Photovoltaic Stormwater Management Research and Testing

A suite of sensors sits on the ground at the base of a solar panel array.

PV-SMaRT field-testing equipment monitoring underground soil moisture at a New York PV facility. Photo from Scott McArt, Cornell University

A diagram lists the 5 steps of the PV-SMaRT program: One – Establish water quality task force; Two - Conduct field research on stormwater quality; Three - Calibrate and validate hydrologic model; Four - Develop PV-specific stormwater management best practices; Five - Provide outreach and education to stakeholders.

The Photovoltaic Stormwater Management Research and Testing (PV-SMaRT) project is developing and disseminating research-based, PV-specific tools and best practices for stormwater management and water quality at ground-mounted PV sites.

To achieve PV-SMaRT's goal, NREL is partnering with the University of Minnesota, Great Plains Institute, and Fresh Energy.

Stormwater Management for PV Facilities

Many jurisdictions treat ground-mounted PV facilities as predominantly impervious surfaces or surfaces that do not allow water to soak into the ground. However, rather than acting like a paved surface, rainwater can generally infiltrate under elevated PV arrays.

Because current stormwater runoff models used by local and state jurisdictions were not designed for ground-mounted PV, the stormwater permitting process can impose costly additional stormwater infrastructure requirements. Often, additional land must be leased or purchased for stormwater mitigation measures, such as detention ponds. The permitting process also often lacks accuracy and leaves unanswered questions for jurisdictions when they attempt to evaluate applications for risks and opportunities associated with ground-mounted PV facilities.

Through its research and analysis, the PV-SMaRT project aims to address the stormwater and water quality challenges facing PV facilities in most jurisdictions.

Research Findings

PV-SMaRT's research and modeling highlight four factors that should be considered in stormwater management and water quality permitting for PV arrays (in order of greatest impact):

Compaction—managing soil compaction and bulk density across the site
Soil depth—including soil depth (rooting depth) in stormwater modeling and design
Ground cover—installing, establishing, and maintaining appropriate vegetated ground cover between and under the arrays to facilitate infiltration
Disconnection—ensuring appropriate distance between arrays for infiltration.

These factors are drawn from the report Best Practices: Photovoltaic Stormwater Management Research and Testing (PV-SMaRT), published by the Great Plains Institute, a PV-SMaRT partner.

Stormwater Runoff Calculator

PV-SMaRT has developed an easy-to-use calculator to estimate stormwater runoff from ground-mounted PV arrays. This calculator is based on research and hydrologic modeling conducted at a set of research sites featuring diverse climatic, topographic, and soil conditions, with either fixed or tracking solar arrays, and vegetation that included pollinators, grass, or cover crops.

Climatic and hydrologic field measurements at each site were used to develop a two-dimensional numerical model for stormwater runoff based on specific combinations of a wide range in 24-hour design storms, soil textures, crop rooting depth, soil bulk densities, presence or absence of solar arrays, spacing of solar arrays, type of ground cover, and slope steepness values.

To learn how to use the calculator, watch this recording of a webinar hosted by Fresh Energy.

PV Field Research Sites

The PV-SMaRT project is using five ground-mounted PV sites in the United States to study stormwater infiltration and runoff. These sites represent a range of elevations, slopes, soil types, and geographical locations. The unique conditions at each site are being characterized, and measurements are being taken of soil infiltration, runoff, site vegetation density, speciation and rooting depth, precipitation, and drip edge runoff.

Minnesota

Minnesota's site has a 3.4-megawatt (MW) DC, fixed-mount, two-in-portrait PV array. It has sandy soil with a pollinator mix dominated by black-eyed Susan daisies and receives 37 in. of annual rainfall. Equipment was installed in June 2020 and will operate for 2 years.

New York

New York's site has an 18-MW DC, fixed-mount, two-in-portrait PV array. It has silt loam soil with a tall grass and clover mix, is ungrazed or grazed by sheep, and receives 49 in. of annual rainfall. Equipment was installed in June 2020 and will operate for 2 years.

Oregon

Oregon's site has a 9.9-MW DC, tracking, two-in-portrait PV array. It's a flat site with clay soil, a diverse pollinator seed mix, and 16 in. of annual rainfall. Equipment was installed in August 2020 and will operate for 2 years.

Colorado

Colorado's site has a 1-MW DC, tracking, one-in-portrait PV array. It has clay soil and pollinator-friendly vegetation and receives 16 in. of annual rainfall. Equipment was installed in September 2020 and will operate for 2 years.

Georgia

Georgia's site has a 1.3-MW DC, tracking, one-in-portrait PV array. It's a flat site with sandy clay soil, mowed cover crops, a high-diversity pollinator mix, and 49 in. of annual rainfall. Equipment was installed in September 2020 and will operate for 2 years.

Water Quality Task Force

The PV-SMaRT water quality task force works closely with the project team to provide feedback and guidance on the technical analysis, modeling, validation, and creation of water quality best practices. The task force is made up of individuals who represent a variety of views and stakeholder groups, including water quality experts, stormwater professionals, and solar industry representatives. Task force members understand the landscape of technical, strategic, permitting, and ground-mounted PV site development issues to meeting water quality goals.

Task force members include:

Jason Bernagros, U.S. Environmental Protection Agency (EPA) Office of Research and Development
Seth Brown, National Municipal Stormwater Association
Veronica Craw, Georgia Department of Natural Resources
Dave Gasper, New York Department of Environmental Conservation
Robert Goo, EPA Office of Wetlands, Watersheds
Britta Hansen, Emmons Olivier Resources
Greg Hoffman, Center for Watershed Protection
Jake Janski, Minnesota Native Landscapes
Gavin Chase Meinschein, Engie North America
David Morley, American Planning Association
Andrew Nelson, Westwood Professional Services
Peter Parkinson, AES Distributed Energy
Sybil Sharvelle, Colorado State InTERFEWS Director.