Overview
Heating is one of the most carbon-intensive sectors in Europe. While electricity systems have integrated renewable sources at scale, heating remains dominated by fossil fuels. Decarbonizing heat is therefore essential to achieving climate goals, and seasonal storage is the key.
In Meldorf, Schleswig-Holstein, Germany’s first commercial pit thermal energy storage (PTES) facility provides that solution. The octagonal basin holds 43 million litres of hot water, storing up to 1,500 MWh of thermal energy at around 80 C. Charged with surplus heat from industrial sources, it allows clean energy to be held over months and released when demand peaks. The result is a system that reduces fossil fuel use and cuts local heating emissions by up to 90 percent, equivalent to around 1,000 tons of CO₂ each year.
Solmax played a central role in the project as part of an innovation consortium led by WIMeG – Wärmeinfrastruktur GmbH & Co. KG. With 68,000 m² of geosynthetics supplied and installed, Solmax ensured the safe and durable containment of the PTES, making Meldorf the first municipality in Germany to operate seasonal heat storage at this scale.
Challenge
Decarbonizing heating requires more than renewable generation. Without storage, most surplus heat is wasted in summer while fossil fuels continue to supply winter demand. To replace gas and oil, communities need large-scale, long-duration storage that is both efficient and durable.
At Meldorf, this ambition was tested by specific technical and environmental constraints. The octagonal geometry of the basin demanded precise detailing to ensure stability and sealing integrity across corners and edges. Marshland conditions near the North Sea presented high groundwater levels just below the surface, requiring continuous management to prevent uplift and seepage.
The thermal environment pushed material performance further. With operating water temperatures above 80 C, conventional liners would degrade quickly, losing flexibility and impermeability. The PTES required a containment system capable of withstanding decades of thermal stress, hydrostatic pressure, and environmental loading without failure.
Finally, the cover system needed to do more than close the reservoir. It had to insulate against heat loss and external loads as well ensure long-term durability under solar radiation. Only a multilayer design, combining sealing, protection, and thermal insulation, could deliver the required performance.
Solution
The Meldorf facility was built around Solmax’s PTES system, designed to make seasonal storage both reliable and effective in reducing emissions.
At its core is the GSE® HDH 2.5 mm geomembrane, a high-temperature liner engineered for continuous exposure to water up to 90 C. This barrier ensures long-term impermeability, protecting both stored energy and surrounding groundwater. Supporting this, FABRINET® geotextiles were installed in multiple configurations within the cover to provide drainage, puncture protection, and stability for insulation layers. GSE HD Green FrictionFlex serves as a final seal layer against rainwater and reduces the risk of slipping in wet conditions thanks to its textured surface.
In total, about 68,000 m² of Solmax geosynthetics were installed: 20,000 m² in the basin lining system and 48,000 m² across six storage lid layers. Together, they form a robust containment system that retains heat, prevents leakage, and extends service life.
Installation was completed by Solmax’s German installation team. Professional welding of the GSE HDH and HD panels and strict quality assurance ensured seam integrity, critical for long-term performance. Special attention was given to the octagonal geometry, where edge transitions required custom detailing.
The outcome of the first project phase is a PTES capable of supplying 1,500 MWh of sustainable heat each year and reducing local CO₂ emissions by around 90 percent. For Meldorf, it is a decisive step toward climate neutrality. For the wider energy transition, it proves that seasonal storage, secured by advanced geosynthetics, is a practical, scalable pathway to decarbonizing heating.
