An alternate to the two-tank molten salt thermal energy storage system using supercritical fluids is presented. This technology can enhance the production of electrical power generation and high temperature technologies for commercial use by lowering the cost of energy storage in comparison to current state-of-the-art molten salt energy storage systems. The volumetric energy density of a single-tank supercritical fluid energy storage system is significantly higher than a two-tank molten salt energy storage system due to the high compressibilities in the supercritical state. As a result, the single-tank energy storage system design can lead to almost a factor of ten decrease in fluid costs. This paper presents results from a test performed on a 5 kWht storage tank with a naphthalene energy storage fluid as part of a small preliminary demonstration of the concept of supercritical thermal energy storage. Thermal energy is stored within naphthalene filled tubes designed to handle the temperature (500 °C) and pressure (6.9 MPa or 1000 psia) of the supercritical fluid state. The tubes are enclosed within an insulated shell heat exchanger which serves as the thermal energy storage tank. The storage tank is thermally charged by flowing air at >500 °C over the storage tube bank. Discharging the tank can provide energy to a Rankine cycle (or any other thermodynamic process) over a temperature range from 480 °C to 290 °C. Tests were performed over three stages, starting with a low temperature (200 °C) shake-out test and progressing to a high temperature single cycle test cycling between room temperature and 480 °C and concluding a two-cycle test cycling between 290 °C and 480 °C. The test results indicate a successful demonstration of high energy storage using supercritical fluids.

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