Novel designs of hybrid thermal energy storage system and operation strategies for concentrated solar power plant

Highlights: • Hybrid thermal energy storage system and operation strategies (HTESS) are proposed. • Utility factor of hybrid systems are improved by 12.5–22.1%. • Unit cost for heat storage is reduced by 8.6% with new designs. • New designs can enhance annual electricity generation up to 14.1%. Packed-bed thermal energy storage (PBTES) has advantage of being relatively low cost, but suffers from low utility factor, compared with two-tank thermal energy storage (TTES). This paper proposes two new designs of hybrid thermal energy storage system (HTESS), consisting of PBTES and TTES, and corresponding operation strategies: HTESS-TS for thermocline storage and HTESS-OTC for outlet temperature control. Firstly, structures and operation strategies of HTESS-TS and HTESS-OTC are described in detail. Then, thermal and economic performances of HTESS and single-tank thermal energy storage system (STESS) only containing PBTES in stand-alone state are compared. Next, effects of cut-off temperature and thermal capacity of TTES are analyzed. Finally, under realistic solar radiation, annual performance of concentrated solar power plant (CSP) with different thermal energy storage systems are compared. Results show that compared with STESS, utility factors of HTESS-TS and HTESS-OTC are improved by 12.5% and 22.1% respectively. Meanwhile, unit cost of HTESS-OTC is 8.6% lower than that of STESS. In addition, for a broad range of outlet temperature limits, HTESS-OTC can maintain more stable outlet temperature, higher utility factor than STESS. Compared with STESS, annual generated electricity induced by HTESS-TS and HTESS-OTC increase by 9.8% and 14.1% respectively.