Overview
In the northern Hessian village of Bracht, Germany, a forward-looking district heating project has now been successfully completed. At its core is a pit thermal energy storage (PTES) system with capacity to hold large volumes of solar heat collected in summer and release it in winter. Together with a solar thermal collector field, heat pumps, and a biomass boiler, the system will supply around 4.200 MWh heat per year for 180 households through 9,7 km of network pipelines. More than 71 percent of local heating demand will be met by solar energy, reducing emissions to near zero about 98 percent. More important than the 71 percent solar share is the fact that the entire system operates on 100 percent renewable energy - making Bracht the first village in Germany to achieve a fully renewable heating supply. By replacing fossil fuels entirely, local emissions are reduced to near zero - approximately 98 percent lower than before. Heating is one of the hardest sectors to decarbonize. Electricity has rapidly integrated renewable sources, but heating still depends largely on fossil fuels. PTES provides a practical solution: it makes seasonal storage of renewable heat possible, turning summer surplus into reliable winter supply. Projects like Bracht show how districts can replace fossil fuels with renewable, affordable heat. Solmax supplied and installed the geosynthetic systems that make the PTES secure and efficient, while also contributing to the research that enables innovation like this to happen.
Challenge
The core challenge in renewable heating is not generation, but storage. Solar energy is abundant in summer but scarce in winter. Without seasonal storage, surplus heat is wasted while fossil fuels continue to cover winter demand. To make renewable heating viable at scale, PTES must deliver long-duration, high-capacity storage with reliable containment. From an engineering perspective, this is demanding. A PTES facility must withstand the hydrostatic pressure of tens of thousands of cubic meters of hot water, while retaining its mechanical integrity under temperatures of up to 90 C. Standard liners degrade under these conditions, leading to leakage or loss of performance. The sealing system must also protect groundwater and minimize thermal losses through the floating lid over decades of use. In Bracht, the challenge was to design a storage that could safely operate at high temperatures for many years, integrated with the wider heating system. This required advanced geomembranes and protection layers able to meet the dual requirements of impermeability and durability.
Solution
The Bracht PTES combines solar collectors, seasonal storage, heat pumps, and a biomass boiler into one system. Solar energy is collected in summer, stored in the 90 C pit, and released through the district heating network in winter. Heat pumps increase efficiency, and biomass provides redundancy during peak demand. To secure the PTES, Solmax supplied and installed two key products:
FABRINET® Base Protect was placed beneath the storage to stabilize the subgrade, protect the geomembrane and provide drainage.
GSE® HDH high temperature geomembrane created the primary barrier, engineered to withstand continuous exposure to high-temperature water.
The floating cover was designed as a multilayer system. A floating GSE HDH geomembrane was combined with additional FABRINET layers, protecting the insulation, providing ventilation and ensuring the cover’s long-term stability. A white GSE HD special UV-stabilized geomembrane ensures permanent tightness when exposed to the local weathering conditions. This configuration with special prefabricated geosynthetic built in parts maintaining the integrity of the storage lid.
Installation was carried out by Solmax’s German Installation team. Seam welding and quality assurance were critical, as long-term performance depends on the integrity of the liner.
The outcome is a storage system capable of covering more than 180 households in Bracht with renewable energy. For the community, this means stable, predictable heating costs and a near-zero emissions footprint. For the energy transition, it demonstrates that reliable geosynthetics make PTES a viable, scalable solution. With GSE HDH, GSE HD and FABRINET, seasonal storage becomes practical, turning solar heat into a year-round resource and advancing the shift toward fossil-free heating.
