A retractable photovoltaic roof is being built above the aeration tanks at the main wastewater treatment plant in Stuttgart-Mühlhausen, Germany. With 5,280 modules and an expected capacity of 2.745 MW, the installation is set to become the country’s largest retractable solar roof when completed in the second quarter of 2027.
The modules will cover approximately 17,493 square meters. Construction began in June, with the project requiring the supporting structures to be integrated into existing infrastructure five meters above the tanks while the plant continues operating.
According to local utility Stadtwerke Stuttgart, the system will consist of 120 roof sections suspended from steel cables. Each section will have its own motor, allowing it to be extended or retracted. The modules can be moved aside during snowfall, strong winds, hailstorms and at night. The system is expected to be connected to the grid this winter.
Swiss solar technology company dhp Technology is implementing the project with its Helioflex retractable solar roof technology. The supporting structure uses approximately 250 metric tons of steel. Preparatory work started in 2025 with modifications to the aeration tanks, followed by installation of the steel structure and retractable sections in mid-2026.
The retractable design will also preserve access to the wastewater treatment infrastructure during crane operations and maintenance. The elevated modules are expected to provide shade for employees working around the aeration tanks in summer. Each section will hold 44 modules, with an average rated output of about 520 watts per module.
Once operational, the installation is expected to generate more than 2.2 GWh of electricity per year. All of that power will be used on site, covering around 9% of the plant’s total electricity demand and helping reduce consumption from energy-intensive equipment, including compressors used in biological wastewater treatment.
Sources: pv magazine.