Solar Thermal - Progetto Formazione

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• frequent and more intense extreme climatic events such like floods, hurricanes,

dryness.

• Increased danger of desertification;

• decrease of glaciers;

• increased sea level up to 0,88 meters within 2010.

First energy crisis of 1973 highlighted that energy sources not only could be considered

inexhaustible but, they are not distributed uniformly both geographically and politically, on

the planet.

The awareness of the need of facing environmental problems and at the same time to

guarantee a fairer social and economical development, has led several countries to undersign

political agreements at international level.

The last one, Kyoto protocol, binds the signatory countries to reduce as a whole by the year

2012, the percentage of main gas emissions, able to change the green house effect of the

planet (natural effect whose intensification and effect of gasses, as explained above,

determines overall increase of the global average temperature) to 5.2% in comparison with

the levels of 1990.

Diffusion of technologies allowing energy production through the use of renewable sources

and wide spread diffusion of techniques allowing energy saving, can contribute to improve

the environment without giving up the development comforts.

Solar energy: 1/5

Solar ThermalCopyright ENEA e-LEARN 2005



Solar Thermal

Solar energy: 2/5

Solar Thermal in Europe

For many years, the electric energy need has been satisfied through large size production

plants, exploiting fossil fuels as primary source and the electric energy has been transported

using grids at different voltage levels. The future energy system will be based on centralised

power generation and its widespread territorial distribution. A method to reduce the negativeconsequences of this centralized way of producing energy could be represented by

“distributed generation”.

The concept of distributed generation will provide us an opportunity to produce and efficient

use of energy both thermal and electrical, near the end-users with numerous advantages such

like reduced grid losses, low economic investment, environmental benefits and, above all,

ever increasing use of renewable energy sources.

Renewable energy sources

Renewable energy sources, different from fossil and nuclear fuels (surely to be depleted

over certain time period) could possibly be considered as virtually inexhaustible.

They include all form of energy generated with principle source of energy as the sun

energy on the earth:

• solar thermal energy;

• photovoltaic energy;

• hydro power;

• wind power;• biomass energy;

• tidal and wave energy;

• geothermal (energy which is dissipated on the coasts by tides);

• energy from biomass wastes.



Solar energy: 2/5




Solar Thermal

Solar energy: 3/5

Sun and solar radiation

The Sun is a star of G class, among thousands of billion of stars composing our galaxy and

originated through gravitational collapse of gasses (above all hydrogen and helium) and

interstellar dusts.

The sun with a diameter of 1.390.000 km, is about 150.000.000 kilometres away from Earth.

Its internal temperature is about 20 millions degrees Kelvin and it decreases up to 5.760 °K

on the surface. It irradiates into space a power of about 3,84 * 1023 kW. This enormous

amount of energy spreads in space in all the directions.

The solar constant is the average amount of solar energy available on a sun-following unit

surafce area just outside the Earth's atmosphere. It is measured by satellite to be roughly 1366

W/m2. The solar constant is not quite constant and it includes all types of solar radiation, not

just the visible light.

The radiation emitted by the sun travel through the space about 150 millions Km before

reaching the earth atmosphere. It further crosses about 15 Km of air and finally reaches on the

earth in the form of light and heat. However, human beings do not avail all the energy

irradiated from the Sun, in fact, it is partly reflected or refracted though the atmosphere.

Direct, diffused and reflected radiation

The radiation that reaches the earth surface can be distinguished as direct, diffused and

reflected. A device that collect and convert solar radiation into useable energy, will receive

not only the direct radiation with a determined angle of incidence, but also diffused onecoming from several angles and a third component being reflected from the ground, water

vapours and others.

The most important component is the direct one, that is incident directly; determined from

angle of inclination of the incident radiation: smaller the angle the radiation makes with a

horizontal surface, more is the air they have to pass through, more absorption and low power

intensity on the surface. Practically, the optimal position of the radiation absorbing surface is

when the angle of inclination is equal to the latitude of the place; orientation to South offers

maximum hours of exposition to the sun.

In addition, the change of location changes the ratio between diffuse and total radiation. Anincrease of inclination of the absorbing surface decreases the diffuse component and

increases the reflected one. It is because of the above-mentioned fact that the inclination that



allow maximum collection of energy, varies from place to place.

Obviously, distance of sun from the earth, affects the intensity of incident radiation: intensity

of incident solar radiation on the most external layer of the atmosphere during the year does

not have a constant value because the earth, in its revolutionary motion around the Sun,

makes an elliptical trajectory, i.e. at a variable distance from Sun.

Taking into account the filter action as a function of terrestrial atmosphere and distance from

the Sun, evaluation of the inclination of the Earth’s axis is important, as well

Let’s consider, for instance, the winter solstice, that is the day of the year when Earth is

nearest to the Sun (vice versa during the summer solstice). It is different from the common

experience: the winter, in our hemisphere, is the coldest season. The above-mentioned can be

explained in terms of the inclination of the sun-rays due to the different position of Earth

during course of the year based upon the inclination of axis.

It is to be noted that only 51% of the total solar radiation incident on the outer surface of the

atmosphere, reaches the Earth surface. This amount is the global radiation. However, the

effective radiation transformed into energy amounts to nearly 47% (as about 4% of the

radiation is reflected off due to the groundwater).

Earth, absorb and convert the energy received from the sun into heat and, consequently, emit

this energy in the infrared region as long wave radiation.

Solar energy: 3/5




Solar Thermal

Solar energy: 4/5

Conversion of solar energy

In order to make direct use of energy available from the sun, it is necessary to convert it

through a series of processes. Most common available technologies are:

• Biological conversion

• Photovoltaic conversion

• Thermal conversion

Biological conversion

The biological conversion of solar

energy occurs through process of

photosynthesis. Such reaction is

fundamental for life, because it allows

conversion of solar energy directly into

the chemical energy. Glucose obtained is

the fuel for the synthesis of high energy

content molecules, necessary for

metabolic processes of all living beings.

Thanks to this mechanism, nowadays,

we find underground fossil fuels; coal

from underground forests, oil and natural

gas from the decomposition of living

organisms.The stored vegetal energy is demonstrated through the growth of plants, used as wood and

supply of various chemicals.

Photovoltaic conversion



The conversion of sunlight both direct and diffused to

electricity through the use of some materials (called

semiconductors) such like silicon and germanium.

Photovoltaic cell, a device able to convert light energy

directly into electrical energy is made of two thin layers of

semiconductor materials of different types, which, whenexposed to sunlight light, produce movement of electrons –

an continuos electric current. Photovoltaic cells, having no

moving components or circulating fluids, do not involve

material consumption and offer a high reliability.

Theoretically, their efficiency is about 24%, however, for commonly used cells, this value is

never more than 12-14%.

Thermal conversion

Thermal conversion of solar energy is based upon two

common processes Depending on the physical intrinsic

features, the first one is based on the ability of an opaque

surface to increase its temperature when exposed to the sun

rays whereas the second one is based on the “green-house

effect” a process that occurs inside the panel. A devices

able to absorb and subsequently convert solar energy into

heat at high efficiency is defined as “solar collector”.

Efforts are made to maximise the amount of solar energy

absorbed and minimise the heat loss from the collector.

Applications of solar thermal energy will be discussed in

the next modules. For additional information on solar photovoltaic please refer course available on line.

Solar energy: 4/5




Solar Thermal

Solar energy: 5/5

From sun to renewable sources

All the energy sources available on the planet have a common origin: solar irradiation.

Fossil fuels (coal, oil, natural gas) made available through the transformation of organic

materials which without sun radiation (main resource for the process of photosynthesis)

would have never existed.

Hydro electrical energy that makes use of the waterfalls, could not be possible without the

evaporating cycle of water, which, in turn, fulfil its energy requirements through solar

energy. Without Sun, no wind and, thus, no wind energy.

So to conclude, all forms of energy, except the geothermal (one related to the tides) and

nuclear energy, directly or indirectly have their origin from star warming our planet.

All forms of energy are renewable in nature; difference lies how fast they are able to

regenerate. The fossil ones need million of years (in a short lapse of some hundreds, we are

consuming those sources whom composition required geological time). The other ones, more

appropriately defined as renewable (solar thermal, photovoltaic, hydroelectric, wind power,

biomass, etc.) being directly dependent upon the solar energy, when compared to the fossil

fuel, are abundant in nature.

Renewable energies available on earth (mainly due to solar radiation; 173.000 Terawatt-year)

is about 12.000 times more than the actual energy consumption worldwide. Nearly half of the

solar energy available is converted into heat on the earth surface and in the oceans, andremaining lost in space as infrared radiation. Most of the remaining flux (on seas and oceans)

is being used for hydrological cycle thus generating Hydro power. Radiation that reaches the

earth surface can be collected in the form of solar thermal and photovoltaic energy.



Energy balance of solar radiation on the Earth

Summary of the module:"Solar energy"

• Energy problem and renewable sources

• Solar Thermal in Europe

• Sun and solar radiation

• Conversion of solar energy

• From sun to renewable sources

Solar energy: 5/5




Solar Thermal

Solar energy: test

It is the moment of the learning test concerning:

"Solar Energy"

Solar energy: test




Solar Thermal

Solar thermal technology: 1/6

Goals of the module:

"Solar thermal technology"

This module is about Solar Thermal system as technology and Solar Cooling. This last one

is a new technology for rooms cooling which turns solar radiation into thermal energy.

Components of the solar thermal system and the running of the plants are also described, as

well as the main typologies of solar collectors, with an analysis of their structure and

running.

Solar thermal: an underestimated resource

Conceptually, solar thermal is the easiest technology for converting solar radiation into

thermal energy.

Exploitation of solar energy for thermal uses, for domestic hot water production or for

heating of buildings, aims at collecting the sunlight and its subsequent conversion into heat.

Talking about the thermal applications of solar energy as a way “to reinvent the wheel”

somehow, can be right. However, this huge resource has not been yet fully exploited. This is

especially true in Italy where the climate, essential for optimum use of this potential resource,

is excellent (a fact demonstrated scientifically). Generally, the amount of solar energy we are

able to collect in thermal form, is very small. In view of the fact that most of us, in practice,

especially in the summer season, make use of the plastic tanks to heat water to take bath or

other useful purposes, the above quantity could be increased by manyfold. However, thoughnew technologies are in course of development, there is a gap between the number of

available technical solutions, (thanks to the research) and the number of plants effectively

built.

A lot of solutions are still at the experimental stage, because, although the method looks so

easy, solar systems require great accuracy for both production and installation and

maintenance. (fare link of reference to successive model).

Solar thermal system devices

There are two types of solar thermal system devices depending on the temperature that can



be obtained by the heat transfer fluid (fare link or active glossario). The break point is the

100 °C temperature.

• low temperature devices work at temperature much lower than 100 °C.

• high temperature devices work higher than this temperature.

Low temperature systems are among the most common ones and they use water or air.

They are used for heating and for producing domestic hot water. On the other hand, high

temperature devices use particular capture systems, which are able to increase the

irradiation level in the absorber. They are made in order to generate electric power (by

means of turbines supplied through the steam produced by the plant) and they are often

used in industries for generating steam to use in mechanical work.





Solar Thermal


How a solar thermal system is made up

A solar thermal system for generating domestic hot water consists essentially of a primary

circuit, made up of a piping system (inside the collector) exposed to the radiation absorbingsolar energy in the thermal form, and of a secondary circuit, where heat is transferred through

an heat exchanger placed in the storage system (boiler).

Usually, a solar thermal system, as listed below, is composed of four main componets, i.e.

• panel or solar collector

• storage tank

• regulation system (not always)

• piping system and a circulating pump (absent, in the case of natural circulation plant)

The “device” for energy conversion is the solar panel that absorb and subsequently convertthe solar energy into heat. The energy from the collector is transferred to the heat transfer

fluid (water) contained in the storage reservoir.



Solar thermal technology: 2/6 Solar ThermalCopyright ENEA e-LEARN 2005



Solar Thermal


Solar system typologies: solar systems with natural and forced circulation systems

A primary distinction among the several technologies for converting energy can be made

depending on the kind of circulation of the fluid (primary circuit).

The primary circuit, in fact, can be a natural circulation system, and in this case the solar

collector has to be placed in a lower level than the storage system, or a forced circulation

system, i.e. mechanically driven.

In natural circulation solar system, circulation of the heat transfer fluid make use of the law

of physics, according to which hot water due to decrease of its density rises up and hence a

natural convection motion. For effective natural circulation, the storage tank should be placed

at higher position than the collector in such a way that because of the temperature difference

in both branches of circuit, fluid movements takes place easily.

Solar panels are connected to a serpentine contained in the storage tank. The heat absorbed by

the heat transfer fluid circulating in the panels warms the water in storage tank, which will be

utilized by users. The circuit between panels and reservoir is a close circuit. The fluid

contained in the circuit circulates as its is pushed upwards beacuse of the heat it contained,

enter in a tank positioned above the solar panels and finally transfer its heat to the water

contained in the storage.

In order to avoid the freezing of the circulating fluid (that can damage the plant) a mixture of

water and glycol, or another anti-frozen, is used (in the close circuit). In such type of plant,

because of natural circulation there is no need of a pumps or control room thus saving lot of money related to maintenance and instrumental expenses.

The difference between a closed circuit system and an open circuit system is based on the

way the heat is transferred to the storage reservoir. In an open circuit system, the heat transfer

fluid that flows in the collector is the water meant for direct use (which reaches directly to the

network). This system is useful when there is no freezing problem.



In close circuit systems, a heat exchanger enables heat transfer of the convector fluid

outgoing from collectors (de-icer liquid) to the water in a storage tank. This system is used

where a de-icer mixture is necessary for facing low temperatures.

In a forced circulation system, where water storage tank is positioned at lower position than

the solar panel, hot fluid in solar panel is unable to provide sufficient energy to createconvective motion to the heat exchanger, which the storage tank contains, and for circulating

the hot fluid, an electric pump controlled by a solar room and some sensors, are used.

In this type of system, the solar panels are connected to a serpentine coil enclosed in a storage

tank (close circuit). The fluid contained in the circuit warms up thus making the control

sensor to work; the sensor send signal to a solar electric exchange power unit controlling an

electrical pump. The pump is activated and it pushes the warm fluid inside the coil that will

heat the water for its subsequent use.

The forced circulation system offers the advantages of a very flexible system, in fact, the

boiler can be inserted anywhere in the house, whereas the solar panels can be anchored in aflat terrace or a pitch, and in other places exposed to South (such as gazebos, car parking

covering); furthermore, it offers the advantage of being suitable for small and big applications

(users) such as hotels, sporting centres, camping, and swimming pools.





Solar Thermal




Typologies of solar collectors

Solar collectors or solar panels can be of two types: Flat and concentrating.

Flat collectors are the simplest and most suitable solar device for for supplying heat at low

temperature (essentially for sanitary applications). They consist of a metal plate, processed in

such a way to guarantee the maximum absorption of incident radiation to heat the fluif

flowing through the collector (water, air, organic fluids or Freon).

The energy collected by the panel (both direct and partly diffused) and subsequently

contained in the circulating fluid is stored in the storage tank (boiler).

The flat plate collector in its easiest configuration consists of the following components:

• a transparent cover consisting of one o more sheet of glass or plastic placed above theabsorbing plate to minimize both thermal convective and the radiative losses between

absoring plate and the atmosphere.

• an absorbing plate consisting of a series of pipes (with convector fluid circulation)

that absorb the solar raditaion.

• a reflecting layer, forexample of Aluminium, to maximize the energy collection.

• Thermal insulation at the back to minimise the heat losses caused due to conduction

from the bottom of the absorber plate.

All these components are enclosed by an envelope – metallic frame, aapropriate measure to

protect the system from dust and humidity and water leakge etc..

Concentrating Collectors use optical systems (rotating

paraboloids) to increase solar radiation intensity on the

absorber to achieve higher temperature than the heat

transfer fluid thus enabling the system to use mechanical

energy either directly or through conversion into electrical

energy (solar thermodynamic system). These systems

(without use of diffused energy) require sun tracking

devices and use of advanced technologies for the

concentring optical system with subsequent increased

overall total cost.

In order to guarantee the maximum absorption of the incident energy, a good collector must

satisfy the following conditions, i.e.

• glas or plastic cover must be ttransparent at wave-length between 0,4 and 2,5 mm

• absorber plate exposed to the solar radiation must absorbs a great deal ofincident

radiation (but inevitably it warms up to temperature t<100°C thus emitting thermal

radiation in the wavelength that corresponds to the infrared;

• in order to reduce the heat loss caused due to convective motion of the air contained

in the solar collector, the distance between the transparent cover and the absorber palte must be approx. 2,5, 4 cm. In fact, the cover mainly because of the greenhouse

effect, contain a part of the energy thus giving back to the plate about the 50% of the



energy which otherwiset would be irradiated totally towards the atmosphere.

• the absorber plate consist of a metallic plate made of selective coating λ(i.e. with low

emission of radiation for wavelength >3mm) from surface exposed to the solar

radiation. This could be achived using selective coating with optimal characteristics

for the absorption of solar radiation.

Following types of collectors are available in the market:

• usually with an absorber surface of about 1,5 – 2,0 mq.

• Plastic solar collectors: they consist of only plastic pipes to unroll in the place

where they are used, they don’t need even a tank; they are the cheapest ones and

they are the most diffused in the market, above all for holidays uses (holidays

houses, swimming pools, camping, bathing establishments)

• Vacuum solar collectors; they usually consist of an absorber surface (made of

copper pipe) inserted in vacuum tubes made from tempered glass; in these pipes a

fluid is inserted (generally ether) that through evaporation/condensation cycle,

releases heat to a convector fluid licking up the evaporator surface; generally each

element has a surface of 0,1 mq.

• Storage solar collectors. the particular aspect of this type is that it has not a true

solar panel, but the storage surface itself works as solar collector since it warms

directly the contained water.

• Air collectors are similar to the glazed collectors except the fact that in solar air

heater, the heat transfer fluid is the air flwing between glass and absorber or

between absorber and panel bottom.





Solar Thermal


Advantages and potentialities of solar thermal energy

Economical and environmental considerations are the important factors for the wide spread

utilisation of a solar plant. Certain advantage such as Less environment pollution and energysaving are the certain advantages a community can generraly benefit while using solar

energy.

Economic advantages

In order to justify both the installation of a solar thermal system and its economic advantage,

it is necessary to determine, above all, the payback period of the investment.

The number of years necessary to recover the investment are calculated dividing the expense

by the maximum yearly saving obtained through the production of hot water using solar

energy.

The saving from a solar thermal plant, in addition, to enviromental benefits depends mainly

on the cost of alternate energy used for water heating and varies according to the kind of

energy used (electric, methane, diesel, carbon, others) national energy policies and the prices

trend.

On average, cost of a single family solar water heater to produce hot water is around 1.500 –

2.600 Euro. The sais cost could be ammortized over a period of 3-5 year whereas the life of

the plant may indicate 15-20 years with annual maintenance cost of the order of 2% of initial

cost of the plant. It is to be remembered that a boiler (electric or methane) nver repays asthere is always a bill to pay. Needless to say that more a solar plant is used more economic it

will be. On the other hand if the hot water requirement is too small perhaps it would be better

to use eithr an electric or gas run boiler.

Advantages for environment

Concerning the environmental advantage, the substitution of an electric water heater with a

solar system allows to avoid the emission of about 2,5 tons of carbon dioxide in a year.

The substitution of a gas water heater with a solar system allows to avoid the emission of

about 2,0 tons of dioxide in a year.



Potentialities

Costs and performance of a solar thermal system vary according to the type of the collector

installed. (The type of solar thermal collectors varies substantially in terms of their overall

performance and cost). collector Furthermore, since solar energy is an aleatory source on

Earth surface, solar thermal collector, realistically should be considered integrative withrespect to the conventional technologies. They must be considered capable to provide directly

only a part of the energy requiremnt by the users, energy otherwise generated by the

conventional boiler. The percentage of thermal energy produced by a solar thermal collector

per year is defined as the recovery factor of thermal energy need.

In Rome, a system with optimum ratio for costs/ energy produced, this factor doesn’t exceed

65%. This is a common limit for most of the technologies based on renewable sources, often

characterized by aleatory or periodic availability.

For this reason, with the increase of overall dimensions of the plant, recovery factor of the

thermal load increases but the ratio between cost of energy and the energy produced remains

to be linear up to 55%÷60%. Exceeding this value, costs increase linearly with the size of the

plant, while the power generated, hence, capacity of the solar system, increases but at a lesser

rate, leading to higher cost for surface unit of collector. For this reason, a solar thermal

collector for DHW, suitably dimesioned, is planned to satisfy about 60÷65% of thermal

needs.





Solar Thermal


When, where and how opting for solar thermal system is advisable

The sun, we can say, reaches everywhere. It does not mean that exploiting all its benefits is

easy. Questions about where and when installing panels for exploiting hot rays, are ceratinlya metter of thorough consideration. That they work better in latitudes near the equator is an

intuitive fact. More, in particular, considering horizontal panels placed in two geographical

extremities, such as, for example, a North European country and one of the deserted African

area, it can be observed that the ratio is 1 to 4. Just to say, if in Sweden available energy is

2kWh/day, in ali they have about 8. The performance of the solar thermal system depends, in

addition, on the temperature to be attained and its use in different seasons.

Concerning the first point, it must be observed that more the temperature of the heated fluid

more are the heat losses to the surrounding. The efficiency of the collector tend sto decrease

and – or to vanish. More the temperature of the fluid, the system will be able to make the

speed of loss equal to the absorbing one. Every collector, then, has its “limit of maximum

temperature”, at which it is able to collect thermal energy.

Instead so far the seasonal use is concerned, the distinction is made between the users who

needs the hot water throughout the year and other who instead make use of the solar thermal

system only in sumer. In the later case, it is advisable to choose simple and thus more

economic solar thermal system.

In a season with plenty of sunshine, glazed collectors, in fact, are not necessary. In view of

the fact that for about half the time solar collectors work at low temperature than the ambient

temperature, experineced shows that simply the plastic collectors can serve the prupose.

The opportunity of using solar panels can be decided according to several situations leading

to adoption of different solutions. Every household or a building can have definite advantages

with possible substitution of existing water heating system powered by electricity or gas with

that of a solar water heater. Considering the simplicity, lower costs and highly efficient

(mainly in summer season with maximum availability of solar radiation) solar collectors will

certainly be a potential option, especially, during summer hilidays in the isolated areas.

Summary of the module:

"Solar energy" • Solar thermal: an underestimated resource



• How a solar thermal system is made up

• Solar system typologies: solar systems with natural and forced circulation systems

• Typologies of solar collectors

• Advantages and potentialities of solar thermal energy

• When, where and how opting for solar thermal system is advisable





Solar Thermal

Solar thermal technology: test


"Solar thermal technology"

Solar thermal technology: test




Solar Thermal

Applications of solar thermal energy: 1/6


"Applications of solar thermal energy"

This module shows the main application areas for solar thermal energy, including the

advantages and the best ways for a proper installation.

Heating and ventilation

Use of solar thermal energy for buildings is becoming more and more common with

numerous applications. Beside its simple application to produce sanitary hot water, largely

practised over the last many years, both air systems for preheating of air in big rooms and

combination of solar systems with heat pump systems for low temperature heating of the

buildings, are other important applications. Such types of plants use floor or a radiant heatingsystem. Development of solar cooling technologies both for cooling (through absorption

refrigeration machine) and treatment and dehumidification of the ventilation air (through a

desiccant system) is the most innovative aspect. In conclusion, use of solar thermal energy

find a variety of applications in different energetic systems with respect to the building, and

its use is spreading even during the periods characterised by high solar insolation radiation,

consequently with improved economic aspects.

Sanitary water heating

With use of solar collectors of the types as mentioned above, it is possible to produce hot

water. Such types of systems are more appropriate for a single-family and, especially, at thecommunity level appears to be a viable application. They can replace electric energy, natural

gas; in the first two cases, solar energy can help and allow considerable savings, while for

natural gas amortization of the investment is a little bit longer.

Domestic hot water



The amount of electric energy used to meet household hot

water requirements could be reduced by possible use of a

solar water heater system. A variety of solar collectors

available in the market can be used to provide most of the

water heating needs. From economic point of view it isadvisable to install a solar water heating unit common for

the whole residential block with number of flats. In fact,

higher is the number of the users, more cheaper will be

the subdivision of the thermal load in several tank. Solar collector installed on a roof

So far residential applications are concerned it is easy to choose a suitable configuration

among the below listed possible options:

• water heater with uncovered collectors;

• water heater with storage tank;• custom built solar water heater;

• factory made solar water heater.

Common buildings and industries

Having a shower or bath?

Certainly, it is not the case with offices without proper and favourable climatic conditions for

the exploitation of solar collector. However, the same could be an potential application in

case of hotels, especially during summer. Holidays and excellent weather conditions certainly

favour to take bath thus offering such applications the maximum benefits possible.Bathhouses, camping, hotels, gyms, sport facilities and old houses have maximum potential

for the use of solar panels. Naturally, the size of these plants is different from the ones for the

family uses. In common buildings, average need (approx. 50 litres per person each day) is

less than the ones of a single house.

But there are cases, such as, luxury hotels, where the use of hot water is four times higher

than the average, or sport facilities that consume 25-30 l per person each day. Showers and

washbasins are not the only elements to be considered. In fact, energy being an important

factor of industrial growth could easily be furnished by solar collector. Also, solar water

heating technology could be used to heat water for cleaning, cooking and washing, inlaunderettes and restaurants.

The problem is that, usually, they are in the ground floor of a more complex building.

Nevertheless, there are other cases such like tanneries or cheese factories which are more

suitable for the use of solar panels which don’t have any space limit. To make leathers and

cheese, in fact, thousands of hectolitres of hot waters are needed. There is no doubt that,

especially, during summertime, the saving due to the thermal solar energy is very important.



Solar Thermal


Types of plants for the production of hot water depending on its use

A large amount of sanitary water heating in the urban areas, is mainly produced by electric

water heaters and gas boilers. The production of sanitary hot water through electric energywhen compared to the production through gas boilers, is certainly an expensive process both

in terms of economy and environment

The introduction of a thermal solar collector to replace part of the heat produced, has many

benefits. The below listed possible options, i.e. the integration of an active solar collector

with an already existing sanitary water heating systems could offer both the energy economic

and environmental benefits.

Three possible solutions:

1. replacement of an electric water heater with an integrated gas/solar system (involve

many small sized domestic and public users which so far have not confronted the

problem and, consequently, could be incentivated financially for the replacement of

the electric water heater).

2. integration of pre-existing gas system with a solar plant (foresee minimum

integration cost). In fact, the users who satisfy their hot water demand using gas,

could easily save up to 60% of the gas consumed annually through use of a solar

thermal system)

3. integration of electric system with a solar plant ( it concerns situations where the

heating system cannot be electric (for instance, nomad settlement or first aidshelter).

For other solutions, an energy and environmental balance (in terms of avoided CO2

emissions) will be made. Energy analysis: calculation of the pro capita energy requirement.

On average, each day 50 l/per capita of sanitary hot water at 45°C is used, in Italy. Assuming

an inlet water temperature of 15°C (from the water supply system), the pro capita thermal

energy (Q) required to satisfy the above-mentioned hot water demand can be calculated using

the expression; Q = G.Cs.(Tu -Ta)= 50.1.30 = 1,500 kcal/day,

where:



• G = amount of water to be heated (litre)

• Cs = specific heat of water (kcal/litre-°C)

• Tu = Outlet temperature of water (°C)

• Ta = inlet temperature of the water from the water supply system (°C).





Solar Thermal


Production of hot water with electric boiler

In this case, use of thermal energy to produce hot water undergoes a double transformation.

During first phase, it is necessary to produce electric energy (generally in thermoelectricstations, rarely in hydroelectric ones). The generated electric energy, then supplied to the

users, again has to be converted into thermal energy.

On average, a family of 4 persons thus uses about 7 kWh/day of electric energy for the

production of hot water. But it is to be noted that to produce each electric kWh, nearly 2,45

kWh of primary energy is used in an electric power station, in Italy.

Considering this double conversion of primary energy into electric energy and from electric

energy into thermal one, it is evident that, for the production of the above-mentioned hot

water, 4,5kWh (3,870 kcal) of primary energy, is necessary.

In this way only 35% of the used primary energy is effectively used by the user.

Production of hot water through gas boiler

Obviously, a gas boiler has higher direct energy yield. It is mainly because of the reason that

it avoids the energy consuming conversion from thermal energy into electric energy (in the

power station). For this reason the global yield is around 85%.

The heat generation and subsequent water heating occurs through the direct combustion of

methane. To generate 1,500kcal/day/person, approx. 1,765 kcal (2,05 kWh) of primaryenergy is needed, each day.

Comparing the energy consumption among the aforementioned cases

The following picture shows the result of the comparison among the energy required for the

production of sanitary hot water through an electric water heater, through a gas boiler,

through a gas boiler/thermal solar collector system and a electric water heater/thermal solar

collector system.

The system with gas boiler integrated with solar collector has 0,5 kWh pro capita energy

consumption, with about 90% reduction in the energy consumption when compared to theelectric water heater one (4,5 kWh).



In comparing the system based on the integration of the solar collector with a gas boiler and

the very boiler, the consumption decreases from 2,05 kWh with boiler alone to nearly 0,5

kWh for the integrated system. A hot water system integrated with solar collector has a

energy consumption of 1,13 kWh (4,5 kWh for the electric water heater).

Applications of solar thermal energy: 3/6 Solar Thermal

Copyright ENEA e-LEARN 2005



Solar Thermal


Rooms heating

Sometimes it can be useful and convenient supporting a conventional heating system with

solar panels.

As the efficiency of a solar panel decreases

with increase in temperature of the heat

transfer fluid, it could contribute in the case

of thermosyphon heating. An higher

efficiency will be possible if solar panels will

be used to supply heat with floor heating

system (where low temperatures are needed)

or through possible heating of inlet air to the

buildings both with the use of solar air heatersand solar water heater with radiator

exchanger.

Functional scheme of a plant for the

generation of medical hot water and heating

Contrary to what occurs with radiators exploiting convective air moment, radiant tubes work

thanks to the thermal exchange due to solar radiation occuring among surfaces with different

temperature. The heat diffusion through radiant terminals provide important advantages and

efficiently combine itself with the low temperature heat generation.

In the residential buildings there is generally a wide choice concerning the radiant surface

among floor, roof and walls. If the thermal solar system , besides providing hot medical

water, has to integrate the heating system of a house, this last one has to be equipped with a

low temperature heating system , built through radiant surfaces: tubes under the floor, in the

walls or with a roof thermostrips where the water flows at 30-35° C. the tubes where the

thermocarrier fluid flows are built with plastic material or copper and have several sizes (

diameters between 15-20 mm for the tubes, about 3 mm for capillary tubes). Their placing

can be made through a fastening on panels for plaster or metal false ceiling, or through the

laying on the surface of intrados of the floor.



Concerning the choice of the size of the

components, in order to get a faster feedback of the

plant and a higher uniformity of emission, close

capillary tubes (about 1 cm of wheel base) are

preferred. Considering a temperature change of 6-

8° between there and back, average heating fluid

temperature is about 33°. With these conditions ,

the yield is about 70 W/m2. For the cooling,

instead, the average temperature is 19° C. In this

case , the thermal power that the system takes away

from the environment is up to 40 W/m2.

The limits on temperature and power one are defined by the possibility of surface

condensation of the radiant surface.

Generally, it is impossible to heat 100% a house only with solar panels: a supporting boiler is

always necessary, but the presence of the solar system allow less gas and gas oilconsumption.

The solar tank (A) has a mushroom shaped internal tank, containing water aimed for sanitary

purposes. The water where the mushroom shaped tank is plunged flows inside the tubes of

the floor heating (B) thus heating the rooms.

Also, it is possible to think of a combination between collector air system (solar wall) and

radiant floors, for example, through an air-water exchanger.

Radiant floor





Solar Thermal


Central heating

In cases where heating is already foreseen with conventional systems, use of solar panels

installed along with centralised thermal production unit, proved to be very affective. Solar panels can meet up to 15-20% of total requirements of heat for a year.

Pools

Irrespective of their both location (outdoor or indoor) and season (summer of winter),

swimming pool heating is an important and wide spread application of solar collectors. This

is mainly because of the reason that such an application coincide with period of maximum

solar energy availability. Having no need of a storage system, low thermal load requirement

and easy to install/dismantle low cost solar panels made of plastic or rubber materials, are

other salient features of the application.

Anyway, the main reason for the wide spread diffusion of this application is the low water

temperature that could easily be obtained using cheap and less complex polypropylene solar

collectors.

For affective and efficient utilisation of solar heating

system it is important that swimming pool be positioned

appropriately. Moreover, due consideration be given to

avoid rapid cooling due to surface evaporation The

performance of a swimming pool located outside can be

affected significantly (rapid cooling of the pool due tosurface evaporation) if due attention is not being paid to the

wind exposure. Water conditions in an indoor pools

remains stable provided the temperature is around 28°,

ventilation on the surface of the water is 0,2 m/s and

humidity in the air is less than 70%.

Once these parameters are satisfied, solar technology with lower operational cost compared to

the conventional system and possible amortisation period of less than tow years, can work

successfully. In the swimming pools, collectors are placed not very far from the pool, with an

extension equal to half the length of the pool.

Agriculture



Solar drying of agricultural products (using hot air from solar air heater) is an easy to use and

economic application of solar energy. This is, especially, true in the countries where process

of slow drying with natural air, depending upon the weather conditions, may last for a

number of days. Also, a considerable amount of conventional energy could be saved by

possible application of solar thermal collectors in the greenhouses, generally, characterised

by high energy consumption.





Solar Thermal


Cooling: solar cooling

It is an application with more complex technologically compared to the previous ones, where

heat generated by collectors able to achieve high efficiency at high temperature, is used by aheat absorption pump to generate cool air.

It is an application “linked” to the best solar radiation, as it is used almost only in summer. At

the same time, using reversible machines, system is able for the space heating in the winter,

as well.

Thermal energy absorption engines for air cooling and conditioning are currently object of

renewed scientific and industrial attention. This is true especially after a period of strong

interest and attention focused on compression machines, generally, characterised by high

energy efficiency. Over the last couple of years, in fact, an energy policy boosting the

exploitation of waste heat available for cogeneration and solar cooling, has got attention of

the scientific community both in Italy and other countries. An incentive for the use of natural

gas as primary energy source has been foreseen, in Italy.

Both circumstances favour use of refrigerating engines with thermal energy absorption.

Particularly, in the case of direct flame engines for summer conditioning, it is possible to

reduce the seasonal imbalance in gas consumption. Another reason for this new attention is

its potential to address the environmental problems: absorption engines can work with fluids

without any negative consequences for the environment, and some of them, particularly, have

almost zero emission of the greenhouse gases.

Generally, they can work using methane, with limited polluting emissions compared to other

fuels or with waste heat or solar energy thus helping to achieve the most important objective

to safeguard our environment.

The results obtained from the research conducted on the absorption refrigerating engines over

the last few years have helped to fill the gap between the efficiency of an absorption cycle

and a compression cycle thus providing the same effect ( quantity of taken away heat). The

choice of the refrigerating-absorbing substances coupling is quite wide. Amongst possible

options, ammonia-salts, methylamine- salts, alcohols- salts, ammonia- organic solvents,

sulphur dioxides- organic solvents, halogenated hidrocarbons- organic solvents, etc., are the

most used coupling. In fact, : lithium bromide-water and water-ammonia have selected for close examination



External conditions having significant affect on the performances of the engine, especially,

for the evaluation of seasonal yield of a refrigerating engine, needs to be evaluated seriously.

This aspect is really important for small sized engines which, for economic reasons, don’t

have adjustment system.

Summary of the module:

"Biomass"

• Heating and ventilation

• Types of plants for the production of hot water depending on its use

• Production of hot water with electric boiler

• Rooms heating

• Central heating

• Cooling: solar cooling





Solar Thermal

Applications of solar thermal energy: test


"Applications of solar thermal energy"

Applications of solar thermal energy: test




Solar Thermal

Construction of a solar thermal plant: 1/20


"Construction of a solar thermal plant"

The module shows the steps concerning the solar thermal plant installation, from the

planning and the analysis of environmental data and energy demand to the solar collectors

installation and plants maintenance. Criteria for the plant design are considered, with

particular attention to analysis method and f-Chart software. Advantages and potentials

concerning thermal solar system use and its obligations and regulations are pointed out

Preliminary phases

After acquiring through knowledge about the know-how and possible uses of solar systems,

we will try to concentrate on their installation and associated relevant phases. Theconstruction of a thermal solar plant must foresee the following phases which are

fundamental for the proper design planning:

• preliminary collection of climatic, geographical and building data;

• analysis and quantification of the energy requirements of the user;

• sizing of the components of the plant.

Elements to know at the moment of the planning of the plant

The right place

For the energy yield, the place where a collector is installed (on the roof or in the garden) is

not very important. On the contrary, orientation and the inclination of the collector has a

direct effect. The standard inclination is 30° at the Southern Europe latitude..

Higher angles increases the energy that the panel is able to receive on its surface during

winter months compared to the summer ones.

It is also important that the panels are not shadowed by chimneys, trees or surrounding

buildings. Problems arising due to unfavourable climatic conditions, no doubt, could be

resolved with suitable increase in the collector surface area but certainly not without

additional economic investment.

To make sure that the offer of the installer is complete. A complete offer includes, besides the



supply and the assembly of the very solar plant, also electrician works (electric connection)

or other works (tinsmith, house painter, bricklayer). Ask for information concerning reference

plants (previous experience of the installer).

Ask for warrant

With a guarantee concerning the performances of the panels, the installer attests the solar

plant ++ rispetta lo stato della tecnica and that the works will be made in conformity with the

requirements of the existing technical set of rules.

Compare the offers

Compare the several offers. A bigger solar collector no doubt cover higher fraction of energy

requirement from solar energy but the fact remains that they are also more expensive. When

comparing, it is important to consider aspects as the guaranteed performance and collateral

works.





Solar Thermal


Initial collection of climatic, geographical and building data

A series of fundamental data for a proper plant project has to be defined:

• data about the solar radiation available on the oriented and inclined surfaces;

• climatic and meteorological data;

• data about the communal urbanistic set of rules: building standards, density;

• data concerning the user;

• data concerning the building where the plant has to be built, architectural, dimensions,

and structural characteristics;

• data about the pre-existent heating system for sanitary hot water;

• taking into consideration the shadows due to surrounding buildings or vegetation

during all seasons;

• analysis of short time local policies for the management of the area close to the

building, as other buildings could later be built , causing problems to the collector

(shadows) or having other negative impacts on the system.





Solar Thermal


Analysis and quantification of the users energy requirements

First of all it is necessary to be aware of the exact consumption of hot water by residents and

have an idea about the wastes, to formulate a hypothesis of energy saving.

The number of people living in a building could be an indication for the hot water

requirements. Generally, the pro capita daily consumption is between 30 and 50 l of hot water

(45°).

Gas bill is another element for checking the consumption. If it is not possible to get a reliable

data from the bill, it is important to find out the habits of the residents concerning the use of

hot water, use times, hours characterised by high consumption, in order to define a trend of

daily and seasonal consumption. If the figures diverge from the foreseen consumption, it is

necessary to understand the possible reasons for the wastes.

The thermal need for hot water generation is not constant during the year and it depends on

the number of the people in the building and on their habits. For a proper planning of the

plant, it is important to know the quantity of hot water used each day and the trend of

consumption in the several periods of the year (load program).

A close analysis of the hot water wastes can be decisive for the reduction of consumption and

consequently, for the reduction of plant costs. After the estimation of the consumption, next

important phase is the system sizing. There are three planning hypothesises:

• to gauge the system on the basis of the coldest month;• to gauge the system on the basis of the intermediate month;

• to gauge the system on the basis of the hottest month.

The three cases are schematised in the diagrams (exposition simplicity, thermal load has been

considered to be constant)



Diagram showing the planning hypothesises

• In the first hypothesis, total coverage of building thermal requirements are guaranteed

from solar plant only. However, it is, however, to be noted that to cover the thermal

load fully, the solar system has to oversized for almost all seasons and part of the

energy generated will be wasted, especially, during the summer.• In the second solution better ratio between the use factor of the solar system and the

installation cost needs to be considered.

• In the third hypothesis, system is sized on the basis of the month with the highest

levels of solar radiation. There is a scarce coverage of the hot water requirements, and

so the total coverage of the thermal requirements through the solar plant is possible

only for a month each year and for the rest an additional systems is to be provided.

Making reference to a month for calculating the area and the sizing of the additional system, a

greater amount of thermal energy will be generated during hottest months, while in the

coldest ones, additional system will be needed to cover the deficit; in this case , the thermal

energy collected using solar collectors will pre-heat the water before its entry into the boiler of the additional system.

The integration factor is the fraction of thermal energy that the additional system must

provide to the system. In the plants for sanitary hot water generation, integration percentage

ranges from 40 to 70%.





Solar Thermal


Planning

The planning phases of the absorbing device, once general scheme of the plant is finalised,

can be represented by the following points:

• energy sizing;

• selection of the type of collector;

• determination of the size of the absorbing surface;

• check on the orientation and inclination of the collector;

• supporting structure;

• hydraulic connection;

• requirements of the components of the collector device;

• graphic printout ;

• project’s verification.

The first four points are going to be dealt in detail while the other five points will be

explained in the following module concerning installation.





Solar Thermal


Dimensioning of a solar water heater

sizing of the plants and thermal loads they have to cover, is the main topic of this paragraph.

In particular , cases of the sanitary hot water production and water for room heating will beconsidered. The goal is to define the area of the solar collector needed to cover the load. The

method of calculation and F-Chart software (which is one of the more spread and validated

method for both check and project calculation) will be mentioned.

Loads determination

In the case of sanitary hot water, it is necessary to have information on its possible uses:

home, hospitals, hotels, barracks, etc. The following table shows reference load for most of

the interesting cases.

Concerning thermal load for rooms heating, a first estimate (it doesn’t consider the

contributions of solar radiation, people, lights, working interruptions, contributions

decreasing the effective load), is given by the following formula:

E = Cg · V · DD · 24 · 3600 (Joule)

where:

• E = heating load

• Cg = global volume dispersion coefficient

• V = heated gross volume• DD = degrees days (°C)

and:

Cg = Cd + Cv

Cd, depending on the heat loss from walls as well as windows, is given by the formula:

( ) progin

c

d

T T V

QC

−=



Where Qc is the building thermal load (W). Inside and outside temperature of the project are

given by Tin and T prog. Cv depends on the air changes and it’s given by

Cv = 0,34 · n

where n stands for the number of time changes. In America CgV replaces UA (W/°C).

DD, the degree days are given by the formula:

( )+∑ −=i aiT DD 20

The summation is for the whole heating period,

ai

T

is the average value of the room temperature of I-DAY and + shows that only the terms with

positive-positive

aiT −20

are considered.





Solar Thermal


Sizing criteria

Sizing the plant and, especially, the collectors, on the basis of the most unfavourable

conditions will lead to an inconceivable overdimensioining of the system, as a systematicunderutilization of the collector area will occur, especially, in winter with minmum solar

energy available. In addition, cost of a basic component of a solar water heating system, i.e.

collector still being too high, from economic point of view, to choose a solar water heating

system compared to an conventional plant, is quite different In fact, in the solar plant, sun

usually doesn’t provide the totality of the energy needed and so the rest needs to be provided

by a standard auxiliary source.

It is therfore evident that the sizing of a solar plant is not only a technical matter rather both

economic and technical matter.

From a technical point of view, it is important to know what fraction of annual thermal load a

solar plant is able to meet . The share of useful energy collected depends on several

parameters, first of all the efficiency of a solar collector which in turn depends on the overall

characteristics of the collector used, temperature of its use, orientation and inclination of the

collector (this last is fundamental to optimize the performance), solar radiation, outside

temperature and wind speed.

A rational planning of a solar plant needs an accurate estimate of the useful energy the plant

can provide, and this calculation can be made through several methods, among which F-

CHART is the most used.

Loads for production

L/DAY MJ/DAY KWh/DAY L/DAY NOTE

HOUSE 50 6,9 1,92 -

HOSPITAL 60 8,29 2,30PER

BEDPLACES

REST HOUSES 40 5,52 1,53 -

SCHOOLS 5 0,69 0,192 -

BARRACKS 30 4,14 1,15 -

INDUSTRIES 20 2,76 0,767 -

OFFICES 5 0,69 0,192 -

CAMPINGS 30 4,14 1,15 PER PERSON

LUXURY 160 22,1 6,14 PER ROOM



HOTELS

CHEAP HOTELS 100 13,82 3,84 PER ROOM

GYMS 35 4,84 1,34 PER USER

LAUNDERETTES 6 0,83 0,23PER WASHED

KG

RESTAURANTS 10 1,38 0,38 PER MEAL

BARS 2 0,27 0,076PER ORDER





Solar Thermal


F-chart method

F-CHART method is a semi-empirical method, which is able to provide good results with

limited calculation time, simulating all the components of the plant (collectors, storage tank,heat exchanger) with dynamic models which solve equations concerning mass and energy

conservation for each component (for a general estimate)

F-CHART method, which is often used because it provides good results for certain types of

solar systems, has been developed at Wisconsin –Madison University (USA) by Klein,

Beckmann and Duffie. It has been obtained on the basis of the results deriving from several

behaviour simulations of some reference plants.

Thanks to this method , it is possible to calculate the f fraction of the monthly thermal

requirements of hot water, supplied by a solar plant with two tanks, a preheating tank and a

service one where the auxiliary energy is the input and for which there are no losses. The

fraction, f, of the monthly thermal requirements supplied by the solar source is defined as:

L

E L f

−=

Where

• L = monthly thermal load

• E = monthly auxiliary energy

• F = fraction is the function of two parameters:

o X = loss parameter (it directly depends on the several system loss factors such like

Fr= heat removal factor, Uc= collector loss coefficient and Fsc= de Winter factor,

considering the reduction of the useful energy collected through the heat exchanger;

o Y = solar parameter (it directly depends on the monthly average energy per day on the

collector , on the effective product (++) and on Fr and Fsc factors.

The relation is the following one:

F = 1,029 Y - 0,065 X - 0,245 Y2 + 0,0018 X2 + 0,0215 Y3

such correlation is valid for the

interval:0 < X ≤ 18 0 < Y ≤ 3 0 < f ≤ 1



and it has been obtained fitting several calculation results provided by a simple simulation

model derived by the TRNSYS codex. The following graph , named F-Chart shows ISO

Fcurves trend.

The monthly F solar fraction foe every month of the year known, annual F fraction is given by:

F f L

L

i i

i

=∑∑

Where

• Fi = solar fraction of a given month

• Li = thermal requirements of a given month

F-CHART is not directly applicable to natural circulation system as it has been developed for

forced circulation systems, with fixed and known capacity inside the collectors and for

which, the high capacity figures inside the tank allow to think that this last one has a uniform

temperature.

On the contrary, in the natural circulation systems, working at low capacities, important

stratification occurs inside the tank with significant improvement of the system performance.

A calculation procedure has been suggested by Malkin and others (reported in [2]) where, to

find the F-CHART for natural circulation systems, effect of stratification is considered

toward the decrease of coefficient loss (Uc) of the collector, which implies a reduction in

parameter X and increase of parameter Y (which depends on Uc through Fr) and instead of

variable flows in a natural circulation plant, an “equivalent average” flow is considered.





Solar Thermal


Selecting a collector

Choice of collector In the case of natural circulation systems is partially linked to the type of

storage used, as the two components are generally sold together. This makes the work of theinstaller designer easier, as he can consider collector- tank as one device.

For almost all the systems, it is possible to vary the number of the collectors feeding a given

storage. The same is true in the case of systems with open circuit and integrated storage

where the storage volume and absorbing surface are inserted in just one element.

In the case of a forced circulation system, collectors can be divided into two types:

• flat plate collectors;

• vacuum systems.

The first ones are generally used to supply sanitary hot water up to 60° C. The main

advantages of this system are low cost, availability in several shapes and sizes, adaptability to

several antifreeze systems, and keep working even if one of the transparent cover break up.

Vacuum sealed collectors instead, can work best when the temperature differences between

collector and external environment are high or when the solar radiation is low. The mainadvantages of vacuum sealed systems are: high efficiency in applications where high



systems (because they are less affected by air temperature), the modules can be freely rotated

to fit the inclination of the surface, etc.





Solar Thermal


Dimensions, number, orientation and inclination of the collectors

Use of big sized collectors reduces the installation costs.

The number of the collectors necessary is given by the ratio between the total absorber area

and the gross area of the single collector provided in the builder’s specifications.

In order to size a collector, for an ideal orientation of 30° inclination facing South, as an

example, following references figures are used, in Italy:

Geographic areas Reference Figures

North 1,2 m2/ person

Centre 0,9 m2/person

South 0,7 m2/person

The optimal inclination (annual average maximum absorption) is equivalent to the latitude

minus a dozen of degrees. Higher inclinations improve winter performances and are

especially recommended for winter heating. Lower inclinations improve summer

performances.

Collector’s arrangement

Collectors have to be installed in a way so as to best exploit the available space and limit the

size of the connection circuit. For the construction of a medium - large sized plants, it is

necessary to install and connect a number of collectors together. These ones could be parallelor series connected.

Series collectors Parallel collectors

It is important:



• Not to connect more than four collectors together;

• Temperature difference between inlet and outlet fluid not to exceed 10-15° C;

• From the hydraulic point of view, plant has to be balanced in every section, without

load losses in the single sections.

Mixed solution





Solar Thermal


Installation

If some fundamental conditions are checked and followed, a collector can be installed

indifferently, on a flat roof, on an inclined roof, on a facade, on a balcony, in a square or in agarden. The surface where collector is to be installed has to bear its weight which,

considering the load, could exceeds even 100kg. During all day and year long, the plant must

not to be shadowed by plants, buildings or mountains and it has to have a good exposure to

South (SE or SW exposure involves a 20%of energy loss compared to 40% if the exposure is

E or W). At the moment of ordering the material, type of the plant has to be specified,

because, according to the boiler, hydraulic circuit of connection between the solar collectors

and the conventional system for sanitary water heating changes.

It is better to ask for an installation kit including, besides the solar collectors, the support for

each collector, fit for the installation surface type, and the devices necessary for the water

connection between the boiler and the collectors, and the user’s manual.

During the course of the installation, in order to avoid that the temperature inside the system

exceeds the operative limits of the materials, it is necessary to avoid exposure of the glazed

collector to the solar raditaion for a longer period until the collectoris filled with water. For

this reason, during installation, it is important to cover the glass surface with opaque

materials (cardboard, fabric, plastic, etc.).

The procedures for collector assemblance and some of its components vary according to the

kind of surface where the systems are installed. In particular, collector’s supports and the

water connection to the pre-existing plant are different if the installation is on a level surfaceor on an inclined one.





Solar Thermal


Installation on a flat surface

For the installation of a collector on a flat surface, it is necessary to use supports fixed to the

supporting surface through 8-12 cm common screw anchors placed just next to the holes of the supports.

The procedure is the following one:

• define the support positions so that the orientation of the absorbing surface is due

South;

• provide supports at a minimum distance of 120-150 cm both horizontaly and

vertically.

• check the horizontalness, as the non horizontalness of the collector provokes a non

uniform distribution of the weights and a partial collection of the very collector, with

consequent loss of yield;

• check the both the bases of the supports uniformously lay on the surface below, for a

good load distribution;

• trace the anchoring holes on the supporting surface just next to the ones existing on

the supports;

• insert the screw anchors in the holes and seal the border with silicon to avoid water

penetrations and humidity;

• place the supports and fix them to the anchoring points;

• install the collector on the supports laying it in centred position.

Installation on an inclined roof

Installation on an inclined tiled roof can occur through supports anchored to the attic or

another fixed structure under the tiles.



careful to the horizontality of the plane ( in the contrary case, the collector causes a

partial filling, with consequent loss of efficiency);

• place the collector in a centre position on the supports and anchor it to them with the

steel cables;

• connect the collector to the waterworks.

Solar panels integrated on a building façade

Construction of a solar thermal plant: 12/20 Solar Thermal




Solar Thermal


Floor installation

In this kind of installation, coils are directly inserted in the structures of the building

(floor).The radiant floor panels are planned for temperatures between 25-35 °C. Thistemperature range is compatible with field at maximum efficiency of the solar collector. The

coil is in plastic or copper or RETICOLATO polyethylene. The load foresees the sizing of the

system for a 24 hours endurance and the annual solar integration is about 40-50%.

Floor radiant panels plant scheme



Floor radiant panels plant section





Solar Thermal


Ceiling installation

Ceiling is the surface characterised by minimum structural obstacles, especially, if compared

to the floor one. Being not subjected to loads, no particular protection is needed againstshocks and stress, maintenance is easy and risk of breakings and fluid losses are very low, as

well. The view factor sup. radient/ambient and sup. radient/user is maximum and it is not

influenced by the arrangement of the fittings. In this way, it is possible to get more uniform

temperature distribution and average radiant temperature of the walls, more suitable to be a

comfort one.

The thermal comfort is higher when the heat comes from the top (as it occurs in nature with

the sun). Ceiling installation with improved hygienic conditions thus reducing the convective

thermal exchange and connected problems (e.g. dust movement) to be minimum. Compared

to the floor heating, there are no problems concerning blood circultaion in the lower limbs.

As the surface of the ceiling is usually wide, ceiling installation offers further advantages.

In fact, more is the available surface, less is the thermal change between surrounding and the

fluid. In order to avoid both the water surface condensation and increase the overall

efficiency to be maximum, the above cited concept is important, especialy, in summer

season.





Solar Thermal


Advantages and potentials of solar thermal: economic and environmental advantages

The main reason for using a solar plant comes from both economic and environmental

considerations. In fact, less environmental pollution and energy saving that could be obtainedusing solar energy, are guranteed overall advantages.

A solar thermal plant allows to save approx. 40-70% energy required to generate sanitary hot

water.

Economic advantages

To figure out the economic advantage of a thermal solar system, it is important to define the

recovery period of overall investment which will allow to evaluate the validity of the

installation. The number of years needed to recover the total investment, in first

approximation, is calculated by dividing the expenses supported for the maximum annual

achievable saving through the production of hot sanitary water with the solar energy.

Besides, the overall saving due to the use of a thermal solar plant apart being dependent on

the above-mentioned considerations and environmental issues, is associated directly with the

cost of energy replaced and it varies in accordance with the kind of energy used (electric

energy, methane, coal, gas oil ) the government’s energy policies and the evolution of the

fuels prices.

In reference to Italian energy market, sanitary hot water requirement at 45-50 °C, is assumed

to be 50 l/days per person. Thus during a year, each person will be using about 18,000 l of sanitary hot water. Now depending on the temperature of the water available, it has been

setimated that nearly 700 kWh of energy will be needed annually to fulfil sanitry hot water

requirement of each person. Considering an average efficiency of the commercial boilers

available locally, each person every year uses about 60 m3 of methane to meet its sanitary hot

water need. Currently the cost of methane is about 0,80€/mc (fixed costs included).

At this point, it has to be considered that a properly sized thermal solar plant can satisfy a

75% share of the complex energy consumption. So, if the annual cost for the fuel is about 50

€ per person (the cost of 60m3), it is possible to save up to 40 € per person while using a solar

thermal plant.

An electric boiler with about 110 € per person annual cost while producing the same quantity

of sanitary hot water, is certainly more expensive. In this case, saving from a possible use of a



solar plant could be of the order of 90€ per person each year.

The cost of a thermal solar plant depends on several aspects. Besides the kind of energy used

and the cost of the components (collectors, load, tubes and fittings) it is necessary to consider

manpower and costs relevant to the support structures of anchorage to the building. Last

variable depends on the placement of the components and on the kind of the works necessaryto make the plant fit for the given case. Approximately, cost of a thermal solar plant can be

evaluated to be more or less 600 €/m2 (surface of the collector).

Data in the following table allows to evaluate the overall expenses necessary to build a plant:

it will be sufficient to multiply the m2 cost for the number of people and absorbing

surface/person. Economic saving and the avoided emissions using a thermal solar plant

instead of electric and methane water heater are considered as well.

For the installation of a solar plant generating hot water

Indicative

average cost forinstalled plant

(€/m2)

Geographical

areas Collectors

inclination

Collector

aperture M2 /

person

Small plants

North 50° 1,2

Centre 45° 0,75Glass flat

collectors South 35° 0,55

620

Solar thermal system for the average family ( 4 members)

Electric water heater

(KWh/year) Methane water heater

(m3/year)

Consumption 2.550 226

Cost of solar thermal plant_(Central Italy - after tax

deduction)

1.306 1.306

Savings (€/year) supposing a

partial

410 136

Recovery time (years, costsexcluded)

3 10

Emissions (T Co2/year) 1,8 0,69

Consumption fulfillment*(percentage) 75%

Environmental advantages

In reference to the environmental advantages, replacement of an electric water heater with a

solar system allows to avoid approximately 2,5 tons of carbon dioxide each year whereas

substitution of a gas water heater with a solar water heater will help to reduce annual

emission of nearly 2,0 tons of carbon dioxide.



Solar Thermal


Advantages of self construction

Using an easy and proper technology, self construction/fabrication of the needed device

especially, in the field of solar thermal system, could be cheaper and more interesting. Insome regions of North Italy , since the end of the 80s, some associations teach the self

fabrication of the solar plants to the citizens, guaranteeing material supply, logistic

arrangement and the technical assistance. Thanks to this unique initiatives, self fabricated

solar plants have got significant success. Compared to the turnkey industrial production, self-

construction have several advantages. In particular, such advantages deal with social, cultural,

economic and environmental aspects compared to the only disadvantage of a possible

reduction in efficiency, which is usually due to not followed building advices provided by the

experts.

From the cultural point of view, getting ready for a self constrcution solar system entails

widening of the technical, scientific and social knowledge. Here, besides issues concerning

solar systems other important aspects dealing with energy and its policies and consequences

are involved, as well. Most important is the fact that beside learning of the techniques

necessary for the construction of solar plants the same is available to a broad number of users

which can be involved in the construction phases for formative and re-educating goals.

Concerning economic aspect , the self built plant is cheaper than the pre-assembled one. For

those built in Northern Europe, the difference between self building and industrial production

is 30% and, in case of Italy, there is a 60% saving.

Besides, self building needs less energy than the industrial production, especially for thematerials that can be reused or be used without wastes (the packaging costs are avoided).

From this point of view, choice of materials is focused to safeguard the environmental aspect

rather than the economic one. On the market, there is a wide availability of materials

workable without complex or expensive apparatus, particularly, fit for the solar panels

construction.

In conclusion, a well built plant can be compared to an industrial product. From the

maintenance point of view, the first one is particularly cheap as the same can be looked after

by the person who has built the system whereas for an industrial product, a skilled labour is

needed.



Construction of a solar thermal plant: 16/20 Solar ThermalCopyright ENEA e-LEARN 2005



Solar Thermal


Potentials

Costs and, above all, the performances of a solar thermal plants changes according to the kind

of collector installed. The types of solar thermal collectors varies a lot in terms of costs and performances.

Solar energy being an uncertain source, solar thermal collectors should be considered as

integrative tools to the standard technologies: directly they can generate only a part of the

energy necessary for the users, energy that otherwise should be generated by a standard

boiler.

The percentage of thermal energy generated in a year by a solar thermal collector is the factor

of the covering of the annual thermal needs.

In Rome, for instance, the ratio between optimized system cost/generated energy ratio,

doesn’t exceed 60%. This limit is common to many renewable energy based technologies,

generally, characterized by uncertain or periodic availability.

It is mainly because of this reason that on one hand when increase in plant size results into

corresponding increase in the thermal load coverage factor at the same time the ratio between

energy cost and energy generated is linear up to 55% - 60%. Once this value is exceeded, cost

keeps growing linearly as a function of plant’s size whereas the energy produced and hence

the potential of the solar plant increases very slowly, with corresponding higher cost for the

surface area of the collector. It is for this reason that a solar thermal collector of appropriate

dimensions meant for the generation of sanitary hot water is planned to satify approximately55-60% of the thermal needs.





Solar Thermal


When, where and how it is good to choose a thermal solar system

Sun, we can say, is everywhere. But this doesn’t means that it is easy to use its potentials for

energy goals. So, both where and when to assemble solar panels for efficient and effectiveexploitation of solar energy potentials, is a good issue. Naturally, solar panels works better in

latitudes closer to the equator. If we consider horizontal panel, one installed in Northern

Europe and the other in the African desert belt, ratio appears to be 1 to 4.

The performance of a solar thermal plant will depend on both the useful temperature required

and the use during the seasons.

Concerning first point, higher is the temperature of the heated fluid, more will be the

convective heat losses to the surrounding environment. The collector output will decrease (or

also canceling out) if the temperature of the fluid will make the rate of heat loss equal to the

absorption one. So, each collector has “maximum temperature limit” at whch is able to

collect thermal energy.

Instead so far seasonal use is concerned, there is a difference between a user that need the

solar thermal plant all year long, and the ones who would like to use the system only during

summertime. The last ones can afford to cheap and simpler panels. During spring and

summer, glazed collectors are not necessary rather plastic ones will be sufficient because with

working temperature of nearly the room temperature (for half of the time ) problems of

thermal losses are significantly reduced.

The choice to use a solar collector can be influenced by several situations leading to differentsolutions. Every house or office can benefit by leaving hot water out of the gas or power

circuit. In the isolated houses, especially if inhabited during summer, solar collectors should

be standard ones, especially, if the simplicity and cheapness due to a use concentrated in the

season of best working of the panel are considered.





Solar Thermal


Laws and rules concerning building

Installation of a solar plant on an already existing building foresees, within the laws

concerning building, a series of administrative acts. Italian law, for example, foresees threedifferent types of acts:

• Work permit - Administrative act (written advise) by the local authority following a

written application (complete with technical records on the works and on the

preliminary project).

• Authorization to work - Administrative act of local authority allowing the works to

be undertaken based upon the previous technical and normative checks (differing

from the permit, which has to be in writing, the authorization is automatically granted

if the Mayor doesn’t give his opinion within 60-90 days from the application).

• Declaration of the beginning of the work - Paper by the local authority reporting the

work, place where the work will be done, fulfilment of communal building

regulations, laws in force concerning historic-artistic and environmental obligations.

For a solar thermal plant installation, in majority of cases, declaration of the

beginning of the work will be sufficient.

Historic - artistic and environmental obligations

The technology of solar thermal system can safeguard environment and reduce the presence

of polluting substances causing negative impacts on the Earth ecosystem .This huge

possibility, together with the care for harmony, ethic considerations and visual impacts can

help to improve the quality life of a human being in every aspect. It is for this reason that the preservation of the historical and environmental heritage came inot force. It is in fact the

same reason that has led to solar thermal system. The main goal of the two aspects is the

health of the human being.

So, before proceeding with the installation, it is important to know the laws protecting

historical-artistic and environmental heritage. It is also important to evaluate the place where

the plant will be installed, as the local governments could thwart the installation of solar

collectors, especially in protected areas or historic city centres.



Solar Thermal


Maintenance

For proper functioning of the plant, it is important to check the working system and its

components, periodically. In particular, it is important to check:

• Hydraulic losses;

• Damages to the collector due to external agents;

• State of the absorbing surface;

• State of the working of the electric and electronic check devices for the level of

limestone deposit;

• Working of the safety devices ( i.e. jolly valves) for the level of heat transfer fluid in

the primary circuit.

To assure continuity of the post-sale assistance service, it is useful to persuade the user to

sign an assurance contract which is free for the first months, as in the case of gas boiler

installation. The writing of a plant booklet including the general characteristics , the results of

the visual check, the preliminary operating tests and the check of performances is extremely

useful for the installer and the user.

Summary of the module:"Biomass"

• Preliminary phases

• Initial collection of climatic, geographical and building data

• Analysis and quantification of the users energy requirements• Planning

• Dimensioning of a solar water heater

• Sizing criteria

• F-chart method

• Selecting a collector

• Dimensions, number, orientation and inclination of the collectors

• Installation

• Installation on a flat surface

• Installation on the ground

• Floor installation

• Ceiling installation• Advantages and potentials of solar thermal: economic and environmental advantages

• Advantages of self construction



• Potentials

• When, where and how it is good to choose a thermal solar system

• Laws and rules concerning building

• Maintenance





Solar Thermal

Construction of a solar thermal plant: test


"Construction of a solar thermal plant"

Construction of a solar thermal plant: test




Solar Thermal

Next thermal solar system: 1/3


"Next thermal solar system"

The module shows the technology of thermodynamic solar system and the three main

typologies of plants. Enea “Archimede Project” is also mentioned.

Thermodynamic solar system

A different kind of solar thermal system is the one of the high temperature devices where the

heat transfer fluid can attain temperature more than 100° C. These devices are called

thermodynamic solar systems. The solar thermodynamic systems uses instruments able to

raise the level of radiation on the absorber. There main objective is to generate electricity and

are used especially in the industries for steam generation to be used later to do the mechanicalwork. To increase the intensity of solar radiation incident on the energy absorbing surface,

solar collectors, in question, works under concentration.

Such type of collectors using optical system (rotating parabolic) made available the heat

absorbing fluid at high temperature thus allowing the mechanical energy obtained to be used

either directly or converted into electric energy (solar thermodynamic). These systems

working without using diffused energy, need devices following the sun (i.e. sun tracking

system) and use of sophisticated technologies for optical concentration system with

subsequent increase in the overall cost.

A thermodynamic solar system allows generation of dynamic energy and electricity through astandard thermodynamic cycle supplied by the heat coming from the concentration of the

solar radiation. To obtain temperature higher than the one generally achieved with a common

solar collector, it is necessary to concentrate the direct solar radiation on the receiver with

subsequent transfer of the energy collected into high temperature heat.

The generated heat can be used in several industrial processes or in the generation of power.

The power generation is the main goal of the concentration solar plants. In this case, solar

heat is used in standard thermodynamic cycles as the ones with steam turbines, gas turbines

or Stirling engines.

In the high solar radiation regions (average direct annual power higher than 300 W/m2), theexploitation of the solar source allows to get, for a year, from a collection surface m2, e

thermal energy equivalent to the one coming from one oil barrel combustion with net



reduction in the emission of 500kg of CO2 in the atmosphere (ENEA-REA 2005 source).

To solve the problem of the variability of the solar source, heat can be accumulated during

the day thus making the system more flexible and meeting the needs of the productive

processes. Different technologies are used for the concentration of the solar radiation in a

solar concentrator plant but following phases of the process could be found in all of them:

• Collection and concentration of solar radiation;

• Conversion of solar radiation into thermal energy;

• Transport and eventual storage of thermal energy;

• Use of thermal energy.





Solar Thermal


Main types of plants

Taking into consideration the geometry and placing of the concentrator with respect to the

receiver, three main types of plants could be defined:

• Parabolic Trough;

• Power Tower;

• Parabolic Dish.

Representation of the three type

Parabolic trough collector

Concentration of the solar radiation in a parabolic trough collector is obtained through

parabolic concave cylindrical surfaces called parabolic troughs. The heat transfer fluid passing through a tube fixed at the focal length of the PTC gets heated up to 500° C. The

successive thermoelectric cycle is merely a water-steam thermal cycle similar to the one in a

standard thermoelectric plant and it includes a steam turbine coupled with an electric

generator, a condenser and preheating system of the water supply. This technology, named

DCS (Distributed Collector System) is quite matured for the generation of electricity on a

large scale.



Scheme of a parabolic trough collector

Parabolic trough collector

Power tower system

In such type of plants, concentration is obtained through the reflection of solar light by

several plain glasses orientated in a way to make the rays converge just at one point, where

thermal receiver is fixed, supported by a high central tower. The primary fluid circulating

through the receiver, depending on the concentration ratio of the plant, can be heated at

temperatures of more than 1000° C. The primary fluid transfer its heat, through an exchanger,

to a secondary fluid that feed the final thermoelectric cycle, composed of an usual turbine-

alternator group, exchangers, etc. which are characteristic of a thermal station. This

technology is named CRS (Central Receiver System) and has demonstrated for 10 MWe

sized plants, a proven total efficiency of approx. 8 - 12%, from the solar radiation absorption

to the electricity fed to the grid between

Scheme of a power tower system



Experimental Solar Two plant (US)

Parabolic Dish Collector

Such type of plants is based on systems composed of independent parabolic concentrators,

where the converter is placed on focus of the dish. They have interesting characteristics, such

as high efficiency, modularity, excellent yields at low powers (generally between 10 and 25kW) and so, the possibility of their use for construction of local or regional systems based on

the distributed generation. Below given pictures are some examples of this type of plants:

Parabolic dishes

Next thermal solar system: 2/3 Solar Thermal




Solar Thermal


ENEA program for the generation of solar heat at high temperature

ENEA has started a triennial research program for the construction of an industrial scale

demonstration plants for the generation of electric energy from solar energy through the production and storage of heat at medium temperature (550° C). The research program is

focused primarily on the more innovative aspects of the technology (collector, receiver,

storage) and foresees experimental circuits ( installed at different centres of ENEA centres)

and a demonstration plant of 0,5 Km2.

Solar thermal concentrator using parabolic trough collectors appears to be the most promising

technology in the very near future. In order to increase efficiency of the system, reduce costs,

widen the possibilities for the application of the energy generated (with guaranteed electric

generation even during the night or partially cloudy days), important planning innovations

have been introduced to the solutions already available in the market.

These innovations have dealt with both the devices for the absorption of the solar radiation,

concentrators and receiver tubes as well as the design aspects of the thermoelectric

conversion part. ENEA project has been inspired by SEGS technology, the technology with

successful working experience of the plants (e.g. hybrid solar-gas plants in America) and

combine both the technologies, i.e. parabolic trough collectors and power tower systems,

after appropriate improvements on the design aspects.

Demonstrative prototype industrial plant : "Archimede project"

Archimede project, result of collaboration between ENEA and ENEL, is the first application

of possible integration between a gas combined cycle and thermodynamic solar plant, basedon the ENEA technology, worldwide. Priolo Gargallo ENEL station (Siracusa) will be the

site of the experimentation. Archimede will use the technology of ENEA, able to generate



power even during the night or with clouds.

Summary of the module:"Next thermal solar system"

• Thermodynamic solar system

• Main types of plants

• ENEA program for the generation of solar heat at high temperature

Next thermal solar system: 3/3 Solar Thermal




Solar Thermal

Next thermal solar system: test


"Next thermal solar system"

Next thermal solar system: test


Solar Thermal - Progetto Formazione

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