This system uses reflecting panels which have a parabolic shape and track the Sun by rotating around two orthogonal axes. These panels also concentrate solar radiation towards a receiver which is installed at the focal point. High tempera­ture heat (>650°C) is normally transferred to a fluid (helium or sodium vapour) and is then used in a motor, which is located above the receiver (see Fig. 95), where mechanical or electrical energy is directly produced. For economic reasons, concentrator dimensions do not exceed a diameter of 15 m, limiting its power to about 25-30 kWe. With a row of these collectors, it is possible to realize sys­tems of any size and power. An interesting application of parabolic dish collec­tors is the one which regards electrical energy production for small communities which are decentralized and distant. These systems have a conversion efficiency which is more than 30% (the highest efficiency among the currently existent solar technologies) [45, 50,51]. This technology has now reached the industrial phase, mostly due to the research which has been developed in Europe, in the USA and in Australia.

Among the described technologies, this system is the one which has the highest electrical energy production cost (in 2004 costs were about 1 €/kW h); neverthe­less, it is interesting for the prospects it offers concerning the drop in this cost [50]. The cost for the construction of a solar thermal electrical system which uses para­bolic dish collectors in 2004 was about 7100-3700 €/kWe with a forecast for the medium term of about 2000-1200 €/kWe.

In the parabolic dish collectors, the thermal vector fluid can reach temperatures which can be even higher than 1000°C, and at such high temperatures it is also possible to produce hydrogen by the dissociation of water. In prospect, this is the most important reason for the interest shown in this technology: in Europe, since 2002 the hydrogen economy has become one of the mainstays of the EU sustain­able energy policy, acknowledging the uniqueness of hydrogen both as a clean fuel and as a high efficiency energy vector [45, 53, 56].

Currently, the most concrete application that CSP plants find use in is the production of electricity.

Also, in the medium to brief term, it is predicted that this will be their main application. Similar to all the other forms of renewable energy that have been introduced recently, to affirm themselves, the CSP plants must face the hard competition as regards the energy generation costs.

The presence of a growing market for ‘green energy’, which calculates a form of economic incentive, allows to overcome, in favourable cases and taking into consideration the growing cost of fossil fuel energy, the competition gap compared to traditional products.

To acquire market quotations it is necessary to reduce the production costs and to improve the market value of the energy produced. As for the cost reduction, two main modalities can be adopted: the reduction in the specific costs of investment and the increment of production efficiency. The improve­ment in the market value can be achieved by making the electrical energy pro­duction less dependent on the solar source variability. The introduction of a storage system or the use of an integrated solar-combustible fossil system is indispensable.

It must be stressed that reduction in the investment costs, improvement in the efficiency and independence from the solar source variability are contrast­ing aspects. Achieving a winning compromise in the course of time is very necessary, but it will necessarily leave space for specific innovations (as it hap­pens in the automobile technology or, in a more persistent way, in the Aeolian technology) [45].

One of the main problems in a future sustainable energy scenario will be the large-scale production of energy at competitive costs and without the production of greenhouse gases, which will be served by transmission vectors such as electri­cal energy and hydrogen. Once the potential of ‘energy transfer’ through electrical interconnection is used up, the new hydrogen vector will allow transferring, over long distances, the potential of the primary sources from the production areas to the consumption areas, as it currently happens for the fossil fuels. Obtaining large quantities of energy to vector as hydrogen without the emission of greenhouse gases means using water as a raw material and as a primary energy source, which does not produce greenhouse gas. The hydrogen production from solar concentra­tion systems, by means of thermochemical processes at high temperatures, prom­ises achieving high earnings in terms of conversion, which are necessary for the process effectiveness. In fact, if presently the hydrogen production by electrolysis is the more mature process to obtain hydrogen from solar source, this process is characterized by a global yield (from hydrogen energetic content radiant energy, passing through the collection and radiation concentration, the conversion into electricity and electrolysis) of the order of 27%. Using photovoltaic conversion for electricity production, followed by electrolysis of water, we do not obtain higher yields, but we typically reach a global yield of the order of 12%. Except the costs, which are currently hard to evaluate, from the energetic point of view they are more useful than those methods in which the heat solar conversion in hydrogen happens in a direct way, based on the scheme represented in Fig. 97; in this way, theoretically it is possible to obtain global conversion yields of the order of 46%.

The thermochemical cycles, comprising oxidation-reduction reaction series that involve different natural intermediate substances, allow the cleavage of the water into hydrogen and oxygen starting from relatively elevated temperatures of heat (800-1500°C), but in solar concentration systems these temperatures are, however, achieved using high concentration systems such as towers or parabolic disk systems. This typology of the process is known since the 1970s, but only dur­ing the last few years it has become the object of renewed interest, driven more and more by the impelling environmental problems. The possibility to thermally feed such cycles by solar energy makes these production systems completely renewable and so perfectly compatible with a sustainable development strategy [45].

Since 2000, ENEA has undertaken research, development and demonstrative pro­duction activity in the field of solar concentration technology that aims at electricity production in a short — and medium-term perspective.

The technology developed by ENEA combines some characteristics of linear parabolic collector systems and tower systems with the aim of creating series of technological innovations that will allow going beyond the critical points of both these systems.

In particular, the features of the ENEA technology are [45, 50, 53]:

• the use of linear parabolic collectors (because it is a more mature technology), but they are renewed compared to the traditional ones (see par. 4.5.2);

• the development of a receiver pipe capable of operating at high temperature (see par. 4.5.2);

• the use of a mixture of molten salts (made of 60% sodium nitrate and 40% potassium nitrate) that is already used in tower plants as the heat transfer fluid in place of the synthetic oil which is used in traditional linear parabolic collectors (e. g. in the SEGS);

• the presence of a thermal storage system, which was also already used in the tower systems but is absent in the traditional linear parabolic collector plants, allows storing the collected thermal energy and making it available continuously at night and during cloudy days or in case of damage to the receiving system.

The working scheme of an ENEA linear parabolic collector plant using molten salts is shown in Fig. 98.

There are two reservoirs (one ‘hot’ and another ‘cold’) that contain the mixture of molten salts, respectively, at temperatures of 550°C and 290°C. From the

Figure 98: ENEA technology scheme for molten salts plant. 1: molten salts;

2: storage tanks; 3: heat generator; 4: turbine and alternator; 5: condenser

reservoirs there are two independent circuits in which the salt is circulated by appropriate circulation pumps. In the circuit of the solar field, in the presence of enough irradiation, the salt taken from the cold tank heats up to 550°C circulating inside the solar collectors and then fills the hot tank. In the circuit of the vapour generator (GV), the salt is taken from the hot tank and after having produced over­heated vapour in the GV it goes back to the cold tank. The vapour produced in the generator feeds a conventional electrical energy production system.

In the limits of the storage capability, the two cycles (one relating to the solar energy consumption and the other relating to vapour production to feed the electri­cal generation system) are completely free, permitting electricity production which is verifiable apart from the availability of solar irradiation [45, 50, 53].

The use of molten salts as the heat transfer fluid, instead of synthetic oil, provides two advantages:

• Thermal storage can be achieved at a low cost, because the salts are economical, not toxic and have limited environmental impact even if there is an accidental outburst.

• The temperature at the exit of the solar field can be raised up to 550°C (Fig. 98), resulting in an improvement in the performance of the thermodynamic cycles involved in electricity productions. In the case of synthetic oil, the highest temperature is, on the contrary, limited to about 390°C (see par. 4.4.1).

Compared to the storage, as already seen, the cost of the fluid, especially the high danger of burning and the major environmental impact in the case of an accidental outburst, makes the realization of thermal storages using synthetic oil impracticable. On the contrary, a thermal storage system with molten salts is more useful from the cost and safety points of view, as is also proposed for oil systems (such as the two AndaSol systems with oil parabolic collectors and a capacity of 50 MWe, in the realization phase in Spain) with the use of appropriate heat exchangers.

In this case, all the features of the molten salts are not exploitable because its temperature is limited compared with the highest temperature of the oil circuit. But by exploiting the highest temperature of the salt, we can obtain a thermal storage density of the order of 0.2 MW h/m3, which is more than double with ref­erence to a molten salts reservoir inserted into an oil circuit. This, combined with the low cost and the high density of the molten salts, allows achieving a cost of 15 €/kW ht for the storage. In terms of electricity production, considering the thermodynamic conversion yield, we obtain a specific cost (cost of investment which is necessary to assure a storage thermal capacity capable of generating 1 kW h electricity) equal to 36 €/kW he.

This storage technology is useful compared to the other forms of storage that are used in the field of electricity production, such as the storage in hydraulic basins (pumping/turbine plants) and electrical storage with batteries, having a lower investment cost: in fact, these systems have costs (for technologies already in commercial use) of the order of 100 €/kW he and 100-1000 €/kW he, depending on the cases [45, 50, 53].

The use of molten salts produces vapour at high temperatures, of the order of 530°C, able to feed vapour cycles with high thermodynamic conversion efficien­cies (42:44% against 37.6% for a vapour feed cycle, which is typical of an oil plant), without the use of a fossil fuel re-heater.

Apart from the advantages described above, the use of molten salts causes more relevant technological problems than the use of synthetic oil. The main problem is that these mixtures are liquids only at temperatures higher than 238°C and hence it is necessary to adopt technical solutions for their usage. In particular, it is neces­sary to have a salt fusion system and preheating electrical pipes system during the ‘the first start’ of the plant (when the pipes have to be filled in with salt) as well as to ensure continuous circulation of the salts in the pipes (even at night) to prevent solidification of the mixture. An alternative to the continuous circulation can be the daily filling and emptying of the circuit, but this operation is only practicable in piping limited extension plants. Another aspect that characterizes the molten salts is the necessity of adopting appropriate materials and constructive technologies for pipes and component production (particularly pumps and valves), particularly for the corrosion behaviour [45].

The solar collector represents the main aspect of the economic analysis that will decide the realization of a solar central plant and hence its cost and efficiency are of particular importance for the diffusion of the concentration of solar tech­nology. For this, ENEA has planned and realized, together with the industry, an original prototype of the linear parabolic collector with the double aim of improving the techno-economic parameters and putting the national industry in a situation to produce this in series, both for the Archimedes Project (see par. 4.5.3) and for making it available in the international market in a competitive manner.

The ENEA collector, shown in Figs 100-102 comprises:

• a structure that supports the mirrors, realizing the parabolic geometry and it allows orienting them to follow the motion of the Sun;

• a series of mirrors of appropriate geometrical design;

• a motion system that is capable of making the structure rotate with the accuracy of required pointing.

• a series of receiver pipes on which the solar rays are concentrated and in which the thermal energy is given to the vector fluid.

The collector was developed in its entirety and tested using a circuit test at the ENEA centre in Casaccia. The length of the prototype collector is equal to 50 m, but the combined length of the series is 100 m. The structure must contemporaneously assure rigidity, geometrical precision and low cost. The main aspect as regards the

Basement

Figure 100: Solar collector scheme used by ENEA.

dimensioning of the structure is the determination of the aerodynamic loads due to the wind action. The solution adopted by ENEA, apart from presenting structural resistance, is characterized by constructive economy and simplicity of assembly. Such a solution based on a central bearing pipe and lateral supports of a variable shape (Fig. 100), heavier than similar concurrent realizations, presents its trump card in terms of the constructive reasons and in the choice of the material, which make it less expensive, easily portable, with quick installation and simple registration, within the required distance from the optical concentrator system. Despite the considerable dimensions (total length of 100 m, width 6 m and height 3.5 m from the rotation axis), after assembly tolerances of 1 mm final can be easily realized [45, 53].

The mirrors are realized with several technologies with the aim of exploring a series of alternatives to achieve a lower final cost and better mechanical character­istics compared with the traditional linear parabolic collector, which uses a thick and hot bent glass mirror. Among the alternatives examined, the better solution is the one which is based on the use of a glass mirror which is sufficiently thin (850 pm) to be cold bent until it assumes the required parabolic shape and to be applied to a conveniently shaped support panel with a structural function (Fig. 103). The sup­port panel is made of an aluminium nucleus with a honeycomb structure, which is often 2.5 cm, and wrapped between two surface layers (leather), 1 mm thick, in composite material [45, 50, 53].

The handling system is made of an autonomous oleo dynamic unit that is capable of moving the whole 100 m collector on the basis of instructions sent to the central supervision system, ensuring that the movement the Sun is followed with a precision of 0.8 mrad.

The system is able to carry the collector safely (when faced with atmospheric events such as strong wind or hail) in the presence of winds with speeds up to 14 m/s; once placed safely, the collector can resist winds with speeds of up to 28 m/s [45].

The receiver pipes (4 m long) are welded to make a line that, in the position of reference during use, must be in axis with the focal line of the parabolic mirrors. The receiver pipeline is held in position by sustaining arms equipped at the extrem­ities with cylindrical hinges which allow the thermal expansion of the pipes when the plant is in use.

The function of the receiver pipes is to transform heat at high temperature and to transfer to the heat transformer fluid the largest quantity, reducing at least the losses of energy by irradiation towards the external environment.

Each receiver pipe (Fig. 106) is made of a stainless steel absorber on whose external surface is deposited, by sputtering technology, a selective spectral cover­ing (coating) made of composite cermet material (CERMET), which is character­ized by an elevated absorbance of the solar radiation and a low emissivity of heat in the infrared region. The stainless steel absorber is capped, vacuum at about 10-2 Pa, in an external borosilicate glass pipe that is coaxial with the receiver pipe; this external glass pipe protects the receiver pipe from the contact with air, reducing at least the thermal exchange for convention between the pipes.

On the surface of the glass pipes, an antiglare treatment is made to improve the transmittance of the solar radiation, reducing the reflected energy. The links between the glass and the steel pipes are realized with two stainless bellows (placed at the extremities of the glass pipe), which are able to compensate the differential between the two materials’ thermal dilatations. To create the vacuum it is necessary to insert in the cavity between the two pipes an appropriate quantity of getter material which is capable of absorbing the gas mixture that could form in the receiver pipe.

A second material absorber, which is very reactive with air (Barium getter), is deposited on the internal surface of the glass pipe, resulting in metal colour scrubs of some cm. When the vacuum is created in the pipe the soaking getter saturates, the scrub becomes white, indicating the loss of heat transmission efficiency to the heat transfer fluid. The receiver pipe is the most delicate element of the solar technology, because it has to grant in time a high energy absorbing coefficient which is concen­trated from the parabolic mirrors, limiting at maximum the losses by irradiation towards the environment. To achieve high reliability, there are two important characteristics:

• the capacity of CERMET to maintain almost unweathered the photo-thermal characteristics at the maximum working temperature of the coating (580°C);

• the capacity of the metal-glass junctions to resist the strains of thermo­mechanical fatigue which originate from the variability in the solar irradiation (the maximum temperature of reference in the proofs of the mechanical characterizations is 400°C).

These characteristics, peculiar to the ENEA project, have led to the development of new technological solutions because the receiver pipes which are available in the market are able to operate up to a maximum coating temperature of 400°C. In the ENEA laboratories in Portici, different CERMET made of metal and ceramic material are being planned, realized and undergoing spectrally selective character­ization until the chemical composition and the optimal physical characteristics to obtain the photo-thermal characteristics required by the ENEA project are attained [45, 53]. The reference parameters of the coating developed in Portici, determined by photo-thermal characterization at the same laboratories, are:

• high photo-thermal efficiency, which means high solar absorbance (>94%) and low emissivity (<14%) up to the temperature of 580°C;

• high chemical and structural stability up to the temperature of 580°C.

A fundamental aspect for the development of the ENEA technology is the real­ization of demonstrative applications on an industrial scale. The realization of a complete solar prototype plant for electrical energy production, linked to the national distribution net, needs the participation of public and private initiatives, as well as adequate investments. In fact, the prototype plant involves high costs because of the essential phase of learning in the setting up and use of new technol­ogies; to be economically viable, plants of this kind need to produce more than 40 MWe of power. But, solar plants can also be integrated with conventional thermo­electrical systems, even those with combined cycles, to improve the total amount electrical energy produced. This possibility allows using, with small changes, already working installations. So we can rely upon the electricity production system on the site and the existing infrastructure, limiting the cost for the con­ventional part of the plant as much as possible and focusing the investment on the innovative components of the new technology. In this case, the improvement in the power can also be widely modulated during the day, making the addi­tional production of the solar plant to happen during the hours when the external users’ demand is higher [53]. In line with what we have just stated, on 26 March

2007, Santo Fontecedro, Director of the General Division and ENEL Energy Man­agement, and Luigi Paganetto, President of ENEA, signed, in the presence of the Environment Minister Alfonso Pecorario Scanio and the Nobel prize winner Carlo Rubbia (former president of ENEA), an agreement protocol to make the Archimedes Project operative. This project located in Sicily at Priolo Gargallo (Siracusa) represents the integration of an ENEL combined cycle thermo-electric solar plant, comprising two sections of 380 MWe each (250 MWe for the turbo gas group and 130 MWe for the vapour group) to produce a total power of 760 MWe, with a thermodynamic solar plant based on the newly elaborated ENEA technology. The Archimedes Project is the main demonstrative realization of the ENEA technology and it will be the first application at a worldwide level of the integration between a combined cycle plant and a thermodynamic solar plant [58].

The choice of Priolo Gallo was made based on the following technical reasons:

• Currently, there is a considerable availability of land that is not being used of almost 60 ha in the central area.

• The site enjoys elevated insulation values, with a medium year direct solar irradiation equal to 1,748 kW h/m2 a year.

• The vapour produced from the solar plant, having practically the same temp­erature and pressure characteristics as that coming from the heat recovering generator of the discharge fumes of the turbo gas, will be directly emitted into the vapour turbine of the existing central part (see Fig. 107), allowing to save

on the entire conventional part (avoiding the installation of a turbo alternator group and electrical instruments for the internet connection).

• The integration with ENEL central will permit the exploitation of a series of technical infrastructures and it will ease the experimental management.

The big solar plant will improve the power from the central of 28 MWe, against an occupied area equal to 37.6 ha. The calculated electrical production net is equal to 54.2 GWe/year, with a primary energy saving equal to 11,835 Tep and the missed emission of 36,306 t of CO2. The global yearly medium earning (from solar energy to electricity) is equal to 17.3% [59].

The realization of a first plant module made of 60 collectors of 100 m, equivalent to about 5 MWe, was already planned. Such a module will permit the production of [58]:

• additional electrical energy from the solar source, which can satisfy the yearly requirements of 4,500 families;

• a saving equivalent to 2,400 t of petrol;

• lower emissions of carbon oxide of about 7,300 t in a year.

Once the demonstrative plant is completed, a more concrete commercial perspec­tive will open up, and some Italian companies have already been authorized to pro­duce components for the emerging Spanish market. In an elevated irradiation site such as in the North African area, the ENEA technology permits, in perspective, the production of 275 GW h/year of electricity at a levelled cost equal to 4.5 c€/kW h for each square kilometre of the territory, with a saving of primary energy which is equal to 60 ktep/year and an avoided emission equal to 185 kt/year of CO2 [59].

4.6 Conclusions

The concentration of solar technology can play a fundamental role in the future of the world’s energy production, allowing the production of large quantities of electricity and hydrogen, which are completely renewable and without emission of greenhouse gases, at competitive costs. The available theoretical potential in the ‘sun-belt’ countries is in fact large enough to ensure a meaningful contribu­tion to the predictable world requirements. The technological maturity regarding electricity production will be realized in the medium to brief term, and regarding hydrogen production in the medium to long term.

Especially the countries facing the southern side of the Mediterranean and the Near East countries have notable powers, with direct insulation characteristics of 50-60%, which is higher than what was found in the most favourable areas, from the southern Europe point of view. This strong insulation and the presence of large areas that are appropriate for the installation of solar concentration plants will result in energy production costs which are really lower than that in Europe.

This fact has lead to a renewed interest in proposing countries with a strong tech­nological background, such as Germany, as candidates for ambitious development plans in collaboration with the middle-southern and Mediterranean area countries.

The presence of areas which are favourable for the concentration of solar tech­nologies, in the southern European countries also (not only in Spain but also in Italy and Germany), has allowed the building of prototype plants to create a solid industrial base so that they can take advantage of, in addition to the energy produc­tion (especially from the manufacturing experience), the huge potential exploita­tion, with returns in terms of supplies for the national industries. In fact, it is evident that, being primarily a free source, the total invoice which is linked to energy production from the solar source is good for those who realize and takes care of the maintenance of production plants, and those who have the know-how are destined to exploit most of the connected businesses.

The solar concentration technology can soon be integrated, even in Italy, with other renewable technologies (Aeolian and solar photovoltaic), which will contribute to the growing European demand of ‘green electricity’ [45].

Let us now consider two other technologies which exploit the solar radiation and are applied as CSP technologies for the generation of electrical energy, but they do not involve the concentration of solar beams. This raises the possibility, in the two case that we are going to discuss, of exploiting not only the direct radiation but also the indirect radiation which, in some seasons and in some countries, has a higher energy than direct radiation.

Solar chimneys, similar to solar ponds, are not characterized by other typical temperatures, whereas, the CSP technology is [7].

4.7.1 Solar chimneys/towers

Solar chimney plants allow producing electrical energy in a renewable way. They are made of a tower that is hollow inside and at the base it has a wide greenhouse, generally circular in shape that covers a notable ground surface. The greenhouse air, heated by the Sun, rises along the chimneys due to two physical phenomena (that function as the tower’s ‘motors’), namely:

• the air rises by floating (based on the phenomenon that hot air tends to rise high);

• the air rises due to the pressure difference between the base and the top of the tower (at the top of the chimney the pressure is lower and so the air is ‘backwashed’ towards the top).

As it rises in the chimney, the hot air accelerates until it reaches a speed of 70 km/h. This flow of air rotates a series of turbines placed at the internal base of the chimney to generate electricity: the turbines transform the kinetic energy and the air potential into electrical energy, as every Aeolian blade. The procedure is made easier from the absolute constancy both in direction and in intensity of the speed vector.

The heat collector in this case is the greenhouse. It can have plastic or glass cov­ers. From the pilot plant at Mazanares (Spain, Fig. 112) we can see that the glass is better because it is more resistant to bad weather. We also observed that if the height of the cover progressively improves towards the centre, the radial flow of the speed is enhanced. The performance directly depends on the chimney height. For this reason, in the current plans, they plan to build chimneys of 1,000 m height.

The main feature that makes the solar chimney/tower particularly interesting is its capacity to work without wind also, 24 hours, 7 days, generating a peak of energy during the hotter days of the year when there is a consumption peak.

The plant can also work at night, due to the ‘pressure gradient’ (i. e. the pressure differential) and, secondly, due to the ground covered by the greenhouse, which heats itself during the day and releases the stored heat during the night. We can easily improve the thermal capacity of the floor by putting a water layer in the greenhouse or using an appropriate arrangement containing water elements that store the heat and release it at night. Obviously, water must be contained and kept; it must not evaporate; otherwise, it consumes the thermal energy absorbed [7, 61, 63].

Among the most ambitious project in terms of dimensions is, without doubt, the solar chimney/tower that to be built in the county of Wentworth in New South Wales, Australia. Figure 114, where the greenhouse cover elements are considered the solar panels, shows the scheme for this project. The numbers of the initial project are as follows [60]:

• The greenhouse should cover an area of about 25,000 acres, which is equal to 5 km2.

• The central tower will be 3,280 feet high, corresponding to 1 km, which would make it the tallest building in the world.

• Inside the tower 32 turbines each of 6.25 MW are placed; every rising hot air motion is estimated to have a maximum speed of the order of 35 miles/hour (<60 km/h); the solar tower will have a total capacity of 200 MW, which is enough to feed almost 200,000 houses.

• The generation of 200 MW of power would allow saving, depending on estimates, between 750,000 and 900,000 t of CO2 per year.

Currently, the project is in the final stage in terms of its feasibility, particularly regarding the economic aspects. In this step, the Guinness dimensions of the initial project have been reduced:

• The use of innovative materials has allowed reducing the height of the tower to 650 m without losing power.

• The power has been reduced to 50 MW.

• At the moment, it is not possible to know the final dimensions of the tower, but it is reasonable to assume that at such levels it should have a height of at least 450 m.

From the technical point of view, the project was already validated, because for 7 years (from 1981 to 1988) a pilot project of 50 kW power was operative at Mazanares. Conceptually, it is not a new technology, but at the moment of its birth, when an oil barrel cost 15 dollars, it did not provoke any particular interest, contrary to the situ­ation today. In fact, the present high price of crude oil and the necessity of reducing greenhouse gas emissions are pushing many countries towards more convenient and cleaner energy sources such as the solar chimney/tower [60].

The highest and most sophisticated solar chimney/tower (750 m) in Europe will be realized at Fuente del Fresno, in the Spanish region of Mancha. This colossal solar system will have a power of 30 MW. This plant will provide electrical energy that is equal to the requirements of 120,000 people and at the same time we will avoid putting into the atmosphere 78 t of CO2 that will be generated from 140,000 oil barrels that could produce the same energy in a year. The construction of this structure will start in 2007 and it will be finished in three years; it will cost 240,000,000 € and it will occupy 350 ha covered with a 3 km diameter crystal

panel. Exploiting the greenhouse effect principle, the overheated air will rise along the tower height, actuating 24 turbines that will produce electricity. A system of storage pipes filled with a gel keeps heat and allows the generators to produce energy even at night and during periods of scarce insulation. The tower has an estimated shelf life of 60 years [65].

4.7.2 Solar ponds

The term ‘solar ponds’ is used to describe a mass contained in a basin of water that also absorbs the solar incident energy and stores it in its interiors. To obtain this performance, three other basic kinds of solar lakes can be named, and they are identified by the terms: salinity gradient solar lake, gel pond and, finally, shallow solar pond. Among the three, the first is the one whose technique was realized for the totalities of the realizations and the management of the physical working studies. This kind of solar lake is realized putting in the bondage a solution of salt in water, e. g. sodium chloride, using filling techniques that allow establishing a growing salt concentration with the depth until the saturation at the bottom layer. Effectively, in the vertical section of the basin (see Fig. 115), which is generally deep 2-3 m, we can find three characteristically superimposed layers: the first layer is high and very slender, it is composed of water with a little quantity of salt (0-35 g/l); the central layer, where we can observe a linear salinity variation; and, finally, the homogeneous and salt saturated bottom layer (200-250 g/l).

Let us now analyse the difference between a normal water basin and a solar pond. In the first case, the solar energy heats water (exposed to the Sun), which,

however, tends to lose this heat. Indeed the water heated by the Sun expands and tends to move higher and higher as it becomes less dense. Convective motions are established and the superficial water is always hotter than the deep water; it rapidly evaporates cooling and giving heat to air. The cold water, which is heavier, moves towards the bottom. In this way, a water basin keeps a relatively low temperature in the deep bottoms and, as it is more radiated, it raises the circulation speed of the water and intensifies the evaporation. But if a system in which the mass of water has a layer shaped salinity is created, with the highest value at the bottom and the lowest value at the surface (solar pond), the convective motions are inhibited. In fact, the hot water specific gravity and high salinity are anyhow bigger than that of the modest salinity cold water, so heat is trapped at the bottom of the solar pond. The absence of convective motions inhibits the mixing of high salinity hot water with the superficial one. The superficial layers of salinity only increase diffusion and this happens over very long periods (years) and so bigger the solar pond spare part time that has to be fed to equalize the losses of evaporation.

When the solar radiation incident on the solar pond surface penetrates through the transparent solution mass, it is absorbed at the bottom and the produced heat transmits itself to the solution for convention. Following mass ascent and energy transfer that could lead to the dissipation of the heat at the surface, it finds a barrier in the interface with the salinity gradient layer and the heat is stored in the pickle at the bottom (where the temperature can also reach 100°C). In fact, the water in the salinity gradient area cannot rise because the water in this layer has a lower salinity content and so it is lighter; for the same reason, the water in the higher layers cannot go down because the water in the lower layer has a salinity content which is lower and heavier and even if its density wanes with the increase in tem­perature, it is always denser than the higher water layers. The intermediate layer acts as a transparent thermal isolator that allows the stored heat in the lower con­vective layer to be extracted with thermal exchange techniques and to be used for thermal purposes [66-68].

Solar ponds are mainly used as energy sources which are appropriate to feed the processes of [7, 66]:

• electrical energy production using organic fluid Rankine cycles; the electrical production yield of the system is very low, but the cost of the storage plant is contained;

• brackish water desalinization;

• agricultural greenhouses and habited environmental heating;

• vegetable drying.

A solar pond can be built using normal intervention techniques used by the building industry, such as digging the basin, covering the basin with an imper­meable membrane and building the structures for housing the devices used for extracting and producing heat. In this way, large heat collection surfaces can be realized, up to thousands of square metres in area with costs for unit area lower than the cost of every other methodology of solar energy exploitation. The big mass for collection and the thermal isolation capability characterize the solar pounds: they can preserve the thermal energy for long periods (seasons) without registering sensible brine temperature decreases.

The construction of a solar lake, in terms of the surface unit, can vary with the basin catchment area. The estimated unitary costs for building different size lakes are listed below:

• surface of 2,000 m2, cost: 150 €/m2;

• surface of 20,000 m2, cost: 95 €/m2;

• surface of 200,000 m2, cost: 70 €/m2.

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.