Performance analysis of the Høje Taastrup water pit thermal energy storage

Pit thermal energy storage (PTES) is a large-scale low-cost storage technology for district heating that has almost exclusively been used in combination with solar thermal collectors for seasonal heat storage. However, PTES systems can also benefit non-solar based district heating systems, which was demonstrated in 2023 with the construction of the first large-scale short-term PTES in Høje Taastrup, Denmark. This storage represents the current state-of-the-art, particularly due to innovations in the lid and diffuser design, the development of a new high-temperature liner material, and the presence of a research-grade monitoring system. This study aimed to present the design improvements of the PTES in Høje Taastrup, analyze the impact of short-term operation, and conduct a performance analysis in terms of storage efficiency and stratification. It was successfully demonstrated that PTES can be used for short-term operation, provided they use liner materials that can withstand exposure to temperatures up to at least 90 °C for long periods. Additionally, the single-plate diffuser solution placed at the edge of the storage did not seem to negatively affect stratification. Overall, the short-term operated PTES in Høje Taastrup featured 89% energy and 75% exergy efficiency, thus performing similarly to the seasonal PTES in Dronninglund.