Organic flash cycles in Rankine-based Carnot batteries with large storage temperature spreads

The defossilization of energy systems by means of renewable energies requires large storage capacities to balance supply and demand. Carnot batteries are an emerging technology which enables base-load capable energy storage with large storage capacities. Low-temperature Carnot batteries by means of heat pump/ORC systems strongly benefit from thermal integration of waste heat sources and allow simple and cheap extension of the storage capacity. However, the temperature mismatch between sensible storage medium and working fluid inherently implies exergy losses which decrease the power-to-power efficiency of the system. Therefore, this simulative study investigates organic flash cycles (OFC) as an alternative to ORCs in Carnot batteries. The six investigated configurations comprise Carnot batteries based on ORC, OFC and two-stage OFC with two-phase expander and intermediate phase separation, each of them with and without recuperation. Pressurized hot water serves as storage medium with a maximum storage temperature of 150°C. The results indicate that low-temperature Carnot batteries generally require waste heat sources to yield feasible power-to-power efficiencies. A storage temperature close to the heat source temperature and a small storage spread are favorable in terms of efficiency but lead to large storage sizes. The organic flash cycle minimizes exergy losses during heat transfer at the cost of throttling losses. Therefore, OFCs necessarily require advanced configurations like the suggested second stage by means of a two-phase expander and intermediate separation in order to generate additional power during the flash step. The two-stage OFC yields significantly higher efficiencies than the ORC, especially for higher storage spreads. For nearly isothermal storage, the efficiencies are comparable. Thus, for an increasing storage temperature spread, the reduced exergy losses during heat transfer outweigh the throttling losses of flash cycles. As a result, organic flash cycles are an efficient alternative to ORCs in Carnot batteries with high storage temperature spreads. Such higher storage temperature spreads allow more compact storages due to a higher volumetric storage density. Finally, a fully reversible two-stage OFC-based Carnot battery concept with two-phase expansion is proposed. The concept combines the advantages of high power-to-power efficiency, high volumetric storage density and reduced investment costs.