Investigation on the relations of operating parameters of a thermodynamic cycle energy storage system

Abstract

In the background of carbon neutrality targets, energy for power generation is being converted from fossil to renewable sources. Energy storage has become particularly more and more important because it is a key technology to solve the instability of renewable energy. An energy storage method coupled with a heat pump and power cycle named thermodynamic cycle energy storage, which uses a heat pump and power cycle to run alternately for energy storage and has attracted the attention of international researchers because of its characteristics of green development, flexible application and convenience for large scale. The relations of operating parameters of the thermodynamic cycle energy storage are very important for operating this system sufficiently. Therefore, an optimization model of thermodynamic cycle energy storage was established for the CO2 transcritical thermodynamic cycle, with hot water as a hot storage medium and NaCl brine as a cold storage medium. The relations of its operating parameters were analyzed by pinch point temperature difference and control variable method. The results showed that turning of some parameter curves occurs with changing the position of pinch point and largest temperature difference points. The round-trip efficiency generally increases firstly and then decreases with increasing the high-temperature side pressure of the power cycle. In the calculation range, when the evaporating pressure of the heat pump is 2.00 MPa, the high-temperature side pressure of the heat pump is 20.00 MPa, and the CO2 temperature of the air cooler outlet is 20 °C, the peak value of round-trip efficiency is the largest, the maximum round-trip efficiency is 56.9 %.