Numerical study of a cascade latent heat energy storage system based on exergy optimization

doi:10.19799/j.cnki.2095-4239.2023.0531

Abstract

Abstract:

Latent thermal energy storage (LTES) technology is employed to rectify the imbalance of time and space in the application of low-grade heat and renewable energy in heat pumps (HPs). The adaptive design of cascade latent thermal energy storage (CTS) devices and the improvement in the thermophysical properties of phase change materials (PCMs) have a crucial effect on the stable and efficient operation of coupled HP drying systems. Based on the exergy optimization of multistage heat engines, a CTS device was designed for a solar HP system, and a three-dimensional shell-tube CTS model was established according to the enthalpy method. Physical constraints such as the phase change temperature of the target material and practical application conditions were comprehensively considered, and the theoretical calculation results provided guidance for the selection and improvement of PCMs, in addition to the adjustment of the heat exchanger size. In this study, sodium acetate trihydrate (SAT)-based composite PCMs were used as the filling PCMs with a cascade layout in the CTS device, which were prepared by the addition of different concentrations of acetamide (AC) to adjust the melting temperatures. To investigate the heat-transfer characteristics and thermoregulation performance of the thermal storage processes, three-stage shell-tube CTS device and single-stage LTES device filled with three ratios of SAT/AC were numerically investigated using constant and variable inlet temperatures of solar hot water. The results revealed that the reduction in the heat transfer between the stages of the CTS device could improve the thermal storage capacity. In the latent heat-storage stage of the three-stage CTS device, the average inlet and outlet temperatures decreased by 4.41 ℃, which could adjust the peak temperature by 0.90% and generate a buffering effect on the inlet temperature. Therefore, the operating temperature range of the three-stage CTS device is wider than that of the single-stage heat-storage unit. In addition, the storage density of the three-stage CTS device was 2.39 times than that of a domestic hot-water storage tank of the same volume, which could effectively store heat from the fluctuating heat source generated by the solar collector. The uniformity of the outlet temperature and heat-transfer power of the CTS device was better than that of the single-stage heat-storage unit. This study can provide not only guidance for the preparation of stepwise PCMs but also new ideas for CTS in terms of the coordination and optimization of devices and materials.

Key words: cascade energy storage, exergic optimization, heat-transfer characteristics

CLC Number:

TK 02

Geng YANG, Xin XIAO, Yunfeng WANG. Numerical study of a cascade latent heat energy storage system based on exergy optimization[J]. Energy Storage Science and Technology, 2023, 12(12): 3770-3779.

Figures/Tables 8

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材料	质量比(SAT∶AC)	4% EG强化热导率k_PCM/[W/(m·K)]	相变温度T_m^[17]/℃	㶲优化理论相变温度T_m_i /℃	相变温度模拟取值/℃	相变潜热h_m^[17]/(kJ/kg)
PCM1	100∶9	1.49	49.07	49.38	49.00	223.56
PCM2	100∶13	1.47	43.85	43.30	43.00	208.43
PCM3	100∶17	1.45	38.63	37.97	39.00	193.31

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