A national laboratory of the U.S. Department of Energy Office of Energy Efficiency & Renewable Energy National Renewable Energy Laboratory Innovation for Our Energy Future Solar Resource Assessment Technical Report NREL/TP-581-42301 D. Renné, R. George, S. Wilcox, T. Stoffel, February 2008 D. Myers, and D. Heimiller NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 Solar Resource Assessment Technical Report NREL/TP-581-42301 D. Renné, R. George, S. Wilcox, T. Stoffel, February 2008 D. Myers, and D. Heimiller Prepared under Task No. PVB7.6401 National Renewable Energy Laboratory 1617 Cole Boulevard, Golden, Colorado 80401-3393 303-275-3000 • www.nrel.gov Operated for the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy by Midwest Research Institute • Battelle Contract No. DE-AC36-99-GO10337 NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Available electronically at http://www.osti.gov/bridge Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone: 865.576.8401 fax: 865.576.5728 email: mailto:[email protected] Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone: 800.553.6847 fax: 703.605.6900 email: [email protected] online ordering: http://www.ntis.gov/ordering.htm Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste Preface Now is the time to plan for the integration of significant quantities of distributed renewable energy into the electricity grid. Concerns about climate change, the adoption of state-level renewable portfolio standards and incentives, and accelerated cost reductions are driving steep growth in U.S. renewable energy technologies. The number of distributed solar photovoltaic (PV) installations, in particular, is growing rapidly. As distributed PV and other renewable energy technologies mature, they can provide a significant share of our nation’s electricity demand. However, as their market share grows, concerns about potential impacts on the stability and operation of the electricity grid may create barriers to their future expansion. To facilitate more extensive adoption of renewable distributed electric generation, the U.S. Department of Energy launched the Renewable Systems Interconnection (RSI) study during the spring of 2007. This study addresses the technical and analytical challenges that must be addressed to enable high penetration levels of distributed renewable energy technologies. Because integration-related issues at the distribution system are likely to emerge first for PV technology, the RSI study focuses on this area. A key goal of the RSI study is to identify the research and development needed to build the foundation for a high-penetration renewable energy future while enhancing the operation of the electricity grid. The RSI study consists of 15 reports that address a variety of issues related to distributed systems technology development; advanced distribution systems integration; system-level tests and demonstrations; technical and market analysis; resource assessment; and codes, standards, and regulatory implementation. The RSI reports are: • Renewable Systems Interconnection: Executive Summary • Distributed Photovoltaic Systems Design and Technology Requirements • Advanced Grid Planning and Operation • Utility Models, Analysis, and Simulation Tools • Cyber Security Analysis • Power System Planning: Emerging Practices Suitable for Evaluating the Impact of High-Penetration Photovoltaics • Distribution System Voltage Performance Analysis for High-Penetration Photovoltaics • Enhanced Reliability of Photovoltaic Systems with Energy Storage and Controls • Transmission System Performance Analysis for High-Penetration Photovoltaics • Solar Resource Assessment • Test and Demonstration Program Definition • Photovoltaics Value Analysis • Photovoltaics Business Models iii • Production Cost Modeling for High Levels of Photovoltaic Penetration • Rooftop Photovoltaics Market Penetration Scenarios. Addressing grid-integration issues is a necessary prerequisite for the long-term viability of the distributed renewable energy industry, in general, and the distributed PV industry, in particular. The RSI study is one step on this path. The Department of Energy is also working with stakeholders to develop a research and development plan aimed at making this vision a reality. iv Executive Summary Study Area #4 focuses on the current availability of solar radiation resource data to support distributed grid-tied PV analyses, and the types of data and products needed to support future analysis and operations of significant grid-tied PV penetration. The study also is the impetus for a 5-year research and development plan to assure delivery of these products to end users. Much of the information derived in this study was obtained through interaction with other study-area authors, and with analysts, utility representatives, and solar resource data providers. Current Knowledge There currently are only a few dozen quality solar measurement stations operating in the United States, and only a subset of these stations has been operating for a period long enough to provide accurate information on the interannual variability and long-term trend of the solar resource. Thus, accurately depicting the spatial extent and time-dependent characteristics of the solar resource in the United States required alternative methods. To accomplish this the National Solar Radiation Data Base (NSRDB) was developed. This tool made use of the meteorological and cloud cover observations at National Weather Service Stations around the country as inputs to models to simulate the solar resource at those sites. The NSRDB originally was published in the early 1990s, and provides estimates of the solar resource at 239 stations for the period 1961–1990. Although all available high-quality solar measurement data were included in the NSRDB, nearly all (> 92%) of the station data were modeled. The NSRDB recently was updated (2006–2007) to cover the period from 1991 to 2005. The update includes a number of enhancements: inclusion of a far greater number of ground stations (a total of 1454 stations that contain hourly solar and meteorological data), and a 10-km gridded satellite-derived database, developed by the Atmospheric Sciences Research Center at the State University of New York–Albany (SUNY), covering the period from 1998 to 2005. This enhancement enables hourly solar data for any grid cell location to be combined with hourly meteorological data from one of the nearby ground stations for use in PV simulations. Results of RSI Solar Resource Assessment Analyses Products from the updated NSRDB have been provided to other members of the RSI Study Team to support their analytical work. For example, NREL analysts have received outputs from the photovoltaic performance model PVWatts, derived from NSRDB data for the period 2001–2005 for a number of sites; and Navigant received data for specific collector types and configurations for the year 2003. We have provided GE with 15- minute measured data from NREL’s Solar Radiation Research Laboratory (SRRL), as well as PVWatts calculations, so that GE can compare these outputs with its model. We also have developed a tool that converts NSRDB data to a format identical with TMY2 data, which very commonly is used in PV performance and buildings load analyses. Because 2003 was chosen as a key study year, we conducted a number of studies to relate the representativeness of that year to longer-term 1998–2005 records available from SUNY, as well as to the original 1961–1990 NSRDB. Some portions of the country, v particularly east of the Rocky Mountains, showed resources during 1998–2005 to be lower than during 1961–1990 (in a few cases, nearly 10% lower), while a few locations in the west showed resources during 1998–2005 to be higher than in 1961–1990 (in some cases, nearly 10% higher). Analyses also were performed for each of the four seasons. More portions of the United States showed greater seasonal differences between 1998 and 2005, and 2003, than in the annual comparisons, especially in the spring. Furthermore, much of the country where the differences were noticeable showed that, in general, 2003 had lower resources, again particularly during the spring. Future Needs and a Multiyear Plan A number of critical solar resource data and information gaps have been identified and need to be filled to support expanded grid-tied