A novel approach to storing thermal energy with supercritical fluids is being investigated, which if successful, promises to transform the way thermal energy is captured and utilized. The use of supercritical fluids allows cost-affordable high-density storage with a combination of latent heat and sensible heat in the two-phase as well as the supercritical state. This technology will enhance penetration of several thermal power generation applications and high temperature water for commercial use if the overall cost of the technology can be demonstrated to be lower than the current state-of-the-art molten salt using sodium nitrate and potassium nitrate eutectic mixtures. An additional attraction is that the volumetric storage density of a supercritical fluid can be higher than a two-tank molten salt system due to the high compressibilities in the supercritical state.
This paper looks at different elements for determining the feasibility of this storage concept — thermodynamics of supercritical state with a specific example, naphthalene, fluid and system cost and a representative storage design. A modular storage vessel design based on a shell and heat exchanger concept allows the cost to be minimized as there is no need for a separate pump for transferring fluid from one tank to another as in the molten salt system. Since the heat exchangers are internal to the tank, other advantages such as lower parasitic heat loss, easy fabrication can be achieved.
Results from the study indicate that the fluid cost can be reduced by a factor of ten or even twenty depending on the fluid and thermodynamic optimization of loading factor. Results for naphthalene operating between 290 °C and 475 °C, indicate that the fluid cost is approximately $3/kWh compared with $25-$50/kWh for molten salt. When the storage container costs are factored in, the overall system cost is still very attractive. Studies for a 12-hr storage indicate that for operating at temperatures between 290–450 °C, the cost for a molten salt system can vary between $66/kWh to $184/kWh depending on molten salt cost of $2/kg or a more recent quote of $8/kg. In contrast, the cost for a 12-hr supercritical storage system can be as low as $40/kWh. By using less expensive materials than SS 316L, it is possible to reduce the costs even further.