Solar Energy

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1INTRODUCTIONSOLAR ENERGY Solar energy is the utilization of the radiant energy from the Sun. Solar power is often used interchangeably with solar energy but refers more specifically to the conversion of sunlight into electricity, either by photovoltaics and concentrating solar thermal devices, or by one of several experimental technologies such as thermoelectric converters, solar chimneys and solar ponds. Solar energy and shading are important considerations in building design. Thermal mass is used to conserve the heat that sunshine delivers to all buildings. Daylighting techniques optimize the use of light in buildings. Solar water heaters heat swimming pools and provide domestic hot water. In agriculture, greenhouses expand growing seasons and pumps powered by solar cells (also known as photovoltaics) provide water for grazing animals. Evaporation ponds are used to harvest salt and clean waste streams of contaminants. Solar energy is the fastest growing form of energy production. Solar distillation and disinfection techniques produce potable water for millions of people worldwide. Family-scale solar cookers and larger solar kitchens concentrate sunlight for cooking, drying and pasteurization. Clotheslines are a common application of solar energy. More sophisticated concentrating technologies magnify the rays of the Sun for high-temperature material testing, metal smelting and industrial chemical production. A range of prototype solar vehicles provide ground, air and sea transportation.

2 ENERGY FROM THE SUN

Solar Energy and its Uses

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Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by the clouds, oceans and land masses. The spectrum of the solar light at the surface of Earth is mostly split between the visible and nearinfrared ranges with a small part in the near-ultraviolet. The absorbed solar light heats the land surface, oceans and atmosphere. The warm air containing evaporated water from the oceans rises driving atmospheric circulation or convection. When the ascending air reaches a high altitude, where the temperature is low, the water vapoir condenses forming various types of clouds. Eventually all evaporated water rains down on to the surface closing what is known as the water cycle. The latent heat of the water condensations amplifies the convection producing such atmospheric phenomena as cyclones and anti-cyclones. The winds are observational manifistation of the atmospheric circulation. Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 C. The conversion of solar energy into chemical energy via photosynthesis produces food, wood and the biomass from which fossil fuels are derived. Solar radiation along with secondary solar resources such as wind and wave power, hydroelectricity and biomass account for over 99.9% of the available flow of renewable energy on Earth. The flows and stores of solar energy in the environment are vast in comparison to current human energy needs. The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850 zettajoules (ZJ) per year, while global wind energy at 80 m, the minimum height of modern large wind turbines, is estimated at 2.25 ZJ per year. Photosynthesis captures approximately 3 ZJ per year in biomass. In contrast, worldwide electricity consumption was approximately 0.0567 ZJ in 2005, and total worldwide primary energy consumption was 0.487 ZJ in the same year. APPLICATIONS OF SOLAR ENERGY TECHNOLOGY Solar energy technologies use solar radiation for practical ends. Technologies that use secondary solar resources such as

biomass, wind, waves and ocean thermal gradients can be included in a broader description of solar energy but only primary resource applications are discussed here. Because the performance of solar technologies varies widely between regions, solar technologies should be deployed in a way that carefully considers these variations. Solar technologies such as photovoltaics and water heaters increase the supply of energy and may be characterized as supply side technologies. Technologies such as passive design and shading devices reduce the need for alternate resources and may be characterized as demand side. Optimizing the performance of solar technologies is often a matter of controlling the resource rather than simply maximizing its collection. ARCHITECTURE AND URBAN PLANNING Darmstadt University of Technology won the 2007 Solar Decathlon with this passive house designed specifically for the humid and hot subtropical climate in Washington, D.C. Sunlight has influenced building design since the beginning of architectural history. Fully developed solar architecture and urban planning methods were first employed by the Greeks and Chinese who oriented their buildings toward the south to provide light and warmth. The elemental features of passive solar architecture are Sun orientation, compact proportion (a low surface area to volume ratio), selective shading (overhangs) and thermal mass. When these features are tailored to the local climate and environment they can produce well-lit spaces that stay in a comfortable temperature range. Socrates' Megaron House is a classic example of passive solar design. The most recent approaches to solar design use computer modeling to tie together solar lighting, heating and ventilation systems in an integrated solar design package. Active solar equipment such as pumps, fans and switchable windows can also complement passive design and improve system performance. Urban heat islands (UHI) are metropolitan areas with higher temperatures than the surrounding environment. These higher temperatures are the result of urban materials such as asphalt and

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Introduction SOLAR LIGHTING

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concrete that have lower albedos and higher heat capacities than the natural environment. A straightforward method of counteracting the UHI effect is to paint buildings and roads white and plant trees. Using these methods, a hypothetical "cool communities" program in Los Angeles has projected that urban temperatures could be reduced by approximately 3 C at an estimated cost of US$1 billion, giving estimated total annual benefits of US$530 million from reduced air-conditioning costs and healthcare savings. AGRICULTURE AND HORTICULTURE Agriculture inherently seeks to optimize the capture of solar energy, and thereby plant productivity. Techniques such as timed planting cycles, tailored row orientation, staggered heights between rows and the mixing of plant varieties can improve crop yields. While sunlight is generally considered a plentiful resource, there are exceptions which highlight the importance of solar energy to agriculture. During the short growing seasons of the Little Ice Age, French and English farmers employed fruit walls to maximize the collection of solar energy. These walls acted as thermal masses and accelerated ripening by keeping plants warm. Early fruit walls were built perpendicular to the ground with a south facing orientation but over time sloping walls were developed to make better use of sunlight. In 1699, Nicolas Fatio de Duillier even suggested using a tracking mechanism, which could pivot to follow the Sun. Solar energy applications in agriculture, aside from growing crops, include pumping water, drying crops, brooding chicks and drying chicken manure. Greenhouses convert solar light to heat enabling year-round production and the growth (in enclosed environments) of specialty crops and other plants not naturally suited to the local climate. Primitive greenhouses were first used during Roman times to produce cucumbers year-round for the Roman emperor Tiberius. The first modern greenhouses were built in Europe in the 16th century to keep exotic plants brought back from explorations abroad. Greenhouses remain an important part of horticulture today, while plastic transparent materials have also been used to similar effect in polytunnels and row covers.

The history of lighting is dominated by the use of natural light. The Romans recognized a right to light as early as the 6th century and English law echoed these judgments with the Prescription Act of 1832. In the 20th century artificial lighting became the main source of interior illumination. Daylighting systems collect and distribute sunlight to provide interior illumination; they are passive systems. These systems directly offset energy use by replacing artificial lighting, and indirectly offset non-solar energy use by reducing the need for airconditioning. The use of natural lighting also offers physiological and psychological benefits compared to artificial lighting, Although difficult to quantify. Daylighting design implies careful selection of window types, sizes and orientation; exterior shading devices may also be considered. Individual features include sawtooth roofs, clerestory windows, light shelves, skylights and light tubes. These features may be incorporated into existing structures, but are most effective when integrated into a solar design package that accounts for factors such as glare, heat flux and time-of-use. When daylighting features are properly implemented they can reduce lighting-related energy requirements by 25%. The most important of the active solar lighting methods is the hybrid solar lighting (HSL). HSL systems collect sunlight using focusing mirrors that track the Sun and use optical fibers to transmit the light into a building's interior to supplement conventional lighting. In single-story applications, these systems are able to transmit 50% of the direct sunlight received. Although daylight saving time is promoted as a way to use sunlight to save energy, recent research has been limited and reports contradictory results: several studies report savings, but just as many suggest no effect or even a net loss, particularly when gasoline consumption is taken into account. Electricity use is greatly affected by geography, climate and economics, making it hard to generalize from single studies. SOLAR THERMAL Solar thermal technologies can be used for water heating, space heating, space cooling and process heat generation.

6 WATER HEATING


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Solar hot water systems use sunlight to heat water. In low geographical latitudes (below 40 degrees) solar heating system can provide from 60 to 70% of domestic hot water use with temperatures up to 60 C. The most common types of solar water heaters are evacuated tube collectors (44%) and glazed flat plate collectors (34%) generally used for domestic hot water; and unglazed plastic collectors (21%) used mainly to heat swimming pools. As of 2007, the total installed capacity of solar hot water systems is approximately 154 GW. China is the world leader in the deployment of solar hot water with 70 GW installed as of 2006 and a long term goal of 210 GW by 2020. Israel is the per capita leader in the use of solar hot water with 90% of homes using this technology. In the United States, Canada and Australia, heating swimming pools is the dominant application of solar hot water, with an installed capacity of 18 GW as of 2005. HEATING, COOLING AND VENTILATION In the United States, heating, ventilation and air conditioning (HVAC) systems account for 30% (4.65 EJ) of the energy used in commercial buildings and nearly 50% (10.1 EJ) of the energy used in residential buildings. Solar heating, cooling and ventilation technologies can be used to offset a portion of this energy. Thermal mass, in the most general sense, is any material that has the capacity to store heat. In the context of solar energy, thermal mass materials are used to store heat from the Sun. Common thermal mass materials include stone, cement and water. These materials have historically been used in arid climates or warm temperate regions to keep buildings cool by absorbing solar energy during the day and radiating stored heat to the cooler atmosphere at night, but they can also be used in cold temperate areas to maintain warmth. The size and placement of thermal mass should consider several factors such as climate, daylighting and shading conditions. When properly incorporated, thermal mass maintains space temperatures in a comfortable range and reduces the need for auxiliary heating and cooling equipment.

A solar chimney (or thermal chimney) is a passive solar ventilation system composed of a vertical shaft connecting the interior and exterior of a building. As the chimney warms, the air inside is heated causing an updraft that pulls air through the building. Performance can be improved by using glazing and thermal mass materials in a way that mimics greenhouses. These systems have been in use since Roman times and remain common in the Middle East. Deciduous trees and plants have often been promoted as a means of controlling solar heating and cooling. When planted on the southern side of a building, the leaves provide shade during the summer while the bare limbs allow light and warmth to pass during the winter. Since bare, leafless trees shade 1/3 to 1/2 of incident solar radiation, there is a balance between the benefits of summer shading and the corresponding loss of winter heating. In climates with significant heating loads, deciduous trees should not be planted on the southern side of a building because they will interfere with winter solar availability but they can be used on the east and west sides to provide a degree of summer shading without appreciably affecting winter solar gain. DESALINATION AND DISINFECTION The production of potable water from saline or brackish water using solar energy is called the solar distillation. The first recorded use was by 16th century Arab alchemists. The first large-scale solar distillation project was constructed in 1872 in the Chilean mining town of Las Salinas. This plant, which had solar collection area of 4,700 m, still could produce up to 22,700 L per day and operated for 40 years. Individual still designs include single-slope, double-slope (or greenhouse type), vertical, conical, inverted absorber, multi-wick and multiple effect. These stills can operate in passive, active or hybrid modes. Double slope stills are the most economical for decentralized domestic purposes while active multiple effect units are more suitable for large-scale applications. Solar water disinfection (SODIS) is a method of disinfecting water by exposing water-filled plastic polyethylene terephthalate (PET) bottles to several hours of sunlight. Exposure times vary depending on weather and climate from a minimum of six hours

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Introduction PROCESS HEAT

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to two days during fully overcast conditions. SODIS is recommended by the World Health Organization as a viable method for household water treatment and safe storage. Over two million people in developing countries use SODIS for their daily drinking water needs. COOKING Solar cookers use sunlight for cooking, drying and pasteurization. These devices can be grouped into three broad categories: box cookers, panel cookers and reflector cookers. The simplest type of solar cooker is the box cooker first built by Horace de Saussure in 1767. A basic box cooker consists of an insulated container with a transparent lid. These cookers can be used effectively with partially overcast skies and will typically reach temperatures of 90-150 C. Panel cookers use a reflective panel to direct sunlight onto an insulated container and reach temperatures comparable to box cookers. Reflector cookers use various concentrating geometries (dish, trough, Fresnel mirrors) to focus light on a cooking container. These cookers reach temperatures of 315 C and above but require direct light to function properly and must be repositioned to track the Sun. The solar bowl is a unique concentrating technology employed by the Solar Kitchen in Auroville, India. The solar bowl is a stationary spherical reflector that focuses light along a line perpendicular to the sphere's interior surface and a computer control system moves the receiver to intersect this line. Steam is produced in the receiver at temperatures reaching 150 C and then used for process heat in the kitchen. A reflector developed by Wolfgang Scheffler in 1986 is used in many solar kitchens. Scheffler reflectors are flexible parabolic dishes that combine aspects of trough and power tower concentrators. Polar tracking is used to follow the Sun's daily course and the curvature of the reflector is adjusted for seasonal variations in the incident angle of sunlight. These reflectors can reach temperatures of 450-650 C and have a fixed focal point which improves the ease of cooking. The world's largest Scheffler reflector system in Abu Road, Rajasthan, India is capable of cooking up to 35,000 meals a day. As of 2008, over 2,000 large Scheffler cookers had been built worldwide.

Solar concentrating technologies such as parabolic dish, trough and Scheffler reflectors can provide process heat for commercial and industrial applications. The first commercial system was the Solar Total Energy Project (STEP) in Shenandoah, Georgia where a field of 114 parabolic dishes provided 50% of the process heating, air conditioning and electrical requirements for a clothing factory. This grid-connected cogeneration system provided 400 kW of electricity plus thermal energy in the form of 401 kW steam and 468 kW chilled water, and had a one hour peak load thermal storage. Evaporation ponds are shallow pools that concentrate dissolved solids through evaporation. The use of evaporation ponds to obtain salt from sea water is one of the oldest applications of solar energy. Modern uses include concentrating brine solutions used in leach mining and removing dissolved solids from waste streams. Clothes lines, clotheshorses, and clothes racks dry clothes through evaporation. These devices use wind and sunlight instead of electricity or natural gas. Florida legislation specifically protects the 'right to dry' and similar solar rights legislation has been passed in Utah and Hawaii. Unglazed transpired collectors (UTC) are perforated sun-facing walls used for preheating ventilation air. UTCs can raise the incoming air temperature up to 22 C and deliver outlet temperatures of 45-60 C. The short payback period of transpired collectors (3 to 12 years) makes them a more cost-effective alternative than glazed collection systems. As of 2003, over 80 systems with a combined collector area of 35,000 m had been installed worldwide, including an 860 m collector in Costa Rica used for drying coffee beans and a 1,300 m collector in Coimbatore, India used for drying marigolds. SOLAR ELECTRICITY Sunlight can be converted into electricity using photovoltaics (PV), concentrating solar power (CSP), and various experimental technologies. PV has mainly been used to power small and medium-sized applications, from the calculator powered by a

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single solar cell to off-grid homes powered by a photovoltaic array. For large-scale generation, CSP plants like SEGS have been the norm but recently multi-megawatt PV plants are becoming common. Completed in 2007, the 14 MW power station in Clark County, Nevada and the 20 MW site in Beneixama, Spain are characteristic of the trend toward larger photovoltaic power stations in the US and Europe. PHOTOVOLTAICS A solar cell (or photovoltaic cell) is a device that converts light into direct current using the photoelectric effect. The first solar cell was constructed by Charles Fritts in the 1880s. Although the prototype selenium cells converted less than 1% of incident light into electricity, both Ernst Werner von Siemens and James Clerk Maxwell recognized the importance of this discovery. Following the fundamental work of Russell Ohl in the 1940s, researchers Gerald Pearson, Calvin Fuller and Daryl Chapin created the silicon solar cell in 1954. These early solar cells cost 286 USD/watt and reached efficiencies of 4.5-6%. The earliest significant application of solar cells was as a backup power source to the Vanguard I satellite, which allowed the satellite to continue transmitting for over a year after its chemical battery was exhausted. The successful operation of solar cells on this mission was duplicated in many other Soviet and American satellites, and by the late 1960s PV had become the established source of power for satellites. Photovoltaics went on to play an essential part in the success of early commercial satellites such as Telstar and continue to remain vital to the telecommunications infrastructure today. The high cost of solar cells limited terrestrial uses throughout the 1960s. This changed in the early 1970s when prices reached levels that made PV generation competitive in remote areas without grid access. Early terrestrial uses included powering telecommunication stations, off-shore oil rigs, navigational buoys and railroad crossings. These and other off-grid applications have proven very successful and accounted for over half of worldwide installed capacity until 2004. The 1973 oil crisis stimulated a rapid rise in the production of PV during the 1970s and early 1980s.

Economies of scale which resulted from increasing production along with improvements in system performance brought the price of PV down from 100 USD/watt in 1971 to 7 USD/watt in 1985. Steadily falling oil prices during the early 1980s led to a reduction in funding for photovoltaic R&D and a discontinuation of the tax crs associated with the Energy Tax Act of 1978. These factors moderated growth to approximately 15% per year from 1984 through 1996. Since the mid-1990s, leadership in the PV sector has shifted from the US to Japan and Germany. Between 1992 and 1994 Japan increased R&D funding, established net metering guidelines, and introduced a subsidy program to encourage the installation of residential PV systems. As a result, PV installations in the country climbed from 31.2 MW in 1994 to 318 MW in 1999, and worldwide production growth increased to 30% in the late 1990s. Germany has become the leading PV market worldwide since revising its Feed-in tariff system as part of the Renewable Energy Sources Act. Installed PV capacity has risen from 100 MW in 2000 to approximately 4,150 MW at the end of 2007. Spain has become the third largest PV market after adopting a similar feed-in tariff structure in 2004, while France, Italy, South Korea and the US have also seen rapid growth recently due to various incentive programs and local market conditions. CONCENTRATING SOLAR POWER Concentrating Solar Power (CSP) systems is divided into Concentrating solar thermal (CST) and Concentrating PV (CPV). CSP use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exist; the most developed are the solar trough, parabolic dish and solar power tower. These methods vary in the way they track the Sun and focus light. In all these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage. A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The reflector is made to follow the Sun during the

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daylight hours by tracking along a single axis. Trough systems are the most mature CSP technology. The SEGS plants in California and Acciona's Nevada Solar One near Boulder City, Nevada are representatives of this technology. EXPERIMENTAL SOLAR POWER A solar updraft tower (also known as a solar chimney or solar tower) consists of a large greenhouse that funnels into a central tower. As sunlight shines on the greenhouse, the air inside is heated and expands. The expanding air flows toward the central tower where a turbine converts the air flow into electricity. A 50 kW prototype was constructed in Ciudad Real, Spain and operated for eight years before decommissioning in 1989. A solar pond is a pool of salt water (usually 1-2 m deep) that collects and stores solar energy. Solar ponds were first proposed by Dr. Rudolph Bloch in 1948 after he came across reports of a lake in Hungary in which the temperature increased with depth. This effect was due to salts in the lake's water, which created a "density gradient" that prevented convection currents. A prototype was constructed in 1958 on the shores of the Dead Sea near Jerusalem. The pond consisted of layers of water that successively increased from a weak salt solution at the top to a high salt solution at the bottom. This solar pond was capable of producing temperatures of 90 C in its bottom layer and had an estimated solar-to-electric efficiency of two percent. Thermoelectric devices convert a temperature difference between dissimilar materials into an electric current. First proposed as a method to store solar energy by solar pioneer Mouchout in the 1800s, thermoelectrics reemerged in the Soviet Union during the 1930s. Under the direction of Soviet scientist Abram Ioffe a concentrating system was used to thermoelectrically generate power for a 1 hp engine. Thermogenerators were later used in the US space program as an energy conversion technology for powering deep space missions such as Cassini, Galileo and Viking. Research in this area is focused on raising the efficiency of these devices from 7-8% to 15-20%. Space solar power systems use a large solar array in geosynchronous orbit to collect sunlight and beam this energy in

the form of microwave radiation to receivers (rectennas) on Earth for distribution. This concept was first proposed by Dr. Peter Glaser in 1968 and since then a wide variety of systems have been studied with both photovoltaic and concentrating solar thermal technologies being proposed. Although still in the concept stage, these systems offer the possibility of delivering power approximately 96% of the time. SOLAR CHEMICAL Solar radiation stimulated chemical processes use solar energy to drive chemical reactions. These processes offset energy that would otherwise be required from an alternate source and can convert solar energy into a storable and transportable fuel. Solar induced chemical reactions are diverse, but can be divided into thermochemical or photochemical. Hydrogen production production technologies involving the use of solar light have been a significant area of research since the 1970s. Aside from electrolysis driven by photovoltaic or photochemical cells, several thermochemical processes have also been explored. The seemingly most direct of these routes uses concentrators to split water at high temperatures (2300-2600 C), but this process has been limited by complexity and low solar-to-hydrogen efficiency (1-2%). A more conventional approach uses the heat from solar concentrators to drive the steam reformation of natural gas thereby increasing the overall hydrogen yield. Thermochemical cycles characterized by the decomposition and regeneration of reactants present another avenue for hydrogen production. The Solzinc process under development at the Weizmann Institute uses a 1 MW solar furnace to decompose zinc oxide (ZnO) at temperatures above 1200 C. This initial reaction produces pure zinc, which can subsequently be reacted with water to produce hydrogen. Sandia's Sunshine to Petrol (S2P) technology uses the high temperatures generated by concentrating sunlight along with a zirconia/ferrite catalyst to break down atmospheric carbon dioxide into oxygen and carbon monoxide (CO). The CO may then be used to synthesize methanol, gasoline and jet fuel. Photoelectrochemical cells or PECs consist of a semiconductor, typically titanium dioxide or related titanates, immersed in an

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electrolyte. When the semiconductor is illuminated an electrical potential develops. There are two types of photoelectrochemical cells: photoelectric cells that convert light into electricity and photochemical cells that use light to drive chemical reactions such as electrolysis. A photogalvanic device is a type of battery in which the cell solution (or equivalent) forms energy-rich chemical intermediates when illuminated. These chemical intermediates then react at the electrodes to produce an electric potential. The ferric-thionine chemical cell is an example of this technology. SOLAR VEHICLES Development of a solar powered car has been an engineering goal since the 1980s. The World Solar Challenge is a biannual solar-powered car race, in which teams from universities and enterprises compete over 3,021 kilometres (1,877 mi) across central Australia from Darwin to Adelaide. In 1987, when it was founded, the winner's average speed was 67 kilometres per hour (42 mph). The 2007 race included a new challenge class using cars which could be a practical proposition for sustainable transport with little modification. The winning car averaged 90.87 kilometres per hour (56.46 mph). The North American Solar Challenge and the planned South African Solar Challenge are comparable competitions that reflect an international interest in the engineering and development of solar powered vehicles. In 1975, the first practical solar boat was constructed in England. By 1995, passenger boats incorporating PV panels began appearing and are now used extensively. In 1996, Kenichi Horie made the first solar powered crossing of the Pacific Ocean, and the sun catamaran made the first solar powered crossing of the Atlantic Ocean in the winter of 2006-2007. Plans to circumnavigate the globe in 2009 are indicative of the progress solar boats have made. In 1974, the unmanned Sunrise II inaugurated the era of solar flight. In 1980, the Gossamer Penguin made the first piloted flights powered solely by photovoltaics. This was quickly followed by the Solar Challenger which demonstrated a more airworthy design with its crossing of the English Channel in July 1981. Developments then turned back to unmanned aerial vehicles (UAV) with the

Pathfinder (1997) and subsequent designs, culminating in the Helios which set the altitude record for a non-rocket-propelled aircraft at 29,524 metres (96,860 ft) in 2001. The Zephyr, developed by BAE Systems, is the latest in a line of record-breaking solar aircraft, making a 54-hour flight in 2007, and month-long flights are envisioned by 2010. A solar balloon is a black balloon that is filled with ordinary air. As sunlight shines on the balloon, the air inside is heated and expands, causing an upward buoyancy force, much like an artificially-heated hot air balloon. Some solar balloons are large enough for human flight, but usage is limited to the toy market as the surface-area to payload-weight ratio is relatively high. Solar sails are a proposed form of spacecraft propulsion using large membrane mirrors to exploit radiation pressure from the sun. Unlike rockets, solar sails require no fuel. Although the thrust is small compared to rockets, it continues as long as the Sun shines onto the deployed sail and in the frictionless vacuum of space significant speeds can eventually be achieved.

ENERGY STORAGE METHODS Storage is an important issue in the development of solar energy because modern energy systems usually assume continuous availability of energy. Solar energy is not available at night, and the performance of solar power systems is affected by unpredictable weather patterns; therefore, storage media or back-up power systems must be used. Thermal mass systems can store solar energy in the form of heat at domestically useful temperatures for daily or seasonal durations. Thermal storage systems generally use readily available materials with high specific heat capacities such as water, earth and stone. Well-designed systems can lower peak demand, shift time-of-use to off-peak hours and reduce overall heating and cooling requirements. Phase change materials such as paraffin wax and Glauber's salt are another thermal storage media. These materials are inexpensive, readily available, and can deliver domestically useful temperatures (approximately 64 C). The "Dover House" (in Dover, Massachusetts) was the first to use a Glauber's salt heating system,

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in 1948. Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are low-cost, have a high specific heat capacity and can deliver heat at temperatures compatible with conventional power systems. The Solar Two used this method of energy storage, allowing it to store 1.44 TJ in its 68 m storage tank with an annual storage efficiency of about 99%. Off-grid PV systems have traditionally used rechargeable batteries to store excess electricity. With grid-tied systems, excess electricity can be sent to the transmission grid. Net metering programs give these systems a cr for the electricity they deliver to the grid. This cr offsets electricity provided from the grid when the system cannot meet demand, effectively using the grid as a storage mechanism. Pumped-storage hydroelectricity stores energy in the form of water pumped when energy is available from a lower elevation reservoir to a higher elevation one. The energy is recovered when demand is high by releasing the water to run through a hydroelectric power generator. DEVELOPMENT, DEPLOYMENT AND ECONOMICS Beginning with the surge in coal use which accompanied the Industrial Revolution, energy consumption has steadily transitioned from wood and biomass to fossil fuels. The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce, but solar development stagnated in the early 20th century in the face of the increasing availability, economy, and utility of fossil fuels such as coal and petroleum. The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies. Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utilization Program in the US and the Sunshine Program in Japan. Other efforts included the formation of research facilities in the US (SERI, now NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE). Between 1970 and 1983, photovoltaic installations grew rapidly, but falling oil

prices in the early 1980s moderated the growth of PV from 1984 to 1996. Since 1997, PV development has accelerated due to supply issues with oil and natural gas, global warming concerns (see Kyoto Protocol), and the improving economic position of PV relative to other energy technologies. Photovoltaic production growth has averaged 40% per year since 2000 and installed capacity reached 10.6 GW at the end of 2007. Since 2006 it has been economical for investors to install photovoltaics for free in return for a long term power purchase agreement. 50% of commercial systems were installed in this manner in 2007 and it is expected that 90% will by 2009. Nellis Air Force Base is receiving photoelectric power for about 2.2 /kWh and grid power for 9 /kWh. Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were gradually replaced by cheaper and more reliable heating fuels. As with photovoltaics, solar water heating attracted renewed attention as a result of the oil crises in the 1970s but interest subsided in the 1980s due to falling petroleum prices. Development in the solar water heating sector progressed steadily throughout the 1990s and growth rates have averaged 20% per year since 1999. Although generally underestimated, solar water heating is by far the most widely deployed solar technology with an estimated capacity of 154 GW as of 2007. Commercial concentrating solar power (CSP) plants were first developed in the 1980s. CSP plants such as SEGS project in the United States have a LEC of 12-14 /kWh. The 11 MW PS10 power tower in Spain, completed in late 2005, is Europe's first commercial CSP system and a total capacity of 300 MW is expected to be installed in the same area by 2013.

CARBON NANOTUBES IN PHOTOVOLTAICS Organic photovoltaic devices (OPVs) are fabricated from thin films of organic semiconductors, such as polymers and smallmolecule compounds, and are typically on the order of 100 nm thick. Because polymer based OPVs can be made using a coating process such as spin coating or inkjet printing, they are an attractive option for inexpensively covering large areas as well as flexible

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plastic surfaces. A promising low cost alternative to silicon solar cells, there is a large amount of research being dedicated throughout industry and academia towards developing OPVs and increasing their power conversion efficiency.

CARBON NANOTUBE COMPOSITES IN THE PHOTOACTIVE LAYER Combining the physical and chemical characteristics of conjugated polymers with the high conductivity along the tube axis of carbon nanotubes (CNTs) provides a great deal of incentive to disperse CNTs into the photoactive layer in order to obtain more efficient OPV devices. The interpenetrating bulk donoracceptor heterojunction in these devices can achieve charge separation and collection because of the existence of a bicontinuous network. Along this network, electrons and holes can travel toward their respective contacts through the electron acceptor and the polymer hole donor. Photovoltaic efficiency enhancement is proposed to be due to the introduction of internal polymer/ nanotube junctions within the polymer matrix. The high electric field at these junctions can split up the excitons, while the SWNT can act as a pathway for the electrons. The dispersion of CNTs in a solution of an electron donating conjugated polymer is perhaps the most common strategy to implement CNT materials into OPVs. Generally poly(3hexylthiophene) (P3HT) or poly(3-octylthiophene) (P3OT) are used for this purpose. These blends are then spin coated onto a transparent conductive electrode with thicknesses that vary from 60 to 120 nm. These conductive electrodes are usually glass covered with indium tin oxide (ITO) and a 40 nm sublayer of (poly (3,4ethylenedioxythiophene) (PEDOT) and poly(styrenesulfonate) (PSS). PEDOT and PSS help to smooth the ITO surface, decreasing the density of pinholes and stifling current leakage that occurs along shunting paths. Through thermal evaporation or sputter coating, a 20 to 70 nm thick layer of aluminum and sometimes an intermediate layer of lithium fluoride are then applied onto the photoactive material. Multiple research investigations with both multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) integrated into the photoactive

material have been completed. Enhancements of more than two orders of magnitude have been observed in the photocurrent from adding SWCNTs to the P3OT matrix. Improvements were speculated to be due to charge separation at polymer-SWCNT connections and more efficient electron transport through the SWCNTs. However, a rather low power conversion efficiency of 0.04% under 100 mW cm-2 white illumination was observed for the device suggesting incomplete exciton dissociation at low CNT concentrations of 1.0% wt. Because the lengths of the SWCNTs were similar to the thickness of photovoltaic films, doping a higher percentage of SWCNTs into the polymer matrix was believed to cause short circuits. To supply additional dissociation sites, other researchers have physically blended functionalized MWCNTs into P3HT polymer to create a P3HT-MWNT with fullerene C60 doublelayered device. However, the power efficiency was still relatively low at 0.01% under 100 mW cm-2 white illumination. Weak exciton diffusion toward the donor-acceptor interface in the bilayer structure may have been the cause in addition to the fullerene C60 layer possibly experiencing poor electron transport. More recently, a polymer photovoltaic device from C60modified SWCNTs and P3HT has been fabricated. Microwave irradiating a mixture of SWCNT-water solution and C60 solution in toluene was the first step in making these polymer-SWCNT composites. Conjugated polymer P3HT was then added resulting in a power conversion efficiency of 0.57% under simulated solar irradiation (95 mW cm-2). It was concluded that improved short circuit current density was a direct result of the addition of SWCNTs into the composite causing faster electron transport via the network of SWCNTs. It was also concluded that the morphology change led to an improved the fill factor. Overall, the main result was improved power conversion efficiency with the addition of SWCNTs, compared to cells without SWCNTs; however, further optimization was thought to be possible. Additionally, it has been found that heating to the point beyond the glass transition temperature of either P3HT or P3OT after construction can be beneficial for manipulating the phase separation of the blend. This heating also affects the ordering of the polymeric chains because the polymers are microcrystalline

20


Introduction

21

systems and it improves charge transfer, charge transport, and charge collection throughout the OPV device. The hole mobility and power efficiency of the polymer-CNT device also increased significantly as a result of this ordering. Emerging as another valuable approach for deposition, the use of tetraoctylammonium bromide in tetrahydrofuran has also been the subject of investigation to assist in suspension by exposing SWCNTs to an electrophoretic field. In fact, photoconversion efficiencies of 1.5% and 1.3% were achieved when SWCNTs were deposited in combination with light harvesting CdS quantum dots and porphyrins, respectively. Among the best power conversions achieved to date using CNTs were obtained by depositing a SWCNT layer between the ITO and the PEDOT : PSS or between the PEDOT : PSS and the photoactive blend in a modified ITO/PEDOT : PSS/ P3HT : (6,6)phenyl-C61-butyric acid methyl ester (PCBM)/Al solar cell. By dip-coating from a hydrophilic suspension, SWCNT were deposited after an initially exposing the surface to an argon plasma to achieve a power conversion efficiency of 4.9%, compared to 4% without CNTs. However, even though CNTs have shown potential in the photoactive layer, they have not resulted in a solar cell with a power conversion efficiency greater than the best tandem organic cells (6.5% efficiency). But, it has been shown in most of the previous investigations that the control over a uniform blending of the electron donating conjugated polymer and the electron accepting CNT is one of the most difficult as well as crucial aspects in creating efficient photocurrent collection in CNT-based OPV devices. Therefore, using CNTs in the photoactive layer of OPV devices is still in the initial research stages and there is still room for novel methods to better take advantage of the beneficial properties of CNTs.

Traditional ITO also has unfavorable mechanical properties such as being relatively fragile. In addition, the combination of costly layer deposition in vacuum and a limited supply of indium results in high quality ITO transparent electrodes being very expensive. Therefore, developing and commercializing a replacement for ITO is a major focus of OPV research and development. Conductive CNT coatings have recently become a prospective substitute based on wide range of methods including spraying, spin coating, casting, layer-by-layer, and Langmuir-Blodgett deposition. The transfer from a filter membrane to the transparent support using a solvent or in the form of an adhesive film is another method for attaining flexible and optically transparent CNT films. Other research efforts have shown that films made of arc-discharge CNT can result in a high conductivity and transparency. Furthermore, the work function of SWCNT networks is in the 4.8 to 4.9 eV range (compared to ITO which has a lower work function of 4.7 eV) leading to the expectation that the SWCNT work function should be high enough to assure efficient hole collection. Another benefit is that SWCNT films exhibit a high optical transparency in a broad spectral range from the UV-visual far into the near IR range. Only a few materials retain reasonable transparency in the infrared spectrum while maintaining transparency in the visible part of the spectrum as well as acceptable overall electrical conductivity. SWCNT films are highly flexible, do not creep, do not crack after bending, theoretically have high thermal conductivities to tolerate heat dissipation, and have high radiation resistance. However, the electrical sheet resistance of ITO is an order of magnitude less than the sheet resistance measured for SWCNT films. Nonetheless, initial research studies demonstrate SWCNT thin films can be used as conducting, transparent electrodes for hole collection in OPV devices with efficiencies between 1% and 2.5% confirming that they are comparable to devices fabricated using ITO. Thus, possibilities exist for advancing this research to develop CNT-based transparent electrodes that exceed the performance of traditional ITO materials.

CARBON NANOTUBES AS A TRANSPARENT ELECTRODE ITO is currently the most popular material used for the transparent electrodes in OPV devices; however, it has a number of deficiencies. For one, it is not very compatible with polymeric substrates due to its high deposition temperature of around 600oC.

CNTS IN DYE-SENSITIZED SOLAR CELLS Due to the simple fabrication process, low production cost, and high efficiency, there is significant interest in dye-sensitized

22


Introduction

23

solar cells (DSSCs). Thus, improving DSSC efficiency has been the subject of a variety of research investigations because it has the potential to be manufactured economically enough to compete with other solar cell technologies. Titanium dioxide nanoparticles have been widely used as a working electrode for DSSCs because they provide a high efficiency, more than any other metal oxide semiconductor investigated. Yet the highest conversion efficiency under air mass (AM) 1.5 (100mWcm2) irradiation reported for this device to date is about 11%. Despite this initial success, the effort to further enhance efficiency has not produced any major results. The transport of electrons across the particle network has been a key problem in achieving higher photoconversion efficiency in nanostructured electrodes. Because electrons encounter many grain boundaries during the transit and experience a random path, the probability of their recombination with oxidized sensitizer is increased. Therefore, it is not adequate to enlarge the oxide electrode surface area to increase efficiency because photo-generated charge recombination should be prevented. Promoting electron transfer through film electrodes and blocking interface states lying below the edge of the conduction band are some of the non-CNT based strategies to enhance efficiency that have been employed. With recent progress in CNT development and fabrication, there is promise to use various CNT based nanocomposites and nanostructures to direct the flow of photogenerated electrons and assist in charge injection and extraction. To assist the electron transport to the collecting electrode surface in a DSSC, a popular concept is to utilize CNT networks as support to anchor light harvesting semiconductor particles. Research efforts along these lines include organizing CdS quantum dots on SWCNTs. Charge injection from excited CdS into SWCNTs was documented upon excitation of CdS nanoparticles. Other varieties of semiconductor particles including CdSe and CdTe can induce charge-transfer processes under visible light irradiation when attached to CNTs. Including porphyrin and C60 fullerene, organization of photoactive donor polymer and acceptor fullerene on electrode surfaces has also been shown to offer considerable improvement in the photoconversion efficiency of solar cells. Therefore, there is an

opportunity to facilitate electron transport and increase the photoconversion efficiency of DSSCs utilizing the electronaccepting ability of semiconducting SWCNTs. Other researchers fabricated DSSCs using the sol-gel method to obtain titanium dioxide coated MWCNTs for use as an electrode. Because pristine MWCNTs have a hydrophobic surface and poor dispersion stability, pretreatment was necessary for this application. A relatively low destruction method for removing impurities, H2O2 treatment was used to generate carboxylic acid groups by oxidation of MWCNTs. Another positive aspect was the fact that the reaction gases including CO2 and H2O were nontoxic and could be released safely during the oxidation process. As a result of treatment, H 2O2 exposed MWCNTs have a hydrophilic surface and the carboxylic acid groups on the surface have polar covalent bonding. Also, the negatively charged surface of the MWCNTs improved the stability of dispersion. By then entirely surrounding the MWCNTs with titanium dioxide nanoparticles using the sol-gel method, an increase in the conversion efficiency of about 50% compared to a conventional titanium dioxide cell was achieved. The enhanced interconnectivity between the titanium dioxide particles and the MWCNTs in the porous titanium dioxide film was concluded to be the cause of the improvement in short circuit current density. Here again, the addition of MWCNTs was thought to provide more efficient electron transfer through film in the DSSC. ENERGY STORAGE Energy storage is the storing of some form of energy that can be drawn upon at a later time to perform some useful operation. A device that stores energy is sometimes called an accumulator. All forms of energy are either potential energy (eg. chemical, gravitational or electrical energy) or kinetic energy (eg. thermal energy). A wind up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to keep a clock chip in a computer running (electrically) even when the computer is turned off, and a hydroelectric dam stores power in a reservoir as gravitational potential energy. Even food is a form of energy storage, chemical in this case.

24


Introduction

25

HISTORY Energy storage as a natural process is as old as the universe itself - the energy present at the initial creation of the Universe has been stored in stars such as the Sun, and is now being used by humans directly (e.g. through solar heating), or indirectly (e.g. by growing crops or conversion into electricity in solar cells). Energy storage systems in commercial use today can be broadly categorized as mechanical, electrical, chemical, biological, thermal and nuclear. As a purposeful activity, energy storage has existed since pre-history, though it was often not explicitly recognized as such. An example of deliberate mechanical energy storage is the use of logs or boulders as defensive measures in ancient forts the logs or boulders were collected at the top of a hill or wall, and the energy thus stored used to attack invaders who came within range. A more recent application is the control of waterways to drive water mills for processing grain or powering machinery. Complex systems of reservoirs and dams were constructed to store and release water (and the potential energy it contained) when required. Energy storage became a dominant factor in economic development with the widespread introduction of electricity and refined chemical fuels, such as gasoline, kerosene and natural gas in the late 1800s. Unlike other common energy storage used in prior use, such as wood or coal, electricity must be used as it is generated and cannot be stored on anything other than a minor scale. Electricity is transmitted in a closed circuit, and for essentially any practical purpose cannot be stored as electrical energy. This meant that changes in demand could not be accommodated without either cutting supplies (eg, via brownouts or blackouts) or arranging for a storage technique. An early solution to the problem of storing energy for electrical purposes was the development of the battery, an electrochemical storage device. It has been of limited use in electric power systems due to small capacity and high cost. A similar possible solution with the same type of problems is the capacitor. Chemical fuels have become the dominant form of energy storage, both in electrical generation and energy transportation. Chemical fuels in common use are processed coal, gasoline, diesel

fuel, natural gas, liquefied petroleum gas (LPG), propane, butane, ethanol, biodiesel and hydrogen. All of these chemicals are readily converted to mechanical energy and then to electrical energy using heat engines (turbines or other internal combustion engines, or boilers or other external combustion engines) used for electrical power generation. Heat engine powered generators are nearly universal, ranging from small engines producing only a few kilowatts to utility-scale generators with ratings up to 800 megawatts. Electrochemical devices called fuel cells were invented about the same time as the battery. However, for many reasons, fuel cells were not well developed until the advent of manned spaceflight (the Gemini Program) when lightweight, non-thermal (ie, efficient) sources of electricity were required in spacecraft. Fuel cell development has increased in recent years to an attempt to increase conversion efficiency of chemical energy stored in hydrocarbon or hydrogen fuels into electricity. At this time, liquid hydrocarbon fuels are the dominant forms of energy storage for use in transportation. However, these produce greenhouse gases when used to power cars, trucks, trains, ships and aircraft. Carbon-free energy carriers, such as hydrogen, or carbon-neutral energy carriers, such as some forms of ethanol or biodiesel, are being sought in response to concerns about the possible consequences of greenhouse gas emissions. Some areas of the world (Washington and Oregon in the USA, and Wales in the United Kingdom are examples) have used geographic features to store large quantities of water in elevated reservoirs, using excess electricity at times of low demand to pump water up to the reservoirs, then letting the water fall through turbine generators to retrieve the energy when demand peaks. Several other technologies have also been investigated, such as flywheels or compressed air storage in underground caverns, but to date no widely available solution to the challenge of mass energy storage has been deployed commercially.

GRID ENERGY STORAGE Grid energy storage lets energy producers send excess electricity over the electricity transmission grid to temporary

26


Introduction

27

electricity storage sites that become energy producers when electricity demand is greater. Grid energy storage is particularly important in matching supply and demand over a 24 hour period of time.

STORAGE METHODS Chemical o Hydrogen o Biofuels Electrochemical o Batteries o Flow batteries o Fuel cells Electrical o Capacitor o Supercapacitor o Superconducting magnetic energy storage (SMES) Mechanical o Compressed air energy storage (CAES) o Flywheel energy storage o Hydraulic accumulator o Hydroelectric energy storage o Spring Thermal o Molten salt o Cryogenic liquid air or nitrogen o Seasonal thermal store o Solar pond o Hot bricks o Steam accumulator o Fireless locomotiveHYDROGEN Hydrogen is a chemical energy carrier, just like gasoline, ethanol or natural gas. The unique characteristic of hydrogen is

that it is the only carbon-free or zero-emission chemical energy carrier. Hydrogen is a widely used industrial chemical that can be produced from any primary energy source. Most of the world's production is by the thermal reformation of natural gas (methane) into hydrogen that is used immediately to refine petroleum into gasoline, diesel fuel and other petrochemicals. The carbon dioxide produced by the reforming process is either captured and processed into liquid carbon dioxide or vented to the atmosphere. Because hydrogen is produced and distributed in such huge quantities, the technology needed to build infrastructure to serve wholesale and retail energy markets is proven, reliable and commercially available. Hydrogen can be used as a fuel for all types of internal and external combustion heat engines and turbines (with adjustments to compensate for the difference between, say, diesel fluid and hydrogen gas). Hydrogen fueled heat engines can be optimized to operate at higher thermal efficiencies than traditional heat engines using traditional hydrocarbon fuels. The increased thermodynamic efficiency, and reduced pollution, would be beneficial, but they are not produced in quantity largely because hydrogen is not industrially available. Sufficiently purified hydrogen can also be used to power electrochemical engines, such as the proton exchange membrane (PEM) fuel cell. Hydrogen fuel cells can be more efficient than hydrogen fueled heat engines, and thus much more efficient than hydrocarbon fuel heat engines. They are also less polluting. Several companies are attempting to develop reliable, inexpensive PEM fuel cells. However, designs are not sufficiently developed to be routinely mass produced. The limited quantities available for purchase are hand made and much more expensive than conventional heat engines. Hydrogen production in quantities sufficient to replace existing hydrocarbon fuels is not possible. Such production will require more energy than is currently being used, and require large capital investment in hydrogen production plants. Because of the increased costs, hydrogen is not yet in widespread use. If the cost of greenhouse gas production is fully included into the market price of hydrocarbon fuels, hydrogen fuels may become more attractive

28


Introduction

29

commercially, providing clean, efficient power for our homes, businesses and vehicles. Disadvantages of hydrogen include a low energy density per volume (even when highly compressed) compared to traditional hydrocarbon fuels, changing such things as the volumes of fuel required for equivalent performance. And, for many hydrogen production methods, there is a significant loss of energy during the conversion. Some production methods, for instance, electrolytic generation from water, are more efficient.

existing fuel distribution infrastructures. Manufacturing synthetic hydrocarbon fuel reduces the amount of carbon dioxide in the atmosphere until the fuel is burned, when the same amount of carbon dioxide returns to the atmosphere. If usable on a wide scale, this approach may help in the long term to avoid some of the deleterious effects of greenhouse gas emission.

BIOFUELS Various biofuels such as biodiesel, straight vegetable oil, alcohol fuels, or biomass can be used to replace hydrocarbon fuels. Various chemical processes can convert the carbon and hydrogen in coal, natural gas, plant and animal biomass, and organic wastes into short hydrocarbons suitable as replacements for existing hydrocarbon fuels. Examples are Fischer-Tropsch diesel, methanol, dimethyl ether, or syngas. This diesel source was used extensively in World War II in Germany, with limited access to crude oil supplies. Today South Africa produces most of country's diesel from coal for similar reasons. A long term oil price above 35 USD may make such synthetic liquid fuels economical on a large scale (See coal). Some of the energy in the original source is lost in the conversion process. Historically, coal itself has been used directly for transportation purposes in vehicles and boats using steam engines. And compressed natural gas is being used in special circumstances fuel, for instance in busses for some mass transit agencies. SYNTHETIC HYDROCARBON FUEL Carbon dioxide in the atmosphere has been, experimentally, converted into hydrocarbon fuel with the help of energy from another source. To be useful industrially, the energy will probably have to come from sunlight using, perhaps, future artificial photosynthesis technology. Another alternative for the energy is electricity or heat from solar energy or nuclear power. Compared to hydrogen, many hydrocarbons fuels have the advantage of being immediately usable in existing engine technology and

BORON, SILICON, AND ZINC Boron, silicon, and zinc have been proposed as energy storage solutions. MECHANICAL STORAGE Energy can be stored in water pumped to a higher elevation, in compressed air, or in spinning flywheels, but mechanical methods of storing energy on a large scale are expensive and water pumping systems require considerable capital investment. Several companies have done preliminary design work for vehicles using compressed air power. INTERMITTENT POWER Many renewable energy systems produce intermittent power. Other generators on the grid can be throttled to match varying production from renewable sources, but most of the existing throttling capacity is already committed to handling load variations. Further development of intermittent renewable power will require some combination of grid energy storage, demand response, and spot pricing. Intermittent energy sources is limited to at most 20-30% of the electricity produced for the grid without such measures. If electricity distribution loss and costs are managed, then intermittent power production from many different sources could increase the overall reliability of the grid. Non-intermittent renewable energy sources include hydroelectric power, geothermal power, solar thermal, tidal power, Energy tower, ocean thermal energy conversion, high altitude airborne wind turbines, biofuel, and solar power satellites. Solar photovoltaics, although technically intermittent, produce electricity

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Solar Variation

31

largely during peak periods (ie, daylight), and hence do reduce the need for peak power generation, though somewhat unreliably in most areas since weather conditions interfere with terrestrially mounted solar cells. On the demand side, demand response programs, which send market pricing signals to consumers (or their equipment), can be a very effective way of managing variations in electricity production. For example, electrically powered hydrogen production can be set to increase when electricity is being produced beyond current demand (and prices will be lowest), and conversely, hot water heaters can be automatically set to a lower temperature when demand is high and pricing is also high.

2SOLAR VARIATIONSolar variations are changes in the amount of solar radiation emitted by the Sun. There are periodic components to these variations, the principal one being the 11-year solar cycle (or sunspot cycle), as well as aperiodic fluctuations. Solar activity has been measured via satellites during recent decades and through 'proxy' variables in prior times. Climate scientists are interested in understanding what, if any, effect variations in solar activity have on the Earth. Effects on the earth caused by solar activity are called "solar forcing". The variations in total solar irradiance (TSI) remained at or below the threshold of detectability until the satellite era, although the small fraction in ultra-violet wavelengths varies by a few percent. Total solar output is now measured to vary (over the last three 11-year sunspot cycles) by approximately 0.1% or about 1.3 W/m peak-to-trough during the 11 year sunspot cycle. The amount of solar radiation received at the outer surface of Earth's atmosphere varied little from an average value of 1,366 watts per square meter (W/m). There are no direct measurements of the longer-term variation and interpretations of proxy measures of variations differ; recent results suggest about 0.1% variation over the last 2,000 years, although other sources suggest a 0.2% increase in solar irradiance since 1675. The combination of solar variation and volcanic effects has very likely been the cause of some climate change, for example during the Maunder Minimum. A 2006 study and review of existing literature, published in Nature, determined that there has been no net increase in solar brightness since the mid 1970s, and that changes in solar output

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Solar Variation

33

within the past 400 years are unlikely to have played a major part in global warming. However, the same report cautions that "Apart from solar brightness, more subtle influences on climate from cosmic rays or the Sun's ultraviolet radiation cannot be excluded, say the authors. They also add that these influences cannot be confirmed because physical models for such effects are still too poorly developed." HISTORY OF STUDY INTO SOLAR VARIATIONS The longest recorded aspect of solar variations are changes in sunspots. The first record of sunspots dates to around 800 BC in China and the oldest surviving drawing of a sunspot dates to 1128. In 1610, astronomers began using the telescope to make observations of sunspots and their motions. Initial study was focused on their nature and behavior. Although the physical aspects of sunspots were not identified until the 1900s, observations continued. Study was hampered during the 1600s and 1700s due to the low number of sunspots during what is now recognized as an extended period of low solar activity, known as the Maunder Minimum. By the 1800s, there was a long enough record of sunspot numbers to infer periodic cycles in sunspot activity. In 1845, Princeton University professors Joseph Henry and Stephen Alexander observed the Sun with a thermopile and determined that sunspots emitted less radiation than surrounding areas of the Sun. The emission of higher than average amounts of radiation later were observed from the solar faculae. Around 1900, researchers began to explore connections between solar variations and weather on Earth. Of particular note is the work of Charles Greeley Abbot. Abbot was assigned by the Smithsonian Astrophysical Observatory (SAO) to detect changes in the radiation of the Sun. His team had to begin by inventing instruments to measure solar radiation. Later, when Abbot was head of the SAO, it established a solar station at Calama, Chile to complement its data from Mount Wilson Observatory. He detected 27 harmonic periods within the 273-month Hale cycles, including 7, 13, and 39 month patterns. He looked for connections to weather by means such as matching opposing solar trends during a month to opposing temperature and precipitation trends

in cities. With the advent of dendrochronology, scientists such as Waldo S. Glock attempted to connect variation in tree growth to periodic solar variations in the extant record and infer long-term secular variability in the solar constant from similar variations in millennial-scale chronologies. Statistical studies that correlate weather and climate with solar activity have been popular for centuries, dating back at least to 1801, when William Herschel noted an apparent connection between wheat prices and sunspot records. They now often involve high-density global datasets compiled from surface networks and weather satellite observations and/or the forcing of climate models with synthetic or observed solar variability to investigate the detailed processes by which the effects of solar variations propagate through the Earth's climate system. SOLAR ACTIVITY

SUNSPOTS Sunspots are relatively dark areas on the surface of the Sun where intense magnetic activity inhibits convection and so cools the surface. The number of sunspots correlates with the intensity of solar radiation. The variation is small (of the order of 1 W/m or 0.1% of the total) and was only established once satellite measurements of solar variation became available in the 1980s. Based on work by Abbot, Foukal et al. (1977) realised that higher values of radiation are associated with more sunspots. Nimbus 7 (launched October 25, 1978) and the Solar Maximum Mission (launched February 14, 1980) detected that because the areas surrounding sunspots are brighter, the overall effect is that more sunspots means a brighter sun. There had been some suggestion that variations in the solar diameter might cause variations in output. But recent work, mostly from the Michelson Doppler Imager instrument on SOHO, shows these changes to be small, about 0.001% (Dziembowski et al., 2001). Various studies have been made using sunspot number (for which records extend over hundreds of years) as a proxy for solar output (for which good records only extend for a few decades). Also, ground instruments have been calibrated by comparison

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Solar Variation

35

with high-altitude and orbital instruments. Researchers have combined present readings and factors to adjust historical data. Other proxy data - such as the abundance of cosmogenic isotopes - have been used to infer solar magnetic activity and thus likely brightness. Sunspot activity has been measured using the Wolf number for about 300 years. This index (also known as the Zrich number) uses both the number of sunspots and the number of groups of sunspots to compensate for variations in measurement. A 2003 study by Ilya Usoskin of the University of Oulu, Finland found that sunspots had been more frequent since the 1940s than in the previous 1150 years. Sunspot numbers over the past 11,400 years have been reconstructed using dendrochronologically dated radiocarbon concentrations. The level of solar activity during the past 70 years is exceptional - the last period of similar magnitude occurred over 8,000 years ago. The Sun was at a similarly high level of magnetic activity for only ~10% of the past 11,400 years, and almost all of the earlier high-activity periods were shorter than the present episode. A list of historical Grand minima of solar activity includes also Grand minima ca. 690 AD, 360 BC, 770 BC, 1390 BC, 2860 BC, 3340 BC, 3500 BC, 3630 BC, 3940 BC, 4230 BC, 4330 BC, 5260 BC, 5460 BC, 5620 BC, 5710 BC, 5990 BC, 6220 BC, 6400 BC, 7040 BC, 7310 BC, 7520 BC, 8220 BC, 9170 BC. SOLAR CYCLES Solar cycles are cyclic changes in behavior of the Sun. Many possible patterns have been suggested; only the 11 and 22 year cycles are clear in the observations. 11 years: Most obvious is a gradual increase and decrease of the number of sunspots over a period of about 11 years, called the Schwabe cycle and named after Heinrich Schwabe. The Babcock Model explains this as being due to a shedding of entangled magnetic fields. The Sun's surface is also the most active when there are more sunspots, although the luminosity does not change much due to an increase in bright spots (faculae).

22 years: Hale cycle, named after George Ellery Hale. The magnetic field of the Sun reverses during each Schwabe cycle, so the magnetic poles return to the same state after two reversals. 87 years (70-100 years): Gleissberg cycle, named after Wolfgang Gleiberg, is thought to be an amplitude modulation of the 11-year Schwabe Cycle (Sonnett and Finney, 1990).Braun, et al, (2005) 210 years: Suess cycle (a.k.a. de Vries cycle). Braun, et al, (2005). 2,300 years: Hallstatt cycle Other patterns have been detected: In carbon-14: 105, 131, 232, 385, 504, 805, 2,241 years (Damon and Sonnett, 1991). During the Upper Permian 240 million years ago, mineral layers created in the Castile Formation show cycles of 2,500 years. The sensitivity of climate to cyclical variations in solar forcing will be higher for longer cycles due to the thermal inertia of the ocean, which acts to damp high frequencies. Scafetta and West (2005) found that the climate was 1.5 times as sensitive to 22 year cyclical forcing relative to 11 year cyclical forcing, and that the thermal inertial induced a lag of approximately 2.2 years in cyclic climate response in the temperature data.

Predictions Based on Patterns A simple model based on emulating harmonics by multiplying the basic 11-year cycle by powers of 2 produced results similar to Holocene behavior. Extrapolation suggests a gradual cooling during the next few centuries with intermittent minor warmups and a return to near Little Ice Age conditions within the next 500 years. This cool period then may be followed approximately 1,500 years from now by a return to altithermal conditions similar to the previous Holocene Maximum. There is weak evidence for a quasi-periodic variation in the sunspot cycle amplitudes with a period of about 90 years. These characteristics indicate that the next solar

36

Solar Energy and its Uses cycle should have a maximum smoothed sunspot number of about 14530 in 2010 while the following cycle should have a maximum of about 7030 in 2023. Because carbon-14 cycles are quasi periodic, Damon and Sonett (1989) predict future climate:

Solar Variation

37

years (see global dimming) possibly caused by increased atmospheric pollution, whilst over roughly the same timespan solar output has been nearly constant.

Cycle length

Cycle name

Last positive carbon-14 anomaly AD 1922 (cool) AD 1898 (cool) AD 1986 (cool)

Next "warming"

232

--?--

AD 2038

MILANKOVITCH CYCLE VARIATIONS Some variations in insolation are not due to solar changes but rather due to the Earth moving closer or further from the Sun, or changes in the relative amount of radiation reaching regions of the Earth. These have caused variations of as much as 25% (locally; global average changes are much smaller) in solar insolation over long periods. The most recent significant event was an axial tilt of 24 during boreal summer at near the time of the Holocene climatic optimum. SOLAR INTERACTIONS WITH EARTH There are several hypotheses for how solar variations may affect Earth. Some variations, such as changes in the size of the Sun, are presently only of interest in the field of astronomy.

208

Suess

AD 2002

88

Gleisberg

AD 2030

Changes in Total Irradiance Overall brightness may change. The variation during recent cycles has been about 0.1%. Changes corresponding to solar changes with periods of 9-13, 18-25, and >100 years have been measured in seasurface temperatures. Since the Maunder Minimum, over the past 300 years there probably has been an increase of 0.1 to 0.6%, with climate models often using a 0.25% increase. One reconstruction from the ACRIM data show a 0.05% per decade trend of increased solar output between solar minima over the short span of the data set. These display a high degree of correlation with solar magnetic activity as measured by Greenwich Sunspot Number. Wilson, Mordvinov (2003)

SOLAR IRRADIANCE OF EARTH AND ITS SURFACE Solar irradiance, or insolation, is the amount of sunlight which reaches the Earth. The equipment used might measure optical brightness, total radiation, or radiation in various frequencies. Historical estimates use various measurements and proxies. There are two common meanings: the radiation reaching the upper atmosphere the radiation reaching some point within the atmosphere, including the surface. Various gases within the atmosphere absorb some solar radiation at different wavelengths, and clouds and dust also affect it. Hence measurements above the atmosphere are needed to observe variations in solar output, within the confounding effects of changes to the atmosphere. Indeed, there is some evidence that sunshine at the Earth's surface has been decreasing in the last 50

Changes in Ultraviolet Irradiance Ultraviolet irradiance (EUV) varies by approximately 1.5 percent from solar maxima to minima, for 200 to 300 nm UV.

38

Solar Energy and its Uses Energy changes in the UV wavelengths involved in production and loss of ozone have atmospheric effects. o The 30 hPa atmospheric pressure level has changed height in phase with solar activity during the last 4 solar cycles. o UV irradiance increase causes higher ozone production, leading to stratospheric heating and to poleward displacements in the stratospheric and tropospheric wind systems. A proxy study estimates that UV has increased by 3% since the Maunder Minimum.

Solar Variation

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Changes in the Solar Wind and the Sun's Magnetic Flux A more active solar wind and stronger magnetic field reduces the cosmic rays striking the Earth's atmosphere. Variations in the solar wind affect the size and intensity of the heliosphere, the volume larger than the Solar System filled with solar wind particles. Cosmogenic production of 14C, 10Be and 36Cl show changes tied to solar activity. Cosmic ray ionization in the upper atmosphere does change, but significant effects are not obvious. As the solar coronal-source magnetic flux doubled during the past century, the cosmic-ray flux has decreased by about 15%. The Sun's total magnetic flux rose by a factor of 1.41 from 1964-1996 and by a factor of 2.3 since 1901.

The Earth's albedo decreased by about 2.5% over 5 years during the most recent solar cycle, as measured by lunar "Earthshine". Similar reduction was measured by satellites during the previous cycle. Merranean core study of plankton detected a solar-related 11 year cycle, and an increase 3.7 times larger between 1760 and 1950. A considerable reduction in cloud cover is proposed. A laboratory experiment conducted by Henrik Svensmark at the Danish National Space Center was able to produce particles as a result of cosmic ray-like irradiation, though these particles do not resemble actual cloud condensation nuclei found in nature.

Other Effects Due to Solar VariationInteraction of solar particles, the solar magnetic field, and the Earth's magnetic field, cause variations in the particle and electromagnetic fields at the surface of the planet. Extreme solar events can affect electrical devices. Weakening of the Sun's magnetic field is believed to increase the number of interstellar cosmic rays which reach Earth's atmosphere, altering the types of particles reaching the surface. It has been speculated that a change in cosmic rays could cause an increase in certain types of clouds, affecting Earth's albedo. GEOMAGNETIC EFFECTS

Solar Particles Interact with Earth's MagnetosphereThe Earth's polar aurorae are visual displays created by interactions between the solar wind, the solar magnetosphere, the Earth's magnetic field, and the Earth's atmosphere. Variations in any of these affect aurora displays. Sudden changes can cause the intense disturbances in the Earth's magnetic fields which are called geomagnetic storms.

EFFECTS ON CLOUDS Cosmic rays have been hypothesized to affect formation of clouds through possible effects on production of cloud condensation nuclei. Observational evidence for such a relationship is, at best, inconclusive. 1983-1994 data from the International Satellite Cloud Climatology Project (ISCCP) showed that global low cloud formation was highly correlated with cosmic ray flux; subsequent to this the correlation breaks down.

Solar Proton EventsEnergetic protons can reach Earth within 30 minutes of a major flare's peak. During such a solar proton event, Earth is showered in energetic solar particles (primarily protons) released

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from the flare site. Some of these particles spiral down Earth's magnetic field lines, penetrating the upper layers of our atmosphere where they produce additional ionization and may produce a significant increase in the radiation environment.

Galactic Cosmic RaysAn increase in solar activity (more sunspots) is accompanied by an increase in the "solar wind," which is an outflow of ionized particles, mostly protons and electrons, from the sun. The Earth's geomagnetic field, the solar wind, and the solar magnetic field deflect galactic cosmic rays (GCR). A decrease in solar activity increases the GCR penetration of the troposphere and stratosphere. GCR particles are the primary source of ionization in the troposphere above 1 km (below 1 km, radon is a dominant source of ionization in many areas). Levels of GCRs have been indirectly recorded by their influence on the production of carbon-14 and beryllium-10. The Hallstatt solar cycle length of approximately 2300 years is reflected by climatic Dansgaard-Oeschger events. The 80-90 year solar Gleissberg cycles appear to vary in length depending upon the lengths of the concurrent 11 year solar cycles, and there also appear to be similar climate patterns occurring on this time scale.

this correlation as statistically significant, and some that do attribute it to other solar variability (e.g. UV or total irradiance variations) rather than directly to GCR changes. Difficulties in interpreting such correlations include the fact that many aspects of solar variability change at similar times, and some climate systems have delayed responses.

Carbon-14 ProductionThe production of carbon-14 (radiocarbon: 14C) also is related to solar activity. Carbon-14 is produced in the upper atmosphere when cosmic ray bombardment of atmospheric nitrogen (14N) changes the Nitrogen into an unusual form of Carbon with an atomic weight of 14 rather than the more common 12. Paradoxically, increased solar activity results in a reduction of cosmic rays reaching the earth's atmosphere and reduces 14C production. This is because cosmic rays are partially excluded from the Solar System by the outward sweep of magnetic fields in the solar wind. Thus the cosmic ray intensity and carbon-14 production vary oppositely to the general level of solar activity. Therefore, the 14C concentration of the atmosphere is lower during sunspot maxima and higher during sunspot minima. By measuring the captured 14C in wood and counting tree rings, production of radiocarbon relative to recent wood can be measured and dated. A reconstruction of the past 10,000 years shows that the 14C production was much higher during the mid-Holocene 7,000 years ago and decreased until 1,000 years ago. In addition to variations in solar activity, the long term trends in carbon-14 production are influenced by changes in the Earth's geomagnetic field and by changes in carbon cycling within the biosphere (particularly those associated with changes in the extent of vegetation since the last ice age). GLOBAL WARMING Researchers have correlated solar variation with changes in the Earth's average temperature and climate - sometimes finding an effect, and sometimes not. Researchers who have found an

Cloud EffectsChanges in ionization affect the abundance of aerosols that serve as the nuclei of condensation for cloud formation. As a result, ionization levels potentially affect levels of condensation, low clouds, relative humidity, and albedo due to clouds. Clouds formed from greater amounts of condensation nuclei are brighter, longer lived, and likely to produce less precipitation. Changes of 3-4% in cloudiness and concurrent changes in cloud top temperatures have been correlated to the 11 and 22 year solar (sunspot) cycles, with increased GCR levels during "antiparallel" cycles. Global average cloud cover change has been found to be 1.52%. Several studies of GCR and cloud cover variations have found positive correlation at latitudes greater than 50 and negative correlation at lower latitudes. However, not all scientists accept

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effect include Willie Soon and Sallie Baliunas or Douglass and Clader, Geophysical Research Letters, 2002. The IPCC questions the magnitude of long-term (last hundred or more years) solar variation in section 6.11 of the TAR and show various results including Lean et al. (1995). However the Lean 1995 value may well be too high: more recently Lean et al (GRL 2002,) say: Our simulation suggests that secular changes in terrestrial proxies of solar activity (such as the 14C and 10Be cosmogenic isotopes and the aa geomagnetic index) can occur in the absence of long-term (i.e., secular) solar irradiance changes. ...this suggests that total solar irradiance may also lack significant secular trends. ...Solar radiative forcing of climate is reduced by a factor of 5 when the background component is omitted from historical reconstructions of total solar irradiance ...This suggest that general circulation model (GCM) simulations of twentieth century warming may overestimate the role of solar irradiance variability. ...There is, however, growing empirical evidence for the Sun's role in climate change on multiple time scales including the 11-year cycle ...Climate response to solar variability may involve amplification of climate modes which the GCMs do not typically include. ...In this way, long-term climate change may appear to track the amplitude of the solar activity cycles because the stochastic response increases with the cycle amplitude, not because there is an actual secular irradiance change. More recently, a study and review of existing literature published in Nature in September 2006 suggests that the evidence is solidly on the side of solar brightness having relatively little effect on global climate, and downplays the likelihood of significant shifts in solar output over long periods of time. Lockwood and Frhlich, 2007, find that there "is considerable evidence for solar influence on the Earth's pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century. Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures." This is now however

disputed by a recent reply by Svensmark and Friis-Christensen which concludes that tropospheric air temperature records, as opposed to the surface air temperature data used by Lockwood and Frhlich, do show a significant negative correlation between cosmic-ray flux and air temperatures up to 2006. They also point out that Lockwood and Frhlich present their data by using running means of around 10 years, which shows a constant temperature rise. This reply has so far not been published in a peer-reviewed journal.

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3SOLAR VARIATION THEORYThere have been proposals that variations in solar output explain past climate change and contribute to global warming. The most accepted influence of solar variation on the climate is through direct radiative forcing. Various hypotheses have been proposed to explain the apparent solar correlation with temperatures that some assert appear to be stronger than can be explained by direct irradiation and the first order positive feedbacks to increases in solar activity. The meteorological community has responded with skepticism, in part because theories of this nature have come and gone over the course of the 20th century. Sami Solanki, the director of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany said: The sun has been at its strongest over the past 60 years and may now be affecting global temperatures... the brighter sun and higher levels of so-called "greenhouse gases" both contributed to the change in the Earth's temperature, but it was impossible to say which had the greater impact. Nevertheless, Solanki agrees with the scientific consensus that the marked upswing in temperatures since about 1980 is attributable to human activity. Just how large this role of solar variation is, must still be investigated, since, according to our latest knowledge on the variations of the solar magnetic field, the significant increase in the Earth's temperature since 1980 is indeed to be ascribed to the greenhouse effect caused by carbon dioxide."

Willie Soon and Sallie Baliunas of the Harvard Observatory correlated historical sunspot counts with temperature proxies. They report that when there are fewer sunspots, the earth cooled (see Maunder Minimum, Little Ice Age) - and that when there are more sunspots the earth warmed. The theories have usually represented one of three types: Solar irradiance changes directly affecting the climate. This is generally considered unlikely, as the amplitudes of the variations in solar irradiance are much too small to have the observed relation absent some amplification process. Variations in the ultraviolet component having an effect. The UV component varies

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