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Spectral Modeling of Solar and Atmospheric Radiation for Solar Power Integration


Atmospheric longwave irradiance (LW, with wavenumbers ranging from 0 to 2,500 cm-1) and solar shortwave irradiance (SW, wavenumbers ranging from 2,500 to 40,000 cm-1) determine the radiative balance in the atmosphere. The balance between these radiative flux contributions is also essential in the design and operation of cooling systems, including evaporative cooling towers, passive dry fans, and optically selective materials, among many other natural and engineered surfaces exposed to solar and atmospheric radiation. Large scale solar farms interact with the local atmosphere through greenhouse gases emission offset and by replacing surface albedo with materials that respond to radiation very differently than soil or vegetation. Solar photovoltaic (PV) farms are highly absorbing (lower albedo) while concentrated solar power (CSP) farms are highly reflective (higher albedo) when compared to the ground where they are usually deployed. This work employs detailed and comprehensive spectral radiative models to calculate LW and SW through the atmosphere for different ground surfaces in order to quantify local interactions caused by the different boundary conditions for a model of the atmosphere.

First, existing data-driven empirical models for determination of the surface downwelling longwave irradiance (DLW) are reviewed and recalibrated, and a more accurate comprehensive empirical model is proposed.

The broadband empirical model then serves as a benchmark to validate a Line-by-Line (LBL) spectral radiative model that is able to capture details of the highly wavenumber-dependent nature of the irradiance fluxes. For the atmospheric longwave spectrum that is emitted and absorbed by gases, aerosols, clouds and the ground, a high-resolution two-flux model with a recursive scattering method is developed. For the shortwave (solar) part of the spectrum, which includes scattering from atmospheric constituents and the ground, comprehensive Monte-Carlo LBL simulations are used.

The LW spectral model is then used to quantify the contribution of each atmospheric constituent to DLW as well as the spectral and vertical distribution of LW irradiance. The SW spectral model is used to quantify the albedo replacement effects of PV and CSP farms on local SW irradiance field. A thermal balance accounting for both longwave and shortwave irradiance is then performed for both PV and CSP surfaces. In general, CSP farms reduce while PV farms increase ground temperatures with respective changes in relative humidity. The change in temperature is a function of solar zenith angle, column water vapor content, aerosol optical depth, cloud optical depth and cloud fraction. This work then quantifies the temperature anomaly due to albedo replacement as function of these parameters. A hybrid solar farm design to minimize both local thermal effects and operational variability is proposed.

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