Window-integrated photobioreactors: a proof-of-concept for green building retrofitting

A central challenge in achieving sustainable urban transformation is reducing the environmental impact of existing buildings, many of which were not designed to meet today’s sustainability standards. While integrating green infrastructure into new construction is increasingly feasible, retrofitting older structures presents significant technical and spatial constraints. We address this challenge by developing a proof-of-concept for integrating modular flat-panel photobioreactors into existing urban buildings. Photobioreactors cultivate photoautotrophic microorganisms that fix atmospheric (Formula presented) through photosynthesis, producing oxygen and biomass. We constructed a window-integrated prototype ((Formula presented) cm) and installed it directly into a standard window frame at the Biology Building, RWTH Aachen University, Germany, without major structural modifications. Window integration offers distinct advantages over facade-mounted systems for retrofit applications which include elimination of structural reinforcements and reduced visual impact on building envelope, as existing structural openings can be used, and stable operating temperatures due to the integration with climate-controlled building interiors. Two sequential cultivation trials using Synechocystis sp. PCC 6803 (5-day window-installed and 20-day stand-alone tests) demonstrated biological activity while revealing critical technical barriers that must be addressed for continuous operation. This proof-of-concept successfully demonstrates the feasibility of window-integrated PBRs for building retrofit applications and establishes a research pathway for system optimization toward practical deployment.