Effects of radiative forcing of building integrated photovoltaic systems in different urban climates

Abstract

Recent years have witnessed a remarkable reduction in solar-panel costs, such that low-efficiency, low-cost photovoltaics (PV) currently prevail over more complex, high-efficiency technologies. Although solar-energy-generating installations provide a renewable energy source often considered emission-free, a number of externalities are frequently ignored that favor technologies with a reduced efficiency as long as they are available at lower cost. Whenever PV systems are installed, the absorption properties of the surface are changed and less sunlight from the Earth's surface is reflected into space. By including this radiative forcing in the form of the Earth's surface reflection coefficient albedo (α), we take these externalities into consideration in the overall equivalent global warming potential (GWP) of a PV system. Three different effects need to be considered when changing the absorption properties of the Earth's surface: (1) global albedo impact, (2) regional atmospheric heat islands, and (3) locally heated surfaces. The unintended radiative forcing adversely affects the net efficiency of building-integrated solar installations in warm urban climates, as more energy is required for cooling to ensure human comfort. The total GWP of four different PV technologies was examined for three different urban climates, temperate, moderate, and warm. To minimize the system energy payback time (EPBT) it is most sensible to install high-efficiency solar-energy systems outside cities and urban developments in locations with high annual irradiance. Only when taking radiative forcing into environmental and economic considerations is it expected that solar-technology development will correct its trajectory away from low-cost systems and toward high-efficiency installations with lower overall GWP.