Preface

Conventional energy sources based on oil, coal and natural gas are damaging economic and social progress, the environment and human life. Many people are concerned about these problems and wish to address the symptoms as a matter of urgency, but not all understand the basic causes and consequently do not realize that not only technological, but also social changes are required. It is now widely acknowledged that renewable energy capacity has to be increased by exploiting its enormous potential.

During the last few years the ‘energy issue’ has been assuming a more and more important role among any other choice, strategy and policy concerning human survival and development.

Nowadays the energy model is almost totally centred (for the 80%) on the exploitation of fossil fuels such as petrol, natural gas and coal. To the industrial-economic costs connected with these fuels, social and environment costs, which cannot be overlooked, have to be added.

First of all, fossil fuels are exhaustible energy sources; their formation time is infinitely lower than the one which refers to their exploitation and for this reason are also defined as ‘non-renewable resources’. Although the level of the world’s fossil fuel supply cannot be considered as worrying in the short term, the increased difficulties in reaching the fields have made the cost-benefit ratio of the extraction processes less and less favourable.

Secondly, the political, social and economic instability, deriving from the world consumption distribution (only 20% of world population consumes the 80% of available resources) and the increasing number of wars connected with the geopolitics of fossil resources and with the control of international supplies, represents a risk for the security and the possible normal development of nations.

Eventually it is necessary to consider the environmental impact caused by the exploitation of fossil energy sources; actually their combustion process brings on the emission of noxious substances such as sulphurous anhydride, nitrogen monoxide and carbon anhydride (15 billion tons of CO2 are poured out into the atmosphere every year). Sulphurous anhydride and nitrogen monoxide contribute to the formation of acid rains while carbon anhydride is the main greenhouse gas which causes global warming (greenhouse effect). So behind fossil fuel exploitation is hidden the risk of worrying consequences regarding both the Earth (desertification, arctic ice melt, sea level rise…) and indirectly human health (rise in respiratory diseases, decrease of drinkable water…) .

The analyzed context brought us to review in a critical way the concepts and models of development which have been taken into consideration to date and which have centred on the massive exploitation of fossil sources. During the last few years this review has led to elaborate the concept of sustainable development, which is based on energy consumption reduction and optimization, and also on the use of renewable energy sources (the Sun, the Wind, hydraulic energy, geothermic resources, tides and wave motion; this definition is completed by the biomasses, although these resources can only be considered as renewable if run with the purpose to make their exploitation time consistent with their renewal time).

In comparison to fossil fuels, renewable sources could contribute to the development of a sustainable energy system and to environment and territorial protection; they could also provide new economic growth opportunities.

Recently, the European Union passed new legislative measures to delineate in a binding manner the plan, from now to 2020, to decrease the climate effects caused by present energy consumption levels; that is to say that at least 20% of primary energy will have to be produced by renewable sources, greenhouse gas emission will have to be reduced of 20% and another 20% will have to be an energy saving which the EU means to reach by a wide energy efficiency recovery.

The importance placed upon renewable energy sources now and in the future inside the world energy panorama led us to focus this study on what can be defined as the most relevant renewable source: the Sun.

A policy of energy sustainability can’t leave solar energy exploitation out of consideration. Actually its incident quota on the terrestrial surface is 10,000 times greater than the yearly energy requirement of the world’s population. Besides being the origin of almost all the other energy sources, renewable and conventional, excluding geothermic, nuclear and gravitational (tides) ones, the energy provided by the Sun is free, endless and clean (the devices used to exploit solar energy are characterized by very low emissions while running). Moreover solar energy is easy to harness and distribute (it is particularly abundant in many world areas with depressed and difficult economic situation).

The first chapter of this study is dedicated to the analysis and calculation of solar radiation incident on an inclined surface at an instantaneous, hourly and daily level.

The second chapter offers a summary and an analysis of all technologies available today to use solar energy: the solar thermal (technologies which exploit solar radiation in order to produce thermal energy that can be used in domestic, civil and productive fields; the differences between low, medium and high temperature solar thermal energy will be identified).

In the last part of the book we judge through a deeper investigation the opportunities offered by the exploitation of biomass energy.

Renewable energy education is a relatively new field and previously it formed a minor part of traditional university courses. However, over the past decade, several new approaches have emerged: we see these in the new literature and, even more clearly, in new books. The present treatise, in the authors’ auspices, represents a contribution to this new ‘incoming science’.

The book is highly recommended to professors, students and professionals in mechanical, civil, environmental, chemical and agricultural engineering. It is also recommended to all the readers interested in the aims, philosophy, structure, design, strategies and overcomes in the use of energy from ‘solar thermal and biomasses’.

The Authors, 2010

Thermal solar power systems use solar radiation to produce heat in place of tradi­tional fossil fuels. To get heat at a temperature higher than 250°C, it is necessary to concentrate the solar radiation. To achieve this concentration, an appropriate opti­cal system (the concentrator) is used. This device gathers and delivers direct solar radiation to another device (the receiver) where it is transformed into high tem­perature heat [50]. The heat produced in this way can then be applied to different industrial processes (such as the desalination of seawater and production of hydrogen using thermal chemical processes) or to electrical energy production.

Currently, electrical energy production is the main purpose for which CSP sys­tems are used. In this case, solar heat is used in traditional thermodynamic cycles such as the Rankine, Stirling and Brayton cycles. Until now, systems that are able to convert about 30% of the solar radiation received on the Earth’s soil into electri­cal energy have been used. The range of power obtainable varies from 10 kW to a few hundreds of megawatts, including more than one modular system [50, 51].

As regards the concentration, solar systems can apply to different technologies; however, it is possible to point out the following processes in every one of these technologies [50]:

• gathering and concentration of solar radiation;

• conversion of solar radiation into thermal energy;

• transport and possible storing of thermal energy;

• thermal energy utilization.

The gathering and concentration of radiation, whose power density is very low by nature, is one of the principal problems of solar systems. As already stated,

this process occurs thanks to a concentrator which is composed of appropriately shaped panels with reflecting surfaces. During the day, the concentrator follows the Sun so as to gather the direct component of its radiation and concentrates it inside the receiver. This latter device transforms solar energy into thermal energy, which is then given to a fluid that flows inside (thermal vector fluid). As we will observe from the analysis of tower systems with a central receiver (par. 4.4.2), the use of a melted salts mixture as the thermal vector fluid lets the system have a system of thermal energy accumulation before its utilization in the production process. This storing is realized by collecting the thermal vector fluid that goes out from the receiver in appropriately insulated storage tanks. In this way, solar energy, which is very variable in nature, can become a thermal energy source that is always available to users whenever required [45, 50].

On the basis of geometry and the concentrator’s position, we can have three kinds of CSP systems [12, 45, 50-53]:

• linear parabolic collector systems;

• tower systems with a central receiver;

• parabolic dish collector systems.

The solar circuit is the connection between the solar collectors and the storage tank. A solar circuit can be made of copper or stainless steel pipes. To restrict heat losses, the pipes which connect the collectors to the tank have to be short and insulated to their maximum. Stainless steel pipes have a surface which is less smooth than that of copper pipes and so they cause bigger load loss. In open systems, it is better to use tubes which have a smooth inner surface (e. g. cop­per) to prevent encrustments. When we choose the circuit components, it is very important to consider the kind of fluid they are going to contain; actually, anti­freeze solutions or products used in the swimming pool impose the utilization of components (taps and fittings, sets) which do not corrode when they come into contact with certain chemical substances. The good processing of a solar system strongly depends on how the solar circuit’s insulation has been carried out. A suf­ficient insulation layer and also a good insulation execution without interruptions or escapes are necessary. This also applies to the circuit’s elbows. Concerning the choice of the insulating materials, it is important to take into consideration their
resistance to high temperatures; for very short periods, the temperature inside the solar circuit’s tubes can reach more than 200°C. Moreover, the insulation should be able to resis atmospheric agents and ultraviolet rays. Suitable materials could be insulating mineral fibres or insulating materials such as Aeroflex and Armaflex HT. On the outside, the insulation can be protected by tube-coverings made of a steel or zinc-plated layer.

In the solar circuit, besides the pipes, there are devices which are necessary to guarantee the fluid’s motion and the security on the basis of its assemblage and utilization (pumps, buffer vessels, expansion vessels, security valves, air discharge valves, etc.). Generally, all the basic hydraulic components offered are already pre-set by a great majority of the manufacturers. Also, the control instruments (manometers and thermometers) are pre-set [5, 6, 9, 17].

Let us now see the elements which characterize the solar circuit, with reference to systems with a forced circulation, because of its complexity. It has been seen

that in the natural circulation system the storage tank can be directly heated up by the natural circulation or by a heat exchanger. Moreover, there is no device which is able to actively regulate the solar circuit [6].

Pump As in the centralized heating systems, in the solar systems, there are ‘lots’ and ‘returns’. The pipe in which the hot thermal vector fluid flows from the col­lector to the storage tank is called ‘lot’, whereas the ‘return’ is the pipe with the colder fluid which flows from the storage tank to the collector. The pump must be installed on the return line, with the motor’s shaft in the horizontal direction. The pump must not be insulated [5, 6, 33].

Non-return valve (or restraint valve) When collectors are installed in an ele­vated position than the storage tank and the fluid inside the tank has a higher temperature than the fluid which flows inside the collectors (especially during the night), the temperature difference between the hot fluid inside the exchanger (in a lower position) and the cold fluid inside the collectors (in a higher position) would start a natural circulation inside the primary circuit causing dispersion of the heat stored in the tank during the day.

To avoid this phenomenon, it is necessary to install a non-return valve between the pump and the collector; this valve has to be well proportioned so as to prevent its opening by the sole thrust force of the thermal vector fluid. In this way, the storage tank will not be cooled by the collector when the pump is not running [5, 9].

Regulating power unit The regulating unit of a thermal solar system controls the running of the circulation pump to exploit the solar energy to its fullest. Often, we talk about simple electronic power units based on the temperature difference. This kind of power unit installed in standard systems (collectors on the roof and storage tanks in cellars) are provided with two temperature sensors. The first sen­sor is installed inside the collector, at the hottest point of the solar circuit, and the second sensor is installed inside the tank, connected with the heat exchanger of the solar circuit.

The temperature values detected by the sensors are compared by a control device: the pump is run by a relay when the intervention temperature is reached. The correct definition of the intervention temperature comes from different factors. Generally, the longer the circuit pipes are, the larger is the temperature difference or the delay in the intervention. To make the pump work, standard directions sug­gest that the temperature difference between the solar collectors and the storage tank should be between 5°C and 8°C. Instead, the pump switches off when the temperature difference reaches 3°C. It is also possible to insert a third optional sensor which measures the temperature of the upper part of the storage tank [33].

Temperature sensors The efficiency of the pump’s intervention mostly depends on the position of the thermal sensors. Each collector’s sensor is positioned on the storage pipe or directly on the absorber (the part of the collector which absorbs solar radiation), near the lot’s exit. However, the thermal sensor has to record the absorber temperature and communicate it to the regulating power unit, even in a stalemate situation, i. e. when the pump is not working.

The storage tank’s sensor has to be placed at the same height as the heat exchanger and it can be an immersion sensor or a contact sensor. The sensor for the auxiliary heating communicates to the power unit when this intervention is necessary and has to be put at the same height as its respective heat exchanger [33].

Expansion vessel It absorbs the thermal vector fluid’s expansion. The size of this component depends on the fluid quantity inside the circuit, since the vessel must be able to contain the fluid’s dilatation between 4°C and 90°C.

Let us assume a system with a 100-l tank and with collectors and pipes which can contain a water volume of 20 l. The expansion vessel’s volume must be able to absorb the dilation of 120 l of water which occurs at the just said temperatures. The pipes which connect the expansion vessel to the system must not be insulated [5, 6, 17, 33].

Security valve Security valves protect the system when the pressure increases because of various reasons, such as superheating. Such circumstances might arise when the circulation pump is broken or is not working due to a power black-out. So the fluid’s temperature inside the collectors and other circuit com­ponents may increase until the formation of steam which is then released by the security valve.

The valve must not operate during the system’s normal processing and therefore it has to be set at a higher pressure than the maximum pressure of the circuit. For example, it is chosen as a pressure of 600 kPa (6 bar) if the circuit’s pressure is set at 550 kPa (5.5 bar) [5, 17].

‘Jolly’ valve To avoid air storage inside the pipe and thereby the reduction of fluid delivery and thermal exchange, there must be vent-holes in the upper parts of the circuit.

These vent-holes (jolly valves) can work automatically or manually. In the open circuit systems, the jolly valve should be left open since air continually enters [5, 33].

Flow regulating valves Especially for medium — and large-sized systems, these valves are inserted in every collector’s row to balance the flows inside the different branches of the circuit. In this way, uniform performance from different parts of the system is guaranteed [5].

Intercepting valves The function of these valves is to interrupt the flow and insulate certain circuit elements (such as valves or pumps), when maintenance is needed or when there are security problems. They are installed at the upper and lower part of each system’s element [5].

Emptying taps Manual emptying taps are installed at different circuit points. Generally, there is one in every collector’s row. To allow the gradual emptying of the fluid contained inside the circuit, it is necessary to ‘fix’ an emptying tap between two intercepting valves [5].

Three-port valves Three-port valves allow combining two flows (mixing valves) or separating a flux into two parts (diverting valves) [5].

The pump, the non-return valve, the expansion vessel and the security valve are offered in the market as a ‘pumps and security’ pre-set group. The expansion ves­sel and the security valve have to be installed in any case to avoid interruption between them and the collector [6].

temperature sensors

Jolly valve

solar

^.collector

insulation

during the summer and use it in the winter (seasonal storage), but as we have already stated in point 3 of par. 2.2.2.2, the technological solutions based on this principle are not convenient for limited domestic use because of its cost and logis­tics. Currently, the most common storage systems are the ones which allow the storage of the heat efficiently for a day or two. Storage tanks can be classified depending on their final utilization and the kind of insulation used [5, 9].

2.2.4.1 Storage tank materials Pressurized tanks are made of stainless steel, enamelled steel or plastic-covered steel. Stainless steel tanks are lighter and last longer, but they are much more expensive than enamelled steel tanks. However, stainless steel is easily corroded by water with a high chlorine content. To reduce the corrosion risks, this kind of tank is generally provided with a magnesium anode which has to be replaced periodically. Non-porous plastic-covered steel tanks are also available in the market and they cost less than the other tanks. However, this storage tank cannot withstand temperatures higher than 80°C. Plastic-covered tanks are characterized, in fact, by a lower reliability level compared with other constructive typologies [9].

2.2.4.2 Sanitary water storage tank Figure 64 shows the storage tank installed in standard solar systems. In this storage device, there are generally two heat exchangers: the solar exchanger, which allows the thermal exchange between the thermal vector fluid inside the solar system and the fluid inside the tank, and the additional exchanger, which allows heat transfer from the integrative heating system (e. g. a central-heating boiler) to the fluid stored inside the tank.

Moreover, in the lower part of the storage tank there is a connection to the water pipes for the supply of cold water. The operating pressure inside the pressurized storage tanks is about 4-6 bar.

As regards the choice of the storage tank’s volume, we generally consider 40-100 l/m2 of flat collector surface. Concerning the proportioning of the solar system, while for the big-sized systems we refer to values that are near the lower limit of the above interval, it is vice versa for small systems. Large-sized storage

tanks can contain larger quantities of energy; however, this choice also involves larger heat losses and frequent starting of the integrative heating system. This hap­pens because the heating of larger quantities of water requires more energy. As regards sanitary water storage tanks, it is important to take into consideration the calcareous encrustment problem, which may form at high temperatures in the exchanger: for this reason, in the solar systems used inside houses the tank temperature should not be above 60-70°C [9].

2.2.4.3 Storage tank shape If the storage tank works properly it should have different water layers inside. The creation of these layers is possible thanks to the variation in fluid density at different temperatures. Actually, hot water which is ‘lighter’ is stored in the upper part of the tank while the ‘heavier’ cold water is stored at the bottom of the tank. This layering effect is an essential requisite for the good processing of the solar system. As soon as the hot water is requested by users, for example, for showering, the cold water flows into the tank from the pipes and mixes with the previously heated water. To limit this undesired effect and to maintain the temperature layering for as long as possible, the storage tank (generally shaped like a cylinder) should be tall and narrow.

These conditions can be realized using vertical storage tanks whose height — diameter ratio is at least 2.5:1 (in monobloc natural circulation systems the storage

Figure 65: Example of a solar storage tank.

tank is generally horizontal because of aesthetic and encumbrance reasons). Low temperatures in the lower part of the tank guarantee a high performance of the solar system even in case of insufficient radiation and low temperature of the thermal vector fluid.

Before installing a vertical tank, it is important to make sure that its height is compatible with the place where the storage system will be placed [5, 9].

2.2.4.4 The cold water inlet device in the storage tank This particular inlet device, when suitably installed in connection with the adduction pipe, weakens the strong motion of cold water flowing into the tank and limits the risk of its mixing with the warmer water layers.

2.2.4.5 Hot water collection In traditional storage tanks, hot water is collected from the upper part of the tank; because of the layers’ phenomenon and the outlet pipe being located in the upper part of the tank, we are always sure to collect the hottest water. After the collection, a part of this hot water stagnates inside the pipes getting cold. It is possible that this cold water can flow back into the upper part of the tank where it mixes with the hot water which is stored there. This causes heat dispersions of the relevant entity (see Fig. 64). To avoid this drawback, it is possible to direct the lot tube downwards making it pass inside the storage tank or outside across the insulated layer which covers the tank [9].

2.2.4.6 Heat exchangers and respective connections The solar circuit’s heat exchanger should be installed in the lower part of the storage tank to ensure that the thermal exchange occurs inside the water volume present at the bottom of the tank. The heat exchanger for the integrative heating system is generally placed in the upper part of the tank, to guarantee quick heating of the water volume at a temperature (corresponding to the daily requirements) without resulting in a tem­perature increase in the lower part of the tank, where the solar circuit’s exchanger is installed. This disposition of the exchanger ensures that the thermal exchange in the lower part of the tank, where the water is the coldest, occurs with the highest efficiency even when the solar circuit fluid does not reach the highest temperature [5, 9, 33].

2.2.4.7 Storage tank insulation The purpose of the storage tank insulation is to reduce the heat dispersions to the outside environment to its minimum. To have a storage tank insulation which efficiently limits heat losses, the following charac­teristics are needed:

• It should be 10 cm thick on the sides and 15 cm thick in the connections with the upper surface.

• It should also cover the bottom of the tank.

• It should be perfectly adherent to tank’s sides to avoid losses by convection.

• It should be made of materials which do not contain CFC and PVC and have low thermal conductivity (<0.035 W/m K).

Thermal dispersions in an insulated storage tank must be lower than 2 W/K. To limit these losses, it is very important to make sure that the thermal covering in connection with flanges and pipe fittings is hermetically sealed.

The tank linings that are currently available in the market can be flexible (expanded polyurethane foam, fibreglass, etc.), inflexible (they could be used out­side for retrofit interventions) or by direct injection with a plastic or metal cover­ing [5, 9, 33].

Preface

Conventional energy sources based on oil, coal and natural gas are damaging economic and social progress, the environment and human life. Many people are concerned about these problems and wish to address the symptoms as a matter of urgency, but not all understand the basic causes and consequently do not realize that not only technological, but also social changes are required. It is now widely acknowledged that renewable energy capacity has to be increased by exploiting its enormous potential.

During the last few years the ‘energy issue’ has been assuming a more and more important role among any other choice, strategy and policy concerning human survival and development.

Nowadays the energy model is almost totally centred (for the 80%) on the exploitation of fossil fuels such as petrol, natural gas and coal. To the industrial-economic costs connected with these fuels, social and environment costs, which cannot be overlooked, have to be added.

First of all, fossil fuels are exhaustible energy sources; their formation time is infinitely lower than the one which refers to their exploitation and for this reason are also defined as ‘non-renewable resources’. Although the level of the world’s fossil fuel supply cannot be considered as worrying in the short term, the increased difficulties in reaching the fields have made the cost-benefit ratio of the extraction processes less and less favourable.

Secondly, the political, social and economic instability, deriving from the world consumption distribution (only 20% of world population consumes the 80% of available resources) and the increasing number of wars connected with the geopolitics of fossil resources and with the control of international supplies, represents a risk for the security and the possible normal development of nations.

Eventually it is necessary to consider the environmental impact caused by the exploitation of fossil energy sources; actually their combustion process brings on the emission of noxious substances such as sulphurous anhydride, nitrogen monoxide and carbon anhydride (15 billion tons of CO2 are poured out into the atmosphere every year). Sulphurous anhydride and nitrogen monoxide contribute to the formation of acid rains while carbon anhydride is the main greenhouse gas which causes global warming (greenhouse effect). So behind fossil fuel exploitation is hidden the risk of worrying consequences regarding both the Earth (desertification, arctic ice melt, sea level rise…) and indirectly human health (rise in respiratory diseases, decrease of drinkable water…) .

The analyzed context brought us to review in a critical way the concepts and models of development which have been taken into consideration to date and which have centred on the massive exploitation of fossil sources. During the last few years this review has led to elaborate the concept of sustainable development, which is based on energy consumption reduction and optimization, and also on the use of renewable energy sources (the Sun, the Wind, hydraulic energy, geothermic resources, tides and wave motion; this definition is completed by the biomasses, although these resources can only be considered as renewable if run with the purpose to make their exploitation time consistent with their renewal time).

In comparison to fossil fuels, renewable sources could contribute to the development of a sustainable energy system and to environment and territorial protection; they could also provide new economic growth opportunities.

Recently, the European Union passed new legislative measures to delineate in a binding manner the plan, from now to 2020, to decrease the climate effects caused by present energy consumption levels; that is to say that at least 20% of primary energy will have to be produced by renewable sources, greenhouse gas emission will have to be reduced of 20% and another 20% will have to be an energy saving which the EU means to reach by a wide energy efficiency recovery.

The importance placed upon renewable energy sources now and in the future inside the world energy panorama led us to focus this study on what can be defined as the most relevant renewable source: the Sun.

A policy of energy sustainability can’t leave solar energy exploitation out of consideration. Actually its incident quota on the terrestrial surface is 10,000 times greater than the yearly energy requirement of the world’s population. Besides being the origin of almost all the other energy sources, renewable and conventional, excluding geothermic, nuclear and gravitational (tides) ones, the energy provided by the Sun is free, endless and clean (the devices used to exploit solar energy are characterized by very low emissions while running). Moreover solar energy is easy to harness and distribute (it is particularly abundant in many world areas with depressed and difficult economic situation).

The first chapter of this study is dedicated to the analysis and calculation of solar radiation incident on an inclined surface at an instantaneous, hourly and daily level.

The second chapter offers a summary and an analysis of all technologies available today to use solar energy: the solar thermal (technologies which exploit solar radiation in order to produce thermal energy that can be used in domestic, civil and productive fields; the differences between low, medium and high temperature solar thermal energy will be identified).

In the last part of the book we judge through a deeper investigation the opportunities offered by the exploitation of biomass energy.

Renewable energy education is a relatively new field and previously it formed a minor part of traditional university courses. However, over the past decade, several new approaches have emerged: we see these in the new literature and, even more clearly, in new books. The present treatise, in the authors’ auspices, represents a contribution to this new ‘incoming science’.

The book is highly recommended to professors, students and professionals in mechanical, civil, environmental, chemical and agricultural engineering. It is also recommended to all the readers interested in the aims, philosophy, structure, design, strategies and overcomes in the use of energy from ‘solar thermal and biomasses’.

The Authors, 2010

Thermal solar power systems use solar radiation to produce heat in place of tradi­tional fossil fuels. To get heat at a temperature higher than 250°C, it is necessary to concentrate the solar radiation. To achieve this concentration, an appropriate opti­cal system (the concentrator) is used. This device gathers and delivers direct solar radiation to another device (the receiver) where it is transformed into high tem­perature heat [50]. The heat produced in this way can then be applied to different industrial processes (such as the desalination of seawater and production of hydrogen using thermal chemical processes) or to electrical energy production.

Currently, electrical energy production is the main purpose for which CSP sys­tems are used. In this case, solar heat is used in traditional thermodynamic cycles such as the Rankine, Stirling and Brayton cycles. Until now, systems that are able to convert about 30% of the solar radiation received on the Earth’s soil into electri­cal energy have been used. The range of power obtainable varies from 10 kW to a few hundreds of megawatts, including more than one modular system [50, 51].

As regards the concentration, solar systems can apply to different technologies; however, it is possible to point out the following processes in every one of these technologies [50]:

• gathering and concentration of solar radiation;

• conversion of solar radiation into thermal energy;

• transport and possible storing of thermal energy;

• thermal energy utilization.

The gathering and concentration of radiation, whose power density is very low by nature, is one of the principal problems of solar systems. As already stated,

this process occurs thanks to a concentrator which is composed of appropriately shaped panels with reflecting surfaces. During the day, the concentrator follows the Sun so as to gather the direct component of its radiation and concentrates it inside the receiver. This latter device transforms solar energy into thermal energy, which is then given to a fluid that flows inside (thermal vector fluid). As we will observe from the analysis of tower systems with a central receiver (par. 4.4.2), the use of a melted salts mixture as the thermal vector fluid lets the system have a system of thermal energy accumulation before its utilization in the production process. This storing is realized by collecting the thermal vector fluid that goes out from the receiver in appropriately insulated storage tanks. In this way, solar energy, which is very variable in nature, can become a thermal energy source that is always available to users whenever required [45, 50].

On the basis of geometry and the concentrator’s position, we can have three kinds of CSP systems [12, 45, 50-53]:

• linear parabolic collector systems;

• tower systems with a central receiver;

• parabolic dish collector systems.

Starting from the experimental values obtained on horizontal surfaces, Liu and Jordan have introduced a widely used method to calculate the monthly average solar radiation on inclined surfaces. This method is based on the division of radia­tion between its direct and diffuse components. Liu and Jordan discovered that the ratio between the monthly average diffuse radiation D and the global radiation H received on a horizontal surface can be correlated to a parameter called monthly clearness index K. This index is obtained by dividing the monthly average terres­trial radiation for every day by the monthly average extraterrestrial radiation for every day both received on a horizontal plane.

K=H/Hm (39)

To compute Hex the solar constant value used is 1394 W/m2 (instead of the more recent value of 1367 W/m2). For this reason, K values should be based on that value. As for the calculation of Hex, it has been suggested that eqn (38) be applied to a specific day of each month. That day must be chosen to get an extraterrestrial radiation Hex on a horizontal surface, which is equal to the monthly average extra­terrestrial radiation Hex.

If B (= H — D) is the monthly average direct component received on a horizontal surface for each day and E is the monthly average global radiation on an arbitrary oriented surface, we have:

E=RbB+RiD+Rr(B+D) (41)

where ha’ is the hour angle calculated using eqn (22); ha and ha are expressed in radians and Rb is the monthly average factor of inclination by direct radiation [1].

Buffer storage tank This kind of storage tank can be made of steel (pressurized tanks) or plastic and it is mainly used for room heating. In this case, the fluid inside the storage tank is withdrawn, heated inside the boiler and put back inside the tank; at this point, the warmed fluid is once again withdrawn from the tank and sent to the radiators. This solution is adopted to improve the boiler working conditions and so the boiler is not forced to work in the stop-and-go mode. Actu­ally the boiler warms up the stand-by water volume which is kept at a certain temperature inside the storage tank and so it can remain switched off for a long period of time [9].

‘Tank in tank’ storage tank The combined solar systems are conjointly used for both warming up sanitary water and room heating. This system often uses a type of storage tank called tank in tank, which consists of a buffer tank inside which there is a storage tank for drinkable water. The latter tank gets the heat through its own sides from the fluid contained inside the outer tank. The storage tank with sanitary water is located in the upper part of the buffer tank where the water is maintained at a certain temperature by an integrative heating system. At the bottom of the buf­fer tank, instead, there is the solar heat exchanger (see Figs 67 and 68) [9].

Figure 68: Scheme of a combined solar system.

1 The solar physics

The Sun is a sphere made up of gaseous elements consisting of 80% hydrogen, 19% helium and 1% of all the well-known substances. It has a diameter of 1.39 ■ 109 m and it is located at a distance of 1.495 ■ 1011 m from the Earth. However, this dis­tance may vary by ±1.7% during the year because of the orbit’s eccentricity.

The Sun is characterized by two motions: a motion of revolution around the centre of the galaxy, which has a linear speed of 300 km/s and takes 200 million years to complete, and a motion of rotation around the axis, which lasts about 4 weeks.

Inside the Sun, numerous fusion reactions take place. The heat produced by these reactions spreads from the inner layers to the outer layers by convection, conduction and radiation. From the outer layers, the heat is transmitted to the surrounding space by radiation. Among the nuclear reactions that occur in the Sun, the most important is the one which converts hydrogen into helium; the mass of a helium nucleus is smaller than that of the four original protons and this mass defect is converted into energy.

The mass of the Sun is roughly 2 ■ 1030 kg. The areas at the centre of the Sun reach temperatures of about 8-40 million kelvin and a density 100 times greater than that of water. However, the density is extremely lower in the outer layers.

It is believed that the region between 0 and 0.23R (R = solar ray), which con­stitutes 40% of the solar mass, produces 90% of the solar energy. The area between 0.7 and 1R is called the convective envelope (temperature 5000 K, den­sity 10-5 kg/m3), because of the importance of convective processes in this layer. The photosphere, the outer layer from the convective envelope, is composed of strongly ionized gases, which are capable of absorbing and emitting through a continuous spectrum of radiation. Over the photosphere, there is the inversion layer, which is hundreds of kilometres wide and is made up of cold gases. Out­side the inversion layer, there is the chromosphere, which is 10,000 km wide, and the corona, characterized by a very low density and high temperatures (106 K) (Fig. 1) [1].

The electromagnetic radiation emitted by the Sun extends over a wide wave­length interval: from 0.1 nm to 104 m; however, the greatest part of that energy falls in the interval between 0.2 and 4 |jm. In particular, 95% of the energy which reaches the Earth is included between 0.3 and 2.4 |jm. The spectrum of the solar radiation is similar to a black body’s spectrum at a temperature of 5780 K, since temperatures at the surface of a star fluctuate between 4000 and 6000 K. Therefore, it is right to assume that the behaviour of the Sun with regard to radiation is similar to the behaviour of a black body at a uniform/regular temperature (Fig. 2). This temperature of 5780 K is calculated using the Stefan-Boltzmann law [1, 3].

Analysing the spectrum more carefully, one can notice that the greatest part of the radiation falls in (1) the ultraviolet band, which extends from 0.20 to 0.38 pm; (2) the visible light band, from 0.38 to 0.78 pm; and (3) the near infrared band until about 4 pm. Only 8-9% of all the solar energy which reaches the Earth falls in the ultraviolet band; 46-47% falls in the visible band while the remaining 45% falls in the infrared band [3].

Currently, the most suitable technology for electrical energy production by ther­modynamic systems is one which uses linear parabolic collectors [45, 50, 53].

These collectors are composed of a linear concentrator with a parabolic profile whose reflecting surface follows the Sun rotating on a single axis. The concentra­tor is fixed on a support structure which guarantees the correct processing during windy conditions and the action of other atmospheric agents.

The reflecting panel is normally composed of a common glass mirror with an appropriate thickness. Solar radiation is focused towards a receiving tube that is placed along the parabolic concentrator’s fire. The energy absorbed by the receiv­ing tube is then transferred to a processing fluid (thermal vector fluid) generally made of synthetic oil which is drawn up inside. The heat gathered is normally used as shown in the Fig. 86 (i. e. for the electrical energy production).

Figure 86 shows the processing scheme of a solar thermal electrical system with linear parabolic collectors using synthetic oil as the thermal vector fluid. In such systems, parabolic collectors are connected in series, generally in two par­allel rows which are a few hundred metres long and form a string that represents the unitary module of the system. The strings as a whole form the so-called solar field (Fig. 85). The synthetic oil pumped towards the receiving tubes comes out from the warmed up solar field at a temperature of about 390°C and then feeds a power unit (which is right at the centre of the solar field): the thermal vector fluid transfers the heat to a steam generator to start the processing of an electrical turbo-generator group. After delivering the heat, the oil (at 290°C) comes back

to the solar field to be warmed up once again. With the linear parabolic collector technology, a maximum system processing temperature of 600°C can be reached (but it also depends on the kind of thermal vector fluid used and on its tempera­ture when coming out from the solar field).

Nevertheless, the use of synthetic oil as the thermal vector fluid, which is the case in almost all solar systems with linear parabolic collectors, does not allow reaching temperatures higher than 390°C (as seen in Fig. 86) which has a nega­tive influence on the thermodynamic performance of the steam generator group. In these systems, the conversion efficiency for the conversion of solar energy directly into electrical energy is 15%. At present, solar thermal electrical sys­tems with linear parabolic collectors have typical dimensions; the capacity of these systmes can be in the range 30-80 MWe and they can also burn a certain quantity of fossil fuel (natural gas) to produce energy when there is a lack of solar energy, so these type of systems are hybrid systems, i. e. solar-fossil fuel systems [45, 50, 51, 53].

The maturity of this technology can be proved using the example of the Kramer Junction in Mojave Desert (California), where in 1984 this kind of solar thermal electrical system (SEGS I, Solar Electric Generating Systems) with a capacity of 14 MWe was realized. This system uses both linear parabolic collectors and natural gas as the fuel for overheating and sustaining the system in case of low radiation or breakdown.

An additional eight systems were constructed from 1984 to 1991, SEGS II-SEGS IX, reaching a total power of 354 MWe. In these systems, the tech­nology had been improved and costs were reduced such that the cost of elec­tricity generated was reduced from about 30 c$/kW h (in the first system) to 8 c$/kW h (in the last system). These systems have all been producing electric­ity and have added more than 13 TW h (billions of kW h) to the electrical net [45, 50, 51, 53].

sulnr

superheater

reheatcr expansion essel

Figure 88: SEGS IX scheme.

Linear parabolic collector systems have shown a few limitations which have not allowed their wider application. The principal problems are [53]:

• electrical energy production depends on the intermittence and variability of the solar source, which necessitates the use of fossil fuels to integrate the thermal energy production and therefore the need for solar-fossil fuel hybrid systems;

• the low conversion efficiency of the systems, which is due to the limited efficiency of solar energy gathering and the low processing temperature of the thermal vector fluid (<400°C);

• the high cost of the electrical energy produced, which is a consequence of the low efficiency of the systems and the high construction cost;

• the dangers posed by the use of the working fluid (synthetic oil) which is toxic and highly inflammable at the processing temperature.

Technological development projects have been planned in many countries. In 2004, the construction cost for a solar thermal electrical system with linear parabolic collectors was about 2500-3500 €/kWe, with a predicted 30% reduction in the medium term [51].

4.4.1 Tower system with a central receiver

This technology has overcome the demonstrative phase as an industrial prototype, but it has not reached the phase of trade maturity yet. The central tower system (see Fig. 89) makes use of flat reflecting panels (which as a whole form the solar field) called heliostats (Fig. 90). These panels track the Sun by rotating on two axes and concentrate the sunlight towards a sole receiver. The receiver is installed at the top of a tower (which is at the centre of the system) and inside a fluid (thermal vector fluid) flows to absorb the solar heat.

The thermal energy which is made available by this process can be used in dif­ferent processes, especially for the production of electrical energy.

In this kind of system, the thermal vector fluid can reach high processing temperatures (>500°C), which allows achieving high efficiencies in the conver­sion of solar energy into electrical energy. Generally, the transformation happens by exploiting the heat in a traditional water-steam thermodynamic cycle (see Fig. 91).

The central tower technology has shown its technological practicability in the production of electrical energy by the realization and the running of numerous small-sized systems (0.5-10 MWe) in different countries all over the world (Spain,

Italy, Japan, France and USA). From this experience, which has come with maturity, it has been seen that the best size for these systems is in the range 50-200 MWe. Different kinds of thermal vector fluid (such as water, air, melted salts) have been experimented with for years; however, until now the most suitable fluid for this technology has been the melted salts mixture composed of 60% sodium nitrate (NaNO3) and 40% potassium nitrate (KNO3).

Compared to synthetic oil, which is used as the thermal vector fluid in most solar thermal electric systems with linear parabolic collectors, the melted salts mixture, used in tower systems with a central receiver, has two important advan­tages: the fluid can reach a higher processing temperature (565°C) and it is possi­ble to install a thermal energy accumulation system, which can be created by piping the mixture heated up in the receiver towards an appropriately insulated storage tank (see Fig. 91). The sodium and potassium nitrate mixture can be heated until a maximum temperature of 565°C is reached (when the temperature is higher than 565°C, nitrates decompose into nitrites causing potential corrosion problems), which is much higher than the temperature of 390°C allowed by synthetic oil; this higher temperature allows achieving a better performance in the thermodynamic cycle for the production of electrical energy as well.

The elevated cost, the environmental risks and the inflammability which character­ize synthetic oil do not allow the storage of this warm liquid in such a volume needed to achieve an efficient thermal accumulation (actually, there is no thermal accumula­tion in the solar systems with linear parabolic collectors). Instead, the cheapness, non-toxicity and low environmental risks typical of the melted salts mixture make this fluid the most suitable for use in a thermal energy accumulation system, which solves the problem of the solar source variability and allows the production of electrical energy on demand so as to make the system more flexible [45, 50, 51, 53].

Figure 91 shows the processing scheme for a tower solar thermal electric sys­tem with a central receiver; it uses the melted salts mixture (described above) as the thermal vector fluid. Heliostats concentrate the sunlight towards the receiver

inside which the melted salts mixture flows; this mixture absorbs the heat and reaches a temperature of 565°C. The warmed thermal vector fluid is then directed to an insulated storage tank where thermal energy accumulation takes place before being picked up for the production of electrical energy. When it comes out from the warm storage tank (565°C), the melted salts mixture delivers heat to the steam generator which feeds an electrical turbo-generator. After delivering the heat, the thermal vector fluid cools down (290°C); at this point, it is accumulated inside another storage tank, waiting to be once again directed to the receiver. Eventually, the most important improvements introduced by this kind of system compared to those brought about by the system with linear parabolic collectors are as follows [53]:

• The thermal vector fluid is more safe since sodium and potassium nitrate (well-known compost) is neither flammable nor toxic (compared to the syn­thetic oil).

• The increase in the processing temperature of the thermal vector fluid from 390 to 565°C improves the performance of the thermodynamic cycle.

• The introduction of thermal accumulation solves the problem of the daily vari­ation in solar intensity. This provides clear advantages as regards the processing continuity of the turbine-alternator group and avoids resorting to the fossil fuel integration. Therefore, these systems are not hybrids but exclusively feed by renewal sources.

• The mirrors are made of compound materials (honeycomb) which are lighter, stronger and cheaper than the glass used in SEGS systems.

Although tower systems with energy accumulation are more efficient as regards conversion and require cheaper initial investment than the systems with linear parabolic collectors, a few disadvantages make their installation difficult on a wide scale and for high power requirements [53]:

• the conspicuous dimensions of the central tower whose height is proportional to the mirrors’ field extension and the system’s power;

• It is very difficult to concentrate solar radiation towards the receiver which is installed at a height of 100 m. By contrast, the focal length of systems with linear parabolic collectors is lesser than 2 m.

Without any doubt, one of the most important examples of this technology is represented by the experimental American system Solar Two of 10 MWe power, which was in operation from 1996 to 1999 in Dagget, California. Solar Two was the first system to use a melted salts mixture composed of 60% sodium nitrate and 40% potassium nitrate as the thermal vector fluid [8, 15, 42].

In Italy, as regards high-temperature solar thermal systems, the most relevant example was in the beginning of the 1980s with the construction of the world’s biggest solar power plant in Adrano, Sicily. This power plant, called Eurelios

(constructed within a CEE research project and thanks to the investment from an Italian-French-German society), has not been in use since an experimental phase which lasted for 6 years, from 1981 to 1987. Eurelios was able to produce a power of only 1 MW [2, 13].

Technological development projects are currently being implemented in USA, Spain (with the collaboration of a few countries), South Africa and Israel. In 2004, the construction cost for a solar thermal electrical system was about 4500 €/kWe, with a predicted reduction to about 2000 €/k in the medium term [51].

Tower systems with a central receiver have and will continue to have great importance in the field of both continental and in world energy in the near future. As proof of this statement is the PS10 system with its capacity of 11 MW power. This system, which began production in January 2007, located in Sanlucar La Mayor (Andalusia) is the biggest European solar thermal power plant. The PS10 Spanish power plant, whose overall cost is 35 million Euro, occupies 60 hectares of land and based on predictions it will produce 23 GW h/year at a cost of 0.1 €/ kW h produced. An interesting result when we consider that the best photovoltaic systems currently produce energy at a cost no cheaper than 0.23 €/kW h, which is close to cost of energy produced by fossil fuel systems (0.06 €/kW h) [25].

Observations on the thermal accumulation The introduction of a thermal accu­mulation system allows the elimination of the short transitory effects due to the irregularity of the solar radiation and also allows the release of the production diagram from the solar radiation diagram, as seen in Fig. 93 (where it is assumed that the electrical power installed is the same in both cases). The presence of the accumulation allows the use of a wider solar field, even if the electrical power is the same, to produce more energy and also a greater number of ‘equivalent annual hours’ of operation. These can go from 1500 h, typical of a system without the accumulation, to 2000-4000 h or more in a system with accumulation. A very big

storage tank would virtually allow the continuous production of energy. Actually, it is more appropriate to limit the accumulation to a storage tank which would allow 5-10 h of nominal power processing. This will allow users to plan the production of electrical energy to its best, concentrating it during periods of high require­ment (also increasing its trade value). In fact, as can be seen from the diagram in Fig. 94, the requirement of electrical energy in Italy has its peak during the evening — night hours and so it is delayed by nearly six hours from the solar radiation peak. This aspect is often more evident in developing countries. Generally, based on the weather forecast for two or three days (which is being introduced in the manage­ment system of the electrical generation park), it is even possible to optimize the energy production to make it available during the hours when energy costs more. An accumulation system also allows production on requirement, contributing to the creation of the required margin for the power stock of the net [45].

load factor: 25%

‘electricity

tidiostats

energy stored

accumulation of heat

Liu and Jordan extended their model of division, which is valid for monthly average radiation values, to the valuation of daily global radiation received on an inclined surface. If K is the daily clearness index, which is equal to H/Hex (i. e. the ratio between daily radiation and extraterrestrial radiation received on a horizontal surface on a specific day), the ratio between diffuse radiation and global radiation received on a horizontal surface on a certain day has to be cal­culated as follows:

D/H = 1.0045+ 0.04349K -3.5227K2 + 2.6313K3 se K < 0.75 (^

D/H = 0.166 seK > 0.75

The direct component is calculated by the difference:

B = H — D (44)

The daily global radiation on an arbitrary inclined surface can be calculated using the following expression:

E = Rb B + Rd D + Rr(B + D) (45)

where Rb, Rd and Rr are the inclination coefficients of direct, diffuse and reflected radiation determined, respectively, by eqns (29), (32) and (34), but for the chosen day [1].