A phase change calcium looping thermochemical energy storage system based on CaCO3/CaO-CaCl2

Abstract

Calcium looping thermal energy storage (CaL-TES) is promising because of its high operating temperature, excellent energy storage density, and low material cost. However, a significant problem with the CaL-TES system is the rapid degradation of CaO. To overcome this degradation problem and also improve the energy storage performance, a novel phase change calcium looping thermal energy storage (PCCaL-TES) process has been developed based on the solid solution system of CaCO3/CaO-CaCl2. In the charging process of the PCCaL-TES system, surplus energy is stored via sensible heat, latent heat, and chemical energy with the calcination and melting of the solid solution CaCO3/CaO-CaCl2. In the discharging process, the carbonation and solidification of CaCO3/CaO-CaCl2 takes place and thus the stored energy is retrieved. Compared to the conventional CaL-TES system, the innovative PCCaL-TES system can help maintain a high activity of CaO over long-term operation due to the enhanced heat and mass transfer in the liquid-state carbonation. In this study, the energy storage performance of PCCaL-TES was assessed using the simulation package Aspen Plus v10. According to the modeling results, the PCCaL-TES system can achieve a round-trip efficiency of up to 49% and an energy storage density of nearly 1.5 GJ/m3, presenting improvements of about 4% and 20%, respectively, compared with the conventional CaL-TES system. Meanwhile, a parametric analysis was carried out to examine the effect of key operating parameters on the system performance of PCCaL-TES. The calculations show that the round-trip efficiency of the PCCaL-TES system can be improved by raising the mole fraction of CaCl2, increasing the carbonator temperature, or employing an optimum carbonator pressure. It was also found that a low mole fraction of CaCl2, a high activity of CaO, or a low temperature of CO2 storage was beneficial to achieving a high energy storage density. Finally, the impact of other operating factors on the round-trip efficiency of PCCaL-TES was summarized in a sensitivity analysis. It revealed the round-trip efficiency of the PCCaL-TES system was dominantly determined by the performance of the CO2 turbine (W-TURB) and compressor (W-COMP), the gas–liquid heat exchanger (HX3) and the exhaust CO2 cooler (CL3).