Location: Livestock Nutrient Management ResearchTitle: Comparison of measured direct normal irradiance data to values calculated using the DISC model for Texas Panhandle Author
Submitted to: National Solar Conference
Publication Type: Proceedings
Publication Acceptance Date: 4/15/2011
Publication Date: 11/1/2011
Citation: Vick, B.D., Myers, D. 2011. Comparison of measured direct normal irradiance data to values calculated using the DISC model for Texas Panhandle. National Solar Conference. 40:103-110. Interpretive Summary: It is important that we convert from fossil energy to wind and solar energy. Wind energy is currently the cheapest form of renewable energy, but it is usually a poor match to utility electrical generation in the Great Plains of the United States. Concentrating solar power (CSP) plants are a better match to utility electrical loading, but so far none of these plants have been constructed in the Great Plains. Part of the reason no CSP plants have been constructed is lack of confidence in the solar resource data required (direct normal irradiance or DNI for short). There are several models for converting global solar radiation to DNI, and one of them is the DISC model. Both DNI data and global radiation data were collected at the USDA-Agricultural Research Laboratory at Bushland, Texas, for the years 2009 and 2010. Using the DISC model and measured global radiation data, DNI was calculated and compared to measured DNI data. The total annual solar energy estimated with the model was 5% greater than what was actually measured which was fairly close. However, on a monthly or hourly basis, the percentage difference could be as high as 25%. Analysis of individual months and days resulted in some recommendations for improving the DISC model. Some of the poor correlations between measured solar energy and calculated solar energy using the DISC model implied that another model (DIRINDEX) might be better. Therefore, additional work is planned to investigate the DIRINDEX model with the same data set to see if the correlation improves. This work may potentially lead to the increased use of solar energy for electrical generation, which will decrease the use of fossil fuels and the environmental pollution from fossil fuel power plants.
Technical Abstract: Direct normal irradiance (DNI) is required in the performance estimation of concentrating solar energy systems. The objective of this paper is to compare measured and modeled DNI data for a site in the Texas Panhandle (Bushland, Texas) to determine the accuracy of the model and where improvements might be required. The DNI model used was the Direct Insolation Simulation Code (DISC) which was developed at the National Renewable Energy Lab (NREL) in Boulder, Colorado. We have determined that a portion of the DISC standard deviation error is due to inaccurate modeling of the first and last hour of the day, but other inaccuracies of the DISC model are also discussed. The accuracy of the DISC model improves as the time scale increases (e.g. hour to month to year) due to errors cancelling, and for Bushland, Texas, the DISC model during 2009 predicted 5% more solar energy than the measured DNI.