Efficient Pit Thermal Energy Storages (PTES) are the key element for a successful heat transition for a green future. Heat storage systems bridge the time gap between heat generation – e.g. from solar thermal energy, industrial waste heat or power-to-heat systems – and the use of heat. Therefore, they are the central element of the heat transition.

The system of PTES has proven their worth in the flexible provision of district heating from renewable energy resources in the last years. PTES must meet many technical requirements such as being temperature resistant, lightweight, cost-effective, stable, well insulated and leak proof.

This is how Pit Thermal Energy Storages are constructed


A PTES can store a very large amount of renewable and climate-neutral heat, enabling a more flexible use of renewable heat sources. 


All components of the storage system must be coordinated to ensure efficient use of the heat. Pit Thermal Energy Storages are usually rectangular basins – in the form of an inverted truncated pyramid – dug into the ground. The slope angle and the maximum construction depth are determined by the nature of the surrounding soil. PTES must be effectively sealed to the base and at the sides and be closed with an airside insulated lid. The basin is sealed with watertight, high-temperature (up to 95°C) resistant geosynthetics and can – if necessary – be insulated all around against the ground.

The PTES is usually filled with water as storage medium due to its high heat capacity. The top of the basin is closed by a floating cover which is thermally insulated in any case. In water-filled storages, heat is fed in and extracted directly via charging and discharging pipes with a diffuser, which slowly feed the hot water into or out of the storage. This minimizes the mixing of the temperature layers.

Solmax´ contribution to a successful heat transition

Solmax not only manufactures innovative geosynthetic products for this application but has also been implementing these PTES with the company´s own installation department for several years. With the appropriate products and experienced own installation department, Solmax offers a sustainable, cost-efficient, and effective overall solution from a single source. 


Since September 2021 Solmax is part of the Efficient Pit research project which focuses on the question: What does the next generation of Pit Thermal Energy Storages look like? Based on a resolution of the German Bundestag, the project is being funded by the German Federal Ministry of Economics and Climate Protection for four years. It involves the development of new materials, components and designs for future heat storage systems that meet the requirements of the German and European district heating industry.