Multi-objective optimisation of a thermocline thermal energy storage integrated in a concentrated solar power plant

Most of CSP plants built today are all equipped with storage devices. Most are sensitive heat storages, in particular 2-tank systems. The aim of this study is to optimise a thermocline storage, designed for the Andasol CSP plant in Spain, on the basis of exergy, environmental and economic criteria. The optimum tank geometry, particle size and heat transfer fluid/filler material pair must be determined. To achieve this, six different fillers operating with Dowtherm A synthetic oil are being investigated: machined ceramics, ceramics obtained from waste materials and natural rock. The NSGA-II multi-objective genetic algorithm is used to solve the problem. This optimisation selects recycled ceramics as best filler materials. The exergy efficiency is not affected by the filler, with more than 99.8%. The application of TOPSIS method combined with weighting derived from Shannon entropy, selects recycled ceramic obtained from electric furnaces as the best filler. This tank has a tapered shape (diameter of 27.4 m, height of 22.2 m) and a very small particle diameter (6 mm). Compared with an earlier design available in literature, the environmental impact is reduced by 19%. The LCOE reaches 10.8 c€/kWhe, a 48% reduction compared with the selling price of heat in Spain (in 2021).