heat flux, ideally from using a hot spring. The next best option is to drill a well into a hot aquifer. If no adequate aquifer is available, an artificial one may be built by injecting water to hydraulically fracture the bedrock. This last approach is called hot dry rock geothermal energy in Europe, or enhanced geothermal systems in North America. Much greater potential may be available from this approach than from conventional tapping of natural aquifers.

Estimates of the electricity generating potential of geothermal energy vary six–fold, from .035 to 2 TW depending on the scale of investments. Upper estimates of geothermal resources assume enhanced geothermal wells as deep as 10 kilometres (6 mi), whereas existing geothermal wells are rarely more than 3 kilometres (2 mi) deep. Wells of this depth are now common in the petroleum industry. The deepest research well in the world, the Kola superdeep borehole, is 12 kilometres (7 mi) deep. This record has recently been imitated by commercial oil wells, such as Exxon's Z-12 well in the Chayvo field, Sakhalin.

Sustainability

Geothermal power is considered to be sustainable because any projected heat extraction is small compared to the Earth's heat content. The Earth has an internal heat content of 1031 joules (3·1015 TW·hr). About 20% of this is residual heat from planetary accretion, and the remainder is attributed to higher radioactive decay rates that existed in the past. Natural heat flows are not in equilibrium, and the planet is slowly cooling down on geologic timescales. Human extraction taps a minute fraction of the natural outflow, often without accelerating it.

Even though geothermal power is globally sustainable, extraction must still be monitored to avoid local depletion. Over the course of decades, individual wells draw down local temperatures and water levels until a new equilibrium is reached with natural flows. The three oldest sites, at Larderello, Wairakei, and the Geysers have experienced reduced output because of local depletion. Heat and water, in uncertain proportions, were extracted faster than they were replenished. If production is reduced and water is reinjected, these wells could theoretically recover their full potential. Such mitigation strategies have already been implemented at some sites. The long-term sustainability of geothermal energy has been demonstrated at the Lardarello field in Italy since 1913, at

the Wairakei field in New Zealand since 1958, and at The Geysers field in California since 1960.

The extinction of several geyser fields has also been attributed to geothermal power development

Depletion

Electricity Generation at Poihipi, New Zealand.

Electricity Generation at Ohaaki, New Zealand.

Electricity Generation at Wairakei, New Zealand.

Falling electricity production may be boosted through drilling additional supply boreholes, as at Poihipi and Ohaaki. The Wairakei power station has been running much longer, with its first unit commissioned in November 1958, and it attained its peak generation of 173MW in 1965, but already the supply of high-pressure steam was faltering, in 1982 being derated to intermediate pressure and the station managing 157MW. At the turn of the century it was managing about 150MW, then in 2005 two 8MW isopentane systems were added, boosting the station's output by about 14MW.

Detailed data are unavailable, being lost due to re-organisations. One such re-organisation in 1996 causes the absence of early data for Poihipi (started 1996), and the gap in 1996/7 for Wairakei and Ohaaki; half-hourly data for Ohaaki's first few months of operation are also missing, as well as for most of Wairakei's history.

Although the usual operation mode of a geothermal generator is steady at full possible power because the fuel is free, Poihipi's operation is constrained by limits on its steam consumption, switching on and off daily as shown by the plot's separation of high and low values. Because the temperature difference between the inlet and outlet of the heat engine is relatively low (as compared to fuel-heated working fluids), power production can be noticeably affected by small variations in temperature of the environmental aspect employed by the cooling unit, especially air-cooled units.

History

The oldest known pool fed by a hot spring, built in the Qin dynasty in the 3rd century BC.

Hot springs have been used for bathing at least since paleolithic times. The oldest known spa is a stone pool on China’s Lisan mountain built in the Qin dynasty in the 3rd century BC, at the same site where the Huaqing Chi palace was later built. In the first century AD, Romans conquered Aquae Sulis, now Bath, Somerset, England, and used the hot

springs there to feed public baths and underfloor heating. The admission fees for these baths probably represent the first commercial use of geothermal power. The world's oldest geothermal district heating system in Chaudes-Aigues, France, has been operating since the 14th century. The earliest industrial exploitation began in 1827 with the use of geyser steam to extract boric acid from volcanic mud in Larderello, Italy.

In 1892, America's first district heating system in Boise, Idaho was powered directly by geothermal energy, and was copied in Klamath Falls, Oregon in 1900. A deep geothermal well was used to heat greenhouses in Boise in 1926, and geysers were used to heat greenhouses in Iceland and Tuscany at about the same time. Charlie Lieb developed the first downhole heat exchanger in 1930 to heat his house. Steam and hot water from geysers began heating homes in Iceland starting in 1943.

Global geothermal electric capacity. Upper red line is installed capacity; lower green line is realized production.

In the 20th century, demand for electricity led to the consideration of geothermal power as a generating source. Prince Piero Ginori Conti tested the first geothermal power generator on 4 July 1904, at the same Larderello dry steam field where geothermal acid extraction began. It successfully lit four light bulbs. Later, in 1911, the world's first commercial geothermal power plant was built there. It was the world's only industrial producer of geothermal electricity until New Zealand built a plant in 1958.

By this time, Lord Kelvin had already invented the heat pump in 1852, and Heinrich Zoelly had patented the idea of using it to draw heat from the ground in 1912. But it was not until the late 1940s that the geothermal heat pump was successfully implemented.

The earliest one was probably Robert C. Webber's home-made 2.2 kW direct-exchange system, but sources disagree as to the exact timeline of his invention. J. Donald Kroeker designed the first commercial geothermal heat pump to heat the Commonwealth Building (Portland, Oregon) and demonstrated it in 1946. Professor Carl Nielsen of Ohio State University built the first residential open loop version in his home in 1948. The technology became popular in Sweden as a result of the 1973 oil crisis, and has been growing slowly in worldwide acceptance since then. The 1979 development of polybutylene pipe greatly augmented the heat pump’s economic viability.

In 1960, Pacific Gas and Electric began operation of the first successful geothermal electric power plant in the United States at The Geysers in California. The original turbine lasted for more than 30 years and produced 11 MW net power.

The binary cycle power plant was first demonstrated in 1967 in the U.S.S.R. and later introduced to the U.S. in 1981. This technology allows the generation of electricity from much lower temperature resources than previously. In 2006, a binary cycle plant in Chena Hot Springs, Alaska, came on-line, producing electricity from a record low fluid temperature of 57 °C (135 °F).

Development around the world Installed geothermal electric capacity

Country Capacity (MW)2007

Capacity (MW)2010

Percentageof nationalproduction

USA 2687 3086 0.3% Philippines 1969.7 1904 27% Indonesia 992 1197 3.7% Mexico 953 958 3% Italy 810.5 843 New Zealand 471.6 628 10% Iceland 421.2 575 30% Japan 535.2 536 0.1% El Salvador 204.2 204 14% Kenya 128.8 167 11.2% Costa Rica 162.5 166 14% Nicaragua 87.4 88 10% Russia 79 82 Turkey 38 82 Papua-New Guinea 56 56 Guatemala 53 52 Portugal 23 29 China 27.8 24

France 14.7 16 Ethiopia 7.3 7.3 Germany 8.4 6.6 Austria 1.1 1.4 Australia 0.2 1.1 Thailand 0.3 0.3

TOTAL 9,731.9 10,709.7

Chapter-3

Enhanced Geothermal System


Enhanced Geothermal Systems (EGS) are a new type of geothermal power technologies that do not require natural convective hydrothermal resources. Until recently, geothermal power systems have only exploited resources where naturally occurring water and rock porosity is sufficient to carry heat to the surface. However, the vast majority of geothermal energy within drilling reach is in dry and non-porous rock. EGS technologies "enhance" and/or create geothermal resources in this hot dry rock (HDR) through hydraulic stimulation.

When natural cracks and pores will not allow for economic flow rates, the permeability can be enhanced by pumping high pressure cold water down an injection well into the rock. The injection increases the fluid pressure in the naturally fractured granite which mobilizes shear events, enhancing the permeability of the fracture system. Water travels through fractures in the rock, capturing the heat of the rock until it is forced out of a second borehole as very hot water, which is converted into electricity using either a steam turbine or a binary power plant system. All of the water, now cooled, is injected back into the ground to heat up again in a closed loop.

EGS / HDR technologies, like hydrothermal geothermal, are expected to be baseload resources which produce power 24 hours a day like a fossil plant. Distinct from hydrothermal, HDR / EGS may be feasible anywhere in the world, depending on the economic limits of drill depth. Good locations are over deep granite covered by a thick (3–5 km) layer of insulating sediments which slow heat loss. HDR wells are expected to have a useful life of 20 to 30 years before the outflow temperature drops about 10 degrees Celsius and the well becomes uneconomic. If left for 50 to 300 years the temperature will recover.

There are HDR and EGS systems currently being developed and tested in France, Australia, Japan, Germany, the U.S. and Switzerland. The largest EGS project in the world is a 25 megawatt demonstration plant currently being developed in the Cooper Basin, Australia. The Cooper Basin has the potential to generate 5,000–10,000 MW.

EGS industry

Commercial projects are currently either operational or under development in Australia, the United States, and Germany.

The largest project in the world is being developed in Australia's Cooper Basin by Geodynamics. The Cooper Basin project has the potential to develop 5–10 GW. Australia now has 33 firms either exploring for, drilling, or developing EGS projects. Australia's industry has been greatly aided by a national Renewable Portfolio Standard of 25% renewables by 2025, a vibrant Green Energy Credit market, and supportive R&D collaboration between government, academia, and industry.

Germany's 23 cent/kWh Feed-In Tariff (FIT) for geothermal energy has led to a surge in geothermal development, despite Germany's relatively poor geothermal resource. The Landau partial EGS project is profitable today under the FIT.

The AltaRock Energy effort is a demonstration project being conducted to prove out the company's proprietary technology at the site of an existing geothermal project owned and operated by NCPA in The Geysers, and does not include power generation. However, any steam produced by the project will be supplied to NCPA's flash turbines under a long-term contract.

Current EGS projects

Project Type Country Size (MW) Plant Type

Depth (km) Developer Status

Soultz R&D France (EU) 1.5 Binary 4.2 ENGINE Operational

Desert Peak R&D United

States 11–50 Binary DOE, Ormat, GeothermEx

Develop- ment

Landau Commercial Germany (EU) 3 Binary 3.3 ? Operational

Paralana (Phase 1)

Commercial Australia 7–30 Binary 4.1 Petratherm Drilling

Cooper Basin Commercial Australia 250–500 Kalina 4.3 Geodynamics Drilling

The Geysers Demonstration United

States (Unknown) Flash 3.5 – 3.8

AltaRock Energy, NCPA

Canceled (Jan 2010)

Bend, Oregon Demonstration United

States (Unknown)

AltaRock Energy, Davenport Power

Permitting (Mar 2010)

Ogachi R&D Japan (Unknown) 1.0 – 1.1 CO2

experimentsUnited Downs, Redruth

Commercial United Kingdom 10 MW Binary 4.5

Geothermal Engineering Ltd

Fundraising

Eden Project Commercial United

Kingdom 3 MW Binary 3–4 EGS Energy Ltd. Fundraising

Research and development

Australia

The Australian government has provided research funding for the development of Hot Dry Rock technology.

On 30 May 2007, then Australian opposition environmental spokesperson and current Minister for the Environment, Heritage and the Arts Peter Garrett announced that if

elected at the 2007 Australian Federal Election, the Australian Labor Party would use taxpayers money to subsidise putting the necessary drilling rigs in place. In an interview, he promised:

"There are some technical difficulties and challenges there, but those people who are keen on getting Australia into geothermal say we've got this great access to resource and one of the things, interestingly, that's held them back is not having the capacity the put the drilling plants in place. And so what we intend this $50 million to go towards is to provide a one for one dollars. Match $1 from us, $1 from the industry so that they can get these drilling rigs on to site and really get the best sites identified and get the industry going."

European Union

The EU's EGS R&D project at Soultz-sous-Forêts, France, has recently connected its 1.5 MW demonstration plant to the grid. The Soultz project has explored the connection of multiple stimulated zones and the performance of triplet well configurations (1 injector/2 producers).

Portugal – Portuguese government has awarded, December 2008, an exclusive license to Geovita Ltd, to prospect and explore geothermal energy in one of the best areas in continental Portugal. An area of about 500 square kilometers that is being studied together by Geovita and Coimbra's University — Science and Technology Faculty — Earth Sciences Department, and foresees the installation of an Enhanced Geothermal System (EGS).

Induced seismicity in Basel led to the cancellation of the EGS project.

United Kingdom

Cornwall is set to host a 3MW demonstration project, based at the Eden Project, that could pave the way for a series of 50MW commercial-scale geothermal power stations in suitable areas across the country.

A commercial-scale project near Redruth is also planned. The plant, which has been granted planning permission, would generate 10MW of electricity, and 55MW of thermal energy, and is scheduled to become operational in 2013–2014.

United States

Early Days—Fenton Hill

The United States pioneered the first EGS effort—then termed Hot Dry Rock—at Fenton Hill, New Mexico with a project run by the federal Los Alamos Laboratory. It was the first attempt anywhere to make a deep, full-scale HDR reservoir, and efforts there

spanned 1974 through 1992, in two phases. Ultimately, the project was unable to generate net energy, and the project was terminated.

Working at the Edges—Using EGS Technology to Improve Hydrothermal Resources

EGS funding languished for the next few years, and by the next decade, U.S. efforts focused on the less ambitious goal of improving the productivity of existing hydrothermal resources. According to the Fiscal Year 2004 Budget Request to Congress from DOE's Office of Energy Efficiency and Renewable Energy,

EGS are engineered reservoirs that have been created to extract heat from economically unproductive geothermal resources. EGS technology includes those methods and equipment that enhance the removal of energy from a resource by increasing the productivity of the reservoir. Better productivity may result from improving the reservoir’s natural permeability and/or providing additional fluids to transport heat.

In Fiscal Year 2002, this vision translated into completing "preliminary designs for five competitively selected projects employing EGS technology," and the selection of one project for "full-scale development" at the Coso Hot Springs geothermal field at the U.S. Naval Weapons Air Station in China Lake, Calif., and two additional projects for "preliminary analysis from a new solicitation" at Desert Peak in Nevada and Glass Mountain in California. Funding for this effort totaled $1.5 million.

In Fiscal Year 2003, $3.5 million was appropriated to launch the Coso project, with the aim of improving the permeability of an existing poorly performing well, and to complete the conceptual design and feasibility studies at the Desert Peak and Glass Mountain sites.

The Fiscal Year 2004 request for $6 million was to "[s]tep up work on EGS cost-shared projects' at the three sites, to include "drilling and reservoir stimulation experiments" at one and drilling a production well at another.

The U.S. Department of Energy USDOE issued two Funding Opportunity Announcements (FOAs) on March 4, 2009 for enhanced geothermal systems (EGS). Together, the two FOAs offer up to $84 million over six years, including $20 million in fiscal year 2009 funding, although future funding is subject to congressional appropriations.

The DOE followed up with another FOA on March 27, 2009, of stimulus funding from the American Reinvestment and Recovery Act for $350 million, including $80 million aimed specifically at EGS proejcts,

Induced seismicity

Some induced seismicity is inevitable and, indeed, expected in EGS, which involves pumping fluids at pressure to enhance or create permeability through the use of hydraulic

fracturing techniques. Depending on the rock properties, and on injection pressures and fluid volume, the reservoir rock may respond with tensile failure, as is common in the oil and gas industry, or with shear failure of the rock's existing joint set, as is thought to be the main mechanism of reservoir growth in EGS efforts. Seismicity events at the Geysers geothermal field in California have been strongly correlated with injection data.

In several cases, significant events have occurred including a magnitude 7.2 event at Baja California in Mexico. The case of induced seismicity in Basel bears special mention because it led the city (which is a partner) to suspend the project and conduct a seismic hazard evaluation, which resulted in the cancellation of the project in December 2009.

Induced seismicity refers to typically minor earthquakes and tremors that are caused by human activity that alters the stresses and strains on the Earth's crust. Most induced seismicity is of an extremely low magnitude. A few sites regularly have larger quakes, such as The Geysers geothermal plant, which in the past 5 years has averaged 2 M4 events and 15 M3 events every year.

Causes

There are a number of ways in which induced seismicity has been seen to occur.

Reservoirs

The mass of water in a reservoir alters the pressure in the rock below, which can trigger earthquakes. Reservoir-induced seismic events can be relatively large compared to other forms of induced seismicity. The first case of reservoir induced seismicity occurred in 1932 in Algeria’s Quedd Fodda Dam. Unfortunately, understanding of reservoir induced seismic activity is very limited. However, it has been noted that seismicity appears to occur on dams with heights larger than 100 meters. The extra water pressure created by vast reservoirs is the most accepted explanation for the seismic activity. Induced seismicity is usually overlooked due to cost cutting during the geological surveys of the locations for proposed dams. Once the reservoirs are filled, induced seismicity could occur immediately or with a small time lag. The 6.3 magnitude 1967 Koynanagar Earthquake occurred in Maharashtra, India with its epicenter, fore and aftershocks all located near or under the Koyna Dam reservoir. 180 people died and 1,500 were left injured. The effects of the earthquake were felt 230 km away in Bombay with tremors and power outages. During the beginnings of the Vajont Dam in Italy, there were seismic shocks recorded during its initial fill. With scares of landslide emerging, the dam was drained and consequently seismic activity was almost non-existent. On August 1, 1975, a magnitude 6.1 earthquake at Oroville, California, was attributed to seismicity from a massive earth-fill dam and reservoir recently constructed and filled there.

The filling of the Katse Dam in Lesotho, and the Nurek Dam in Tajikistan is an example. In Zambia, Kariba Lake may have provoked similar effects.

Some experts worry that the Three Gorges Dam in China may cause an increase in the frequency and intensity of earthquakes.

Mining

Mining leaves voids that generally alter the balance of forces in the rock. These voids may collapse producing seismic waves and in some cases reactivate existing faults causing minor earthquakes. Natural cavern collapse forming sinkholes would produce an essentially identical local seismic event.

Extraction of fossil fuel and groundwater

Subsidence caused by fossil fuel (oil and/or natural gas) and groundwater extraction can generate seismic waves and minor earthquakes.

Geothermal energy

Enhanced geothermal systems (EGS), a new type of geothermal power technologies that do not require natural convective hydrothermal resources, are known to be associated with induced seismicity. EGS involves pumping fluids at pressure to enhance or create permeability through the use of hydraulic fracturing techniques. Hot dry rock (HDR) EGS actively creates geothermal resources through hydraulic stimulation. Depending on the rock properties, and on injection pressures and fluid volume, the reservoir rock may respond with tensile failure, as is common in the oil and gas industry, or with shear failure of the rock's existing joint set, as is thought to be the main mechanism of reservoir growth in EGS efforts.

HDR and EGS systems are currently being developed and tested in Soultz-sous-Forêts (France), Desert Peak and the Geysers (U.S.), Paralana and Landau (Germany), and Cooper Basin (Australia). Induced seismicity events at the Geysers geothermal field in California has been strongly correlated with injection data. The test site at Basel, Switzerland, has been shut down due to induced seismic events.

Largest Events at EGS Sites Worldwide Site Maximum Magnitude

Cerro Prieto, Baja California, Mexico 7.2 The Geysers, United States 4.6

Cooper Basin, Australia 3.7 Basel, Switzerland 3.4

Rosemanowes, United Kingdom 3.1 Soultz-sous-Forêts, France 2.9

Researchers at MIT believe that seismicity associated with hydraulic stimulation can be mitigated and controlled through predictive siting and other techniques. With appropriate

management, the number and magnitude of induced seismic events can be decreased, significantly reducing the probability of a damaging seismic event.

Induced seismicity in Basel led to suspension of its HDR project. A seismic hazard evaluation was then conducted, which resulted in the cancellation of the project in December 2009.

CO2 EGS

The recently established Center for Geothermal Energy Excellence at the University of Queensland, has been awarded $18.3 million (AUS) for EGS research, a large portion of which will be used to develop CO2 EGS technologies.

Research conducted at Los Alamos National Laboratories and Lawrence Berkeley National Laboratories examined the use of supercritical CO2, instead of water, as the geothermal working fluid with favorable results. CO2 has numerous advantages for EGS:

1. Greater power output 2. Minimized parasitic losses from pumping and cooling 3. Carbon sequestration 4. Minimized water use

EGS potential in the United States

Geothermal power technologies.

A 2006 report by MIT, and funded by the U.S. Department of Energy, conducted the most comprehensive analysis to date on the potential and technical status of EGS. The 18-member panel, chaired by Professor Jefferson Tester of MIT, reached several significant conclusions:

1. Resource Size: The report calculated the United States total EGS resources from 3–10 km of depth to be over 13,000 zettajoules, of which over 200 ZJ would be extractable, with the potential to increase this to over 2,000 ZJ with technology improvements — sufficient to provide all the world's current energy needs for several millennia. The report found that total geothermal resources, including hydrothermal and geo-pressured resources, to equal 14,000 ZJ — or roughly 140,000 times the total U.S. annual primary energy use in 2005.

2. Development Potential: With a modest R&D investment of $1 billion over 15 years (or the cost of one coal power plant), the report estimated that 100 GWe (gigawatts of electricity) or more could be installed by 2050 in the United States. The report further found that the "recoverable" resource (that accessible with today's technology) to be between 1.2–12.2 TW for the conservative and moderate recovery scenarios respectively.

3. Cost: The report found that EGS could be capable of producing electricity for as low as 3.9 cents/kWh. EGS costs were found to be sensitive to four main factors: 1) Temperature of the resource, 2) Fluid flow through the system measured in liters/second, 3) Drilling Costs, and 4) Power conversion efficiency.

Chapter-4

Ground-coupled Heat Exchanger

A ground-coupled heat exchanger is an underground heat exchanger loop that can capture or dissipate heat to or from the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses. If building air is blown through the heat exchanger for heat recovery ventilation, they are called earth tubes (also known as earth cooling tubes or earth warming tubes) in Europe or earth-air heat exchangers (EAHE or EAHX) in North America. These systems are known by several other names, including: air-to-soil heat exchanger, earth channels, earth canals, earth-air tunnel systems, ground tube heat exchanger, hypocausts, subsoil heat exchangers, underground air pipes, and others.

Earth tubes are often a viable and economical alternative or supplement to conventional central heating or air conditioning systems since there are no compressors, chemicals or burners and only blowers are required to move the air. These are used for either partial or full cooling and/or heating of facility ventilation air. Their use can help buildings meet the German Passive House standards or the North American LEED's (Leadership in Energy and Environmental Design) Green Building rating system.

Earth-air heat exchangers have been used in agricultural facilities (animal buildings) and horticultural facilities (greenhouses) in the United States over the past several decades and have been used in conjunction with solar chimneys in hot arid areas for thousands of years, probably beginning in the Persian Empire. Implementation of these systems in Austria, Denmark, Germany, and India has become fairly common since the mid-1990s, and is slowly being adopted into North America.

Ground-coupled heat exchanger may also use water or antifreeze as a heat transfer fluid, often in conjunction with a geothermal heat pump. See, for example downhole heat exchangers.

Design

Heat recovery ventilation, often including an earth-to-air heat exchanger, is essential to achieve the German passivhaus standard

Earth pipe before being covered with ground

Earth-air heat exchangers can be analyzed for performance with several software applications using weather gage data. These software applications include GAEA, AWADUKT Thermo, EnergyPlus, L-EWTSim, WKM, and others. However, numerous earth-air heat exchanger systems have been designed and constructed improperly, and failed to meet design expectations. Earth-air heat exchangers appear best suited for air pretreatment rather than for full heating or cooling. Pretreatment of air for an air-source heat pump or ground-source heat pump often provides the best economic return on investment, with simple payback often achieved within one year after installation.

Most systems are usually constructed from 100 to 600 mm (4 to 24 inch) diameter, smooth-walled (so they do not easily trap condensation moisture and mold), rigid or

semi-rigid plastic, plastic-coated metal pipes or plastic pipes coated with inner antimicrobial layers, buried 1.5 to 3 m (5 to 10 ft) underground where the ambient earth temperature is typically 10 to 23 °C (50-73 °F ) all year round in the temperate latitudes where most humans live. Ground temperature becomes more stable with depth, and between about 3 m and 12 m (10 ft and 40 ft) the soil is steadily at - or close to - the median annual air temperature.

Smaller diameter tubes require more energy to move the air and have less earth contact surface area. Larger tubes permit a slower airflow, which also yields more efficient energy transfer and permits much higher volumes to be transferred, permitting more air exchanges in a shorter time period, when, for example, you want to clear the building of objectionable odors or smoke. It is more efficient to pull air through a long tube than to push it with a fan. A solar chimney can use natural convection (warm air rising) to create a vacuum to draw filtered passive cooling tube air through the largest diameter cooling tubes. Natural convection may be slower than using a solar-powered fan. Sharp 90-degree angles should be avoided in the construction of the tube - two 45-degree bends produce less-turbulent, more efficient air flow. While smooth-wall tubes are more efficient in moving the air, they are less efficient in transferring energy.

There are three configurations, a closed loop design, an open 'fresh air' system or a combination:

• Closed loop system: Air from inside the home or structure is blown through a U-shaped loop(s) of typically 30 to 150 m (100 to 500 ft) of tube(s) where it is moderated to near earth temperature before returning to be distributed via ductwork throughout the home or structure. The closed loop system can be more effective (during air temperature extremes) than an open system, since it cools and recools the same air.

• Open system: outside air is drawn from a filtered air intake (Minimum Efficiency Reporting Value MERV 8+ air filter is recommended). The cooling tubes are typically 30 m (100 ft) long (or more) of straight tube into the home. An open system combined with energy recovery ventilation can be nearly as efficient (80-95%) as a closed loop, and ensures that entering fresh air is filtered and tempered.

• Combination system: This can be constructed with dampers that allow either closed or open operation, depending on fresh air ventilation requirements. Such a design, even in closed loop mode, could draw a quantity of fresh air when an air pressure drop is created by a solar chimney, clothes dryer, fireplace, kitchen or bathroom exhaust vents. It is better to draw in filtered passive cooling tube air than unconditioned outside air.

Single-pass earth air heat exchangers offer the potential for indoor air quality improvement over conventional systems by providing an increased supply of outdoor air. In some configurations of single-pass systems, a continuous supply of outdoor air is

provided. This type of system would usually include one or more ventilation heat recovery units.

Safety

If humidity and associated mold colonization is not addressed in system design, occupants may face health risks. At some sites, the humidity in the earth tubes may be controlled simply by passive drainage if the water table is sufficiently deep and the soil has relatively high permeability. In situations where passive drainage is not feasible or needs to be augmented for further moisture reduction, active (dehumidifier) or passive (desiccant) systems may treat the air stream.

Formal research indicates that earth-air heat exchangers reduce building ventilation air pollution. Rabindra (2004) states, “The tunnel [earth-Air heat exchanger] is found not to support the growth of bacteria and fungi; rather it is found to reduce the quantity of bacteria and fungi thus making the air safer for humans to inhale. It is therefore clear that the use of EAT [Earth Air Tunnel] not only helps save the energy but also helps reduce the air pollution by reducing bacteria and fungi.” Likewise, Flueckiger (1999) in a study of twelve earth-air heat exchangers varying in design, pipe material, size and age, stated, “This study was performed because of concerns of potential microbial growth in the buried pipes of ground-coupled air systems. The results however demonstrate, that no harmful growth occurs and that the airborne concentrations of viable spores and bacteria, with few exceptions, even decreases after passage through the pipe-system”, and further stated, “Based on these investigations the operation of ground-coupled earth-to-air heat exchangers is acceptable as long as regular controls are undertaken and if appropriate cleaning facilities are available”.

Whether using earth tubes with or without antimicrobial material, it is extremely important that the underground cooling tubes have an excellent condensation drain and be installed at a 2-3 degree grade to ensure the constant removal of condensed water from the tubes. When implementing in a house without a basement on a flat lot, an external condensation tower can be installed at a depth lower than where the tube enters into the house and at a point close to the wall entry. The condensation tower installation requires the added use of a condensate pump in which to remove the water from the tower. For installations in houses with basements, the pipes are graded so that the condensation drain located within the house is at the lowest point. In either installation, the tube must continually slope towards either the condensation tower or the condensation drain. The inner surface of the tube, including all joints must be smooth to aid in the flow and removal of condensate. Corrugated or ribbed tubes and rough interior joints must not be used. Joints connecting the tubes together must be tight enough to prevent water or gas infiltration. In certain geographic areas, it is important that the joints prevent Radon gas infiltration. Porous materials like uncoated concrete tubes cannot be used. Ideally, Earth Tubes with antimicrobial inner layers should be used in installations to inhibit the potential growth of molds and bacteria within the tubes.

Effectiveness

Implementations of earth-air heat exchangers for either partial or full cooling and/or heating of facility ventilation air have had mixed success. The literature is, unfortunately, well populated with over-generalizations about the applicability of these systems - both supportive and unsupportive. A key aspect of earth-air heat exchangers is the passive nature of operation and consideration of the wide variability of conditions in natural systems.

Earth-air heat exchangers can be very cost effective in both up-front/capital costs as well as long-term operation and maintenance costs. However, this varies widely depending on the location latitude, altitude, ambient Earth temperature, climatic temperature-and-relative-humidity extremes, solar radiation, water table, soil type (thermal conductivity), soil moisture content and the efficiency of the building's exterior envelope design / insulation. Generally, dry-and-low-density soil with little or no ground shade will yield the least benefit, while dense damp soil with considerable shade should perform well. A slow drip watering system may improve thermal performance. Damp soil in contact with the cooling tube conducts heat more efficiently than dry soil.

Earth cooling tubes are much less effective in hot humid climates (like Florida) where the ambient temperature of the earth approaches human comfort temperature. The higher the ambient temperature of the earth, the less effective they are for cooling and dehumidification. However, they can be used to partially cool and dehumidify the replacement fresh air intake for passive-solar thermal buffer zone areas like the laundry room, or a solarium / greenhouse, especially those with a hot tub, swim spa, or indoor swimming pool, where warm humid air is exhausted in the summer, and a supply of cooler drier replacement air is desired.

Not all regions and sites are suitable for earth-air heat exchangers. Conditions which may hinder or preclude proper implementation include shallow bedrock, high water table, and insufficient space, among others. In some areas, only cooling or heating may be afforded by earth-air heat exchangers. In these areas, provision for thermal recharge of the ground must especially be considered. In dual function systems (both heating and cooling), the warm season provides ground thermal recharge for the cool season and the cool season provides ground thermal recharge for the warm season, though overtaxing the thermal reservoir must be considered even with dual function systems.

Renata Limited, a prominent pharmaceutical company in Bangladesh, tried out a pilot project trying to find out whether they could use the Earth Air Tunnel technology to complement the conventional air conditioning system. Concrete pipes (total length 60 feet, inner diameter 9 inches, outer diameter 11 inches) were placed at a depth of 9 feet underground and a blower of 1.5 kW rated power was employed. The underground temperature at that depth was found to be around 28°C. The mean velocity of air in the tunnel was about 5 m/s. The Coefficient of Performance (COP) of the underground heat exchanger thus designed was poor ranging from 1.5-3. The results convinced the authorities that in hot and humid climates, it is unwise to implement the concept of Earth-

Air heat exchanger. The cooling medium (earth itself) being at a temperature approaching that of the ambient environment happens to be the root cause of the failure of such principles in hot, humid areas (parts of Southeast Asia, Florida in the U.S.A. etc.). However, investigators from places like Britain and Turkey have reported very encouraging COPs-well above 20. The underground temperature seems to be of prime importance when planning an Earth-Air heat exchanger.


In the context of today's diminishing fossil fuel reserves, increasing electrical costs, air pollution and global warming, properly-designed earth cooling tubes offer a sustainable alternative to reduce or eliminate the need for conventional compressor-based air conditioning systems, in non-tropical climates. They also provide the added benefit of controlled, filtered, temperate fresh air intake, which is especially valuable in tight, well-weatherized, efficient building envelopes.

Water to earth

An alternative to the earth-to-air heat exchanger is the "water" to earth heat exchanger. This is typically similar to a geothermal heat pump tubing embedded horizontally in the soil (or could be a vertical sonde) to a similar depth of the earth-air heat exchanger. It uses approximately double the length of pipe of 35 mm diameter, e.g., around 80 m compared to an EAHX of 40 m. A heat exchanger coil is placed before the air inlet of the heat recovery ventilator. Typically a brine liquid (heavily salted water) is used as the heat exchanger fluid.

Many European installations are now using this setup due to the ease of installation. No fall or drainage point is required and it is safe because of the reduced risk from mold.

Heat Recovery Ventilation Heat recovery ventilation, also known as HRV, Mechanical ventilation heat recovery, or MVHR, is an energy recovery ventilation system, using equipment known as a heat recovery ventilator, Heat exchanger, air exchanger or air-to-air exchanger, that employs a counter-flow heat exchanger (countercurrent heat exchange) between the inbound and outbound air flow. HRV provides fresh air and improved climate control, while also saving energy by reducing the heating (or cooling) requirements.

Energy recovery ventilators (ERVs) are closely related, however ERVs also transfer the humidity level of the exhaust air to the intake air.

Benefits

As building efficiency is improved with insulation and weatherstripping, buildings are intentionally made more air-tight, and consequently less well ventilated. Since all

buildings require a source of fresh air, the need for HRVs has become obvious. While opening a window does provide ventilation, the building's heat and humidity will then be lost in the winter and gained in the summer, both of which are undesirable for the indoor climate and for energy efficiency, since the building's HVAC systems must compensate. HRV technology offers an optimal solution: fresh air, better climate control, and Energy efficiency - Sustainability.

Technology

Heat recovery ventilation-HRVs and ERVs can be stand-alone devices that operate independently, or they can be built-in, or added to existing HVAC systems. For a small building in which nearly every room has an exterior wall, then the HRV/ERV device can be small and provide ventilation for a single room. A larger building would require either many small units, or a large central unit. The only requirements for the building are an air supply, either directly from an exterior wall or ducted to one, and an energy supply for air circulation, such as wind energy or electricity for a fan. When used with 'central' HVAC systems, then the system would be of the 'forced-air' type.

Air to air heat exchanger

A Heat exchanger.

There are a number of heat exchangers used in Heat recovery ventilation-HRV devices, as diagrammed to the right :

• cross flow heat exchanger up to 60% efficient (passive)

• countercurrent heat exchanger up to 99% efficient (passive) • rotary heat exchanger (requires motor to turn wheel) • heat pipes • thin multiple heat wires (Fine wire heat exchanger)

Incoming air

The air coming into the heat exchanger should be at least 0°C. Otherwise humidity in the outgoing air may condense, freeze and block the heat exchanger.

A high enough incoming air temperature can also be achieved by

• recirculating some of the exhaust air (causing loss of air quality) when required, • by using a very small (1 kW) heat pump to warm the inlet air above freezing

before it enters the HRV. (The 'cold' side of this heatpump is situated in the warm air outlet.)

• using a heating "battery" supplied by heat from a heat source eg hot water circuit from a wood fired boiler, etc.

Earth-to-air heat exchanger

Heat recovery ventilation, often with an earth-to-air heat exchanger, is essential to achieve German Passivhaus standards.

This can be done by an earth warming pipe ("ground-coupled heat exchanger"), usually about 30 m to 40 m long and 20 cm in diameter, typically buried about 1.5 m below ground level. In Germany and Austria this is a common configuration for earth to air heat exchangers.

In high humidity areas where internal condensation could lead to fungal / mould growth in the tube leading to contamination of the air, several measures exist to prevent this.

• Ensuring the tube drains of water. • Regular cleaning • Tubes with an imbedded bactericide coating such as silver ions (non-toxic for

humans) • Air filters F7 / EU7 (>0,4 micrometres) to traps mould (of a size between 2 & 20

micrometres). • UV air purification • Use a earth to "water" heat exchanger.

The pipes may be either corrugated/slotted to enhance heat transfer and provide condensate drainage or smooth/solid to prevent gas/liquid transfer.

Air quality

This is highly site dependent.

Radon

One critical problem being located in soils with underlying rock strata which emit radon. In these situations the tube needs to be airtight from the surrounding soils, or an Air to Water heat exchanger be used.

Bacteria and fungi

Formal research indicates that Earth-Air Heat Exchangers reduce building ventilation air pollution. Rabindra (2004) states, “The Earth-Air Tunnel is found not to support the growth of bacteria and fungi; rather it is found to reduce the quantity of bacteria and fungi thus making the air safer for humans to inhale. It is therefore clear that the use of EAT (Earth-Air Tunnel) not only helps save the energy but also helps reduce the air pollution by reducing bacteria and fungi.”

Likewise, Flueckiger (1999) in a study of twelve Earth-Air Heat Exchangers varying in design, pipe material, size and age, stated, “This study was performed because of concerns of potential microbial growth in the buried pipes of 'ground-coupled' air

systems. The results however demonstrate, that no harmful growth occurs and that the airborne concentrations of viable spores and bacteria, with few exceptions, even decreases after passage through the pipe-system”, and further stated, “Based on these investigations the operation of ground-coupled earth-to-air heat exchangers is acceptable as long as regular controls are undertaken and if appropriate cleaning facilities are available”.

Earth-to-Water heat exchanger

An alternative to the earth to air heat exchanger is the earth to "water" heat exchanger. This is typically similar to a geothermal heat pump tubing embedded horizontally in the soil (or could be a vertical pipe/sonde) to a similar depth of the EAHX. It uses approximately double the length of pipe Ø 35 mm ie around 80 metres compared to an EAHX. A heat exchanger coil is placed before the air inlet of the HRV. Typically a brine liquid (heavily salted water) is used as the heat exchange fluid which is slightly more efficient and environmentally friendly than polypropylene heat transfer liquids.

In temperate climates in an energy efficient building, such as a passivhaus, this is more than sufficient for comfort cooling during summer without resorting to an airconditioning system. In more extreme hot climates a very small air to air micro-heat pump in reverse (an air conditioner) with the evaporator (giving heat) on the air inlet after the HRV heat exchanger and the condensor (taking heat) from the air outlet after the heat exchanger will suffice.

Seasonal bypassing

At certain times of the year it is more thermally efficient to bypass the Heat recovery ventilation-HRV heat exchanger or the earth to air heat exchanger (EAHX).

For example, during the winter, the earth at the depth of the earth to air heat exchanger is ordinarily much warmer than the air temperature. The air becomes warmed by the earth before reaching the air heat exchanger.

In the summer, the opposite is true. The air becomes cooled in the earth to air exchanger. But after passing through the EAHX, the air is warmed by the heat recovery ventilator using the warmth of the outgoing air. In this case, the HRV can have an internal bypass such that the inflowing air bypasses the heat exchanger maximising the cooling potential of the earth.

In autumn and spring there may be no thermal benefit from the EAHX—it may heat/cool the air too much and it will be better to use external air directly. In this case it is helpful to have a bypass such that the EAHX is disconnected and air taken directly from outside. A differential temperature sensor with a motorized valve can control the bypass function.

Chapter-5

Geothermal Heating

Geothermal heating is the direct use of geothermal power for heating applications. Humans have taken advantage of geothermal heat this way since the Paleolithic era. Approximately seventy countries made direct use of a total of 270 PJ of geothermal heating in 2004. As of 2007, 28 GW of geothermal heating capacity is installed around the world, satisfying 0.07% of global primary energy consumption. Thermal efficiency is high since no energy conversion is needed, but capacity factors tend to be low (around 20%) since the heat is mostly needed in the winter.

Geothermal energy originates from the heat retained within the Earth's core since the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface. Most high temperature geothermal heat is harvested in regions close to tectonic plate boundaries where volcanic activity rises close to the surface of the Earth. In these areas, ground and groundwater can be found with temperatures higher than the target temperature of the application. However, even cold ground contains heat, below 10' or 3 Meters, the ground is consistently 12.8°C (55°F), and it may be extracted with a geothermal heat pump. Due to recent advances in heat pump performance, this is now a rapidly growing market in the US.

Briefly and Simply Explained

Geothermal energy is not from a reasonably constant earth core temperature of 10,000º to 13.3°C (54º to 56°F) . Running a thermal loop to this constant core temperature by drilling wells vertically or horizontally allows for this heat transfer to a medium in pipes. These pipes contain fluid from which either cooling or heating is extracted. Cooling is easier to extract as general refrigerant in a typical home conventional cooling system is at 12.2°C (54°F). All that is required is to pass warm air over cooled pipes in a heat exchange. Heating can be more complicated depending upon the ambient average outside temperature in the region. While geothermal can cope with average ambient temperatures around the same temperatures as a conventional heat pump, it will need to be able to provide a supplemental source if temperatures dip below freezing.

Applications Top countries using the most geothermal heating in 2005

Country Production PJ/yr

CapacityGW

CapacityFactor

Dominant applications

China 45.38 3.69 39% bathing Sweden 43.2 4.2 33% heat pumps USA 31.24 7.82 13% heat pumps Turkey 24.84 1.5 53% district heating Iceland 24.5 1.84 42% district heating Japan 10.3 0.82 40% bathing (onsens) Hungary 7.94 0.69 36% spas/greenhouses Italy 7.55 0.61 39% spas/space heatingNew Zealand 7.09 0.31 73% industrial uses 63 others 71 6.8

Total 273 28 31% space heating

There are a wide variety of applications for cheap geothermal heat. In 2004 more than half of direct geothermal heat was used for space heating, and a third was used for spas. The remainder was used for a variety of industrial processes, desalination, domestic hot water, and agricultural applications. The cities of Reykjavík and Akureyri pipe hot water from geothermal plants under roads and pavements to melt snow. Geothermal desalination has been demonstrated.

Geothermal systems tend to benefit from economies of scale, so space heating power is often distributed to multiple buildings, sometimes whole communities. This technique, long practiced throughout the world in locations such as Reykjavik, Iceland, Boise, Idaho, and Klamath Falls, Oregon is known as district heating.

Extraction

Some parts of the world, including substantial portions of the western USA, are underlain by relatively shallow geothermal resources. Similar conditions exist in Iceland, parts of Japan, and other geothermal hot spots around the world. In these areas, water or steam may be captured from natural hot springs and piped directly into radiators or heat exchangers. Alternatively, the heat may come from waste heat supplied by co-generation from a geothermal electrical plant or from deep wells into hot aquifers. Direct geothermal heating is far more efficient than geothermal electricity generation and has less demanding temperature requirements, so it is viable over a large geographical range. If the shallow ground is hot but dry, air or water may be circulated through earth tubes or downhole heat exchangers which act as heat exchangers with the ground.

In areas where the shallow ground is too cold to provide comfort directly, it is still warmer than the winter air. The thermal inertia of the shallow ground retains solar energy accumulated in the summertime, and seasonal variations in ground temperature disappear completely below 10m of depth. That heat can be extracted with a geothermal heat pump more efficiently than it can be generated by conventional furnaces. Geothermal heat pumps are economically viable essentially anywhere in the world. One geothermal district heating system at Drake Landing enhances storage of solar energy in the ground to such an extent that no heat pumps are needed.

Geothermal heat pumps

Even in regions without large high temperature geothermal resources, a geothermal heat pump can still provide space heating and air conditioning. Like a refrigerator or air conditioner, these systems use a heat pump to force the transfer of heat from the ground to the application. In theory, heat can be extracted from any source, no matter how cold, but a warmer source allows higher efficiency. A ground-source heat pump uses the shallow ground or ground water (typically starting at 10–12 °C, 50–54 °F) as a source of heat, thus taking advantage of its seasonally moderate temperatures. In contrast, an air-source heat pump draws heat from the colder outside air and thus requires more energy.

Closed loop geothermal heat pumps circulate a carrier fluid (usually a water/antifreeze mix) through pipes buried in the ground. As the fluid circulates underground it absorbs heat from the ground and, on its return, the now warmer fluid passes through the heat pump which uses electricity to extract the heat from the fluid. The re-chilled fluid is sent back through the ground thus continuing the cycle. The heat extracted and that generated by the heat pump appliance as a byproduct is used to heat the house. The addition of the ground heating loop in the energy equation means that more heat is generated than if electricity alone had been used directly for heating. Switching the direction of heat flow, the same system can be used to circulate the cooled water through the house for cooling in the summer months. The heat is exhausted to the same relatively cool soil (or groundwater) rather than delivering it to the hot outside air as an air conditioner does. As a result, the heat is pumped across a smaller temperature difference and this leads to higher efficiency and lower energy use.

This technology makes geothermal heating economically viable in any geographical location. In 2004, an estimated million geothermal heat pumps with a total capacity of 15 GW extracted 88 PJ of geothermal energy for space heating. Global geothermal heat pump capacity is growing by 10% annually.

History

The oldest known pool fed by a hot spring, built in the Qin dynasty in the 3rd century BC.

Hot springs have been used for bathing at least since Paleolithic times. The oldest known spa is a stone pool on China's Lisan mountain built in the Qin dynasty in the 3rd century BC, at the same site where the Huaqing Chi palace was later built. In the first century AD, Romans conquered Aquae Sulis and used the hot springs there to feed public baths and underfloor heating. The admission fees for these baths probably represents the first commercial use of geothermal power. The world's oldest geothermal district heating system in Chaudes-Aigues, France, has been operating since the 14th century. The earliest industrial exploitation began in 1827 with the use of geyser steam to extract boric acid from volcanic mud in Larderello, Italy.

In 1892, America's first district heating system in Boise, Idaho was powered directly by geothermal energy, and was soon copied in Klamath Falls, Oregon in 1900. A deep geothermal well was used to heat greenhouses in Boise in 1926, and geysers were used to heat greenhouses in Iceland and Tuscany at about the same time. Charlie Lieb developed the first downhole heat exchanger in 1930 to heat his house. Steam and hot water from the geysers began to be used to heat homes in Iceland in 1943.

By this time, Lord Kelvin had already invented the heat pump in 1852, and Heinrich Zoelly had patented the idea of using it to draw heat from the ground in 1912. But it was not until the late 1940s that the geothermal heat pump was successfully implemented. The earliest one was probably Robert C. Webber's home-made 2.2 kW direct-exchange system, but sources disagree as to the exact timeline of his invention. J. Donald Kroeker designed the first commercial geothermal heat pump to heat the Commonwealth Building (Portland, Oregon) and demonstrated it in 1946. Professor Carl Nielsen of Ohio State University built the first residential open loop version in his home in 1948. The technology became popular in Sweden as a result of the 1973 oil crisis, and has been growing slowly in worldwide acceptance since then. The 1979 development of polybutylene pipe greatly augmented the heat pump’s economic viability. As of 2004, there are over a million geothermal heat pumps installed worldwide providing 12 GW of thermal capacity. Each year, about 80,000 units are installed in the USA and 27,000 in Sweden.

Economics

Geothermal drill machine.

Geothermal energy is a type of renewable energy that encourages conservation of natural resources. According to the U.S. Environmental Protection Agency, geo-exchange systems save homeowners 30-70 percent in heating costs, and 20-50 percent in cooling costs, compared to conventional systems. Geo-exchange systems also save money

because they require much less maintenance. In addition to being highly reliable they are built to last for decades.

Some utilities, such as Kansas City Power and Light, offer special, lower winter rates for geothermal customers, offering even more savings.

Subsidence

In geothermal heating projects the underground is penetrated by trenches or drillholes. Large projects may cause problems if the geology of the area is poorly understood as with all underground work. In connection with a geothermal heating project for the historical city hall of Staufen im Breisgau, Germany, subsidence of the ground up to eight millimeters has occurred while other areas have been uplifted by a few millimeters. A relation to the geothermal wells is suspected. The subsidence has caused considerable damage to buildings in the city center.

Chapter-6

Geothermal Heat Pump

Ground source heating and cooling

Ground source heating and cooling

A geothermal heat pump or ground source heat pump (GSHP) is a central heating and/or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). This design takes advantage of the moderate temperatures in the ground to boost efficiency and reduce the operational costs of heating and cooling systems, and may be combined with solar heating to form a geosolar system with even greater efficiency. Geothermal heat pumps are also known by a variety of other names, including geoexchange, earth-coupled, earth energy or water-source heat pumps. The engineering and scientific communities prefer the terms "geoexchange" or "ground source heat pumps" to avoid confusion with traditional geothermal power, which uses a high temperature heat source to generate electricity.

Ground source heat pumps harvest a combination of geothermal power and heat from the sun when heating, but work against these heat sources when used for air conditioning.

Depending on latitude, the upper 3 metres (9.8 ft) of Earth's surface maintains a nearly constant temperature between 10 and 16 °C (50 and 60 °F). Like a refrigerator or air conditioner, these systems use a heat pump to force the transfer of heat from there. Heat pumps can transfer heat from a cool space to a warm space, against the natural direction of flow, or they can enhance the natural flow of heat from a warm area to a cool one. The core of the heat pump is a loop of refrigerant pumped through a vapor-compression refrigeration cycle that moves heat. Heat pumps are always more efficient at heating than pure electric heaters, even when extracting heat from cold winter air.. But unlike an air-source heat pump, which transfers heat to or from the outside air, a ground source heat pump exchanges heat with the ground. This is much more energy-efficient because underground temperatures are more stable than air temperatures through the year. Seasonal variations drop off with depth and disappear below seven meters due to thermal inertia. Like a cave, the shallow ground temperature is warmer than the air above during the winter and cooler than the air in the summer. A ground source heat pump extracts ground heat in the winter (for heating) and transfers heat back into the ground in the summer (for cooling). Some systems are designed to operate in one mode only, heating or cooling, depending on climate.

The geothermal pump systems reach fairly high Coefficient of performance (CoP), 3-6, on the coldest of winter nights, compared to 1.75-2.5 for air-source heat pumps on cool days. Ground source heat pumps (GSHPs) are among the most energy efficient technologies for providing HVAC and water heating. Actual CoP of a geothermal system which includes the power required to circulate the fluid through the underground tubes can be lower than 2.5. The setup costs are higher than for conventional systems, but the difference is usually returned in energy savings in 3 to 10 years. System life is estimated at 25 years for inside components and 50+ years for the ground loop. As of 2004, there are over a million units installed worldwide providing 12 GW of thermal capacity, with an annual growth rate of 10%.

Differing terms and definitions

There is a great deal of controversy and confusion with regard to exactly what geothermal heat pumps do. There are several concepts commonly attached to the idea of geothermal:

• Utilizing geologically hot rocks, which have little relationship to the surface climate and derive their heat from deep in the earth, to run a heat engine which produces electricity. Such a system can be operated only until the rock around the bore cools, then it gradually loses its generating ability. All of these systems are in tectonically or volcanically active areas. Most people are pretty clear that this should be called "geothermal power".

• Utilizing geologically hot rocks to heat some type of liquid or gas which is pumped up to be used to heat a building is often called "geothermal heating".

• Utilizing a heat exchanger with a finite amount of external material to incorporate additional thermal mass to a building. This makes the building change temperature slowly, and allows the inhabitants to go through a time period with less overall temperature variation.

Builders may try to smooth out the indoor climate over surface temperature variations resulting from the day-night cycle, variations due to short-term weather patterns, or variations due to entire seasons. The amount of thermal mass incorporated is on a spectrum, so one cannot say their system addresses any of these cycles specifically – a system sized for day-night cycling will still help somewhat in a week-long blizzard. Such a system requires power to pump the coolant, but can be operated indefinitely.

To further complicate things, even though most home-sized systems termed "geothermal" operate primarily on the former principle, the thermal mass in such systems is rarely perfectly finite and closed. Groundwater flows through the area, and heat leaks out and warms/cools the surrounding area. True geothermal heat may play a small or large role in such systems.

When trying to explain this subject, experts may go through a series of explanations and divisions.

First, people separate out terms for geothermal electricity generation:

• geothermal power

Then, they split out geothermal heating, which is commonly used in tectonically or volcanically active regions:

• geothermal heating

Then, they explain the traditional concept of a heat pump which uses only inside and outside air:

• heat pump

After that, they try to identify simple systems in which the coolant is air which is pumped directly out of and back into the building, going through a simple hole in the ground:

• earth tube or earth air heat exchanger • ground-coupled heat exchanger

After that, they remove systems which depend on large quantities of water or wet ground, primarily for cooling:

• lake water cooling • deep water source cooling

At this point they may explain the concept of a seasonal thermal store or a thermal mass climate control strategy:

• trombe wall • seasonal thermal store • thermal mass

Then, they may try to figure out the size of the system. Is it targeted at a home? A building? Is it a full-scale district heating system?

Then they go into the specifics of the system. First, is the coolant water, and if so is it "open loop" – exposed to groundwater – or "closed loop" – not exposed.

Are other energy sources helping? Is solar absorbed from the house or from a dedicated thermal collector?

• annualized geothermal solar or annualized geo solar • geosolar or solar combisystem

After this they concentrate on the specific form factor of the system. Is it a grid of pipes buried 3 feet (0.91 m) underneath the owner's garden? Does it consist of a hundred-foot borehole? A thousand-foot borehole? Dozens of 8-foot (2.4 m) boreholes?

• downhole heat exchanger or borehole heat exchanger

Finally they may try to decide what the locals call the system, as identical systems are often called different things in different countries, and in some countries generic terms may be trademarked in others:

• Geoexchange is a trademarked product in the US, but is a standards coalition in Canada.

• Earth tubes, Air-earth heat exchangers and "heat exchanger" in general, appear to be primarily used in the UK.

History

The heat pump was described by Lord Kelvin in 1853 and developed by Peter Ritter von Rittinger in 1855. After experimenting with a freezer, Robert C. Webber built the first direct exchange ground-source heat pump in the late 1940s. The first successful commercial project was installed in the Commonwealth Building (Portland, Oregon) in 1946, and has been designated a National Historic Mechanical Engineering Landmark by ASME. The technology became popular in Sweden in the 1970s, and has been growing slowly in worldwide acceptance since then. Open loop systems dominated the market until the development of polybutylene pipe in 1979 made closed loop systems economically viable. As of 2004, there are over a million units installed worldwide providing 12 GW of thermal capacity. Each year, about 80,000 units are installed in the

USA (geothermal energy is used in all 50 U.S. states today, with great potential for near-term market growth and savings) and 27,000 in Sweden.

Ground heat exchanger

Loop field for a 12-ton system (unusually large for most residential applications)

Heat pumps provide wintertime heating by extracting heat from a source and transferring it to the building. In theory, heat can be extracted from any source, no matter how cold, but a warmer source allows higher efficiency. A ground source heat pump uses the shallow ground as a source of heat, thus taking advantage of its seasonally moderate temperatures.

In the summer, the process can be reversed so the heat pump extracts heat from the building and transfers it to the ground. Transferring heat to a cooler space takes less energy, so the cooling efficiency of the heat pump gain benefits from the lower ground temperatures.

Shallow horizontal heat exchangers experience seasonal temperature cycles due to solar gains and transmission losses to ambient air at ground level. These temperature cycles lag behind the seasons because of thermal inertia, so the heat exchanger can harvest heat deposited by the sun several months earlier. Deep vertical systems rely heavily on

migration of heat from surrounding geology, unless they are recharged annually by exhaust heat from air conditioning.

Ground source heat pumps must have a heat exchanger in contact with the ground or groundwater to extract or dissipate heat. This component accounts for a third to a half of the total system cost. Several major design options are available for these, which are classified by fluid and layout. Direct exchange systems circulate refrigerant underground, closed loop systems use a mixture of anti-freeze and water, and open loop systems use natural groundwater.

Direct exchange

The Direct exchange geothermal heat pump is the oldest type of geothermal heat pump technology. It is also the simplest and easiest to understand. The ground-coupling is achieved through a single loop circulating refrigerant in direct thermal contact with the ground (as opposed to a combination of a refrigerant loop and a water loop). The refrigerant leaves the heat pump appliance cabinet, circulates through a loop of copper tube buried underground, and exchanges heat with the ground before returning to the pump. The name "direct exchange" refers to heat transfer between the refrigerant and the ground without the use of an intermediate fluid. There is no direct interaction between the fluid and the earth; only heat transfer through the pipe wall. Direct exchange heat pumps are not to be confused with "water-source heat pumps" or "water loop heat pumps" since there is no water in the ground loop. ASHRAE defines the term ground-coupled heat pump to encompass closed loop and direct exchange systems, while excluding open loops.

Direct exchange systems are significantly more efficient and have potentially lower installation costs than closed loop water systems. Copper's high thermal conductivity contributes to the higher efficiency of the system, but heat flow is predominantly limited by the thermal conductivity of the ground, not the pipe. The main reasons for the higher efficiency are the elimination of the water pump (which uses electricity), the elimination of the water heat exchanger (which is a source of heat losses), and most importantly, the latent heat phase change of the refrigerant in the ground itself.

While they require much more refrigerant and their tubing is more expensive per foot, a direct exchange loop is shorter than a closed water loop for a given capacity. A direct exchange system requires only 15 to 30% of the length of tubing and half the diameter of drilled holes, and the drilling or excavation costs are therefore lower. Refrigerant loops are less tolerant of leaks than water loops because gas can leak out through smaller imperfections. This dictates the use of brazed copper tubing, even though the pressures are similar to water loops. The copper loop must be protected from corrosion in acidic soil through the use of a sacrificial anode or cathodic protection.

Direct exchange geothermal heat pump

A direct exchange (DX) geothermal heat pump system is a geothermal heat pump system in which the refrigerant circulates through copper tubing placed in the ground. The refrigerant exchanges heat directly with the soil through the walls of the copper tubing. This eliminates the plastic water pipe and water pump to circulate water found in a water-source geothermal heat pump. This simplicity allows the system to reach high efficiencies while using a relatively shorter and smaller set of buried tubing, reducing installation cost. DX systems, like water-source systems, can also be used to heat water in the house for use in radiant heating applications and for domestic hot water, as well as for cooling applications.

History

The first geothermal heat pump was a DX system built in the late 1940s by Robert C. Webber. Later designs incorporated an additional plastic pipe loop to circulate water in deep wells in an effort to gather sufficient heat for large industrial applications such as cement plants. Thus water-source technology advanced due to industrial interest while DX, more suited to smaller projects such as small businesses and private homes, lagged behind.

Gradually developing since the 1970s, DX technology is now experiencing a surge in popularity among homeowners and small businesses due to high energy costs. There is also increasing awareness of environmental and energy issues among urban and suburban residents with limited space in which to install a system.

Applications

Typical drill rig for DX installation, length 8 ft

Typical drill rig for water source installation, length 22 ft

Because of their small earth loop size, DX systems can be installed in relatively small areas and in relatively shallow soil. This provides a flexibility of installation that is useful in allowing many properties to be served by geothermal that could not be served otherwise. A direct exchange system ground loop can be drilled with a small drill rig that can fit into small side yards and gardens under existing trees. It can be drilled in areas where rock is found 50 foot (15 m) to 100 foot (30 m) below ground without the need for actually drilling into rock.

Because DX derives its efficiency from the direct heat exchange between refrigerant and ground, the compressor unit cannot be placed at great distance from the earth loops. This can limit some DX applications. However, the use of multiple distributed compressor units on a single project can allow DX systems to serve large buildings.

Ground loop configuration

The copper tubing consists of a line set, a pair of manifolds, and several earth loops. The line set is the pair of main copper pipes coming from the heat pump compressor unit, usually located indoors. One line is for the liquid refrigerant, the other is for gaseous refrigerant. The line set runs through the building wall and runs underground to the

location of the manifolds. Each manifold (one for gas and one for liquid) serves to allow a main pipe to be attached to the earth loops which exchange heat with the ground.

The earth loops can be installed vertically, diagonally or horzontally 6 foot (1.8 m) deep and laying the earth loops on the bottom of the pit before installation is done by drilling several boreholes radiating outward from the manifolds and placing an earth loop into each of the boreholes. After the earth taps are placed, the boreholes are then filled with grout for good thermal contact between loop and soil.

The boreholes are drilled to a length of 50, 75 or 100 ft (15, 22 or 30 m) with a diameter of 3 inches (76 mm). A total of 100 feet (30 m) to 140 feet (43 m) of drilling is needed for each ton (3.5 kWth) of system capacity.

Because copper is a naturally-occurring metal that survives in the ground for thousands of years in most soil conditions, the copper loops have a very long lifetime in most soil conditions. Corrosion of the copper earth loop in acidic soil can be eliminated through installation of a sacrificial anode.

System sizing

DX systems are currently manufactured in sizes from 1.5 tons (5.25 kWth) to 6 tons (21 kWth). Larger projects can be accomplished through installation of multiple units.

Closed loop

Most installed systems have two loops on the ground side: the primary refrigerant loop is contained in the appliance cabinet where it exchanges heat with a secondary water loop that is buried underground. The secondary loop is typically made of High-density polyethylene pipe and contains a mixture of water and anti-freeze (propylene glycol, denatured alcohol or methanol). After leaving the internal heat exchanger, the water flows through the secondary loop outside the building to exchange heat with the ground before returning. The secondary loop is placed below the frost line where the temperature is more stable, or preferably submerged in a body of water if available. Systems in wet ground or in water are generally more efficient than drier ground loops since it is less work to move heat in and out of water than solids in sand or soil. If the ground is naturally dry, soaker hoses may be buried with the ground loop to keep it wet.

An installed liquid pump pack

Closed loop systems need a heat exchanger between the refrigerant loop and the water loop, and pumps in both loops. Some manufacturers have a separate ground loop fluid pump pack, while some integrate the pumping and valving within the heat pump. Expansion tanks and pressure relief valves may be installed on the heated fluid side. Closed loop systems have lower efficiency than direct exchange systems, so they require longer and larger pipe to be placed in the ground, increasing excavation costs.

Closed loop tubing can be installed horizontally as a loop field in trenches or vertically as a series of long U-shapes in wells(see below). The size of the loop field depends on the soil type and moisture content, the average ground temperature and the heat loss and or gain characteristics of the building being conditioned. A rough approximation of the initial soil temperature is the average daily temperature for the region.

Vertical

A vertical closed loop field is composed of pipes that run vertically in the ground. A hole is bored in the ground, typically 75 to 500 feet (23–150 m) deep. Pipe pairs in the hole are joined with a U-shaped cross connector at the bottom of the hole. The borehole is commonly filled with a bentonite grout surrounding the pipe to provide a thermal connection to the surrounding soil or rock to improve the heat transfer. Thermally enhanced grouts are available to improve this heat transfer. Grout also protects the ground water from contamination, and prevents artesian wells from flooding the property. Vertical loop fields are typically used when there is a limited area of land available. Bore holes are spaced at least 5–6 m apart and the depth depends on ground and building characteristics. For illustration, a detached house needing 10 kW (3 ton) of heating capacity might need three boreholes 80 to 110 m (260 to 360 ft) deep. (A ton of heat is 12,000 British thermal units per hour (BTU/h) or 3.5 kilowatts.) During the cooling season, the local temperature rise in the bore field is influenced most by the moisture travel in the soil. Reliable heat transfer models have been developed through sample bore holes as well as other tests.

Horizontal

A 3-ton slinky loop prior to being covered with soil. The three slinky loops are running out horizontally with three straight lines returning the end of the slinky coil to the heat pump

A horizontal closed loop field is composed of pipes that run horizontally in the ground. A long horizontal trench, deeper than the frost line, is dug and U-shaped or slinky coils are placed horizontally inside the same trench. Excavation for horizontal loop fields is about half the cost of vertical drilling, so this is the most common layout used wherever there is adequate land available. For illustration, a detached house needing 10 kW (3 ton) of heating capacity might need 3 loops 120 to 180 m (390 to 590 ft) long of NPS 3/4 (DN 20) or NPS 1.25 (DN 32) polyethylene tubing at a depth of 1 to 2 m (3.3 to 6.6 ft).

As an alternative to trenching, the horizontal loop field may be laid by mini horizontal directional drilling. (mini-HDD) This technique can lay piping under yards, driveways or other structures without disturbing them, with a cost between those of trenching and vertical drilling.

A slinky (also called coiled) closed loop field is a type of horizontal closed loop where the pipes overlay each other (not a recommended method). The easiest way of picturing a slinky field is to imagine holding a slinky on the top and bottom with your hands and then move your hands in opposite directions. A slinky loop field is used if there is not adequate room for a true horizontal system, but it still allows for an easy installation. Rather than using straight pipe, slinky coils, use overlapped loops of piping laid out horizontally along the bottom of a wide trench. Depending on soil, climate and your heat pump's run fraction, slinky coil trenches can be anywhere from one third to two thirds shorter than traditional horizontal loop trenches. Slinky coil ground loops are essentially a more economic and space efficient version of a horizontal ground loop.

Pond

12-ton pond loop system being sunk to the bottom of a pond

A closed pond loop is not common because it depends on proximity to a body of water, where an open loop system is usually preferable. A pond loop may be advantageous where poor water quality precludes an open loop, or where the system heat load is small. A pond loop consists of coils of pipe similar to a slinky loop attached to a frame and located at the bottom of an appropriately sized pond or water source.

Open loop

In an open loop system (also called a groundwater heat pump), the secondary loop pumps natural water from a well or body of water into a heat exchanger inside the heat pump. ASHRAE calls open loop systems groundwater heat pumps or surface water heat pumps, depending on the source. Heat is either extracted or added by the primary refrigerant loop, and the water is returned to a separate injection well, irrigation trench, tile field or body of water. The supply and return lines must be placed far enough apart to ensure thermal recharge of the source. Since the water chemistry is not controlled, the appliance may need to be protected from corrosion by using different metals in the heat exchanger and pump. Limescale may foul the system over time and require periodic acid cleaning.

Also, as fouling decreases the flow of natural water, it becomes difficult for the heat pump to exchange building heat with the groundwater. If the water contains high levels of salt, minerals, iron bacteria or hydrogen sulfide, a closed loop system is usually preferable.

Deep lake water cooling uses a similar process with an open loop for air conditioning and cooling. Open loop systems using ground water are usually more efficient than closed systems because they are better coupled with ground temperatures. Closed loop systems, in comparison, have to transfer heat across extra layers of pipe wall and dirt.

A growing number of jurisdictions have outlawed open-loop systems that drain to the surface because these may drain aquifers or contaminate wells. This forces the use of more environmentally sound injection wells.

Standing column well

A standing column well system is a specialized type of open loop system. Water is drawn from the bottom of a deep rock well, passed through a heat pump, and returned to the top of the well, where traveling downwards it exchanges heat with the surrounding bedrock. The choice of a standing column well system is often dictated where there is near-surface bedrock and limited surface area is available. A standing column is typically not suitable in locations where the geology is mostly clay, silt, or sand. If bedrock is deeper than 200 feet (61 m) from the surface, the cost of casing to seal off the overburden may become prohibitive.

A multiple standing column well system can support a large structure in an urban or rural application. The standing column well method is also popular in residential and small commercial applications. There are many successful applications of varying sizes and well quantities in the many boroughs of New York City, and is also the most common application in the New England states. This type of ground source system has some heat storage benefits, where heat is rejected from the building and the temperature of the well is raised, within reason, during the Summer cooling months which can then be harvested for heating in the Winter months, thereby increasing the efficiency of the heat pump system. As with closed loop systems, sizing of the standing column system is critical in reference to the heat loss and gain of the existing building. As the heat exchange is actually with the bedrock, using water as the transfer medium, a large amount of production capacity (water flow from the well) is not required for a standing column system to work. However, if there is adequate water production, then the thermal capacity of the well system can be enhanced by discharging a small percentage of system flow during the peak Summer and Winter months.

Since this is essentially a water pumping system, standing column well design requires critical considerations to obtain peak operating efficiency. Should a standing column well design be misapplied, leaving out critical shut-off valves for example, the result could be an extreme loss in efficiency and thereby cause operational cost to be higher than anticipated.

Building distribution

Liquid-to-air heat pump

The heat pump is the central unit that becomes the heating and cooling plant for the building. Some models may cover space heating, space cooling, (space heating via conditioned air, hydronic systems and / or radiant heating systems), domestic or pool water preheat (via the desuperheater function, demand hot water, and driveway ice melting all within one appliance with a variety of options with respect to controls, staging and zone control. The heat may be carried to its end use by circulating water or forced air. Almost all types of heat pumps are produced for commercial and residential applications.

Liquid-to-air heat pumps (also called water-to-air) output forced air, and are most commonly used to replace legacy forced air furnaces and central air conditioning systems. There are variations that allow for split systems, high-velocity systems, and ductless systems. Heat pumps cannot achieve as high of a fluid temperature as a conventional furnace, so they require a higher volume flow rate of air to compensate. When retrofitting a residence, the existing duct work may have to be enlarged to reduce the noise from the higher air flow.

Liquid-to-water heat pump

Liquid-to-water heat pumps (also called water-to-water) are hydronic systems that use water to carry heating or cooling through the building. Systems such as radiant underfloor heating, baseboard radiators, conventional cast iron radiators would use a liquid-to-water heat pump. These heat pumps are preferred for pool heating or domestic hot water pre-heat. Heat pumps can only heat water to about 50 °C (122 °F) efficiently, whereas a boiler normally reaches 65–95 °C (149–203 °F). Legacy radiators designed for these higher temperatures may have to be doubled in numbers when retrofitting a home. A hot water tank will still be needed to raise water temperatures above the heat pump's maximum, but pre-heating will save 25-50% of hot water costs.

Ground source heat pumps are especially well matched to underfloor heating and baseboard radiator systems which only require warm temperatures (40 °C) to work well. Thus they are ideal for open plan offices. Using large surfaces such as floors, as opposed to radiators, distributes the heat more uniformly and allows for a lower water temperature. Wood or carpet floor coverings dampen this effect because the thermal transfer efficiency of these materials is lower than that of masonry floors (tile, concrete). Underfloor piping, ceiling or wall radiators can also be used for cooling in dry climates, although the temperature of the circulating water must be above the dew point to ensure that atmospheric humidity does not condense on the radiator.

Combination heat pumps are available that can produce forced air and circulating water simultaneously and individually. These systems are largely being used for houses that have a combination of air and liquid conditioning needs, for example central air conditioning and pool heating.

Seasonal thermal storage

A heat pump in combination with heat and cold storage

The efficiency of ground source heat pumps can be improved by using seasonal thermal storage. If heat loss from the ground source is sufficiently low, the heat pumped out of the building in the summer can be retrieved in the winter. Heat storage efficiency increases with scale, so this advantage is most significant in commercial or district heating systems. Geosolar combisystems further augment this efficiency by collecting extra solar energy during the summer (more than is needed for air conditioning) and concentrating it in the store.

Such a system has been used to heat and cool a greenhouse using an aquifer for thermal storage. In summer, the greenhouse is cooled with cold ground water. This heats the water in the aquifer which can become a warm source for heating in winter. The combination of cold and heat storage with heat pumps can be combined with water/humidity regulation. These principles are used to provide renewable heat and renewable cooling to all kinds of buildings.

Also the efficiency of existing small heat pump installations can sometimes be improved a lot by adding large, cheap, water filled solar collectors. These may be integrated into a to be overhauled parking lot, or in walls or roof constructions simply by putting lots of one inch PE pipes into the outer layer. A very simple option is to add a large mechanically ventilated out door water-air heat exchanger (like the one that is in front of your car engine, but larger). In the summer they allow to pump lots of heat, almost free of running cost, into the ground. This only works well when ground water mobility is not too high, and it works better when more houses install this system next to each other. (In the winter such outdoor components have to be drained of water.)

A seasonal thermal store (also known as a seasonal heat store or inter-seasonal thermal store) is a store designed to retain heat deposited during the hot summer months for use during colder winter weather. The heat is typically captured using solar collectors, although other energy sources are sometime used separately or in parallel.

Types of seasonal thermal storage system

Seasonal (or "annualized") thermal storage can be divided into three broad categories:

• Low-temperature systems use the soil adjoining the building as a low-temperature seasonal heat store (reaching temperatures similar to average annual air temperature), drawing upon the stored heat for space heating. Such systems can also be seen as an extension to the building design (normally passive solar building design), as the design involves some simple but significant differences when compared to 'traditional' buildings.

• Warm-temperature interseasonal heat stores also use soil to store heat, but employ active mechanisms of solar collection in summer to heat thermal banks in advance of the heating season.

• High-temperature seasonal heat stores are essentially an extension of the building's HVAC and water heating systems. Water is normally the storage medium, stored in tanks at temperatures that can approach boiling point. Phase change materials (which are expensive but which require much smaller tanks) and high-tech soil heating systems (remote from the building) are occasionally used instead. For systems installed in individual buildings, additional space is required to accommodate the size of the storage tanks.

In all cases, very effective above-ground insulation / superinsulation of the building structure is required to minimize heat-loss from the building, and hence the amount of heat that needs to be stored and used for space heating.

Despite the differences in design that they involve, low-temperature systems tend to offer simple and relatively inexpensive implementations which are less vulnerable to equipment failure. They do, however, require the site of the building to be clear of the water table, bedrock and existing buildings, and are limited to temperate (or warmer) climate zones and to space heating only. High-temperature systems share the same vulnerabilities as conventional space and water heating systems due to their 'active' mechanical and electrical components, as well as their advantage of enabling greater control. They can also be employed in colder climates.

Low-temperature seasonal heat stores

One of the original motivations of early man's movement into caves was probably the ability of the earth to naturally even out variations in temperature. At depths of about 20 feet (6m) temperature is naturally “annualised” at a stable year-round temperature.

With the development of modern passive solar building design, during the 1970s and 1980s a number of techniques were developed in the US that enabled thermally and moisture-protected soil to be used as an effective seasonal storage medium for space heating, with direct conduction as the heat return method.

Two basic techniques can be employed:

• In the Passive Annual Heat Storage (PAHS) and similar direct solar gain systems, solar heat is directly captured by the structure's spaces (through windows and other surfaces) in summer and then passively transferred (by conduction) through its floors, walls (and, sometimes, roof) into adjoining thermally-buffered soil. It is then passively returned (by conduction and radiation) as those spaces cool in winter. These techniques were advocated in Daniel Geery's 1982 book Solar Greenhouses: Underground and John Hait's 1983 Passive Annual Heat Storage.

• The Annualized Geothermal Solar (AGS) concept involves the capture of heat by isolated solar gain devices (rather than the building structure). From here it is deposited in the earth (or other storage masses or mediums) adjoining the building using active or passive technology. The depth at which the heat is deposited is calculated (according to soil type) to provide a controlled 6-month heat-return time-lag to the building through conduction as the building cools. This alternative was posed by Don Stephens.

Warm-temperature seasonal heat stores

Warm-temperature heat stores are a development of low-temperature stores in that solar collectors are used to capture surplus heat in summer and actively raise the temperature of large thermal banks of soil so that heat can be extracted more easily (and more cheaply) in winter. Interseasonal Heat Transfer uses water circulating in pipes embedded in asphalt solar collectors to transfer heat to Thermal Banks beneath the insulated foundation of buildings. A ground source heat pump is used in winter to extract the warmth from the Thermal Bank to provide space heating via underfloor heating. A high Coefficient of Performance is obtained because the heat pump starts with a warm temperature of 25°C (77°F) from the thermal store, instead of a cold temperature of 10°C (50°F) from the ground.

High-temperature seasonal thermal stores

High-temperature seasonal thermal stores are found on a variety of scales, from those installed in individual houses to those serving neighbourhoods via district heating.

Individual structures

Although the use of high-temperature seasonal thermal stores within individual buildings dates back to at least 1939 (MIT Solar House #1), the United States, Switzerland and Germany have all been notable pioneers in this field.

One example of this active approach is the experimental “Jenni-Haus” built in 1989 in Oberburg, Switzerland. This has three tanks storing a total of 118m³ (4,100 cubic feet) providing far more heat than is required to heat the building.

The more recent “Zero Heating Energy House”, completed in 1997 in Berlin as part of the IEA Task 13 low energy housing demonstration project, stores water at temperatures up to 90 °C (195 °F) inside a 20m³ (700 cubic feet) tank in the basement , and is now one of a growing number of similar properties.

Another similar example was set up in Ireland in 2009. The solar seasonal store consists of a 23m³ (23,000 liters) tank, filled with water , which was installed in the ground, heavily insulated all around, to store heat from evacuated solar tubes during the year. The system was installed as an experiment to heat the world's first standardised pre-fabricated passive house in Galway, Ireland. The aim was to find out if this heat would be sufficient to eliminate the need for any electricity in the already highly efficient home during the winter months. The system is monitored and documented by a research team from The University of Ulster and the results will be included in part of a PhD thesis.

Neighbourhoods

At the neighbourhood level, the Wiggenhausen-Süd solar development at Friedrichshafen has received international attention. This features a 12,000 m³ (424,000 cubic feet) reinforced concrete thermal store linked to 4,300m² (46,000 square feet) of solar collectors, which will supply the 570 houses with around 50% of their heating and hot water .

A different approach is illustrated by the Drake Landing Solar Community development in Okotoks, Alberta. This community consists of 52 houses built to the stringent R-2000 building code. Here the store is created from the ground itself, with solar heated water pumped into a Borehole Thermal Energy Storage (BTES) system. It consists of 144 boreholes, each 37 m (121 ft) deep, which heat the ground to a maximum of around 90 °C (195 °F) . During the winter, the hot water flows from the BTES field to the houses through a distribution network. Once inside the house, it flows through coil units, over which air is blown. The hot air then heats the house. Each house also has an independent solar thermal system installed on its sloped roof to provide domestic hot water. This system has a 90% solar fraction, meaning 90% of the energy required to heat the air and water within the community is provided by the sun. This results in a reduction of over 5 tonnes of CO2 equivalent, per house.

Greenhouses

A heat pump in combination with heat and cold storage

Thermal storage (sometimes referred to as heat and cold storage) is also used extensively for applications as the heating of greenhouses. In summer, the greenhouse is cooled with ground water, pumped from an aquifer, which is the cold source. This heats the water, which is then stored by the system in a warm source. In winter, the warm water is pumped up to supply heat. The now cooled water is returned to the cold source. The combination of cold and heat storage with heat pumps has an additional benefit for greenhouses, as it may be combined with humidification. In the (closed circuit) system, the hot water is stored in one aquifer, while the cold water is stored in another. The water is used to heat or cool the air, which is moved by fans. Such a system can be completely automated.

Thermal efficiency

The net thermal efficiency of a heat pump should take into account the efficiency of electricity generation and transmission, typically about 40%. Since a heat pump moves 3 to 5 times more heat energy than the electric energy it consumes, the total energy output is much greater than the input. This results in net thermal efficiencies greater than 100% for most electricity sources. Traditional combustion furnaces and electric heaters can never exceed 100% efficiency, but heat pumps provide extra energy by extracting it from the ground.

Geothermal heat pumps can reduce energy consumption— and corresponding air pollution emissions—up to 44% compared to air source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment.

The dependence of net thermal efficiency on the electricity infrastructure tends to be an unnecessary complication for consumers and is not applicable to hydroelectric power, so performance of heat pumps is usually expressed as the ratio of heating output or heat removal to electricity input. Cooling performance is typically expressed in units of BTU/hr/watt as the Energy Efficiency Ratio, (EER) while heating performance is typically reduced to dimensionless units as the Coefficient of Performance. (COP) The conversion factor is 3.41 BTU/hr/watt. Performance is influenced by all components of the installed system, including the soil conditions, the ground-coupled heat exchanger, the heat pump appliance, and the building distribution, but is largely determined by the "lift" between the input temperature and the output temperature.

For the sake of comparing heat pump appliances to each other, independently from other system components, a few standard test conditions have been established by the American Refrigerant Institute (ARI) and more recently by the International Organization for Standardization. Standard ARI 330 ratings were intended for closed loop ground-source heat pumps, and assumes secondary loop water temperatures of 77 °F (25 °C) for air conditioning and 32 °F (0 °C) for heating. These temperatures are typical of installations in the northern USA. Standard ARI 325 ratings were intended for open loop ground-source heat pumps, and include two sets of ratings for groundwater temperatures of 50 °F (10 °C) and 70 °F (21 °C). ARI 325 budgets more electricity for water pumping than ARI 330. Neither of these standards attempt to account for seasonal variations. Standard ARI 870 ratings are intended for direct exchange ground-source heat pumps. ASHRAE transitioned to ISO 13256-1 in 2001, which replaces ARI 320, 325 and 330. The new ISO standard produces slightly higher ratings because it no longer budgets any electricity for water pumps.

Efficient compressors, variable speed compressors and larger heat exchangers all contribute to heat pump efficiency. Residential ground source heat pumps on the market today have standard COPs ranging from 2.4 to 5.0 and EERs ranging from 10.6 to 30. To qualify for an Energy Star label, heat pumps must meet certain minimum COP and EER ratings which depend on the ground heat exchanger type. For closed loop systems, the ISO 13256-1 heating COP must be 3.3 or greater and the cooling EER must be 14.1 or greater.

Actual installation conditions may produce better or worse efficiency than the standard test conditions. COP improves with a lower temperature difference between the input and output of the heat pump, so the stability of ground temperatures is important. If the loop field or water pump is undersized, the addition or removal of heat may push the ground temperature beyond standard test conditions, and performance will be degraded. Similarly, an undersized blower may allow the plenum coil to overheat and degrade performance.

Soil without artificial heat addition or subtraction and at depths of several meters or more remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air-temperature of the chosen location, usually 7–12 °C (45–54 °F) at a depth of six meters in the northern USA. Because this temperature remains more

constant than the air temperature throughout the seasons, geothermal heat pumps perform with far greater efficiency during extreme air temperatures than air conditioners and air-source heat pumps.

Standards ARI 210 and 240 define Seasonal Energy Efficiency Ratio (SEER) and Heating Seasonal Performance Factors (HSPF) to account for the impact of seasonal variations on air source heat pumps. These numbers are normally not applicable and should not be compared to ground source heat pump ratings. However, Natural Resources Canada has adapted this approach to calculate typical seasonally adjusted HSPFs for ground-source heat pumps in Canada. The NRC HSPFs ranged from 8.7 to 12.8 BTU/hr/watt (2.6 to 3.8 in nondimensional factors, or 255% to 375% seasonal average electricity utilization efficiency) for the most populated regions of Canada. When combined with the thermal efficiency of electricity, this corresponds to net average thermal efficiencies of 100% to 150%.

The thermal efficiency ( ) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.

When talking about the efficiency of heat engines and power stations the convention should be stated ie HHV (aka Gross Heating Value etc) or LCV (AKA Net Heating value) AND whether Gross output (at the generator terminals or output shaft) or Net Output (at the power station fence) are being considered. The two are of course separate but both must be stated. Failutre to do so causes endless confusion.

Overview

Output energy is always lower than input energy

In general, energy conversion efficiency is the ratio between the useful output of a device and the input, in energy terms. For thermal efficiency, the input, , to the device is

heat, or the heat-content of a fuel that is consumed. The desired output is mechanical work, , or heat, , or possibly both. Because the input heat normally has a real financial cost, a memorable, generic definition of thermal efficiency is

From the first law of thermodynamics, the energy output can't exceed the input, so

When expressed as a percentage, the thermal efficiency must be between 0% and 100%. Due to inefficiencies such as friction, heat loss, and other factors, thermal engines' efficiencies are typically much less than 100%. For example, a typical gasoline automobile engine operates at around 25% efficiency, and a large coal-fueled electrical generating plant peaks at about 46%. The largest diesel engine in the world peaks at 51.7%. In a combined cycle plant, thermal efficiencies are approaching 60%. Such a real-world value may be used as a figure of merit for the device.

There are two types of thermal efficiency–indicated thermal efficiency and brake thermal efficiency.

Heat engines

Heat engines transform thermal energy, or heat, Qin into mechanical energy, or work, Wout. They cannot do this task perfectly, so some of the input heat energy is not converted into work, but is dissipated as waste heat Qout into the environment

The thermal efficiency of a heat engine is the percentage of heat energy that is transformed into work. Thermal efficiency is defined as

The efficiency of even the best heat engines is low; usually below 50% and often far below. So the energy lost to the environment by heat engines is a major waste of energy resources, although modern cogeneration, combined cycle and energy recycling schemes are beginning to use this heat for other purposes. Since a large fraction of the fuels produced worldwide go to powering heat engines, perhaps up to half of the useful energy produced worldwide is wasted in engine inefficiency. This inefficiency can be attributed to three causes. There is an overall theoretical limit to the efficiency of any heat engine due to temperature, called the Carnot efficiency. Second, specific types of engines have lower limits on their efficiency due to the inherent irreversibility of the engine cycle they

use. Thirdly, the nonideal behavior of real engines, such as mechanical friction and losses in the combustion process causes further efficiency losses.

Carnot efficiency

The second law of thermodynamics puts a fundamental limit on the thermal efficiency of all heat engines. Surprisingly, even an ideal, frictionless engine can't convert anywhere near 100% of its input heat into work. The limiting factors are the temperature at which the heat enters the engine, , and the temperature of the environment into which the engine exhausts its waste heat, , measured in an absolute scale, such as the Kelvin or Rankine scale. From Carnot's theorem, for any engine working between these two temperatures:

This limiting value is called the Carnot cycle efficiency because it is the efficiency of an unattainable, ideal, reversible engine cycle called the Carnot cycle. No device converting heat into mechanical energy, regardless of its construction, can exceed this efficiency.

Examples of are the temperature of hot steam entering the turbine of a steam power plant, or the temperature at which the fuel burns in an internal combustion engine. is usually the ambient temperature where the engine is located, or the temperature of a lake or river that waste heat is discharged into. For example, if an automobile engine burns gasoline at a temperature of and the ambient temperature is , then its maximum possible efficiency is:

It should be kept in mind that, due to the other causes detailed below, practical engines have efficiencies far below the Carnot limit; for example the average automobile engine is less than 35% efficient.

As Carnot's theorem only applies to heat engines, devices that convert the fuel's energy directly into work without burning it, such as fuel cells, can exceed the Carnot efficiency.

It can be seen that since is fixed by the environment, the only way for a designer to increase the Carnot efficiency of an engine is to increase , the temperature at which the heat is added to the engine. This is a general principle that applies to all heat engines: the efficiency increases with operating temperature. For this reason the operating temperatures of engines have increased greatly over the long term, and new materials such as ceramics to enable engines to stand higher temperatures are an active area of research.

Engine cycle efficiency

The Carnot cycle is reversible and thus represents the upper limit on efficiency of an engine cycle. Practical engine cycles are irreversible and thus have inherently lower efficiency than the Carnot efficiency when operated between the same temperatures

and . One of the factors determining efficiency is how heat is added to the working fluid in the cycle, and how it is removed. The Carnot cycle achieves maximum efficiency because all the heat is added to the working fluid at the maximum temperature

, and removed at the minimum temperature . In contrast, in an internal combustion engine, the temperature of the fuel-air mixture in the cylinder is nowhere near its peak temperature as the fuel starts to burn, and only reaches the peak temperature as all the fuel is consumed, so the average temperature at which heat is added is lower, reducing efficiency.

• Automobiles: Otto cycle The Otto cycle is the name for the cycle used in spark-ignition internal combustion engines such as gasoline and hydrogen fueled automobile engines. Its theoretical efficiency depends on the compression ratio r of the engine and the specific heat ratio γ of the gas in the combustion chamber.

The higher the compression ratio, the higher the temperature in the cylinder as the fuel burns and so the higher the efficiency. However the maximum compression ratio usable is limited by the need to prevent preignition (knocking), where the fuel ignites by compression before the spark plug fires. The specific heat ratio of the air-fuel mixture γ varies somewhat with the fuel, but is generally close to the air value of 1.4. This standard value is usually used in all the engine cycle equations below, and when this approximation is used the cycle is called an air-standard cycle.

• Trucks: Diesel cycle In the Diesel cycle used in diesel truck and train engines, the fuel is ignited by compression in the cylinder. The efficiency of the Diesel cycle is dependent on r and γ like the Otto cycle, and also by the cutoff ratio, rc, which is the ratio of the cylinder volume at the beginning and end of the combustion process:

The Diesel cycle is less efficient than the Otto cycle when using the same compression ratio. However, practical Diesel engines are 30% - 35% more efficient than gasoline engines. This is because, since the fuel is not introduced to the combustion chamber until it required to ignite, the compression ratio is not limited by the need to avoid knocking, so higher ratios are used than in spark ignition engines.

• Power plants: Rankine cycle The Rankine cycle is the cycle used in steam turbine power plants. The overwhelming majority of the world's electric power is produced with this cycle. Since the cycle's working fluid, water, changes from liquid to vapor and back during the cycle, their efficiencies depend on the thermodynamic properties of water. The thermal efficiency of modern steam turbine plants with reheat cycles can reach 47%, and in combined cycle plants it can approach 60%.

• Gas turbines: Brayton cycle The Brayton cycle is the cycle used in gas turbines and jet engines. It consists of a compressor turbine that increases pressure of the incoming air, then fuel is continuously added to the flow and burned, and the hot exhaust gasses are expanded in a turbine. The efficiency depends largely on the ratio of the pressure inside the combustion chamber p2 to the pressure outside p1

Other inefficiencies

The above efficiency formulas are based on simple idealized mathematical models of engines, with no friction and working fluids that obey simple thermodynamic rules called the ideal gas law. Real engines have many departures from ideal behavior that waste energy, reducing actual efficiencies far below the theoretical values given above. Examples are:

• friction of moving parts • inefficient combustion • heat loss from the combustion chamber • departure of the working fluid from the thermodynamic properties of an ideal gas • aerodynamic drag of air moving through the engine • energy used by auxiliary equipment like oil and water pumps • inefficient compressors and turbines • imperfect valve timing

Another source of inefficiency is that engines must be optimized for other goals besides efficiency, such as low pollution. The requirements for vehicle engines are particularly stringent: they must be designed for low emissions, adequate acceleration, fast starting, light weight, low noise, etc. These require compromises in design (such as altered valve timing) that reduce efficiency. The average automobile engine is only about 35% efficient, and must also be kept idling at stoplights, wasting an additional 17% of the energy, resulting in an overall efficiency of 18%. Large stationary electric generating plants have fewer of these competing requirements as well as more efficient Rankine cycles, so they are significantly more efficient than vehicle engines, around 50% Therefore, replacing internal combustion vehicles with electric vehicles, which run on a battery that is charged with electricity generated by burning fuel in a power plant,has the theoretical potential to increase the thermal efficiency of energy use in transportation, thus decreasing the demand for fossil fuels, although practical problems of energy losses

from long transmission lines and the additional multiple energy conversions required between the power plant and the vehicle driving wheels will reduce any potential fuel saving and may even require increased fuel consumption compared to local use of fuel in the more directly coupled powertrains of traditionally engined vehicles.

Energy conversion

For an energy conversion device like a boiler or furnace, the thermal efficiency is

.

So, for a boiler that produces 210 kW (or 700,000 BTU/h) output for each 300 kW (or 1,000,000 BTU/h) heat-equivalent input, its thermal efficiency is 210/300 = 0.70, or 70%. This means that the 30% of the energy is lost to the environment.

An electric resistance heater has a thermal efficiency of at or very near 100%, so, for example, 1500W of heat are produced for 1500W of electrical input. When comparing heating units, such as a 100% efficient electric resistance heater to an 80% efficient natural gas-fueled furnace, an economic analysis is needed to determine the most cost-effective choice.

Effects of fuel heating value

The heating value of a fuel is the amount of heat released during the combustion of a specified amount of it. The heating value is a characteristic for each substance. It is measured in units of energy per unit of the substance, usually mass, such as: kcal/kg, kJ/kg, J/mol, Btu/m³.

The heating value for fuels is expressed as the HHV, LHV, or GHV:

• Higher heating value (HHV) is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapor produced. This is the same as the thermodynamic heat of combustion.

• Lower heating value (LHV) (or net calorific value) is determined by subtracting the heat of vaporization of the water vapor from the higher heating value. The energy required to vaporize the water therefore is not realized as heat.

• Gross heating value accounts for water in the exhaust leaving as vapor, and includes liquid water in the fuel prior to combustion. This value is important for fuels like wood or coal, which will usually contain some amount of water prior to burning.

Which definition of heating value is being used significantly affects any quoted efficiency. Not stating whether an efficiency is HHV or LHV renders such numbers very misleading.

Heat pumps and refrigerators

Heat pumps, refrigerators and air conditioners use work to move heat from a colder to a warmer place, so their function is the opposite of a heat engine. The work energy (Win) that is applied to them is converted into heat, and the sum of this energy and the heat energy that is moved from the cold reservoir (QC) is equal to the total heat energy added to the hot reservoir (QH)

Their efficiency is measured by a coefficient of performance (COP). Heat pumps are measured by the efficiency with which they add heat to the hot reservoir, COPheating; refrigerators and air conditioners by the efficiency with which they remove heat from the cold interior, COPcooling:

The reason for not using the term 'efficiency' is that the coefficient of performance can often be greater than 100%. Since these devices are moving heat, not creating it, the amount of heat they move can be greater than the input work. Therefore, heat pumps can be a more efficient way of heating than simply converting the input work into heat, as in an electric heater or furnace.

Since they are heat engines, these devices are also limited by Carnot's theorem. The limiting value of the Carnot 'efficiency' for these processes, with the equality theoretically achievable only with an ideal 'reversible' cycle, is:

The same device used between the same temperatures is more efficient when considered as a heat pump than when considered as a refrigerator:

This is because when heating, the work used to run the device is converted to heat and adds to the desired effect, whereas if the desired effect is cooling the heat resulting from the input work is just an unwanted byproduct.

Energy efficiency

The 'thermal efficiency' is sometimes called the energy efficiency. In the United States, in everyday usage the SEER is the more common measure of energy efficiency for cooling devices, as well as for heat pumps when in their heating mode. For energy-conversion heating devices their peak steady-state thermal efficiency is often stated, e.g., 'this furnace is 90% efficient', but a more detailed measure of seasonal energy effectiveness is the Annual Fuel Utilization Efficiency (AFUE).

Energy Efficiency of heat exchangers

A counter flow heat exchanger is generally, effectively 100% efficient in transferring heat energy from one circuit to the other, albeit at a slight loss in temperature.


The U.S. Environmental Protection Agency (EPA) has called ground source heat pumps the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available. Heat pumps offer significant emission reductions potential, particularly where they are used for both heating and cooling and where the electricity is produced from renewable resources.

Ground-source heat pumps have unsurpassed thermal efficiencies and produce zero emissions locally, but their electricity supply almost always includes components with high greenhouse gas emissions. Their environmental impact therefore depends on the characteristics of the electricity supply. The GHG emissions savings from a heat pump over a conventional furnace can be calculated based on the following formula:

Annual greenhouse gas savings from using a ground source heat pump instead of a high-efficiency furnace in a detached residence

GHG savings relative to Country Electricity CO2

Emissions Intensity natural gas heating oil electric heating

Canada 223 ton/GWh 2.7 ton/yr 5.3 ton/yr 3.4 ton/yr

Russia 351 ton/GWh 1.8 ton/yr 4.4 ton/yr 5.4 ton/yr

USA 676 ton/GWh -0.5 ton/yr 2.2 ton/yr 10.3 ton/yr

China 839 ton/GWh -1.6 ton/yr 1.0 ton/yr 12.8 ton/yr

• HL = seasonal heat load ≈ 80 GJ/yr for a modern detached house in the northern USA

• FI = emissions intensity of fuel = 50 kg(CO2)/GJ for natural gas, 73 for heating oil

• AFUE = furnace efficiency ≈ 95% for a modern condensing furnace • COP = heat pump coefficient of performance ≈ 3.2 seasonally adjusted for

northern USA heat pump • EI = emissions intensity of electricity ≈ 200-800 ton(CO2)/GWh, depending on

region

Ground-source heat pumps always produce less greenhouse gases than air conditioners, oil furnaces, and electric heating, but natural gas furnaces may be competitive depending on the greenhouse gas intensity of the local electricity supply. In countries like Canada and Russia with low emitting electricity infrastructure, a residential heat pump may save 5 tons of carbon dioxide per year relative to an oil furnace, or about as much as taking an average passenger car off the road. But in countries like China or USA that are highly reliant on coal for electricity production, a heat pump may result in 1 or 2 tons more carbon dioxide emissions than a natural gas furnace.

The fluids used in closed loops may be designed to be biodegradable and non-toxic, but the refrigerant used in the heat pump cabinet and in direct exchange loops was, until recently, chlorodifluoromethane, which is an ozone depleting substance. Although harmless while contained, leaks and improper end-of-life disposal contribute to enlarging the ozone hole. This refrigerant is being phased out in favor of ozone-friendly R410A for new construction.

Open loop systems that draw water from a well and drain to the surface may contribute to aquifer depletion, water shortages, groundwater contamination, and subsidence of the soil. A geothermal heating project in Staufen im Breisgau, Germany, is suspected to have caused considerable damage to buildings in the city center. The ground has subsided by up to eight millimeters under the city hall while other areas have been uplifted by a few millimeters.

Ground-source heat pump technology, like building orientation, is a natural building technique (bioclimatic building).

Economics

Ground source heat pumps are characterized by high capital costs and low operational costs compared to other HVAC systems. Their overall economic benefit depends primarily on the relative costs of electricity and fuels, which are highly variable over time and across the world. Based on recent prices, ground-source heat pumps currently have lower operational costs than any other conventional heating source almost everywhere in the world. Natural gas is the only fuel with competitive operational costs, and only in a handful of countries where it is exceptionally cheap, or where electricity is exceptionally expensive. In general, a homeowner may save anywhere from 20% to 60% annually on utilities by switching from an ordinary system to a ground-source system. However, many family size installations are reported to use much more electricity then their owners had expected from advertisements. This is often partly due to bad design or installation: Heat exchange capacity with groundwater is often too small, heating pipes in house floors are often too thin and too few, or heated floors are covered with wooden panels or carpets.

Capital costs and system lifespan have received much less study, and the return on investment is highly variable. One study found the total installed cost for a system with 10 kW (3 ton) thermal capacity for a detached rural residence in the USA averaged $8000–$9000 in 1995 US dollars. More recent studies found an average cost of $14,000 in 2008 US dollars for the same size system. The US Department of Energy estimates a price of $7500 on its website, last updated in 2008. Prices over $20,000 are quoted in Canada, with one source placing them in the range of $30,000-$34,000 Canadian dollars. The rapid escalation in system price has been accompanied by rapid improvements in efficiency and reliability. Capital costs are known to benefit from economies of scale, particularly for open loop systems, so they are more cost-effective for larger commercial buildings and harsher climates. The initial cost can be two to five times that of a conventional heating system in most residential applications, new construction or existing. In retrofits, the cost of installation is affected by the size of living area, the home's age, insulation characteristics, the geology of the area, and location of the home/property. Proper duct system design and mechanical air exchange should be considered in the initial system cost.

Payback period for installing a ground source heat pump in a detached residence

Payback period for replacing Country

natural gas heating oil electric heating

Canada 13 years 3 years 6 years

USA 12 years 5 years 4 years

Germany net loss 8 years 2 years

Notes:

• Highly variable with energy prices. • Government subsidies not included. • Climate differences not evaluated.

Capital costs may be offset by substantial subsidies from many governments, for example totaling over $7000 in Ontario for residential systems installed in the 2009 fiscal year. Some electric companies offer special rates to customers who install a ground-source heat pump for heating/cooling their building. This is due to the fact that electrical plants have the largest loads during summer months and much of their capacity sits idle during winter months. This allows the electric company to use more of their facility during the winter months and sell more electricity. It also allows them to reduce peak usage during the summer (due to the increased efficiency of heat pumps), thereby avoiding costly construction of new power plants. For the same reasons, other utility companies have started to pay for the installation of ground-source heat pumps at customer residences. They lease the systems to their customers for a monthly fee, at a net overall savings to the customer.

The lifespan of the system is longer than conventional heating and cooling systems. Good data on system lifespan is not yet available because the technology is too recent, but many early systems are still operational today after 25–30 years with routine maintenance. Most loop fields have warranties for 25 to 50 years and are expected to last at least 50 to 200 years. Ground-source heat pumps use electricity for heating the house. The higher investment above conventional oil, propane or electric systems may be returned in energy savings in 2–10 years for residential systems in the USA. If compared to natural gas systems, the payback period can be much longer or non-existent. The payback period for larger commercial systems in the USA is 1–5 years, even when compared to natural gas.

Ground source heat pumps are recognized as one of the most efficient heating and cooling systems on the market. They are often the second-most cost effective solution in extreme climates, (after co-generation), despite reductions in thermal efficiency due to ground temperature. (The ground source is warmer in climates that need strong air conditioning, and cooler in climates that need strong heating.)

Commercial systems maintenance costs in the USA have historically been between $0.11 to $0.22 per m2 per year in 1996 dollars, much less than the average $0.54 per m2 per year for conventional HVAC systems.

Governments that promote renewable energy will likely offer incentives for the consumer (residential), or industrial markets. For example, in the United States, incentives are offered both on the state and federal levels of government.

Installation

Because of the technical knowledge and equipment needed to properly design and size the system (and install the piping if heat fusion is required), a GSHP system installation requires a professional's services. The International Ground Source Heat Pump Association (IGSHPA), Geothermal Exchange Organization (GEO) and the Canadian GeoExchange Coalition maintain listings of qualified installers in the USA and Canada.

Chapter-7 Geothermal Power in Different Countries

Geothermal Power in the United Kingdom The potential for exploiting geothermal energy in the United Kingdom on a commercial basis was initially examined by the Department of Energy in the wake of the 1973 oil crisis. Several regions of the country were identified, but interest in developing them was lost as petroleum prices fell.

Aquifer based schemes

Southampton District Energy Scheme

Despite this Southampton took the decision to create the UK's first geothermal power scheme as part of a plan to become a ‘self sustaining city’ in energy generation, promoted by then leader of the city council Alan Whitehead. Turned down for funding by the Department of Energy, the scheme was eventually developed in conjunction with French-owned company Utilicom Ltd and the Southampton Geothermal Heating Company was then established. Construction started in 1987 on a well to draw water from the Wessex Basin aquifer at a depth of 1,800 metres and a temperature of 76 °C.

The scheme now heats a number of buildings in the city centre, including the Southampton Civic Centre and the WestQuay shopping centre, by providing 8% of the heat distributed by a larger city centre district heating system that includes other combined heat and power sources. Geothermal energy provides 16 GWh of heat per year.

Another area with great potential for geothermal energy is in the North Sea, on the continental shelf where the Earth's crust is thin (less than 10 kilometres). The offshore platforms extract hydrocarbons from this region, but each year the output falls by 5% and soon it will be uneconomic to continue using these platforms for fossil fuel extraction. An alternate use could be geothermal power generation. A 1986 pioneering work on this was undertaken by Total Energy Conservation and Management Co. Ltd. An overview document was produced called Single Borehole Geothermal Energy Extraction System for Electrical Power Generation

Hot rock schemes

In addition to using geothermally heated aquifers, Hot-Dry-Rock geothermal technology can be used to heat water pumped below ground onto geothermally heated rock. Starting in 1977, trials of the technology were undertaken at Rosemanowes Quarry, near Penryn, Cornwall.

In 2008 a planning application was submitted for a hot rocks project on the site of a former cement works at Eastgate, near Stanhope in County Durham. The geothermal plant will heat the UK's first geothermal energy model village.

In 2010 planning permission for a commercial-scale geothermal power plant was granted by Cornwall Council. The plant will be constructed on the United Downs industrial estate near Redruth by Geothermal Engineering Ltd. The plant will produce 10MW of electricity and 55MW of renewable heat.

On the 18th of December 2010 The Eden Project in Cornwall was given permission to build a Hot Rock Geothermal Plant. Drilling is expected to in 2011 with electricity being produced from the second half of 2013. The plant will be on the north side of the Eden Project, a showcase for environmental projects at Bodelva, near St Austell. It should produce up to 4 megawatts of electricity for use by Eden with a surplus, enough for about 5,000 houses, going in to the National Grid.

Geothermal Power in the Philippines

Geothermal power plant in Valencia, Negros Oriental, Philippines

The Geothermal Education Office and a 1980 article entitled "The Philippines geothermal success story" by Rudolph J. Birsic published in the journal Geothermal Energy (vol. 8, Aug.-Sept. 1980, p. 35-44) note the remarkable geothermal resources of the Philippines. During the World Geothermal Congress 2000 held in Beppu, Ōita Prefecture of Japan (May-June 2000), it was reported that the Philippines is the largest consumer of electricity from geothermal sources and highlighted the potential role of geothermal energy in providing energy needs for developing countries.

According to the International Geothermal Association (IGA), worldwide, the Philippines ranks second to the United States in producing geothermal energy. As of the end of 2003, the US had a capacity of 2020 megawatts of geothermal power, while the Philippines generated 1930 megawatts. (Mexico was third with 953 MW according to IGA). Early statistics from the Institute for Green Resources and Environment stated that Philippine geothermal energy provides 16% of the country's electricity. By 2005, geothermal energy

accounted for 17.5% of the country's electricity production. More recent statistics from the IGA show that combined energy from geothermal power plants in the islands of Luzon, Leyte, Negros and Mindanao account for approximately 27% of the country's electricity generation. Leyte is one of the islands in the Philippines where the first geothermal power plant started operations in July 1977.

Geothermal Power in Japan

Matsukawa geothermal power station

Japan has favorable sites for geothermal power because of its proximity to the Izu-Bonin-Mariana Arc. In 2007, Japan had 535.2 MW of installed electric generating capacity, about 5% of the world total.

There are geothermal electric power plants at the following locations:

• Mori • Ohnuma

• Matsukawa • Kakkonda • Uenotai • Onikoube • Yanaizu-Nishiyama • Hachijo-Jima

Geothermal Power in New Zealand

New geothermal drilling north of Taupo (2007).

Geothermal power in New Zealand is a small but significant part of the energy generation capacity of the country, providing approximately 10% of the country's electricity with installed capacity of over 700 MW. New Zealand, like only a small number of other countries worldwide, has numerous geothermal sites that could be developed for exploitation, and also boasts some of the earliest large-scale use of geothermal energy in the world.

Geothermal energy has been described as New Zealand's most reliable renewable energy source, above wind, solar and even water power, due to its lack of dependence on the weather.

Geothermal fields

The exploration of New Zealand's geothermal fields has been very extensive, and by the 1980s, most fields were considered mapped, with 129 found, of which 14 are in the 70-140 °C range, 7 in the 140-220 °C range and 15 in the >220 °C range. Currently, some potential new geothermal fields are being surveyed that have no surface expression.

New Zealand's high-temperature geothermal fields are mostly concentrated around the Taupo Volcanic Zone (which also has most of the currently operating generation capacity), in the central North Island, with another major field at Ngawha Springs in Northland. However, more systems (some of them potentially exploitable) are scattered all over the country, from the Hauraki Plains to the Bay of Plenty to numerous hot springs in the South Island, most of them associated with faults and other tectonic features.

Many applications of geothermal energy in New Zealand reinject the cooled steam / fluid back into the underground fields, to extend or infinitely use the fields as power sources.

History

The Wairakei geothermal power plant.

Geothermal energy use in New Zealand is strongly tied to Wairakei, where the first geothermal plant was opened in 1958. At that time, it was only the second large-scale plant existing worldwide (the first being the Valle del Diavolo 'Devil's Valley' plant in Larderello, Italy opened in 1911). Several new plants and efficiency-enhancing second-stage equipment have been added since, though there is also some loss of steam generation due to the decade-long drawdown. Some plants are therefore capped in steam extraction volumes to allow the fields to regenerate, and a percentage of the steam/water is reinjected.

The Ngawha geothermal plant was the first to come into operation via a resource consent applied for and issued under the Resource Management Act. Recent geothermal developments include an upgrade of this plant as well as the commissioning of the Kawerau Power Station in 2008.

Geothermal energy is expected to contribute an increasing proportion of the nation's electricity in the future, with several large geothermal projects underway or recently completed. The 220 MW Te Mihi Power Station is expected to begin operation in 2011.

Recently commissioned geothermal projects include the 140 MW Nga Awa Purua Power Station and the 23 MW Te Huka Power Station, a binary plant.

Research

Considerable geothermal research expertise exists at New Zealand's Crown Research Institutes and universities. In particular, at GNS Science , Industrial Research Ltd , and the Geothermal Program at the University of Auckland. New Zealand is also one of the partner nations of the International Partnership for Energy Development in Island Nations (EDIN). As part of EDIN , New Zealand is involved in international research projects to evaluate and increase geothermal power generation domestically as well as in 18 Pacific Island nations.

Laws and regulations Geothermal Energy Act 1953

The Geothermal Energy Act 1953 was made redundant by the Resource Management Act 1991 (RMA). The Geothermal Energy Act granted water rights, which have generally been replaced by RMA resource consents.

Geothermal Energy Regulations 1961

The Geothermal Energy Regulations 1961 define the role of "geothermal inspectors" and specifies processes for applications for authorities and licences.

Rotorua City Geothermal Energy Empowering Act 1967

The Rotorua City Geothermal Energy Empowering Act 1967 is an Act to enable the Rotorua City Council to make provisions for the control of the tapping and use of geothermal energy in the city of Rotorua.

Resource Management Act 1991

The Resource Management Act 1991 (RMA) is a significant, and at times, controversial Act of Parliament passed in 1991. The RMA regulates access to natural and physical resources such as land, air and water, with sustainable use of these resources being the overriding goal. New Zealand's Ministry for the Environment describes the RMA as New Zealand's principal legislation for environmental management.

The Resource Management Act is the principal legislation controlling the use of geothermal resources in New Zealand. The New Zealand Geothermal Association considers the procedures which are currently being adopted under the RMA as the single largest obstacle to further geothermal development, holding that "the regulatory process leads to long delays which impose a significant up-front cost on projects, reducing their financial viability".

List of geothermal power stations

• Kawerau Power Station • Mokai Power Station • Nga Awa Purua • Ngawha Power Station • Ohaaki Power Station • Poihipi Power Station • Rotokawa Power Station

Geothermal Power in the United States

One of 21 power plants at The Geysers, California, the largest geothermal development in the world.

Geothermal power in the United States continues to be an area of considerable activity. In 2010, the United States led the world in geothermal electricity production with 3,086 megawatts (MW) of installed capacity from 77 power plants; the largest group of geothermal power plants in the world is located at The Geysers, a geothermal field in California. The United States generates an average of 15 billion kilowatt hours of geothermal power per year, comparable to burning some 25 million barrels (4,000,000 m3) of oil or 6 million short tons of coal per year.

Geothermal power plants are largely concentrated in the western states. They are the fourth largest source of renewable electricity, after hydroelectricity, biomass, and wind power. A geothermal resource assessment shows that nine western states together have the potential to provide over 20 percent of national electricity needs.

History

According to archaeological evidence, geothermal resources have been in use on the current territory of the United States for more than 10,000 years. The Paleo-Indians first used geothermal hot springs for warmth, cleansing, and minerals.

The first commercial geothermal power plant producing power to the U.S. utility grid opened at The Geysers in California in 1960, producing eleven megawatts of net power. The Geysers system continues to operate successfully today, and the complex has grown into the largest geothermal development in the world, with an output of 750 MW.

Plants

Map showing geothermal power capacity by state as of 2000 (USGS).

The largest dry steam field in the world is the Geysers, 116 km (72 miles) north of San Francisco. It was here that Pacific Gas and Electric began operation of the first successful geothermal electric power plant in the United States in 1960. The original turbine lasted for more than 30 years and produced 11 MW net power. The Geysers has 1517 megawatt (MW) of active installed capacity with an average capacity factor of 63%. Calpine Corporation owns 15 of the 18 active plants in the Geysers and is currently the United States' largest producer of geothermal energy. Two other plants are owned jointly by the Northern California Power Agency and the City of Santa Clara's municipal Electric Utility (now called Silicon Valley Power). The remaining Bottle Rock Power plant owned by the US Renewables Group has only recently been reopened. A nineteenth plant is now under development by Ram Power, formerly Western Geopower. Since the activities of one geothermal plant affects those nearby, the consolidation plant ownership at The Geysers has been beneficial because the plants operate cooperatively instead of in their own short-term interest. The Geysers is now recharged by injecting treated sewage effluent from the City of Santa Rosa and the Lake County sewage treatment plant. This

sewage effluent used to be dumped into rivers and streams and is now piped to the geothermal field where it replenishes the steam produced for power generation.

Another major geothermal area is located in south central California, on the southeast side of the Salton Sea, near the cities of Niland and Calipatria, California. As of 2001, there were 15 geothermal plants producing electricity in the area. CalEnergy owns about half of them and the rest are owned by various companies. Combined the plants have a capacity of about 570 MW.

The Basin and Range geologic province in Nevada, southeastern Oregon, southwestern Idaho, Arizona and western Utah is now an area of rapid geothermal development. Several small power plants were built during the late 1980s during times of high power prices. Rising energy costs have spurred new development. Plants in Nevada at Steamboat Springs, Brady/Desert Peak, Dixie Valley, Soda Lake, Stillwater and Beowawe now produce about 235 MW.

Production and development

US electricity generated from geothermal sources 1960-2008 (blue), and as a percentage of total US electricity (red).

With 3,040.27 MW of installed geothermal capacity, the United States remains the world leader with 30% of the online capacity total. The future outlook for expanded production from conventional and enhanced geothermal systems is positive as new technologies promise increased growth in locations previously not considered.

As of August 2008, 103 new projects are underway in 13 U.S. states. When developed, these projects could potentially supply up to 3,979 MW of power, meeting the needs of about 4 million homes. At this rate of development, geothermal production in the United States could exceed 15,000 MW by 2025.

The most significant catalyst behind new industry activity is the Energy Policy Act of 2005. This Act made new geothermal plants eligible for the full federal production tax credit, previously available only to wind power projects and certain kinds of biomass. It also authorized and directed increased funding for research by the Department of Energy, and enabled the Bureau of Land Management to address its backlog of geothermal leases and permits.

Estimated subterranean temperatures at a depth of 6 kilometers

In April 2008, exploratory drilling began at Newberry Volcano in Oregon.

In 2009, investment bank Credit Suisse calculated that geothermal power costs 3.6 cents per kilowatt-hour, versus 5.5 cents per kilowatt-hour for coal."

In 2009 Raser Technologies (nyse:RZ) put a 8.5 MW called Thermo 1 in Utah the facility uses a process called bottom cycling to produce power from a lower heat level. This plant was 2009's power plant of the year. It was built in only 6 months. They have just begun drilling for their next 15Mw project called Lighting Dock.

Reliability

Geothermal has a higher capacity factor (a measure of the amount of real time during which a facility is used) than many other power sources. Unlike wind and solar resources, which are dependent upon weather, geothermal resources are available 24 hours a day, 7 days a week. While the carrier medium for geothermal electricity (water) must be properly managed, the source of geothermal energy, the Earth's heat, will be available, for most intents and purposes, indefinitely.

In 2008 the USDOE funded research in Enhanced Geothermal Systems (EGS) to learn more about the fracture systems in geothermal reservoirs and better predict the results of reservoir stimulation.

Environmental effects

Acquiring steam from geysers, volcanoes, and hot springs is a process that isn't harmful to the environment. However, the actual steam that is being collected and transferred into energy contains chemicals that contribute to air pollution, and water mixed with the steam consists of dissolved salts that can damage pipes and harm aquatic ecosystems.

In addition to hazardous salts in our environment, some waters that are collected with the process of geothermal energy have contained high concentrations of toxic elements such as boron, lead, and arsenic. A gas that has been found in geothermal water and steam is hydrogen sulfide, which has a bad odor of rotten eggs, and is toxic in high concentrations.

Geothermal Power in China History

Geothermal energy exploitation in China started approximately around 1970. In the socialist planned economy geothermal exploration was handled by national bodies with public investments. Drilled productive wells were transferred free of charge to the final user. Since the mid-'80s, under the framework of privatization and liberalization of the economy, national investment in exploration has been reduced. No new plants have been commissioned in the period 2000-2005 (Zheng et al., 2005; Battocletti et al., 2000). The only electricity producing fields are located in Tibet. According to the "2005 Chinese Geothermal Environment Bulletin" by China's Ministry of Land and Resources, the direct utilization of geothermal energy in China will reach 13.76 cubic meters per second, and the geothermal energy will reach 10,779 megawatts, ranking first in the world . However such programme has not been started so far.

The most important field is Yangbajain, with eight double flash units for a total capacity of 24 MW, fueled from a water dominated shallow reservoir at 140–160 °C with 18 wells of average depth 200 m. The field extension is only 4 km2, although there are clear indications of a total thermal anomaly of 15 km2. The annual energy production is

approximately 100 GW·h, about 30% of the needs of the Tibetan capital, Lhasa. A deep reservoir has been discovered beneath the shallow Yangbajain field. It is characterized by high temperatures (250–330 °C has been measured at 1,500–1,800 m depth). The field potential is estimated at about 50–90 MW. It is still un-exploited. A 2,500 m deep well has been drilled in 2004, reaching the deep reservoir at 1,000–1,300 m. Its evaluation is ongoing.

Additional plants are installed in Langju, West Tibet (two double flash unit, 1 MW each, 80–180 °C) and a 1 MW binary power station (60–170 °C) is operating in Nagqu. Two small 300 kW plants are operating in Guangdong and Hunan.

In Taiwan, a 3 MW single flash unit operated at Qingshui field from 1981 (the reservoir is shallow, less than 500 m depth, at 150–220 °C). A 300 kW binary unit (Tu Chang) was installed in the same field, but exploiting fluid with a maximum temperature of 170 °C. In 1994 these plants stopped their operational activities.

Taken from Ruggero Bertani's paper, " World Geothermal Generation 2001-2005: State of the Art", published in Proceedings of the World Geothermal Congress 2005, Antalya, Turkey, 24–29 April 2005.

Direct uses

Total thermal installed capacity in MWt: 3,687.0

Direct use in TJ/yr: 45,373.0

Direct use in GW·h/yr: 12,604.6

Capacity factor: 0.39

This country is again one of the major users of the direct-use of geothermal energy. Zheng et al. (2005) discusses the latest developments. It appears that along with the restructuring of the economy, national investment in geothermal has decreased. However, as the living standard of the population has risen, geothermal has found favor in that the waters are used more for health, tourism, and balneology in various hot springs. Investors are looking to increase their investment, which has led to an upsurge in geothermal drilling and utilization particularly in the coastal regions of Beijing and Tianjin. The management of the resource also plays a big role particularly in the large cities. Here, efficiency in utilization has improved dramatically and environmental concerns are being addressed. For example, in Beijing the total rate of extraction of hot water has been kept stable and has even decreased slightly but energy utilization in terms of GWh produced has increased significantly. The data of Zheng et al. (2005) show that for the whole of China the installed capacity has risen to 3,687 MWt with an annual energy use of 45,373 TJ/yr (including 15 heat pump units ranging from 220 to 760-kW in capacity operating at an equivalent 2,880 full-load hours annually), from the 2000 (Lund and Freeston, 2001) figures of 2,282 MWt and 37,908 TJ/yr an increase in annual energy use of about 20%.

Geothermal space heating covers 12.7 million m2 and greenhouse heating cover about 1.33 million m2. There are about 1,600 public hot spring bathing houses and swimming pools, including about 430 where balneology and medical practices prevail in the country. The details of the specific uses are as follows: district heating (550 MWt and 6,391 TJ/yr); greenhouse heating (103 MWt and 1,176 TJ/yr); fish farming (174 MWt and 1,921 TJ/yr); agricultural drying (80 MWt and 1,007 TJ/yr); industrial process heat (139 MWt and 2,603 TJ/yr); bathing and swimming (1,991 MWt and 25,095 TJ/yr); other uses (monitoring) (19 MWt and 611 TJ/yr); and heat pumps (631 MWt and 6,569 TJ/yr).

Geothermal Power in Australia Geothermal power in Australia is little used but growing. There are known and potential locations near the centre of the country that have been shown to contain hot granites at depth which hold good potential for development of geothermal energy. Exploratory geothermal wells have been drilled to test for the presence of high temperature geothermal reservoir rocks and such hot granites were detected. As a result, projects will eventuate in the coming years and more exploration is expected to find new locations.

Exploration

Exploration involves finding vast blocks of "hot rocks" with fracture systems that could generate electricity through water being injected, circulated through the fractures, and being returned to surface as steam.

There are vast deep-seated granite systems in Central Australia that have high temperatures at depth and these are being drilled by companies such as Geodynamics Ltd, Petratherm, Green Rock Energy and Pacific Hydro to depths of more than four kilometres.

South Australia has been described as "Australia's hot rock haven" and this renewable energy form could provide an estimated 6.8% of Australia's base load power needs by 2030. According to a conservative estimate by the Centre for International Economics, Australia has enough geothermal energy to contribute electricity for 450 years.

Parts of central Tasmania have been identified by KUTh Energy as having the potential to generate up to 280MW of power. Such a resource would be able to supply 25% of Tasmania's electricity needs.

Projects

Paralana

The 30 MW Paralana project is located adjacent to the Beverley Uranium Mine. It is an engineered geothermal system (EGS) project, based on Petratherm’s "heat exchanger within insulator" model.

Cooper Basin

The 25 MW Cooper Basin demonstration project will demonstrate the potential of hot-rock geothermal energy for zero-emission, base-load power. The project is owned by Geodynamics and will be the world’s first multi-well hot fractured rock power project. Geodynamics has assessed its resource as holding geothermal energy sufficient to support several thousand megawatts of electricity generating capacity.

Jurien-Woodada

The Jurien-Woodada project, owned by New World Energy Limited, is the most advanced geothermal play in Western Australia for electricity production. The project is adjacent to transmission infrastructure and large resource-driven energy markets in the mid-west region. The project area has the potential to contain both hot sedimentary aquifer and EGS styles and is being assessed for delivery of electricity into Western Australia's South West Interconnected System.

Introduction to Geothermal Power

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