Radiation Incident on Tilted Collectors

Abstract

For solar energy system design purposes, observations of solar radiation on a horizontal surface must be converted to values on a tilted energy collector. An empirical conversion relationship, introduced by Liu and Jordan (1960) and based on short-term data for a single station, has been widely adopted throughout the nation. The spatial variations of the coefficients of this relationship and their stability with record length are investigated using the SOLMET radiation data for four stations in the southeast United States centered on North Carolina. Minor variations in coefficients result from changes in record length, but the differences have little impact on the design or performance of a solar energy system. Similarly, minor variations occur between coastal and inland sites but are insufficient to alter system designs. Hence a single relationship is appropriate for the area investigated. These SOLMET results indicate a greater proportion of direct radiation for a given total horizontal radiation amount than do Collares-Pereira and Rabl (1979), who used short period records from a few widely scattered stations to refine the original national relationship. The difference may reflect variations in data quality or regional differences in atmospheric transmission characteristics.