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2.1 Solar Measurements A significant part of the test case effort was to find, acquire, and quality assess surface solar measurements to form a data set for model evaluation. Thirty-three sites with nearby meteorological stations were identified and data acquired from various solar measurement networks, including SURFRAD, ISIS, University of Oregon, and the NREL HBCU network. Data were downloaded to NREL computers, imported to an interim database, and then evaluated with several quality assessment tools. Five of the measurement sites used for evaluation produce data collected by rotating shadowband radiometers (RSR), a low-cost, single sensor device that outputs global, direct, and diffuse irradiance using an automated shade-unshade device. While this method produces fairly accurate values of global and direct beam irradiance (roughly within 5% on a daily total) the diffuse values are systematically low by as much as 40% on clear days because of the spectral characteristics of a photodiode-based sensor. However, these shortcomings in RSR measurements can be partially overcome through characterization and post-processing (3). Using RSR data from the University of Oregon Solar Monitoring Laboratory, an analysis of the measurement error produced a method that corrects the sensor’s global and diffuse measurements independently. However, additional work is required to validate the method on other sites. 2.2 ASHRAE/NRCC Model Recently the NRCC, through funding from American Society of Heating, Refrigerating and Air-Conditioning Engineers, produced a solar radiation model aimed primarily at supporting applications for building energy design (4). That model computes global and direct horizontal irradiance from the product of extraterrestrial direct beam, modified by the cosine of the zenith angle, and transmittance functions for typical atmospheric attenuation, Rayleigh scattering, gas and water vapor absorption, aerosol absorption, and cloud absorption. The NRCC ran this model using 1999 and 2000 meteorological input data provided by NREL from the test data set, then sent the output data back to NREL for evaluation. 2.3 SUNYA Satellite Model One goal of the updated NSRDB is a spatial resolution greater than the ancillary interpolated products that were originally produced from the 239 NSRDB meteorological stations. Toward that end, the project evaluated a model from the Atmospheric Sciences Research Center (ASRC) at SUNYA (5). This SUNYA model derives 10-km pixel solar estimates based on differences between a pixel’s clear-sky reflectance as seen by the satellite and the brighter values that occur with increasing cloud reflectance of incoming solar radiation. During this year’s work, the model was modified to accept aerosol optical depth (AOD)-based clear sky limits rather than Linke-based model limits. The AOD-based models currently used are a simplified version of the SOLIS model (6) for clear-sky global horizontal irradiance (GHI) and the Bird model for clear-sky direct normal irradiance (DNI). Evaluation of this model used special model runs for 1999 and 2000 for the 33 sites, in addition to full resolution grids for the 48 lower states, Hawaii and the portion of Alaska below 60o latitude available in our archive. Figure 1 shows a map of annual direct normal averages from this data set. Fig. 1. Annual direct normal irradiance on 10 km grid, derived from the SUNYA satellite model. The ASRC has satellite imagery archived since mid 1998, leaving a large data gap for most of the 1990s. Access to older satellite imagery is possible, but not access to older snow cover data (a model input parameter), implying less accurate winter model performance in northern states. Another approach that may be considered for prior years would be to relate monthly-averaged post-1998 model runs to time-coincident NASA Solar Meteorology and Solar Energy (SSE) gridded data. The SSE data from prior years could be used to generate high-resolution irradiances (assuming that the micro-structure within the SSE grid cells remains constant). Two problems exist for satellite modeling of solar radiation for Alaska above 60o latitude. The SUNYA archive does not include imagery above 60o (although GOES effectively extends to about 70o in Alaska), and the angular view of the GOES satellite above 60o may adversely affect the performance of the model. A possible solution could be to increase the northern border to 70o in future archival data and pattern microstructures to the SSE grid as mentioned above. 2PDF Image | Progress on an Updated National Solar Radiation Data Base for the United States
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