Unpacking the Power Law: Decomposing solar irradiance for a better understanding of geographic smoothing limitations

Temporal fluctuations of solar irradiance are important for renewable-energy harvesting, as well as for climate and ecological-systems analyses. Environmental studies usually focus on slow fluctuations of solar irradiance, assuming that fast fluctuations are averaged out, while studies of solar power generation emphasize the role of faster fluctuations, due to their effect on grid stability. Several analyses have reported a power-law dependence of the generated-power spectrum for frequencies ranging from 1/min to 1/year. However, both the origin of this power law and its geographic dependence are not fully understood, an issue of significance when one considers smoothing of solar-photovoltaic-power fluctuations by combining geographically distributed generation sources.

In this talk, we discuss the origin of the power law and its latitudinal dependence.

For most locations, the spectral power-law dependence of the intermediate (1/day$<f<$1/h) frequencies is different from that of the high ($f>$1/h) frequencies.

Following the insights gained from the clear sky irradiance analyses, we propose a principled approach to decompose the solar-irradiance time series into the deterministic location-dependent clear sky and the stochastic environmental attenuation signals. Our analysis for two different locations shows that the stochastic environmental factors determine the spectral power-law slope in the high-frequency range, while the deterministic clear-sky signal plays an important role in the intermediate frequency range. If time permits, we will also discuss the role of spatial and temporal correlations in wind power generation and their implications for geographic smoothing of wind generated power.