Assessment of the off-design performance of a solar thermally-integrated pumped-thermal energy storage

Long-duration storage (i.e., 6 + h) could foster the power sector transitions towards renewable energy sources. Pumped thermal energy Storage (PTES) recently gained much attention. Thermally-Integrated PTES (TI-PTES) exploiting additional thermal energy sources to boost electric roundtrip efficiency are promising and can exploit low-temperature heat sources, including solar thermal energy. Since the solar resource availability substantially varies along the year, assessing the yearly system performance is mandatory to estimate the solar TI-PTES practical potential. The paper studies a solar TI-PTES plant based on low-concentration Solar Thermal Collec-tors (STC), a High-Temperature Heat Pump (HTHP), and an Organic Rankine Cycle (ORC). In the paper, the HTHP and ORC off-design performance are mapped for a set of representative conditions through an off-design model. Such maps are used to simulate the system operation when coupled with a solar thermal energy pro-duction profile for some periods considered representative of the whole year. Results show that the average round trip efficiency is always between 0.85 and 0.87, and the system can operate in various conditions, even in the cold seasons. While the system performance is almost constant over the year, the operating charging and discharging hours and the stored energy are more than doubled from January to July.