Green heat from the lake

Where excavators once mined coal, Lake Hainer near Leipzig now stretches across the landscape. But the lake is not only known for relaxation and idyllic nature, it also represents an important step forward in the energy transition: since 2024, an aquathermal plant has been supplying the harbor village of Hain with climate-neutral heating and cooling.

A lake as a heating system

The system is based on aquathermy, the use of natural heat stored in water. Lake Hainer’s relatively stable temperature throughout the year makes it an effective energy source. Heat is captured through exchangers beneath the marina, transferred to a central heat pump system, and distributed through a local network to supply the village. In summer, the process can be reversed to provide cooling with far less energy than conventional systems.

A distinctive feature at Hafendorf Hain is the use of six Oloid stirrers, installed alongside the heat exchangers under the jetty. These slowly rotating bodies keep the water moving, which improves heat transfer while also enriching the lake with oxygen. Their energy demand is minimal, comparable to that of a household kettle.The system was planned and implemented by Quartiersenergie GmbH, a joint venture between Leipziger Stadtwerke and Tilia. In August 2024, the project entered trial operation, supplying the holiday village with renewable heat for the first time.

A Model for the Future

Hafendorf Hain is one of only five pilot projects supported by Germany’s Unternehmen Revier program, which funds innovative solutions in former coal-mining regions. By turning a flooded mine into a reliable source of renewable energy, the project demonstrates how structural change can take shape at the local level. For visitors, the technology remains in the background,  but for energy planners and municipalities, it offers a model for how lakes created by mining can become part of a sustainable energy future. Read more about this project here.