Deriving in-field light transmittance in a tower CSP plant

A considerable fraction of the solar energy that “fuels” a tower CSP plant is lost between the heliostats and the receiver due to scattering and absorption by atmospheric gases and aerosols. Measuring this effect in the field is challenging, but quantifying it is important in the planning phase of the plant as well as during operations and performance assessment. BrightSource have developed the DAST system to derive the transmittance of the solar energy in real time and operates it at the 121 MW Ashalim tower CSP plant in Israel. The system consists of an integrating nephelometer and an aerosol monitor and it accounts for additional physical factors to convert the raw measurements to transmittance data. The DAST system is portable and modular, it can run online or offline, it operates in all weather conditions – day or night and it requires very little maintenance. The uncertainty in the derived transmittance values is typically below ∼2%, and they are in-line with the measurements of a long path transmissometer.