Most natural ground surfaces have low values of albedo for bifacial PV projects compared to ice, snow, and water, or artificial surfaces in buildings or in industrial areas. For this reason, calculating the right albedo value of a ground surface can make a big difference for a solar project developer in deciding whether to use bifacial or monofacial panels in a ground-mounted array.
In order to make this choice possible, a researcher from the U.S. Department of Energy's National Renewable Energy Laboratory (NREL), Bill Marion, has created a series of datasets of measured ground albedo values using measurement network data in the U.S. and data contributed by the PV industry. A source of satellite-derived albedos is the National Solar Radiation Database (NSRDB). The NSRDB contains solar radiation and supplementary meteorological data with a spatial resolution of 4km. It provides hourly and half-hourly values of meteorological data and the three most common measurements of solar radiation: global horizontal, direct normal, and diffuse horizontal irradiance. “Albedo is the main factor for differences between monofacial and bifacial,” Marion told pv magazine. “The albedo errors we saw in the NSRDB equate to an energy production error of about 1.5%.”
The researcher compared the satellite-derived albedo from the NSRDB with the measured albedo data from the Surface Radiation budget (SURFRAD) network which is operated by the National Oceanic and Atmospheric Administration (NOAA) and which is generally used by U.S. researchers to support satellite retrieval validation; modeling; and climate, hydrology, and weather research. “The purpose of the comparison is to understand if the satellite-derived albedos from the NSRDB are equivalent to measuring the albedo at a location or to what extent they differ, and with consideration for the uncertainty of the measured albedos,” the academic explained. “The uncertainties of albedo measurements are influenced by the spatial homogeneity of the ground surface, the spectral and angular response of the pyranometers and their calibration, and the shading of the ground surface by the pyranometers and their support structure.”
The comparison between the albedo values from the NSRDB and those from the SURFRAD network locations showed a bias difference varying from minus-0.044 to 0.056. “These differences are greater than the albedo measurement uncertainty of [plus or minus] 0.02; consequently, the NSRDB albedos should be used with caution for estimating the performance of bifacial PV systems,” Marion explained.
For other sources of measured data, the NREL scientists also relied on the Ameriflux Network, which is a collection of long-term, eddy flux measurement stations located across the Western Hemisphere. “The AmeriFlux Network is intended to represent major climate and ecological biomes including tundra, grasslands, savanna, crops, and forests,” the scientists specified, noting that additional data for albedo measurements were also provided by Chinese-Canadian module manufacturer Canadian Solar, U.S. panel maker SunPower Corporation, the Technical University of Denmark, and 7X Energy, which is a U.S.-based large scale solar project developer.
Canadian Solar Inc. provided albedo data for two locations in China: a project located in Changshu, in Jiangsu province, and a project in the desert of Inner Mongolia. SunPower provided data for a PV system with a light-to-medium gray gravel ground surface located in California, and the Technical University of Denmark took albedo data for a grass surface in the Scandinavian country. Moreover, 7X Energy presented data for PV projects located in Ohio and Texas. “The data are not representative for all kinds of projects but the type of ground cover is described so users can judge if it is applicable to their situation,” Marion specified.
The measured albedo datasets and a user’s guide describing the data can be seen on a dedicated online platform of the NREL’s DuraMAT website. The measurement techniques adopted in the research are presented in the paper Measured and satellite-derived albedo data for estimating bifacial photovoltaic system performance, published in Solar Energy.
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