Seasonal solar storage brings fossil-free heating to Tibet

Overview

Built at an altitude of 4,600 m on the Tibetan Plateau, this was Tibet’s first large solar thermal district heating system. The system includes a 22,000 m² solar collector field and a 15,000 m³ pit thermal energy storage (PTES) reservoir designed to store 700 MWh of thermal energy. In a region where winter temperatures can stay below freezing all day and drop as low as –20 C, reliable heating is essential.

The PTES stores hot water at up near to 85 C, making it possible to bridge the seasonal gap between solar production and heating demand. Excess solar heat is transferred into the reservoir via heat exchangers, and in winter the stored energy is discharged back into the district heating network. This system supplies more than 90 percent of the heating demand for the town of Langkazi.

At high altitude, renewable heating projects face extreme conditions. The successful delivery of this PTES shows that even in one of the harshest climates in the world, seasonal storage can make renewable heating a dependable, year-round solution. 

Challenge

The challenge was twofold. First, the PTES needed a reliable sealing system to prevent leakage and retain heat over long periods. Without a secure geomembrane system, the storage would lose water and energy, undermining both efficiency and safety. Second, the system had to withstand Tibet’s extreme climate, where hot summers and freezing winters create repeated thermal stress. Any weakness in the liner would quickly compromise the storage function.

Beyond containment, all system components needed to work seamlessly together to optimize heat use. The storage had to be integrated with the collector field, heat exchangers, and district heating grid, requiring precise planning and execution. Given the scale and location of the project, the geosynthetics had to provide both durability and resistance to high temperature, ultraviolet radiation, and mechanical stress.

The broader challenge reflects a global issue: renewable heat can be produced in abundance, but without seasonal storage, much of it is wasted. PTES provides a solution, but only if the containment system is engineered to endure. 

Solution

Solmax delivered the complete lining system in just seven months, from design and material selection through to prefabrication and installation. Coordination with other trades was continuous to ensure precise execution, with welding, seam quality, and liner placement verified throughout the process.

At the heart of the PTES, GSE^® HDH 2.5 mm geomembrane provided the primary sealing layer. Engineered for high-temperature exposure, this liner maintains impermeability at up to 90 C. Beneath it, FABRINET^® Base Protect 50 5000 created a stable protective base, shielding the geomembrane from subgrade stresses and providing drainage. Additional reinforcement was achieved with FABRINET Base Protect 50 2000, a multifunctional geocomposite offering puncture protection and durability under varying field conditions. 

During the water filling phase to avoid dirt into the storage a GSE HD 1.0 mm was used. Finaly the lid was sealed with the GSE HDH 2.5 mm and as top cover a GSE HD color 1.5 mm (light grey) geomembrane was installed. This geomembrane combines impermeability with UV resistance and reduced heat absorption thanks to its light color, protecting the thermal insulation in the storage lid from water ingress by rain as well solar radiation and environmental loading. 

In total, approximately 32,000 m² of Solmax materials were installed by the Solmax installation team from Germany, forming a multilayer system of sealing, protection, and drainage. Together, these layers ensure the PTES can retain heat efficiently while resisting the combined effects of high temperature, ultraviolet exposure, and mechanical stress.

The result is a facility that meets more than 90 percent of Langkazi’s heating demand through renewable energy. The project proves that PTES is bnot limited to mild climates, it can also deliver reliable fossil-free heating in some of the world’s most demanding environments. For Solmax, it demonstrates how advanced geosynthetics enable seasonal storage, making renewable heating a practical reality in the global transition away from fossil fuels.