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Balancing Renewables and Natural Resources

February 12, 2010

Photo of newly installed blue and silver solar panels on a field of freshly graded dirt with a wind turbine standing prominently behind the solar array. A mountain range in late afternoon shadows can be seen in the background. Enlarge image

Newly installed solar panels at NREL's National Wind Technology Center face skyward to follow the sun. NREL biologists are experimenting with native seed mixes to reestablish plants and wildlife habitat on the graded site.
Credit: Joe Verrengia

NREL's newest source of clean energy — a 1 megawatt solar array — is supplying laboratory researchers with more than carbon-free electricity. It's also an opportunity to examine a sensitive topic — the potential environmental impacts of large-scale renewable energy projects.

That a solar array might raise environmental concerns seems ironic.

Nationwide, three-quarters of the states in the U.S. have adopted aggressive renewable energy mandates to reduce carbon emissions and diminish the environmental damage from more than a century's reliance on fossil fuels.

But utility-scale renewable energy installations also can be significant construction projects — some proposed sites in California and the desert Southwest may cover up to 5,000 acres apiece.

In the West, the U.S. Bureau of Land Management alone has at least 31 renewable energy projects under "fast track" permitting review. And recently, the Secretary of the Interior identified proposed Solar Energy Study Areas where photovoltaic (PV) and solar thermal projects in six Western states are the subject of a special interdepartmental review known as the Solar Energy Development Programmatic EIS.

Some of the nation's best locations for renewable energy are in open, sunny ecosystems such as deserts and shortgrass prairies. These are arid areas where, once disturbed, the vegetation and wildlife may be slow to recover.

"Most plant species in the West have evolved to grow in direct sunlight, and that's where the best resource is for generating solar power," NREL Senior Biologist Brenda Beatty said. "We need to know how shade from PV panels may affect native plant growth and whether those effects will impact wildlife."

Collaborating on a Sustainable Installation

Photo of two workmen dressed warmly while working outside on a winter day. The man on left is dressed in a black parka and black ball cap and he is holding construction plans printed on wide sheets of white paper. The man on the right is dressed in a bright yellow safety parka and he is wearing safety glasses and a red hard hat.  Enlarge image

NREL Site Operations Project Manager Sal Sferrazza and Senior Safety Specialist Michael Stewart of Environment, Health & Safety confer during the final inspection of the solar array field at NREL's National Wind Technology Center.
Credit: Joe Verrengia

At NREL, where construction of both buildings and renewable energy systems is occurring at a rapid pace, environmental experts, construction engineers, and security staff are collaborating to meet NREL's commitment to sustainability and environmental stewardship, while still advancing the U.S. Department of Energy's goals to develop and deploy renewable energy technologies.

PV array fields typically are graded flat and the vegetation removed before installation begins. Soils under PV arrays frequently are sterilized to prevent weed growth, a step that not only introduces chemical agents into the environment, but also prevents the natural revegetation of native plants that could minimize erosion and provide wildlife habitat.

Alternatively, installers spread gravel beneath the solar panels to inhibit weed growth. But gravel can trap heat, which not only can reduce an array's operating life, but also can stress any plants that might grow there.

NREL's newest PV array at the National Wind Technology Center near Boulder, Colo., is an opportunity to study how ecosystems respond to renewable energy development and develop best management practices that re-establish habitat, minimize weed invasion, prevent erosion and protect wildlife.

The steps also may help PV companies and utilities reduce long-term operating and maintenance costs.

"The NWTC array field provides a unique opportunity to study ways in which renewable energy and natural resources can co-exist," Beatty said.

The NWTC's 1 megawatt array sits on nearly eight acres near the center's western boundary for which the U.S. Department of Energy has provided an easement to SunEdison. NREL purchases the electricity from SunEdison through a Solar Power and Services Agreement. It's the Laboratory's third sizeable solar array - NREL also has a 94 kilowatt array on the roof of the Science & Technology Facility and a 720 kilowatt array on South Table Mountain, near the lab's Golden, Colo., campus.

Combined, they are forecast to produce 2.738 gigawatt-hours per year, or about 15 percent of the electricity that NREL is expected to use in fiscal year 2010. The solar renewable energy certificates (RECs) are sold by SunEdison to Xcel Energy to meet Xcel's Renewable Energy Standard solar requirement; NREL purchases replacement RECs, helping NREL to exceed the Energy Policy Act of 2005's goal of 7.5 percent of a federal facility's electricity coming from renewable sources by 2013.

More than a Meadow

Photo of a woman with a field of photovoltaic panels and a wind turbine in the background. Enlarge image

NREL senior biologist Brenda Beatty is supervising revegetation and wildlife studies at the new solar array field at NREL's National Wind Technology Center.
Credit: Lee Jay Fingersh

At the NWTC, most of the vegetation is native shortgrass prairie that includes grasses such as big bluestem, blue gramma and various wheat grasses, forbs such as blanket flower, gayfeather and Penstemon, and shrubs such as, wild rose, hawthorn, and chokecherry.

By studying previous vegetation surveys and walking the site, Beatty and others realized that the NWTC also contains other habitat types that provide critical food and cover for native wildlife species. As part of NREL's sustainability and environmental stewardship commitments, some of these habitats are part of NREL's Conservation Management Area, and are protected from development activities. For example:

  • At the northwest corner of the NWTC, there's a thin ridge of Ponderosa Pine and other montane vegetation typically found at higher elevations.
  • Closer to the array field, a teardrop-shaped seep where groundwater rises and rainwater collects nurtures cattails, rushes, and other wetland plants.
  • On the south edge of the site, there is even a small patch of remnant tallgrass prairie. Only one-tenth of one percent of original tallgrass prairie remains in central North America.

Site engineers and Beatty collaborated on ways to adjust the array's layout to avoid these sensitive areas.

In the center of the eight-acre array field, construction crews also left a total of two acres ungraded. This parcel of intact vegetation is meant to serve as a natural source of native seeds, which should help in revegetation efforts.

Crews also will manually reseed graded areas beneath the solar panels in early spring with a blend of native grasses. The composition of the seed blend was formulated to include plants that can tolerate some shading, to help curtail erosion and weed invasion, and to be somewhat fire-resistant.

To develop a clearer understanding of the types of plants that can successfully be grown under solar panels, NREL's Solar Energy Technology Program, in coordination with the Strategic Energy Analysis Center, provided funding to create vegetation test plots under the solar array. Under Beatty's supervision, an independent botanist will establish one acre of test plots beneath solar panels, as well as two control plots on similar land away from the solar panels. Data from the test plots will be used to evaluate vegetation success under varying conditions of moisture, seed mixes, mulching, and other treatments.

"The experiments will begin to give us a handle on how PV installations and operations affect vegetation in our portion of the arid West, and the information obtained may be useful for other NREL projects, and for revegetation efforts at other solar installations," Beatty said.

Wildlife Considerations

Solar Power at the Wind Site

  • System Size: 1.16 megawatt DC/ 1 megawatt AC
  • Characteristics: Single axis tracker photovoltaic, ground-mounted
  • Location: National Wind Technology Center
  • Land: 8 acres
  • Start of Operation: December 2009
  • Modules: 6,000
  • Tracker Posts: 2,500
  • System Ownership: SunEdison financed, built, owns, operates and maintains the system
  • Solar Power and Services Agreement: 20 years with SunEdison, Western Area Power Administration, NREL and the U.S. Department of Energy

The plant communities at the NWTC attract a variety of animal life from rodents to raptors, including bald and golden eagles. Deer and coyote are the largest mammals commonly found roaming the site, but records show black bear and even mountain lion have appeared at the adjacent Rocky Flats National Wildlife Refuge.

Before the array's installation, Beatty supervised the humane relocation of a black-tailed prairie dog colony from the southwestern corner of the NWTC to another onsite location, in prairie habitat. The relocation was conducted in collaboration with DOE, SunEdison and other stakeholders.

The prairie dog is a keystone species whose activities are critical to the health of the shortgrass prairie; its burrowing stimulates plant growth and attracts other wildlife, and the colonies provide food for predators.

An independent biologist spent several days mapping the colony and recognizing family groups among the 70 animals. At the new colony, the biologist created artificial burrows by digging trenches four feet deep and 20 feet long and installing irrigation piping to serve as tunnels. After backfilling with dirt, the animals were relocated in family groups. The biologist provided food and water during the two weeks it took the prairie dogs to become acclimated to their new site.

PV isn't the only renewable energy technology drawing the attention of NREL biologists.

At the NWTC, the recent installation of the two largest wind turbines ever tested at the center — a 1.5 megawatt General Electric and a 2.3 megawatt Siemens — have prompted a study of whether individual wind turbines and meteorological towers have an impact on birds and bats there. The program includes weekly bird surveys, surveys of nesting and breeding birds and weekly searches for dead birds or bats.

At the Laboratory's main campus in Golden, Colo., NREL senior biologist Tom Ryon is collaborating with stakeholders to establish a wildlife corridor. Such a corridor would allow wildlife to safely move from the top of South Table Mountain to a water source in Lena Gulch, located south of the campus.

— Joe Verrengia

—Heather Lammers