Deriving solar direct normal irradiance using lidar-ceilometer

Direct Normal Irradiance (DNI) is a fundamental gauge in the study of solar irradiation and the main parameter for CSP and CPV applications. However, it is not often measured due to practical reasons, and is derived instead from measurements of global irradiance or satellite-based models. DNI received at ground level depends on the extinction of solar radiation along the line to the sun, and in cloudless conditions, day to day DNI variations are mainly due to variations in aerosol loading in the atmosphere. In this paper, the use of a lidar-ceilometer is investigated for the estimation of DNI by establishing a relation between the daily variations of DNI and backscatter atmospheric extinction around solar noon, under cloud-free conditions. The experiment was carried out in Doha, Qatar for a period of 12months, from December 2012 to November 2013. Atmospheric backscattter profiles were obtained with a Vaisala CL51 ceilometer, which is based on the elastic lidar technique, and DNI was measured at the same time and location with a Kipp and Zonen pyrheliometer. To obtain the correlation between both measurements, the hourly DNI clearness index, K_n, is related to the hourly backscatter measurements integrated over a column of 5km height. An exponential function was found to closely fit K_n to backscatter coefficient; using the results of this fit, DNI is extracted with a relative bias of 1.979% and relative RMSE of 10.869%, statistical values comparable to those obtained from the derivation of DNI from satellite observations. Although elastic lidar-ceilometers operate at one wavelength, this paper shows the potential use of aerosols information as measured by lidar-ceilometer for solar radiation modeling.