Urban Photosynthesis; three functions on a single roof

Urban Photosynthesis involves research in the urban environment on an integrated solution, which closes as many water loops as possible at the building level, combined with the generation of solar energy on green roofs with water storage (blue-green roofs). In this way, we can simultaneously contribute to three functions in dense urban areas: greening, water reuse and energy generation.

With a growing worldwide population density, and more intensive rainfall and periods of extreme drought and heat resulting from climate change, cities are confronted with both flooding and water shortages. Solutions are often sought in water storage and vegetation on building rooftops and for instance on underground parking roofs. There is, however, little or no room left for blue-green solutions, because PV panels are being installed on roofs to meet the current sustainability requirements. Moreover, these PV panels can perform poorly when they are placed on conventional dark roofs. Urban Photosynthesis wants to offer a single solution which addresses all these problems simultaneously.


In densely populated cities there is little room and water available for optimal cooling, green spaces and the generation of (solar) energy. The use of drinking water to make up for shortages of irrigation water for vegetation is not desirable from either a cost or a sustainability perspectiveas. The objective of this research project is to come up with a solution so that the development of multi-functional roofs in cities no longer requires choosing between either (blue)green or PV panels, because there is a reliable way of having both. A system that can convert available grey wastewater naturally into irrigation water, which is usable for cooling to supplement the available rainwater. The hypothesis is that this will make it possible for the roof’s cooling function to remain at its maximum level during lengthy drought periods, without the need for drinking water supplementation. Another hypothesis is that a constant evaporation will increase the efficiency of PV panels, so that the sustainability requirements can be satisfied earlier. This means that more space will become available for other roof functions, such as greenary and/or water storage, and possibly terrace space. The final hypothesis to be tested relates to whether there is enough greywater available, and whether it can be adequately treated in a thin substrate layer on the roof with a specific type of vegetation.


The project involves several challenges. The first has to do with the availability of water. Climate conditions in the Netherlands are such that in the summers there is not enough rainwater available for weeks at a time. In countries located further south, the situation is much more extreme. This is why an alternative, continuous source of water is needed. Our intention is to meet this need with shower water, which would be filtered via a biologically-compact treatment method, integrated in a blue-green roof system. This system first needs to be developed and tested. The treatment itself is therefore a component of the blue-green roof system, with plants that release the water through evaporation. The filtered water then flows to the drainage and water retention layer under the vegetation on the rest of the roof, where it provides for cooling and evaporation. An important focus of attention is the system’s efficiency in the removal of any organic micropollutants from the grey feed water.

The second challenge is to integrate the PV panels into the roof system in such a way that the cooling action of the vegetation is optimised, while the plants are not set too much in the shade. To achieve this, the temperature and production of the panels on a blue-green roof need to be compared to those on a conventional roof.

With the outcomes of the research it will also be important to work towards a different policy with regard to sustainability requirements. At the moment, the effect of a (blue) green roof is not yet taken into account in the sustainability assessment of a building project. This assessment is adversely affected whenever fewer PV panels are installed with a view to providing space for water storage and green. Working with our partners, we therefore want to investigate the extent to which the project results can have a desirable influence on this policy.

Schematic representation of Urban Photosynthesis, with the reuse of shower water, its treatment on the roof and use for the vegetation, as well as PV panels op top of the vegetation.


With Urban Photosynthesis we aim to make a contribution to a sustainable and climate-robust urban living environment. In the roof system before us, we see a greywater treatment integrated into blue-green roofs, so that sufficient irrigation water is availabe for the multi-functional roof with water storage, PV panels and green vegetation, the balcony planters and the blue-green roof of the parking garage. Ultimately, we aspire to develop an optimally combined roof system for PV panels, together with vegetation and water storage.

Concretely, we are working on the following outcomes:

  • Development of a combination-design for the placement and securing of a PV system, together with a blue-green roof system with greywater supplementation.
  • Development and testing of a nature-based greywater system for the generation of sufficient irrigation water.
  • Insight into the provision of information for future residents, to ensure the safe operation of the greywater system.
  • Quantification of the efficiency of the energy generation, the impact on cooling and the quality of the generated irrigation water.
  • Development of knowledge with a view to defining a policy framework for the integration of the sustainability aspects of the blue-green roof (evaporation in particular), and of increased PV output, into the sustainability requirements for buildings.
Artist impression van het Mannoury project in Amsterdam. Deze appartementencomplexen zijn de testlocatie van het Urban Photosynthesis-project (bron: Aedes)

Artist impression of the Mannoury project in Amsterdam. These appartment complexes are the test site for the Urban Photosynthesis project (source: Aedes).