Medium-temperature technology includes systems which are able to reach temper­atures between 100°C and 250°C. The most common application of the medium — temperature solar thermal system is represented by the solar oven (see Fig. 77): a parabolic reflector (composed of aluminium sheets mounted on a zinc-plated steel structure) concentrates the solar radiation towards a single point which works as a cooking-stove. At this point, a pot is placed which warms itself and cooks the food contained inside. Using a solar oven it is possible to reach the same temperature as a traditional cooking-stove (about 200°C).

A solar oven with a diameter of 1 m takes nearly 18 minutes to boil 1 L of water, while it takes only 9 minutes if the diameter is wider (1.4 m). The reflector can be oriented on the basis of the Sun’s position so that it is possible to cook from morn­ing to afternoon and even to exploit the shortest moment of radiation. In Italy, the use of solar ovens is not common; they represent a very small slice of the market and their use is restricted to the those who consider it a hobby. In countries where lack of energy resources is a daily problem (such as Africa), the solar ovens can have good applications [12, 13, 18, 41].

In spite of the various advantages offered by thermal solar systems, their great potential has not been exploited much in the industrial sector. Thermal solar sys­tems can partially meet the heat demand for low — and medium-temperature (up to 250°C) processes, which are typical of a few industrial sectors such as the chemi­cal, food and textile industry. The thermal solar collectors that are now available now in the market, which we analysed when we talked about low-temperature solar thermal systems (par. 2.2), can reach temperatures of 100°C. As regards applications which need higher temperature (up to 250°C), the experiences are limited and suitable collectors do not exist. In 2003, the International Energy Agency (IEA) started a research project called Task 33/IV which aims to find more promising industrial applications in the thermal solar field and also to calculate the overall potential of thermal solar applications for the production of medium-tem­perature process heat. One of the Task 33/IV activities is research, developed together with the industry, on new collectors which can produce processed heat between 100°C and 250°C (a temperature range that is consistent with several industrial processes) [42-44].

At present, the collector typologies which are more promising in the medium- temperature field are:

• high efficiency glazed flat plate collectors: these are flat collectors with double

antireflection glass;

• linear parabolic collectors, similar to the ones used in the high temperature field

but much smaller (these will be analysed in par. 4.4.1).

• static concentration solar collectors: these are flat plate collectors or more frequently evacuated tube collectors characterized by static mirrors (fixed) for the concentration of solar radiation.

Figure 78: Double-glazed flat plate collector with antireflection glass.

The instantaneous global power which weighs on an oriented surface is given by the sum of the direct component that is obtained from the eqn (28), the diffuse component, which comes from the celestial vault portion seen from the surface, and the part reflected by the soil and nearby objects towards the same surface.

If the sky’s behaviour is assimilated to that of an isotropic spring of diffuse radiation, it is possible to determine the diffuse component which reaches the surface as:

Gd=/do Rd (31)

where

Rd = cos2 (b/2) = (1+cos b)/2 (32)

where Ido is the diffuse radiation on the horizontal plane and Rd is the inclination factor of diffuse radiation.

We can express the radiation (direct and diffuse) reflected by the soil on a certain surface as:

(Ibo +Ido)Rr (33)

Rr, the inclination factor of reflected radiation, is equal to:

Rr = psen2 (b/2) = p(1 — cos b)/2 (34)

p is the soil’s reflection coefficient and it can assume values between 0.2 (grass, concrete) and 0.7 (snow). Therefore, the instantaneous solar power, which is received on a arbitrary oriented surface, in the case of isotropic sky, is equal to [1]:

G = Ibo Rb+Ido Rd+(Ibo+Ido)Rr (35)

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.

The rational justification for a low-temperature thermal system derives from eco­nomic and environmental reasons. The reduction of environmental pollution and the saving of energy which can be obtained using solar energy represent a solid advantage for the community.

The Sun can normally give us 80-95% of the hot water that we use daily to wash our hands, take a shower, do the dishes or also wash clothes, if we manage to connect the solar system to the dishwasher’s or washing machine’s hot water entrance. If the products are of good quality and the system is well sized, the outcomes are very good even in northern Italy. While in southern Italy the out­comes are excellent in winter also, in northern Italy we can achieve good results only if one more panel is installed or if a capacious tank is provided to make up for the lack of sun during cloudy days, since a good tank keeps water at a constant temperature for a few days. It is not easy to know the exact saving that could be obtained from a sanitary water solar system, as it depends a lot on the habits of families. So it depends on how much water is used for personal hygiene or for dishwashing and also on the kind of central-heating boiler or water heater the family possesses. Using an approximate calculation, which is sufficiently reli­able, we can indicate a yearly methane saving of 100-180 m3 per person, with a yearly lack of carbon dioxide emission (the main cause of greenhouse effect) of 230-400 kg per person. To these quantities we have to add the wastes generated by the accumulation of the central-heating boiler and water heater, for example, with a simple pilot flame or with intermittent running of the water heater to keep the temperature of a huge quantity of water constant during the more unthinkable hours of the day and night. These methane wastes will be 150-200 m3 per year and they have to be added to the real savings calculated on the basis of the number of persons in a family. The methane cost per m3 or the cost for the fuel used by our own central-heating boiler has to be taken into consideration to calculate cor­rectly the saving in conformity with taxes, the additional tax when exceding the permitted use of energy, the IVA, the inflation, the rise which will take place in the next few years, etc. Since solar systems are capable of producing more than 90% of the hot water requirements between April and October, the central-heating boiler or the water heater could be turned off during this period, resulting in a significant reduction of wastes.

It is important to determine the regeneration time of the investment, which may be important to justify a solar system installation. The number of years taken to regain the investment can be obtained by dividing the cost paid by the yearly max­imum money saving obtained by the use if sanitary hot water produced by solar energy. On average, if we talk about a single family system, the hot water cost alone will be about 1500-2600 €, amortizable within 3-5 years, while the sys­tem’s useful technical life span is about 20 years and the maintenance costs are 1% of the system’s original cost.

A traditional water heater (electrical or methane one) never regains its cost because there is always a bill cost, whereas the more a solar energy system is used, the more it is convenient.

Only when the use of water is reduced, it is more convenient to have a small electrical or gas water heater [16, 18].

3.1 Concentratiing solar power technology: clean energy for power tenability

Energy availability has always been an essential component of human civilization. In the last 150 years, the yearly average rate of world energy consumption has grown by about 2.3%. The energy requirement of human beings, mostly satis­fied by fossil fuel, has grown so much that it has overcome sum of the thermal energy coming from the Earth’s core and from the ties induced by the Sun and the Moon. The endogenous energy of the Earth has been more than doubled by human activities. However, it is important to underline that the overall human energy con­sumption is only 1/10,000 of the energy received on the Earth from the Sun. Solar radiation, despite its scarce density, remains the most rich and clean energy source on the terrestrial surface [45-47].

This statement, together with the exhaustion of fossil fuels and the growing environmental risks, leads us to seriously consider solar energy as one of the most important candidates for the planet’s energy tenability project.

The greatest part of the solar source’s potential can be found in the so-called ‘sun belt’, which is the area of the planet that receives the highest quantity of solar radiation, as shown in Fig. 81. In particular, Northern Africa and the Middle East have large areas with a high level of solar radiation which is suitable for the installation of a large number of solar thermal systems as they cannot be used in any other way. These countries spontaneously stand out as candidates for the intensive development of solar energy [45].

From the recent MED-CSP research, commissioned by the German Ministry of Environment Policies and carried out at the Aerospatiale Centre DLR, it has

. •V’V-V-V — ‘• V-V’V-Y

• • •*.* • • •: ■: • V • .* •: • • * •: •’••••

ж

-•

Smtahiliiy for the implemcntatioii of solar concentration

Мй excellent = tnod

Figure 81: World map of solar radiation underlining the sun belt.

been shown that the solar energy potential available in the countries on the Mediterranean coast is much larger than the actual electrical energy consump­tion of the area which includes Southern Europe, the Middle East and Northern Africa. Besides the DLR and other German organizations, other centres of research such as NERC (Jordan), CNRST (Morocco), NREA (Egypt) and NEAL (Algeria) contributed to the development of this study. This project shows that a common interest could link European countries (energy importers and technol­ogy exporters) with the Northern Africa countries (owners of fossil fuel resources and solar energy who will see a significant growth in their energy consumption in the next few years) [45, 48]. To exploit this huge solar energy potential, the concentrating solar power (CSP) technology is very useful. This technology exploits solar radiation, concentrating it using mirrors for electricity production and also for the realization of high-temperature chemical processes, for example, the production of hydrogen. A more detailed description of CSP technology is given in par. 4.4. This technology can be considered as a competitor of the photovoltaic technology, which is already common and is growing in Europe (actually, this is only partially true). On this matter, two aspects have to be considered: first, the photovoltaic technology exploits both the direct radiation and the spread (diffuse) radiation and so it is also suitable in areas such as Northern Europe where direct solar radiation is scarce; second, it is fit for many different applications (from a few watts of a mobile phone’s solar battery char­ger, to the megawatts of dedicated solar systems, passing through a few or tens of kilowatts used in many applications in the residential or civil field). CSP tech­nology, on the other hand, exploits only the direct radiation and lends itself poorly, excepting in a particular situation or only in thermal applications, to the realization of small dimension systems. As regards systems that have a capacity of about or more than a megawatt and that are used in areas with strong direct radiation, CSP technology allows achieving a cost for electrical energy produc­tion which is much less than that for the energy produced using photovoltaic technology, and this advantage, in this specific case, is destined to last for a long time unless there is a radical technological improvement in the photovoltaic field [45, 48].

Considering only the European part of the Mediterranean area, we can observe a sort of integration between the two technologies: the photovoltaic technology is installed where there is lesser direct radiation and in applications which require from a few kilowatts to a hundred kilowatts of power. On the other hand, CSP technology is installed in areas with stronger direct radiation and in medium-large power systems (starting from a megawatts of power). It is also possible to think of a situation where Europe increases its own consumption of ‘green electricity’ by taking it both from the different renewal sources available in the area and from the solar energy imported from the most suitable regions.

To highlight the significance of this project, which has got many prospects, it is necessary to point out that the density of the solar energy received in the Mediterra­nean southern coast and its territorial characteristics allow to cut the solar energy production costs, which are then invested to produce solar energy in the southern

European area; moreover, large areas which cannot in any case be used for agricultural purposes are available on the coasts of Northern Africa and the Middle Eastt. Since the cost for high-tension and continuous line electricity transmission for a distance of about 1000 km, of which 100 km is by submarine cable, is about 0.7-1.5 c$/kW h, it is not illogical to think of realizing in those regions — inside a wider project of social and economic integration — solar energy production capable of satisfying the growing requirement for electricity in Northern Africa and also a part of Europe [45-47].

With the regard to this matter it is important to underline how, despite mere energy considerations, the so-called ‘Mediterranean electric belt’ has been studied for a year, which will shortly allow the complete electrical interconnection between the Mediterranean countries and the European electric net.

Considering what we have just stated, it is clear that the wide exploitation of the solar source in the Mediterranean area is a very important topic with a high politi­cal and economic agenda since it has important consequences in terms of bringing about the integration of the northern and southern parts of the world and also in terms of development of pacific relationships.

As regards future prospects, the direct production of hydrogen, mainly by CSP technology, will allow the sun belt to increase the production of energy. Presently, a few Southern European areas, Spain in particular and also southern Italy, have a fairly good potential, which allows them to exploit the CSP technologies to increase the renewal quantity of electricity production.

Spain in particular is in a favourable situation because of both the presence of a huge energy potential and the remarkable experience it has with experimental activities developed since 1981 at the Plataforma Solar De Almeira.

As regards Italy, currently, there are no accurate studies on the energy potential which can be exploited using the CSP technology. The MED-CSP study [48] estimates that the ‘technical exploitable’ potential is about 88 TW h/year while the ‘economically exploitable’ potential is about 5TW h/year; actually, these figures refer to very approximate values. Despite this, it is clear that the principal aim of

using the CSP technology is to obtain advantages, in addition to economical advantages, by the exploitation of the energy potential in the areas that are rich in the solar source. This is more justified if we consider the case of Germany which has been pursuing the development of this technology for many years although its economically exploitable energy potential is almost non-existent. Since the primary source is free, it is also important to underline that the total turnover related to energy production from solar energy benefits those who realize this and take care of the production systems; those who own the know-how are then designated to exploit the biggest part of the business connected with solar energy production [45].

The direct solar energy received at a certain time interval on an oriented surface situated on the Earth is given by the expression:

rt0 +At

Eb = J Ibn(t )cos idt (36)

Jt0

where At (time interval) may vary (an hour, a day, a month, etc.) and Ibn(0 stands for the normal direct radiation.

As a rule, it is not possible to use this equation to calculate Eb since Ibn(t) depends on local atmospheric conditions which cannot be known in advance. It is possible, instead, to calculate that quantity per surface on soil during clear sky days using one of the models described in par. 5.

It is often useful to calculate using eqn (36) the radiation received during the day on a horizontal surface placed outside the atmosphere, which is equal to:

Г h

Hex = Г Icse(t)sen a dt (37)

J ha

where ha and ht are, respectively, the hour angles at dawn and sunset. Through a few passages we get:

Hex = 24/nIcs[1 + 0.0033cos(2n«/365)]

• (cos L cos d sen ha + hasen L send) (38)

where ha, in the last term, should be expressed in radians. The daily extraterrestrial radiation calculated using eqn (38) is expressed in watt-hour per square metres (W-h/m2) [1].

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].

At our latitudes, low-temperature solar thermal systems are more common than the medium and high temperature ones (especially among private users, who want to save money on their energy bills). They are usually used [1-5, 17]:

• to heat sanitary water for domestic, hotel and hospital use;

• to heat water for showers (bathing establishments, camping, etc.);

• to heat rooms;

• to heat water used in processes at a low temperature;

• to dry foodstuffs;

• to refresh rooms (although it is still too expensive).

A solar thermal system always works in the same way, although there may be slight changes according to its application and use. First, there are a few solar collectors which absorb large quantities of sunlight and then convert it into heat.

The collectors are crossed by a fluid (thermal vector fluid) which removes the absorbed heat; this fluid, crossing the so-called solar circuit, arrives at an accumu­lator which stores the large quantities of thermal energy to be used in the future when there will be a real need [17].

We will now analyse the low-temperature solar thermal system starting with its most important element: the collector.

The predicted development of CSP applications follows on after 20 years of the development of the Aeolian applications; a possible trend could be the world achiev­ing 5000 MW by 2015. The overall ‘portfolio’ of the CSP systems planned at different levels in the world accounts for about 1562 MW; adding to it both the predicted 28 MW coming from the Italian Archimedes Project (described in par. 4.5.3) and the portfolio of the Global Environment Facility (GEF, an independent financial orga­nization connected with the World Bank and also with the Environment Project of the United Nations; it was founded in 1991 to help developing countries with proj­ects and programmes aimed at protecting the world environment) projects which are currently foreseen will add another 130 MWe, which results in a potential world portfolio for the short to medium term of over 1700 MW.

Of this 1700 MW, 300 MW is considered to be surely realized.

The factors which have resulted in a reduction of the levelled electrical energy costs produced by these systems, valued by the GEF, are shown in Fig. 83; it has been forecast that the levelled energy cost (LEC) will be reduced from the current 16 c$/kW h to about 6 c$/kW h by the year 2025, also reaching by that date the predicted cost for the fossil fuel systems. Other organizations have predicted even lower costs (till 3.5 c$/kW h).

The fulfilment of these developmental forecasts will mostly depend on the polit­ical and economic situation in the next few years. However, it is clear that the knowledge and diffusion of CSP technology is currently at the same stage as that of the Aeolian ones during the mid-1980s; at that time, nobody would have bet on the Aeolian energy; instead, we have currently reached only in Europe an installed Aeolian power of 34,000 MW [45, 49].

The hour angles which correspond to the different positions of the Sun in the sky concern the true solar time and not the conventional time measured by a clock. However, true solar time is determined by the Sun’s position in the sky: for example, when the Sun is on the meridian of a certain place, the hour angle is nil because of the solar midday; instead, the clock will indicate a different time.

To convert clock time to true solar time, first, it is necessary to correct the lon­gitude difference between the local meridian and the standard time zone meridian, considering that any grade of longitude difference corresponds to a four-minute correction; second, because the angle speed of the Earth is not constant during the year, it varies positively or negatively as regards the average conventional value of 360/24 grad/h, we have to correct it using the equation of time (ET).

We use the relation [1]:

True solar time = Conventional time + 4′ (Longitude of the local meridian — Longitude of the standard time zone meridian) + ET

If we apply legal time, the conventional hour must not be the legal one, but the one for the standard time zone meridian. Longitudes are considered to be positive if they are eastward from Greenwich. As regards Italy, the datum meridian is at 15° east of Greenwich and it passes by the volcano Etna. Figure 10 shows the ET trends during the year.

Some values of ET are shown in Table 2.