Considering Global Dimming and Brightening in Solar Resource Assessments

by Jincy Varughese

The planning process for a solar power system requires a solar resource assessment which involves predicting the annual long-term average solar radiation for the lifetime of the solar system. These values are determined by using long-term data previously collected at other solar power systems. However, the period from the 1950s to 1980s saw a decrease in solar radiation, called global dimming, and since the mid 1980s, the opposite change-global brightening, has occurred. In “Rethinking Solar Resource Assessments in the Context of Global Dimming and Brightening”, Müller and his team from Germany analyze the effect of these solar radiation trends on solar resource assessments and explore the effectiveness of other solar radiation prediction methods.

Müller et al. (2014) utilize data from 30 German observation stations, collected over more than 40 years. Thermopile pyranometers measured global horizontal irradiance (GHI) and diffuse horizontal irradiance (DIF). From these values, DHI, direct horizontal irradiance was calculated and the effectiveness of utilizing a 10 year, 20 year, and 30 year reference period was analyzed. The researchers found that actual solar radiation during the dimming period was less than would have been predicted using any of the above reference periods and that during the global brightening period, actual solar radiation was greater than would have been predicted. Additionally, during the brightening period, DIF trends are negative while DHI trends are positive. The authors attribute this to a decrease in air pollution and aerosols, which contribute to global brightening but which reduces scattering and absorption that would contribute to diffuse radiation.

Muller et. al concluded that although the 10 year reference period underestimates future irradiance, it resulted in the most conservative estimate and thus is the best predictor during periods of global brightening or dimming.

Müller, Björn, Martin Wild, Anton Driesse, and Klaus Behrens. “Rethinking Solar Resource Assessments in the Context of Global Dimming and Brightening.” Solar Energy: 272-82. [GSSS: Rethinking solar resource assessments dimming]

TAGS: Björn Müller, Martin Wild, Anton Diresse, Klause Behrens, solar energy, solar resource assessment, global dimming, global brightening

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