The ENEA technological proposal for solar electricity: the use of molten salts in parabolic collector systems

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.

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Figure 97: Hydrogen production scheme from the solar source by a thermochemical process.

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

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1- molten salts

2- storage tanks

3- heat generator

4- turbine and alternator

5- condenser

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].

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