Combining green roofs and renewable energy – the photovoltaic rooftop garden

Abstract

Due to climate change, the urban heat island effect and associated change in the microclimate becomes more and more of an issue in our cities. As the density of the built environment increases, green spaces are often reduced or altogether neglected, even though it is well known, that these green spaces are an effective way to improve the microclimate in urban areas. Adding green facades and rooftops thus represent viable ways to actively integrate plants into the building skin. At the same time the share of renewable energy systems, such as photovoltaics, must be increased, with buildings offering ample space for these applications. Thus often the green building skin competes for space with building integrated photovoltaics (BIPV). The paper describes the results of the research project “PV-Rooftop Garden – Innovative Systems for the Future” which focused on how photovoltaics (PV) and green spaces could be combined in a single rooftop sharing the same floor area. The objective was to combine roof greening, energy generation by photovoltaic panels, development of a recreational area for the building occupants and retention of storm water in one single system. Green roof technology and semi-transparent photovoltaic panels were combined in pergola systems to generate power and at the same time improve the microclimate and provide shading to the users of the roof. Different prototype systems of the PV-pergola were developed and studied and their structural dimensions calculated. For designing the green rooftop garden a comprehensive analyses regarding the user’s needs as well as limitations due to legal barriers was carried out. The shading of the PV modules improves the perceived temperature by about 3-5°C during summer and winter. This is not only perceptible for humans but it also has an impact on the plants. In the research projects various plants and their behavior under the PV-shaded pergola have been analyzed in order to select those species, which are most suitable for this specific application. The system also offers the opportunity to store rainwater from the PV modules and no electric power is needed to pump the water up to the roof. With special plant troughs, it is even possible to water the plants automatically. Based on the assessment of various scenarios, a solution for the PV rooftop garden has been developed which avoids penetration of the roof and sealing layer and subsequent inhibits sound transfer from the steel construction to the building below. Because of the absence of a direct connection to the building structure, it can also be used on existing buildings. The PV rooftop garden provides a comfortable and enjoyable atmosphere, the PV-pergola with semi-transparent glass-glass modules offers shading, provides a garden and produces green electricity. It thus offers a potential solution for renewable power generation and green roof in one single floor area which can be applied both for existing and new buildings.