Parametric study of building-integrated photovoltaic windows

Building integrated photovoltaic (BIPV), as a distributed energy resource, can cover a part of the building energy demands and even help achieve the idea of net-zero energy buildings. By connecting with energy storage and grid, the entire BIPV systems have a high demand flexibility potential and can improve building resilience against power outages. Roof BIPVs, though considered as the mainstream, have limited area in high-rise buildings compared with windows, where semi-transparent PVs can play a significant role of energy resources given the considerable vertical window areas in modern urban environment. Material scientists have developed various semi-transparent solar cells with a wide range of power conversion efficiencies (PCEs), and solar and visible transmittance. However, it is not clear about the optimal configurations of semi-transparent solar cells for different types of buildings and climates. To tackle this problem, we conducted a parametric study on PV windows in a reference commercial building considering variables including PCE, solar transmittance, solar absorptance, U factor, daylighting control, window orientations, and climate types. Our model considers the thermal effects of PV windows, i.e., a load or grid connected PV window turns partial solar absorption into electricity instead of heat. The first finding, which differs from roof PVs, is that the vertical solar radiation on east and west facing windows is comparable to that on the south facing windows because the special 90° title angle results in more uniform POA irradiance in different orientations. It means the combination of PV windows in different orientations provide more stable power generation for the building. Results show that the PCE of PV windows dominates the energy saving despite other variables. The balance between solar transmittance and absorptance is also important for energy saving. Slightly higher visible transmittance (0.1) benefits the building energy saving when daylighting control is applied.