First analysis to detail unique characteristics of solar development that improve water quality
FEBRUARY 15, 2023, MINNEAPOLIS—Stormwater management practices, permitting, and regulations at solar sites often do not account for the unique characteristics of large-scale solar installations, a new U.S. Department of Energy (DOE) funded study has found.
Land change due to photovoltaic solar installations can differ considerably from residential and commercial development regulated by federal Clean Water Act stormwater standards. A new study by the Great Plains Institute (GPI), DOE’s National Renewable Energy Laboratory (NREL), University of Minnesota, and Fresh Energy found that regulation did not account for key drivers of stormwater runoff —including soil compaction, soil depth, ground cover establishment, and the distance between solar arrays.
The Best Practices: Photovoltaic Stormwater Management Research and Testing (PV-SMaRT) analysis, financed by a grant from DOE’s Solar Energy Technology Office and overseen by NREL, is the first scientific study to detail and document the unique characteristics of solar development that affect water quality. The analysis offers best practices for four key elements of solar development that have a large impact on managing stormwater and improving water quality outcomes:
- Managing soil compaction and bulk density, which modeling results indicate is the most important consideration in determining stormwater runoff. Looser soils allow much greater water infiltration and allow improved and more rapid vegetation growth, which can further help maintain lower bulk densities.
- Understanding the importance of soil depth on the site. Deep soil allows more water to infiltrate the ground, while shallow soil can lead to more runoff.
- Installing, establishing, and maintaining appropriate vegetated ground cover between and under the arrays to facilitate water infiltration in the ground. That includes utilizing or accounting for ground cover types, including bare dirt, turf grass, and native prairie.
- Designing the distance between arrays on the solar farm to increase infiltration. If there is more distance between arrays, there is more soil to infiltrate water. This could make a difference for sites with shallow soil depth that could be compensated for by widening distances between arrays.
“One of the permitting challenges is uncertainty and concern about how solar farms affect surface and groundwater,” said Brian Ross, GPI’s vice president of renewable energy. “But if we follow a portfolio of best practices in solar development, we can usually both improve water quality and help meet our nation’s climate goals.”
The PV-SMaRT research findings demonstrate that additional stormwater infrastructure may not be needed under some site conditions when using good site design considerations, even for 100-year frequency design storms. When best practices are not followed, significant additional stormwater management may be needed.
The research findings also demonstrate that runoff estimates are, on average, 38 percent lower using PV-SMaRT methods relative to the Natural Resources Conservation Service runoff curve number method for a 100-year storm. Overestimating runoff can have a cost impact on solar projects, which would need more land for stormwater basins or for additional best management practices.
“This research, best practices, and field calibrated stormwater runoff calculator will help create a more transparent and predictable permitting process for large-scale solar developers and ensure that solar designs incorporate scientific findings from real-world observation,” said James McCall, NREL analyst and PV-SMaRT principal investigator. “We are simultaneously removing barriers to accelerated solar deployment by reducing stormwater permitting uncertainty and creating host community benefits from better stormwater outcomes at solar sites.”
As part of the PV-SMaRT program, the University of Minnesota developed an easy-to-use stormwater runoff calculator, which can be freely downloaded and allows users to estimate runoff amounts for specific types of conditions. The calculator also comes with a user manual to guide users on correct and productive use of the calculator.
“This ground-breaking research provides the first scientific foundation for understanding the hydrology and water quality risks of solar development,” said Dr. David Mulla, University of Minnesota lead scientist. “We are seeing significant interest in the new calculator tool and the best practices by users that include academia, government, engineering firms, and the solar industry.”
Note: Though the PV-SMaRT project is complete, the Great Plains Institute continues to explore co-benefits of solar development through the DOE-funded PhotoVoltaics Supporting Cultural and Community EcoSystem Services (PV-SuCCESS) project. PV-SuCCESS will culminate in a decision-making tool that communities can use to quantify and prioritize co-benefits, especially those related to ecosystems.
About the Great Plains Institute
A nonpartisan, nonprofit organization, the Great Plains Institute (GPI) is transforming the energy system to benefit the economy and environment. Working across the US, we combine a unique consensus-building approach, expert knowledge, research and analysis, and local action to find and implement lasting solutions. Our work strengthens communities and provides greater economic opportunity through creation of higher-paying jobs, expansion of the nation’s industrial base, and greater domestic energy independence while eliminating carbon emissions. Learn more at www.betterenergy.org.
About the University of Minnesota
The University of Minnesota (UMN) is a world-class academic institution, and major contributions to this project were made by scientists in the Dept. Soil, Water & Climate and the Institute on Environment. These contributions include pioneering research that advances our understanding of the factors that affect stormwater runoff at ground-mounted solar photovoltaic (PV) facilities across the U.S. Through better science, our research will help accelerate the planning, design, evaluation and control of stormwater runoff for a wide range of mitigation practices at PV facilities.
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