On the definition of the direct normal irradiance

The direct irradiance received on a plane normal to the sun rays is of particular relevance to concentrated solar systems, including concentrating photovoltaic (CPV). This irradiance is also called direct beam (ASTM G173) or direct normal solar irradiance (ISO9488), or normal incident beam. Stricto sensu the direct normal irradiance (DNI) is the radiant flux collected by a surface normal to the direction of the Sun, within the extent of the solar disk only, i.e., a half-angle  = 0.266°. However, the DNI definition following WMO CIMO and ISO9059, ISO9060, ISO9488, ISO9846, is relating DNI to the direct irradiance on a normal plane measured with an opening angle of say, 5° to 6°. For technologies with high concentration factors, the acceptance angle narrows significantly and the need for methods calculating DNI for smaller acceptance angles is recognized. There is a disagreement between the various definitions of DNI, from what is used within a numerical model simulating the radiative transfer in the atmosphere to the DNI related to a specific technology that depends upon acceptance angle. The circumsolar irradiance adds itself to the stricto sensu DNI. It is far from being negligible in standard measurements of DNI, especially when cirrus are present. These issues as well as more specific ones have been identified in a brainstorming session held within the International Energy Agency (Task SHC 46). They are currently discussed and the present state of this on-going work will be presented. - Is there a "good" and unique definition of DNI as a reference? - Can we establish a generic definition of DNI as a function of the acceptance angle? - Are models to assess the circumsolar and compensate it in DNI available and accurate? - Do all satellite-derived assessments of DNI refer to the same quantity? - The need of best practices for DNI processing from other solar components by transpositions models where the projection into the normal direction needs standardization on the sampling of solar zenithal angle and the quantification of the oversampling effect.