Germany's Largest Solar Folding Roof Is Being Built At Stuttgart's Main Wastewater Treatment Plant

Germany’s largest folding solar roof is being built in Stuttgart-Mühlhausen. Starting in 2027, it will generate over 2.2 million kilowatt-hours of solar power annually.

Germany’s largest solar retractable roof is currently being built at the Stuttgart-Mühlhausen main wastewater treatment plant. It will also be the largest photovoltaic system in Stuttgart: The retractable roof is as large as two soccer fields—it spans the entire length of the aeration tanks in the biological wastewater treatment stage.

What makes the system at the treatment plant unique is its mobility. The module groups are suspended from steel cables and can be retracted using 120 motors. In the event of snow, strong winds, or hail, the modules are moved out of the danger zone; this is also possible at night.

The structure is located about five meters above the aeration tanks. This ensures that the tanks and technical equipment remain accessible for ongoing treatment plant operations and maintenance work. At the same time, the existing space is being put to dual use: it continues to serve biological wastewater treatment and, in the future, will also generate solar power. “The solar folding roof is therefore not simply a large photovoltaic system, but a smart solution built on existing infrastructure,” explains Jürgen Mutz, head of the Civil Engineering Office.

About 9 percent of the facility’s electricity needs will be met by solar energy
Installation of the system at the Stuttgart-Mühlhausen main wastewater treatment plant has been underway since June 2026. Across an area of 17,493 square meters, 120 so-called folding-roof groups—comprising a total of 5,280 solar modules—are being installed. Starting in spring 2027, the movable solar folding roof is expected to generate more than 2.2 million kilowatt-hours of electricity annually with a capacity of 2,745 kWp. The innovative design thus increases the plant’s self-generated electricity production and reduces its carbon footprint.

The main wastewater treatment plant is one of the city’s largest energy consumers. With the new solar folding roof, Stuttgart’s municipal wastewater management expects an annual electricity yield of more than 2.2 million kilowatt-hours. This will cover approximately nine percent of the main wastewater treatment plant’s on-site electricity needs.

The electricity generated can be used directly on the treatment plant grounds. In particular, the compressors used in biological wastewater treatment require large amounts of electrical energy. The solar power is thus used directly where energy demand is high.

Technical Challenges During Ongoing Treatment Plant Operations
Integrating a photovoltaic system of this scale into the existing infrastructure is technically challenging. In 2025, demolition work was carried out and necessary adjustments were made to the existing tanks. Installation of the steel structure and the folding roof assemblies has been underway since mid-2026. Approximately 250 metric tons of steel were used in the construction. Lars Färber, Chief Operating Officer of dhp Technology, the company responsible for the technical implementation of the system, emphasizes: “The challenge lies in integrating a photovoltaic system of this size into the existing wastewater treatment plant infrastructure. The project illustrates the potential inherent in large existing infrastructures when energy production, plant operations, and structural conditions are considered together.”

Boris Diehm, Head of the Wastewater Treatment Plants and Sewer Operations Department, emphasizes: “The solar folding roof is an example of how planning, construction, and operation can work together. Together with Stuttgart Municipal Sewerage and the Office for Environmental Protection, we are contributing our expertise to test new solutions for the energy transition. The project demonstrates how existing infrastructure can also be utilized for the generation of renewable energy. The lessons learned from this may also be of interest to other cities.”

The technical connection of the system is scheduled for the winter of 2026/27. The solar folding roof is expected to generate electricity for the first time in the second quarter of 2027.

Facts & Figures at a Glance

• System area: 17,493 m^²
• Installed capacity: 2,745 kWp
• Annual output: > 2,200,000 kWh
• Contribution to the main wastewater treatment plant’s electricity needs: approx. 9%
• Number of solar modules: 5,280
• Commissioning: 2nd quarter of 2027

For more information, visit https://www.stuttgart-stadtentwaesserung.de/solar-faltdach/