Bridging Seasonal Mismatches: Solar Integration with Pit Thermal Energy Storage in District Heating

Decarbonizing district heating (DH) necessitates effective integration of variable renewable energy sources, such as solar energy. Pit thermal energy storage (PTES) is a promising technology to bridge temporal mismatches between solar energy supply and heat demand. However, it remains unclear how photovoltaic (PV) electricity and solar thermal (ST) heat shape the optimal sizing and operation of PTES. Using the sector-coupled energy system model Enertile, we analyze PTES in Germany, disaggregated into 11 model regions, within a climate-neutral European energy system for 2045. Five scenarios systematically vary European PV and ST deployment. Our results show that PTES is cost-optimal in all scenarios, but capacity and operation vary: More solar energy leads to larger PTES with fewer equivalent full cycles per year, reflecting a shift toward seasonal operation; less solar energy leads to smaller PTES with more frequent charging and discharging. A 20% increase in PV capacity raises optimal PTES capacity by 84% and lowers cycling frequency by 45%, whereas halving PV capacity reduces PTES capacity by 26% in Germany. ST variations yield similar but less pronounced dynamics. To understand regional differences in PTES expansion, we conduct a correlation analysis. Results show that PTES expansion correlates with electricity price spreads, underscoring the role of price volatility in electrified DH systems. PTES expansion is further shaped by the DH generation mix, as dispatchable technologies such as biomass reduce storage needs. Overall, the value of PTES is highly system-dependent, highlighting the need to align storage planning with regional solar deployment and alternative flexibility options.