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

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

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

Figure 85: Solar field.

Figure 86: Schema of a thermal electrical system with linear parabolic collectors.

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

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

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

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

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

sulnr

superheater

reheatcr expansion essel

Figure 88: SEGS IX scheme.

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

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

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

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

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

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

4.4.1 Tower system with a central receiver

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

Figure 89: Solar tower system.

Figure 90: Heliostats.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Figure 92: Impianto Solar Two.

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

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

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

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

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

load factor: 25%

‘electricity

tidiostats

energy stored

accumulation of heat