Multi-stage water vapor high-temperature heat pump with latent thermal storage for extending temperature range
An advanced multi-stage high-temperature heat pump (HTHP) was developed by adding two compression stages to an existing three-stage system, increasing the supply temperature from approximately 200 °C to 250 °C and extending its applicability for the effective electrification of industrial process heat. In addition, two latent thermal energy storage (LTES) units, one for each temperature level, were integrated to mitigate mismatches between heat supply and demand, which are expected to become more pronounced as the share of renewable energy increases, by providing peak-shaving capability. The HTHP coupled by split valves between the existing three stage compression cycle and an additional two stage cycle achieved a COP of 2.4-3.2 depending on the split ratio and consistently delivered over 1 MW of total heat output under the rated temperature conditions. When the HTHP operated under various temperature conditions, while maintaining identical temperature lifts between the existing and additional compression stages, both total heat output and compressor power consumption increased as the temperature level dropped. The required heat storage capacities of the LTES systems were estimated assuming each unit stores sufficient heat for up to 6 hours of peak-shaving operating at nominal HTHP conditions. Increasing split ratio reduces storage needed for the lower-temperature LTES, around 200 °C, but raises it for the higher temperature, around 250 °C. In particular, even when all main flow goes to the additional compression stages, the lower-temperature LTES must can store certain amount energy because the existing HTHP cycle requires heat-sink flow to limit superheat between its compression stages. The integrated system, comprising the advanced multi-stage HTHP and dual LTES configuration, is expected to improve the feasibility of industrial heat electrification and provide demand-side flexibility by supplying heat with higher efficiency and enabling effective peak-shaving.