Carbonate molten salt solar thermal pilot facility: Plant design, commissioning and operation up to 700 °C

Concentrated solar power (CSP) is not currently cost competitive with conventional power generation or other solar energy technologies, but it is attractive because it integrates commercially viable large-scale thermal energy storage (TES). To improve performance and reduce costs, supercritical carbon dioxide (s-CO2) power cycles have been proposed to increase the thermal-to-electric conversion to values above 50%. However, to achieve this target, the solar energy collected by the receiver and stored in the TES must be delivered to the power turbine at a temperature at or above 700 °C, while current advanced plants only provide thermal energy at 565 °C. For the first time, a high temperature (700 °C) molten salt experimental pilot plant for CSP was designed, built, and tested by Abengoa using carbonate salts as the heat transfer fluid. We demonstrate the feasibility of large-scale, industrially relevant operation between 400 and 700 °C, thus providing a solution to integrate higher efficiency s-CO2 power cycles. We address the key points in material compatibility, component design, instrumentation and system integration. Operational aspects such as preheating and filling procedures, start-up, maximum temperature reached, and the main challenges faced are described in detail. The heat losses in the loop and components are analyzed. We show that with appropriate electrical heat tracing elements, operation with carbonates is feasible and that the main subsystems that must be redesigned are the high temperature tanks and pumps. Lessons learned from the plant operation are summarized here to guide the design of future commercial scale high temperature molten salt plants.