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].

Many factors affect the positioning of the solar system’s intercepting surfaces. Among them, the most important is the study of the place and users’ requirements; actually, it is important to examine carefully the consumptions trend during the year. Another factor that has a strong impact on the positioning of the intercepting surfaces is the shading phenomenon at the installation site. To determine accu­rately the shades which can appear on a certain surface, we can use the solar tra­jectories diagram or the Sun’s position diagram (par. 10), which provides precise information on the Sun’s position in the celestial vault during the day and the year at a certain place.

It is clear that the best orientation for an intercepting surface is the one that is orthogonal to the solar rays. The fixed intercepting surfaces (i. e. the ones that do not have automatic Sun chasing devices) meet that orthogonality condition once a day. Hose surfaces are normally installed southward to maximize the energy received during the day. However, this is not a strict norm, especially where the roof is not north-south oriented. Panels which are eastward or south-eastward ori­ented favour the morning running while the westward or south-westward oriented panels favour the afternoon running.

Choosing the best inclination is not easy and immediate; generally, it is chosen such that it is equal to the latitude L decreased by about 10° to maximize the energy collected during the year (e. g. in Rome since the latitude is 42°, the best inclination will be 30°).

If users require the system to work especially in winter months, this value will not be satisfactory. During winter, the apparent trajectory of the Sun in the celestial vault is on average low so that the average inclination of solar radiation reaches the minimum yearly values. A panel inclination higher than the mentioned 30° (e. g. 60° for a hotel located in a skiing resort) will be necessary to favour the intercept­ing surfaces’ exposure to direct radiation.

On the contrary, in summer (e. g. for an open air swimming pool) users can maximize the service with an inclination of about 10°.

The last factor which contributes to the correct positioning of surfaces is the economic result of the investment: the right dimensions and the correct realization of the system minimize the need for the active surface and therefore the number of collectors to be bought and the overall cost of the operation. Eventually, it is necessary to point out that small positioning variations compared to the best panel positioning can lead to negligible loss of energy received [2, 5].

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A solar greenhouse, which is set against a building or is made out of a building, consists of a closed glazed space located on the south side of a house which is separated from it by a thermal accumulator wall (Fig. 74). The greenhouse can be used as both a direct gain non-warmed space and an indirect system since the rooms next to it receive heat through the intermediate wall which works as a storage. It is also possible that the rooms receive heat from the air in the greenhouse through a natural or forced ventilation system. Solar greenhouse planning can follow different criteria: if it is considered as a cheap extension of the house where people live for the greatest part of the year, it will be necessary to employ a big storage mass placed both on the walls and also on the floor and some movable insulation panels for the night. Instead, if the greenhouse is seen as a solar wall system, with an air space which is a few metres wide rather than a few centimetres, it should be planned to ensure that the greatest quantity of intercepted energy will be taken from the air space to heat up the adjacent rooms. In this case, a forced air change using the greenhouse to pre-heat the incoming air could be also planned (Fig. 75).

Solar greenhouses can be realized in a wide range of geometrical configura­tions. It can be considered as a simple addition to a wall, as a semi-jutting out element or as an element which is set in a building (with three of its sides sur­rounded by living spaces). Moreover, the solar greenhouse can be considered as a structure which covers the entire width of the house and is a single storey or two storeys. Even a greenhouse which is isolated from the building structure can supply thermal energy to the building through a system of ventilators and grooves.

Eventually, a correct solar greenhouse plann has to restrict the inner overheating phenomenon during the summer to its minimum. The simplest technique is one which allows ventilation directly from outside by opening the glazed windows, but the use of screenings or glazed surfaces fitted with sun block control is also possible [1, 3, 4].

The solar energy which reaches Earth’s surface is much smaller than that which reaches a surface situated outside the atmosphere. This happens because of the phenomenon of diffusion and absorption of solar radiation by components of the atmosphere. The collision with molecules of air, steam and atmospheric dust results in scattered reflection because of which a part of the radiation is sent back to outer space. Absorption, instead, is principally due to ozone (O3), steam (H2O) and carbon dioxide (CO2). O3 absorbs mainly in the ultraviolet region while H2O absorbs in the infrared region.

Figure 7 shows the spectral distribution of solar radiation when the Sun is at the zenith.

The part of the solar radiation which reaches the Earth’s surface following the direction of the solar rays without being absorbed and reflected is called directed radiation (on soil), while the part that reaches the Earth’s surface from all direc­tions (because of the scattering) is called scattered radiation. Global radiation on soil refers to the sum of directed and scattered radiation.

Diffuse radiation can be picked up almost entirely by flat panels since glass is actually transparent to all solar radiation which arrives with an angle of incidence i (i. e. the angle between a solar ray and a normal surface) smaller than the maxi­mum value of reflection (70-80°). On the other hand, concentrators, assuming that they work in conformity with the rules of geometrical optics, have to be oriented towards directed radiation; they do not pick up diffuse radiation.

If we do not consider horizontal surfaces, which are inclined in any manner, besides directed and diffuse radiation, it is necessary to take into consideration a third kind of radiation: the reflected solar radiation, the radiation reflected from the soil or from the objects near the given surface; its intensity is influenced by the albedo of those objects. Albedo is the fraction of solar radiation that is received

and suddenly reflected by a surface. Every kind of soil and vegetation has its own value of albedo [1-3].

Albedo can also be defined as a transmission coefficient of the atmosphere, which depends on the wavelength and the route of the solar rays in the atmosphere, besides depending on atmospheric composition, which varies with local weather conditions. In the case of clear sky days, the transmission coefficient of directed radiation, given by the ratio between directed radiation on the soil and extraterrestrial radiation on the orthogonal surface, can be calculated using the following equation:

ть = 0.5{exp[ -0.65m(z, a)] + exp[ -0.95m(z, a)]} (9)

We can assume:

m(z, a) = m(0,a)p(z )/p (0) (10)

where p(z) and p(0) are the atmospheric pressures at level z and sea level, respectively.

The adimensional parameter m(z, a) is the air mass for an altitude z above the sea level. This parameter is defined as the ratio between the effective route length of solar rays and their shortest route length, with the Sun at the zenith; a is the angle formed by solar rays with a horizontal plane (see Fig. 8).

The air mass m(0,a) for the sea level can be calculated using the approximated equation:

m(0,a) = 1/sen a = cosec a (11)

which gives an error percentage of 1% per a > 15°, or it can be calculated with the exact formula, taking into consideration the Earth’s and the atmosphere’s bending:

m(0,a) = [1229 + (614sen a)2]05 -614sena (12)

The angle a determines the Sun’s position in space at any time; the relative air mass m has a certain value; therefore, we calculate tb.

Directed radiation is then given by:

I = 11

bn 10 b (13)

Hottel’s model is the second way to calculate the radiation on soil during clear sky days. This model estimates direct radiation on clear sky days for a standard atmosphere with 23 km visibility and four kinds of climate.

The transmission coefficient of normal direct radiation (7bn//0) is calculated using these relations/equations, which are valid for altitudes lower than 2.5 km:

tb = a0 + a^xp( — k/ sena) a0 = r0[0.4237 — 0.00821(6 — Z )2] a1 = r1[0.5055 + 0.00595(6.5 — Z )2] k = rk [0.2711 + 0.01858(2.5 — Z )2]

where Z is the observer altitude expressed in km and r0, r1 and rk are adimensional corrective coefficients.

To achieve global radiation on soil it is also necessary to determine diffuse radi­ation. Liu and Jordan developed an empirical relation between the coefficient of direct radiation tb and that of diffuse radiation td during clear sky days:

td =0.2710 — 0.2939tb (15)

td is the ratio between diffuse radiation on soil over a horizontal plane and extra­terrestrial radiation over a horizontal plane (I0 sen a) [1, 3].

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].

1 Introduction

There are two principal ways of solar energy exploitation:

• heat production (for use in the domestic, civil and production fields; in this case, we talk about thermal solar energy);

• electricity production by the direct conversion of energy (photovoltaic solar energy).

Thermal solar technologies are divided into low, medium and high temperature

ones.

The low temperature technology includes systems which, thanks to appropriate devices (solar collectors, see par. 2.2.2.1), are able to heat fluids at temperatures less than 100°C. These systems are generally installed to produce sanitary hot water (for domestic use, collective users, sport centres, etc.), to produce domestic heating and, in general, other room heating, to heat water in swimming pools, to produce heat at a low temperature for industrial utilization (usually to warm the water used to swill machines or to preserve different kinds of fluids at a certain temperature inside tanks, etc.).

The medium temperature technology includes systems, which allow reaching temperatures of more than 100°C and less than 250°C. Currently, medium — temperature solar thermal power systems are not widespread; among their applica­tions, the most common application is the one represented by simple devices which use solar radiation to cook food (the so-called solar ovens, see par. 2.3).

The high temperature technology includes systems which, thanks to appropriate devices that are able to concentrate solar radiation to a thermal receiver (in this case we talk about concentrating solar power (CSP) technology, see par. 2.4), allow heating a fluid at temperatures more than 250°C.

Concentrated solar technology has its application in electricity production (in this case, we talk about ‘solar thermodynamics’ where the heat at a high tempera­ture is exploited in thermodynamic cycles for electricity production) and in the

fulfilment of chemical processes at high temperatures, such as production of hydrogen. [2, 12-15].

The third and last approach to passive heating is the isolated gain system. In this system, the solar collector and the storage are thermally insulated from the rooms that are to be heated up. The system can run apart from the building which draws energy only when heat is required. Where systems are completely passive, the energy transfer from the collector to the room or to the storage and from the storage to the room happens only by natural processes and not by forced processes such as convection and radiation. The most common technique is the one to create natural circulation systems composed of a flat plane collector and a thermal accumulator tank. The thermal vector fluid is normally air. An air radiator system (Fig. 76) uses a glazed collector located in the most suitable position to get the greatest quantity of the Sun’s radiation, but it must be distant and below the thermal storage tank.

The absorbed heat warms up the air which, because of the density gradient, moves up and enters the storage (made of either a compact conglomerate mass or an incoherent bed of stone) thereby heating it up. The stored heat is then distrib­uted over the air in the room by convection. The thermal storage mass can be put below the floor of the building, below the windows or inside pre-fabricated plug­ging elements. The space orientation of the building is less important for the sys­tem’s efficiency compared with the other kinds of solar gain [1, 3, 4].

An inclined surface situated on the terrestrial plane is characterized by two geo­metrical quantities: inclination b, the inclination of the surface compared to the horizontal, and surface’s azimuth aw, the angle that the projection on the normal to the surface’s horizontal plane has to rotate to superimpose itself on the southern direction. If that rotation is counter clockwise, angle aw is considered to be posi­tive. The angle between the solar rays and the normal to the surface is called the angle of incidence i.

The direct radiation intercepted by a surface is:

Gb = Ibn cos i

The general expression for cos i:

cos i = cos(a — aw )cos a sen b + sen a cos b (17)

or, as a function of the fundamentals angles L, d and h:

cos i = send (sen L cos b — cos L sen b cos aw)

+ cos d cos h(cosL cos b + sen L sen bcos aw)

+ cos d sen b senawsenh (18)

This expression indicates three cases of particular interest:

• For a horizontal surface (b = 0°), we have:

cos i = sen d sen L + cos d cos L cos h = sen a (19)

• For a vertical surface facing south (aw = 0°, b = 90°), we have:

cos i = — sen d cos L + cos d sen L cos h (20)

The introduction of the surface’s azimuth aw results in a remarkable complication when compared with the case where aw = 0°. In that case, using geometrical demon­stration, it can be easily shown that radiation on an inclined surface of angle b at latitude L is equal to the radiation on latitude (L — b), these being surfaces parallels. Therefore, for an inclined surface facing south, we have:

cos i = sen(L — b)sen d + cos(L — b)cos h cos d (21)

Often, it is useful to know when the Sun rises and sets as regards an oriented surface: the surface ‘sees’ the Sun when the angle of incidence is lower than 90° and the solar altitude is more than 0° at the same time. The Sun rises and sets on
the surface connected with the minimum hour angle ha’ (and ht’), between the absolute value which is calculated by ignoring sen a (hour angle of dawn and sun­set on the horizon), and the absolute value is obtained by ignoring cos i (i. e. con­sidering i = 90°).

As a rule, in the northern hemisphere for southward oriented surfaces, when it is winter and days are short, it is sufficient to ignore sen a; however, during sum­mer, when the angle of incidence is more than 90° and the Sun has already risen and before it sets, it is enough to ignore cos i. The general rule, valid only if the surface actually sees the Sun, is given by the following equations:

min ha(a = 0°), ha(i = 90°)

= min ht (a = 0°), ht(i = 90°)|

In the simple case of inclined surfaces facing south, we have: cos i = cos 90° = 0

= sen(L — b)sen d + cos (L — b)cos d cos h

I ha (i = 90°) = I ht (i = 90°) =arcos [ — tg (L — b) tg d] (25)

|ha (a = 0°) = |ht(a = 0°) = arcos [ — tg L tg d] (26)

For northward oriented surfaces, there could be both two dawns and two sunsets (in spring and in summer, in the northern hemisphere) and no dawns and no sun­sets, that is, absence of direct lightening (in autumn and winter).

When the surface is not oriented southward, it is not possible to get simple closed — form expressions for the hour angles on the surface at dawn and sunset.

Assuming that 4o is the instantaneous direct radiation on a horizontal plane, linked to normal direct radiation by the relation:

hn=ho/sena (27)

and applying the (16), we get:

G = /bocos i/sen a = I bo R (28)

where

Rb = cos i/sen a (29)

Rb is the inclination factor for direct radiation; remembering that for a horizontal surface it is sufficient to put b = 0°, expression (28) states that the direct radiation Gb on a surface that is inclined and oriented in any direction is equal to the product of direct radiation Ibo on a horizontal plane and the inclination factor [1, 3].

For a southward oriented surface, we have:

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].

Based on thermodynamic considerations, the use of electrical energy to produce hot water is not recommended, since a prized kind of energy is used and also because the global efficiency of the water heating process is lower than the produc­tion of many other direct water heating processes. In fact, heat is a kind of energy which we inevitably find in every real process as consequence of the irreversibility of this process. So it does not make sense to degrade completely a noble form of energy to obtain heat, without getting the mechanical work which can be obtained from that energy.

An alternative way to produce hot water involves the exploitation of solar energy, which represents a form of clean and inexhaustible energy, by low — temperature thermal solar technologies [16].

These technologies include systems using a solar collector to heat a fluid or the air. The aim of these low-temperature thermal solar systems is to intercept and transfer solar energy to produce hot water or heat buildings. By low temperature we mean the heating of fluids at a temperature of less than 100°C (it rarely reaches 120°C) [1, 2, 5, 13, 17].

The detailed description of low-temperature thermal solar systems can be found in par. 2.2.2.