This paper describes a thermodynamic model that simulates the discharge cycle of a single-tank thermal energy storage (TES) system using supercritical fluid in a concentrating solar power plant.

Current state-of-the-art TES design utilizes a two-tank system with molten nitrate salts; one major problem is the high cost of the fluid. The alternate design explored here involves the use of less expensive fluids at supercritical temperatures and pressures. By cycling the storage fluid between a relatively low temperature two-phase state and a high temperature supercritical state, a large excursion in internal energy can be accessed which includes both sensible heat and latent heat of vaporization.

Supercritical storage allows for the consideration of fluids that are significantly cheaper than molten salts; however, a supercritical TES system requires high pressures and temperatures that necessitate a relatively high cost containment vessel that represents a large fraction of the system capital cost. To mitigate this cost, the proposed design utilizes a single-tank TES system, effectively halving the required wall material. A single-tank approach also significantly reduces the complexity of the system in comparison to the two-tank systems, which require expensive pumps and external heat exchangers. However, a single-tank approach also results in a loss of turbine power output as the storage fluid temperature declines over time during the discharge cycle.

The thermodynamic model is used to evaluate system performance; in particular it predicts the reduction in energy output of the single-tank system relative to a conventional two-tank storage system. Tank wall material volume is also presented and it is shown that there is an optimum average fluid density that generates a given turbine energy output while minimizing the required tank wall material and associated capital cost.

Overall, this study illustrates opportunities to further improve current solar thermal technologies. The single-tank supercritical fluid system shows great promise for decreasing the cost of thermal energy storage, and ensuring that renewable energy can become a significant part of the national and global energy portfolio.

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