View Factors of Flat Collectors, Including Photovoltaics, Visible to Partial Sky

Abstract

The sky view factor of collectors in photovoltaic (PV) fields is a parameter that determines the amount of incident diffuse radiation. The diffuse radiation may contribute significantly to the generated electric energy. PV systems are deployed in multiple rows and separated from each other and from nearby structures. Rows in front and nearby structures may block part of the visible sky to the collectors, thus decreasing the sky view factor. The distance between collectors and objects is an important parameter for the design of photovoltaic systems. The present study develops the expression for sky view factors for cases encountered in deployments of PV fields and presents numerical values for the sky view factors for distances between collectors and from obscuring structures. Sky view factors for flat collectors in the presence of adjacent collectors or structures have not been treated in the literature, besides for simple or common cases. Based on the values of the sky view factor, the PV system designer may assess the diffuse radiation losses, which are a dominant component of the global radiation losses. For example, for a collector installed at a distance 2.75 m from a building, the sky view factor is reduced from 0.97 to 0.85, i.e., a reduction of 12.4% in the incident diffuse radiation on the collector.