Estimating Atmospheric Attenuation in Central Receiver Systems

Atmospheric attenuation loss between a heliostat field and receiver has been recognized as a significant source of loss in Central Receiver System. Methods that can improve estimation of attenuation loss using available measurements will be useful in reducing uncertainty in estimation of CSP plant production, particularly in locations and climates that differ in atmospheric composition from typical arid desert locations. In clear sky situations, Direct Normal Irradiance (DNI) is primarily impacted by aerosols in the atmosphere. Aerosols extinct direct radiation with the photons either being absorbed or scattered based on the aerosols optical characteristics. As aerosol loading is high close to the surface, the attenuation loss between heliostat and receivers is significantly influenced by amount of aerosols present on a particular day. The purpose of the study is to understand the impact of aerosols on attenuation loss and model this loss as a function of the ratio of measured DNI to a calculated DNI for an "aerosol-free" atmosphere. The assumption here is that the reduction in clear sky DNI due to aerosols when compared to a theoretical "clean environment" value can provide valuable information about aerosol loading at the surface and therefore attenuation loss between heliostat and receiver. Preliminary analysis shows that such an approach is viable. Historically, human observers have measured visibility on a daily basis. While these observations are subject to varying levels of uncertainty they may be a good indicator of atmospheric attenuation between heliostat and receiver. In this paper we will review historical and recent publications to show how visibility observations contain useful information for estimating attenuation loss in central receiver systems. We will also present a simple relationship that uses visibility observations to estimate heliostat to receiver attenuation for varying separation distances.