How do atmospheric aerosols affect direct normal irradiance (DNI) compared to global horizontal irradiance (GHI), and how does this difference impact the performance of concentrating solar power (CSP) systems?

Atmospheric aerosols are tiny particles suspended in the air, such as dust, smoke, and pollutants. They affect solar radiation by scattering and absorbing sunlight. Direct normal irradiance (DNI) is the amount of solar radiation received directly from the sun on a surface perpendicular to the sun's rays. Global horizontal irradiance (GHI) is the total amount of solar radiation received on a horizontal surface, including both direct and diffuse radiation. Aerosols have a much stronger impact on DNI than on GHI. They scatter and absorb direct sunlight, reducing the amount of direct radiation reaching the surface. Some of this scattered radiation is redirected towards the surface as diffuse radiation, which contributes to GHI. While aerosols also absorb some radiation, the overall effect is a greater reduction in DNI compared to GHI. This difference significantly impacts the performance of concentrating solar power (CSP) systems. CSP systems rely on mirrors or lenses to focus sunlight onto a receiver, which heats a working fluid to generate electricity. These systems require high DNI levels to operate efficiently because they can only utilize direct sunlight. Aerosols reduce DNI, decreasing the amount of focused sunlight and therefore reducing the power output of CSP plants. In contrast, photovoltaic (PV) systems can utilize both direct and diffuse radiation, so the impact of aerosols on PV system performance is less severe than on CSP systems. Therefore, locations with high aerosol concentrations are less suitable for CSP plants, even if GHI levels are relatively high.