(415g) Combining Thermochemical with Sensible Heat Storage for Increasing the Stored Energy Density and Providing Stable Heat Transfer Fluid Outflow Temperature

Concentrated solar power (CSP) plants with integrated thermal energy storage (TES) are considered to be a promising option for the cost-effective electricity generation and dispatchability because they allow the temporal decoupling of electricity generation and solar insolation. TES systems can be classified as sensible heat, latent heat, or thermochemical storage (TCS) systems [1]. In a TCS, the heat is looped via reversible chemical reactions: it is stored as the enthalpy of the endothermic reaction and recovered as sensible heat through the exothermic recombination of the reactants.

A competitive advantage of TCS compared to sensible or latent TES is the ability to control the charging and discharging temperatures through an adjustment of the equilibrium temperature of the reaction [2]. This requires a physical separation of the chemical reactants from the heat transfer fluid (HTF), e.g., by placing the participants of a gas-solid reaction in metal tubes. By connecting the tubes to an external gas compression unit, the partial pressure of the gas reactant can be continuously adjusted to maintain the reaction at the desired rate and temperature and thereby heat the HTF passing the tubes to the desired temperature.

The ability to control the temperature of the HTF passing a TCS section is expected to provide several advantages compared to current state-of-the-art sensible and latent heat storage systems:

• By heating the HTF to a constant outflow temperature during discharging, the power block can be operated at its design point and therefore at a high thermal-to-electricity efficiency.
• By cooling the HTF to a constant outflow temperature during charging that is equal to the temperature of the HTF leaving the power block, these two HTF streams can be merged and recirculated to the solar field without creating exergy losses due to mixing.
• By distributing multiple TCS sections along the TES axis, the axial temperature gradient can be increased, leading to increased energy storage densities.

To reduce the overall material costs of the TES, the TCS sections may be combined with sections comprised of packed low-cost sensible-heat storage (SHS) materials such as rocks or steel slag. If the HTF and the storage and encapsulation materials in the TCS and SHS sections are selected accordingly, the operating temperature can exceed the limit of ~600°C that is typically encountered in state-of-the-art two-tank TES systems.

Whereas the adjustment of the gas reactant pressure in a TCS offers some distinct advantages compared to other TES concepts, it introduces an additional energy penalty. Future work will focus on (1) reducing the losses due to adjusting the gas-reactant pressure, (2) identifying suitable TCS materials, and (3) optimizing the combined storage performance through a variation of the TCS materials, storage geometry and/or operating conditions.

References

[1] S. Kuravi, J. Trahan, D. Y. Goswami, M. M. Rahman, and E. K. Stefanakos, â??Thermal energy storage technologies and systems for concentrating solar power plants,â? Prog. Energy Combust. Sci., vol. 39, no. 4, pp. 285â??319, Aug. 2013.

[2] Y. Q. Yu, P. Zhang, J. Y. Wu, and R. Z. Wang, â??Energy upgrading by solid-gas reaction heat transformer: A critical review,â? Renewable Sustainable Energy Rev., vol. 12, no. 5, pp. 1302â??1324, 2008.