A retractable solar roof comprising 5,280 photovoltaic modules is under construction above the aeration tanks at the main wastewater treatment plant in Stuttgart-Mühlhausen, Germany. According to local utility Stadtwerke Stuttgart, the system will have an installed capacity of 2.745 MW upon completion in the second quarter of 2027, making it Germany’s largest retractable solar roof.

The modules will cover a total area of 17,493 m2. Installation work began in June. One of the main challenges is integrating the supporting structures and framework into the existing infrastructure at a height of five meters while the wastewater treatment plant remains fully operational.

Stadtwerke Stuttgart said the system will comprise 120 retractable roof sections suspended from steel cables. Each section will be equipped with an individual motor, allowing the modules to be extended or retracted as needed. The modules can be retracted during snowfall, strong winds or hailstorms, as well as at night. The system is expected to be connected to the grid this winter.

Swiss solar technology supplier dhp Technology is implementing the project using its Helioflex retractable solar roof technology. The supporting structure comprises approximately 250 metric tons of steel. Preparatory work began in 2025, including modifications to the existing aeration tanks, before installation of the steel structure and retractable roof sections started in mid-2026.

The retractable design also allows the solar modules to be moved aside during crane operations and maintenance work, ensuring continued access to the wastewater treatment infrastructure. In addition to generating electricity, the elevated installation will provide shade for employees working around the aeration tanks during summer. Each of the 120 retractable sections will accommodate 44 solar modules, with an average rated output of approximately 520 W per module.

Once operational, the installation is expected to generate more than 2.2 GWh of electricity annually, all of which will be consumed on site. This will cover approximately 9% of the wastewater treatment plant’s total electricity demand. The electricity will help offset consumption from energy-intensive equipment, particularly the compressors used in the biological wastewater treatment process.