The advantages of molten salts

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.

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Figure 99: Storage reservoirs.

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].

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