Optimal design and thermal performance study of a two-stage latent heat thermal energy storage technology for heating systems

Power-to-heat coupled with latent heat thermal energy storage can improve the economy and flexibility of heating systems and also is one of the key technologies used for promoting the balance between power grid supply and demand. However, the existing latent heat thermal energy storage heating systems are primarily based on a single-stage device filled with a single phase change material. Their exergy and energy utilization efficiencies are limited. Moreover, it is difficult to apply the existing multistage devices filled with cascaded phase change materials to building heating because of their small capacity. Therefore, in this study, a novel twostage latent heat thermal energy storage device was developed. First, a numerical simulation method was used to optimize the cascade mode and select phase change material. Second, experiments were conducted to determine how the flow rate, inlet temperature, and number of stages of the devices affected the thermodynamic performance. Finally, heating effects were investigated. Compared with the single-stage device, the total heat storage and release of the two-stage device increased by up to 53.0% and 66.3%, respectively. The most noteworthy finding was that the thermal performance of the device was significantly improved by increasing the flow rate of the heat transfer fluid only when the flow regime changed from laminar flow to critical flow or from critical flow to turbulent flow. In addition, the two-stage device can support a 3.0-kW heat demand for 7.2 h. This study provides theoretical guidance for engineering applications of cascaded latent heat thermal energy storage heating systems.