Experimental investigation on charging performance of a thermally integrated pumped thermal energy storage system incorporating a two-stage heat pump

Efficient grid-scale energy storage is essential to buffer the intermittency of renewable power, and thermally integrated pumped thermal energy storage (TI-PTES) offers flexible scalability without geological constraints. In this paper, the charging dynamics and performance of a TI-PTES prototype combining a two-stage high-temperature heat pump with a cascaded thermal storage module of pressurized water, paraffin wax and erythritol were experimentally characterized. At a baseline heat source temperature of 55 °C, four distinct charging periods (sensible-heat dominant, transition, latent-heat dominant and thermal sprint) were identified from temperature and COP profiles. Over the first three periods, COP of stage 1 declined from 8.4 to 6.6 and stage 2 from 4.6 to 2.0, highlighting heat pump stage 2's key role in overall system efficiency. Varying the heat source temperature from 45 °C to 65 °C reduced total charging time by 58.8 % (25.89 h–10.66 h), while average COP fluctuated non-linearly between 2.23 and 2.33. Total energy consumption ranged from 310.91 kWh to 116.60 kWh, driven by dynamic interactions between heat-transfer rates, material phase changes and heat dissipation. These findings provide valuable insights into optimizing TI-PTES design and operation in large-scale renewable energy storage.