Modeling of the rock bed thermal energy storage system of a combined cycle solar thermal power plant in South Africa

• A heat transfer model of a rock bed thermal storage system was created.
• Performance simulations of a combined cycle CSP plant with storage were run.
• Impact of storage dimensions on the LCOE are shown.
• A minimum storage size and diameter should be maintained.
• Rock bed storage system costs are marginal.

A thermocline-based rock bed thermal energy storage system potentially offers a cheap and simple way of achieving dispatchability in an air-cooled central receiver CSP plant. In order to efficiently match heliostat field size, storage dimensions, back-up fuel consumption and turbine sizes for non-stop power generation and economic feasibility, year-long power plant simulations have to be run. This paper focuses on the storage as the center of in- and outgoing thermal energy. The derived storage model has one spatial dimension which is justified by the high tube-to-particle diameter ratio and because yearly aggregated – and not momentary – values are of interest. A validation of the correlations with data from the literature shows acceptable agreement. Sensitivity analyses indicate that, due to low costs of the storage system, above certain minimum storage dimensions, the influence on energetic and monetary performance indicators is marginal. The calculated LCOE is in the range of 0.11–0.18 EUR/kW h and in agreement with other studies on combined cycle CSP plants.