Concrete as a thermal energy storage medium for thermocline solar energy storage systems

• Concrete bricks can potentially replace aggregates as a thermal energy storage medium.
• Concrete and mortars exhibited strength reduction when cycled in molten salt.
• Conditioning of concrete improved refractoriness.
• Supplementary cementitious materials improved refractoriness.
• Calcium aluminate cement/fly ash mixtures are not compatible with molten salt.

Rising energy costs and the adverse effect on the environment caused by the burning of fossil fuels have triggered extensive research into alternative sources of energy. Harnessing the abundance of solar energy has been one of the most attractive energy alternatives. However, the development of an efficient and economical solar energy storage system is of major concern. According to the Department of Energy (DOE), the cost per kilowatt hour electric from current technologies which utilize solar energy is high, estimated at approximately $0.15–$0.20/kW helectric, while the unit cost to store the thermal energy is approximately $30.00/kW hthermal. Based on traditional means of producing electricity (through burning fossil fuels), the unit cost of electricity is $0.05–$0.06/kW h. Clearly, current solar energy technologies cannot compete with traditional forms of electricity generation. In response, the DOE has established a goal of reducing the cost of solar generated electricity to $0.05–$0.07/kW helectric and achieving thermal storage costs below $15.00/kW hthermal. Reduction in the cost of the storage medium is one step in achieving the stated goal. In this research program economical concrete mixtures were developed that resisted temperatures up to 600 °C. This temperature level represents a 50% increase over the operating temperature of current systems, which is approximately 400 °C. However, long-term testing of concrete is required to validate its use. At this temperature, the unit cost of energy stored in concrete (the thermal energy storage medium) is estimated at $0.88–$1.00/kW hthermal. These concrete mixtures, used as a thermal energy storage medium, can potentially change solar electric power output allowing production through periods of low to no insolation at lower unit costs.