Dynamic simulation of integrated rock-bed thermocline storage for concentrated solar power

In contrast to wind and photovoltaic, concentrated solar power plants can be equipped with thermal energy storage in order to decouple intermittent energy supply and grid feed-in. The focus of this study is the technical evaluation of a cost-efficient storage concept for solar tower power plants. Consisting of a quartzite-rock bed that is charged with a hot air flow and discharged by cold air counter-flow, the storage essentially operates like a regenerator. For such systems, the discharge temperature typically declines with time. Furthermore, the use of a randomly packed bed results in considerable pressure loss. In order to describe the relevant flow and heat transfer mechanisms in rock beds used for thermal storage, a mathematical model written in the modelling language Modelica is developed and validated. Good agreement with experimental data from literature is obtained. With the aid of the validated model, a rock-bed thermal storage for application in a semi-industrial scale solar power plant (1.5 MWel) is designed and optimised with respect to electrical efficiency of the plant during the charge and discharge cycle. The storage capacity is equivalent to four hours of full-load operation. Results show that compressor work should be considered directly in the selection of packed-bed geometry in order to minimise the efficiency penalty of storage integration in the solar plant.