A Novel Compression-assisted Absorption Thermal Energy Storage Heat Transformer for Low-grade Energy Utilization

To deal with serious environmental issues, such as global warming and extreme climates, structural reforms on the energy supply side are imperative. Many countries recently announced targets for reaching net-zero emissions. Increasing the proportion of renewable and sustainable energy on the energy supply side is an important means of reducing greenhouse gas emissions. Thermal energy storage technology, i.e., thermal battery, is a promising method to balance the mismatch between the intermittent renewable energy sources and time-variable user loads. However, for the existing thermal batteries, the charging temperature is usually required to be higher than the discharging temperature, which results in the underutilization of low-grade energy sources. Absorption thermal energy storage heat transformer can achieve temperature upgrading with satisfactory storage performance. To further improve the system performance, enhance the output temperature, and thus facilitate the application of low-grade renewable energy, a novel compression-assisted thermal energy storage heat transformer is proposed. A dynamic simulation is conducted using MATLAB to compare the charging and discharging characteristics of the energy storage heat transformer cycles with and without compression assistance. Then the storage performance of the two cycles, including energy storage efficiency and energy storage density, under various discharging temperatures (temperature lifts) are investigated. Results show that significant enhancements are achieved by the compression-assisted energy storage heat transformer for both energy storage performance and temperature upgrading ability. With the compression assisted, the maximum temperature lift is increased from 30 ºC to 60 ºC. Besides, with the same temperature lift, the compression-assisted cycle has higher energy storage efficiencies and densities. Under a discharging temperature of 90 ºC (temperature lift of 30 ºC), the energy storage efficiency and density are greatly improved from 0.25 to 0.40 and 41.9 kWh/m³ to 264.9 kWh/m³, respectively. The proposed cycle demonstrates the advantages of enhanced storage performance and large temperature lift.