An Energy Balance Model of Green Roof Integrated Photovoltaics: a Detailed Energy Balance including Microclimatic Effects

Photovoltaic (PV) panels get hot in the sun. This decreases the operating efficiency through various means. Several approaches have been proven effective at cooling panels. Green Roofs are thermally cool, outperforming even white roofs over their life. On a hot summer day, a green roof can be 30◦C cooler than a typical roof, potentially improving the efficiency of a photovoltaic (PV) panel by as much as sixteen percent. Other methods that are typically used to cool PVs include wicking, spraying, and combined PV/Thermal systems to remove or capture and utilize excess heat from the PVs. These systems typically require complex mechanical systems. Through the employment of a green roof to create a cool microclimate for the PVs, a quantifiable benefit can be obtained by passive means. This system is termed Green Roof Integrated Photovoltaics (GRIPV): coupled systems via thermal and diffuse-reflective relationship between the roof and the PVs. The amount of cooling from a green roof is highly dependent on climate. Depending on the amount of rainfall, relative humidity, and typical ambient temperature of a region, the evapotranspiration from the leaves and the variety of plants that will survive on a green roof will vary significantly. To quantify the thermal benefit of the plants, based on both the mass transfer from evapotranspiration of water from the leaves as well as the convective effects of the roof, computational fluid dynamics models are developed in conjunction with transient system simulations to determine the latent and sensible heat removal based on climate, geographic location, type of PV material, and geometric configuration. System integrative photovoltaic (SIPV) design contains integration at the systems level by considering thermal effects of each configuration. Because systems can be pessimized if the design is not approached from this perspective, it is crucial to be able to quantify both the active and passive thermal benefits of various systems. A PV system that is integrated with a green roof is a SIPV design because the PVs benefit from the thermal properties of the green roof while the green roof simultaneously benefits by reduced scorching from the partial shading of the array. The model that has been developed in this study enables comprehensive design and economic decision making for green roof integrated photovoltaics. Figure 10 shows the total energy balance of a GRIPV system. In essence, this is a green roof model coupled with a thermally integrated BIPV model. The resulting relationship between the PVs and the microclimate created by the green roof increases the roof’s value proposition and is potentially beneficial to the performance of the PVs and could be economically viable in certain geographic regions based on an increase in PV performance during the summer.