Proposal for a solar and aerosol optical depth network for the United States

The solar energy received at the earth's surface in the U.S. has been measured intermittently, most often with second-class instrumentation, and, generally, with no separate measurements of the direct and diffuse components. The best effort to produce a solar atlas for the U.S., using the small amount of reliable data that exist, has been the National Renewable Energy Laboratory's National Solar Radiation Data Bases (NSRDB) 1 and 2, which cover the periods 1960-1990 and 1991-2005, respectively. Over 97% of the data used for the atlases were modeled based on a few network measurements, many of which have since been discontinued. Estimated root-mean-square errors are ~ 100 W/m2 and ~ 200 W/m2 for global horizontal and direct beam irradiance, respectively. The GOES satellites present an alternate means of estimating downwelling irradiance at the surface; 30-minute samples are available. The root-mean-square-error for instantaneous 30-minute samples is ~ 84 W/m2 for global horizontal and ~ 200 W/m2 for direct beam irradiance. Clearly, there is a need to dramatically improve the uncertainty in the solar resource. Further, the increasing use of photovoltaic and concentrating solar power systems to generate a greater fraction of our nation's electricity is demanding solar resource data with higher time and space resolution than is available. A national database of solar irradiance and aerosol optical depth from research-quality ground stations is needed to improve solar radiation resource assessments. These observations would serve to better anchor both satellite- and model-based resource assessments. This paper outlines a plan that would equip the established Climate Reference Network (CRN) sites in the U.S. with a direct and diffuse solar measurement capability at relatively low cost; further, the instrument proposed would measure aerosol optical depth and water vapor, which are the two principal sources of extinction in direct beam radiation when skies are clear. The first deployments would take place in the high solar regions of the southwestern U.S. The proposed instrumentation is described and the sites delineated.