Can we store heat underground?

The MAHLE GmbH production site in Vaihingen/Enz was previously heated primarily using natural gas. In order to achieve the company's climate targets and reduce its dependence on fossil fuels, MAHLE was looking for a solution that would switch both the site's electricity and heat supply to renewable sources – while also meeting the particular challenges of an ongoing production operation.

In spring 2025, 720 PVT modules were installed on the roof of the production hall, generating electricity and heat simultaneously. The modules are supplied by Sunmaxx PVT GmbH, in which MAHLE is an investor and development partner. The rooftop system is complemented by a further 280 PVT modules installed above an adjacent geothermal field with 60 ground heat probes.

The geothermal field serves as a seasonal heat store: excess heat generated by the PVT modules in summer is fed into the ground and made available in winter. A 500 kW brine-to-water heat pump using propane as a natural refrigerant raises the stored temperature to a level suitable for use in the production halls.

The system supplies the buildings and production operations with electricity from renewable sources on a net annual basis (around 5,000 MWh/year). In milder winters, the heat supply can be fully covered by geothermal energy and the heat pump (around 1,100 MWh/year), without the use of natural gas. An overview of the annual benefits:

• Reduction in electricity demand: 428 MWh/year
• Reduction in heat demand (natural gas): 1,086 MWh/year
• Additional reduction in heat consumption: 600 MWh/year
• Natural gas savings: more than 90%
• Total CO₂e savings: around 523 tonnes/year

As Germany’s first large-scale PVT system for decarbonizing an industrial building, the project demonstrates that a carbon-neutral energy supply is economically viable. The hybrid energy generation enabled by PVT modules can be applied to industrial and commercial operations, hotels, outdoor and indoor swimming pools, as well as residential and multi-family buildings. It therefore represents a reference solution for residential construction, SMEs, and large-scale industry alike.