Development of a utility-scale solar photovoltaic project involves management of various potential environmental impacts, including impacts on wildlife and habitat. Although solar facility construction activities do involve short-term disturbance, responsibly developed solar power plants can provide shelter, protection, and stable use of land to support biodiversity. Land use practices and their relationship to biodiversity are examined at one of the world’s largest solar facilities, the 550 MW Topaz Solar Farms project in San Luis Obispo County, CA, USA. Pre- and postconstruction biological monitoring data indicate similar to higher vegetation productivity on-site compared to reference sites. Postconstruction monitoring has documented the presence of dozens of wildlife species, including several with special conservation status. Best practices in responsible land use utilized in the Topaz project are specified in the categories of community, biology, water, design and construction, and end of life. These practices, as well as future solar project development innovations that reduce ground disturbance, can be applied to enhance biodiversity at other solar facilities.

KEY MESSAGE

Readers of this case will gain a basic understanding of biological monitoring methods to evaluate impacts on wildlife and habitat at utility-scale solar facilities, best practices in responsible land use that can enhance biodiversity at those facilities, and opportunities for improvement from reducing ground disturbance during project development.

INTRODUCTION

Solar energy is one of the fastest growing energy technologies in the world, accounting for nearly one-third of new net electricity capacity additions globally in 2016 [1]. Industry growth has been led by development of utility-scale solar photovoltaic (PV) facilities, which provide high capacity factors [2], economies of scale [3], and advanced grid integration capabilities [4]. Utility-scale PV facilities are multimegawatt (MW) in scale, ground-mounted, and occupy approximately 2.5–3.5 hectares per MWac [5]. In addition to being technically feasible and economically competitive, large-scale grid penetration of solar PV provides considerable climate benefits (~2 ¢/kWh) and air quality and public health benefits (~1.4 ¢/kWh) [6].

As with other large-scale construction projects, development of a utility-scale PV project involves management of various potential environmental impacts, including impacts on wildlife and habitat [7, 8]. Although solar facility construction activities do involve short-term disturbance to existing land and wildlife habitats, responsibly developed PV power plants can create new habitats and help protect sensitive animal and plant species. These biodiversity concepts were first evaluated in large-scale solar facilities in Europe.

A 2010 review published by the German Renewable Energies Agency considered biodiversity in over 10 large-scale solar projects located in arable and brownfield sites in Germany [9]. In addition to providing best practices for design, construction, and operation of solar facilities, the study found that solar projects can help conserve and promote biodiversity by providing a refuge for plants and animals.

A 2015 study of 11 large-scale solar facilities and control sites in the United Kingdom found that an increase in biodiversity can be detected across a number of different species groups [10]. In particular, increased botanical biodiversity results from various microclimates within the solar facilities, with shaded and unshaded areas or wetter and drier environments. This botanical biodiversity can lead to greater abundance of invertebrates and higher diversity of bird species. The relationship between botanical biodiversity at solar plants and invertebrate abundance includes pollinators, such as bees and butterflies, which were found in greater abundance at solar facilities than control sites. Pernicious weeds are managed at these facilities under the United Kingdom’s Weeds Act, with spot treatment with herbicides as a common approach [10].

A 2017 study of native vegetation productivity was conducted by the National Renewable Energy Laboratory under a solar PV array at the National Wind Technology Center in Jefferson County, CO, USA [11]. Taking into consideration factors such as shading and moisture availability under solar panels, an extensive plant cover with limited presence of noxious weeds was observed under arrays within a 3-year period, sufficient to control erosion and begin to restore wildlife habitat.

Although utility-scale solar facilities share common features with regards to scale, infrastructure, and proximity to transmission, they also have site-specific impacts that depend on the landscape in which they are sited and the potential for long-term disturbance if not developed responsibly [12, 13]. Impacts on biodiversity are presented here as a case study to reflect site-specific aspects while also serving as an example that other utility-scale solar facilities may follow.

The purpose of this case study is to examine land use practices and their relationship to biodiversity at one of the world’s largest solar facilities, the 550 MW Topaz Solar Farms project (Topaz project) in San Luis Obispo County, CA, USA. A metaphor of a “solar reef” communicates the concept that responsibly developed large-scale PV facilities can provide shelter, protection, and stable use of land to support biodiversity while also generating renewable energy. The Topaz project case is used to demonstrate how solar developers can work toward minimizing negative consequences on biodiversity, including promising approaches, monitoring methods, and opportunities for improvement.

CASE EXAMINATION

Project History

The Topaz project is located on 4,700 acres (25 km2) of private land in San Luis Obispo County in the northwestern corner of the Carrizo Plains (annual grasslands). It was developed and constructed by First Solar and is owned by BHE Renewables [14]. Project planning began in 2008–2009 followed by permitting from 2009 to 2011, including development of a draft and final environmental impact statement [15]. Construction began in 2011 and was completed in 2014. The plant began providing energy to the grid in 2013 and was commissioned and fully operational in 2015, producing over 1 TWh of electricity annually.

The 550 MWac project deploys approximately 9 million fixed-tilt thin film PV panels mounted south-facing at a 25° angle, with the tallest component of the array and mounting structure less than 2 m above grade. The PV panels have dimensions of 1.2 m (length) × 0.6 m (width), mounted in a landscape orientation, with four rows of panels in each array. The row-to-row spacing (distance between two consecutive posts on which arrays are mounted) is 4.3 m. In addition to the PV panels and mounting structures, other components of the project include an electrical substation, switching station, overhead collector lines, a monitoring and maintenance facility, and perimeter fencing.

Pacific Gas and Electric Company (PG&E) purchases the electricity generated by the Topaz project under a 25-year power purchase agreement. The site is adjacent to the existing 230-kV Morro Bay-to-Midway transmission line, which provides access to transmission capacity. The Topaz project is built on previously disturbed agricultural land within PG&E’s service territory with global horizontal irradiation of ~2,000 kWh/m2/year. The Mediterranean climate of the area includes average summer high temperatures between 30 and 40°C and average annual precipitation of ~25 cm, with annual rainfall occurring primarily between October and April [16].

Throughout the general project region, there are agricultural, ranching, petroleum development, mining, and federal land uses. The specific project site is surrounded by privately owned agricultural lands for grazing to the east of the project and for grazing and dry farming to the north, south, and west [15]. The Topaz project was largely constructed on active and fallowed dry-land farmed fields (barley farming) [17]. The habitat replaced by the Topaz project was actively farmed for grain (dryland cropping requires tilling each year and planting in alternate years to conserve soil moisture accumulated during the off year). The habitat was treated annually with fertilizers, herbicides, and rodenticides. The Topaz project eliminated the annual disturbance regime, fertilizers, and use of rodenticides.

Biological Monitoring

In 2010, prior to the onset of construction, two test PV arrays and two adjacent controls were established to test the effects of PV arrays on vegetation growth and species composition. In 2011, total vegetation production and species composition were measured in 0.093 m2 plots and compared between the lowest edge of the PV array table, directly under the PV array tables, in alleys between PV tables, and in the controls [18]. Analysis of variance (ANOVA) tests showed no statistically significant difference in mean vegetation productivity (grams per unit area dry weight) between test array and control locations (Figures 1 and 2).

FIGURE 1.

2011 vegetation under PV test arrays and in alley between arrays.

FIGURE 1.

2011 vegetation under PV test arrays and in alley between arrays.

FIGURE 2.

Preconstruction (2011) vegetation productivity (grams per 0.093 m2 plot; n=20; ±standard deviation) in test array and control sites.

FIGURE 2.

Preconstruction (2011) vegetation productivity (grams per 0.093 m2 plot; n=20; ±standard deviation) in test array and control sites.

During winter months, researchers observed that vegetation in the controls and along the lowest edge of the tables dried faster between rain events than under PV panels and within the alleys between PV tables. More direct sunlight reached the ground below the lowest edge of the panels in the late fall and winter than the shaded alleys. Even though control and edge treatments visibly appeared to produce less (shorter) vegetation, productivity was not statistically different between treatments.

In 2015, after the completion of construction, vegetation sampling was again conducted at 22 on-site PV array blocks and in surrounding Stewardship lands [19]. Data were collected from 1 m2 quadrats at three sampling points per block using a random point generator (Figure 3). For each sample point, two quadrat locations were sampled: one under the PV panels and one within the alley between the PV panel rows. Reference samples were also collected from Stewardship Lands adjacent to blocks, with three sampling points generated in each Stewardship Land section affected by the blocks.

FIGURE 3.

2015 vegetation sampling quadrat (1 m2) locations (in green squares) within PV array blocks (B1–B22) and Stewardship Land sections (ST01–ST24).

FIGURE 3.

2015 vegetation sampling quadrat (1 m2) locations (in green squares) within PV array blocks (B1–B22) and Stewardship Land sections (ST01–ST24).

In spring 2015, percent live cover, litter cover from the previous growing season, and bare ground were quantitatively assessed. All plant species occurring in each quadrat were identified to the lowest taxonomic group feasible. The average and maximum height of vegetation were also measured. In fall 2015, live vegetation cover, litter cover, bare ground, total cover, average height, and maximum height were measured, with individual species not identified in each quadrat because most species had senesced and/or gone to seed. Sites were grazed between spring and fall sampling, which obscured the identity of many species. ANOVA tests were performed to detect significant differences between on-site areas (in alleys and under panels) and Stewardship Lands.

ANOVA tests indicated that the solar plant generally had greater percent live cover, less bare ground, and higher species diversity than the surrounding Stewardship Lands (Tables 1 and 2). Overall, over 120 botanical species were observed on-site during biological monitoring in 2015. Approximately 40 wildlife species (70% avian, 20% mammal, 10% reptile) were observed on-site and in temporary disturbance areas and surrounding Stewardship Lands during biological monitoring in 2015, including several with special conservation status (California horned lark, Ferruginous hawk, Loggerhead shrike, Prairie falcon, American badger, San Joaquin kit fox) (Table 3).

TABLE 1.

Postconstruction (2015) vegetation sampling summary (1 m2 quadrats; n=93; see Figure 3 for sampling locations)

ParameterSpringFallANOVA statistics (significance threshold p<0.05)
On-site (alley)On-site (under array)Stewardship landOn-site (alley)On-site (under array)Stewardship land
Mean total percent cover (live vegetation plus litter) 57 54 41 48 51 38 On-site and stewardship land means are significantly different 
Base ground estimate (percent bare) 43 46 59 52 49 63 
Thatch/litter estimate (percent cover) 48 51 37 
Average height (cm) 23 21 16 On-site and stewardship land means are significantly different in spring and not in fall 
ParameterSpringFallANOVA statistics (significance threshold p<0.05)
On-site (alley)On-site (under array)Stewardship landOn-site (alley)On-site (under array)Stewardship land
Mean total percent cover (live vegetation plus litter) 57 54 41 48 51 38 On-site and stewardship land means are significantly different 
Base ground estimate (percent bare) 43 46 59 52 49 63 
Thatch/litter estimate (percent cover) 48 51 37 
Average height (cm) 23 21 16 On-site and stewardship land means are significantly different in spring and not in fall 
TABLE 2.

Dominant species of vegetation by quadrat sampling location in 2015, tallied by species and number of quadrats in which it was dominant (1 m2 quadrats)

Scientific nameCommon nameOn-site (alley)On-site (under array)Stewardship landTotal
Achyrachaena mollis Blow-wives   
Amsinckia intermedia Common fiddleneck  15 
Avena fatua Wild oat 10 12 26 
Brassica nigra Black mustard  
Bromus diandrus Ripgut grass 11  15 
Bromus hordeaceus Soft chess  10 
Bromus madritensis subsp.rubens Red brome 12 15  27 
Calandrinia menziesii Red maids   
Capsella bursa-pastoris Shepherd’s purse   
Deinandra fasciculata Clustered tarweed   
Descurainia sophia Tansy mustard  
Erodium cicutarium Redstem filaree 35 12 20 67 
Festuca microstachys Small fescue  
Hordeum murinum Foxtail barley 17 18  35 
Lamium amplexicaule Henbit   
Lasthenia gracilis Common goldfields   
Lepidium nitidum Peppergrass  
Lupinus bicolor Miniature lupine   
Phacelia ciliata Great valley phacelia   
Plagiobothrys canescens Valley popcornflower   
Plagiobothrys nothofulvus Rusty popcornflower   
Poa secunda subsp.secunda One-sided blue grass  
Scientific nameCommon nameOn-site (alley)On-site (under array)Stewardship landTotal
Achyrachaena mollis Blow-wives   
Amsinckia intermedia Common fiddleneck  15 
Avena fatua Wild oat 10 12 26 
Brassica nigra Black mustard  
Bromus diandrus Ripgut grass 11  15 
Bromus hordeaceus Soft chess  10 
Bromus madritensis subsp.rubens Red brome 12 15  27 
Calandrinia menziesii Red maids   
Capsella bursa-pastoris Shepherd’s purse   
Deinandra fasciculata Clustered tarweed   
Descurainia sophia Tansy mustard  
Erodium cicutarium Redstem filaree 35 12 20 67 
Festuca microstachys Small fescue  
Hordeum murinum Foxtail barley 17 18  35 
Lamium amplexicaule Henbit   
Lasthenia gracilis Common goldfields   
Lepidium nitidum Peppergrass  
Lupinus bicolor Miniature lupine   
Phacelia ciliata Great valley phacelia   
Plagiobothrys canescens Valley popcornflower   
Plagiobothrys nothofulvus Rusty popcornflower   
Poa secunda subsp.secunda One-sided blue grass  
TABLE 3.

Wildlife species observed on-site and in temporary disturbance areas and surrounding Stewardship Lands in 2015

Common nameScientific nameConservation status
Avifauna 
American crow Corvus brachyrhynchos  
American kestrel Falco sparverius  
Ash-throated flycatcher Myiarchus cinerascens  
Black phoebe Sayornis nigricans  
Brewer’s blackbird Euphagus cyanocephalus  
California horned lark Eremophila alpestris actia State: watch list (nesting) 
Common raven (nesting) Corvus corax  
Dark-eyed junco Junco hyemalis  
European starling (nesting) Sturnus vulgaris  
Ferruginous hawk Buteo regalis State: watch list (wintering) 
House finch Haemorhous mexicanus  
Lark sparrow Chondestes grammacus  
Lesser goldfinch Spinus psaltria  
Loggerhead shrike (nesting) Lanius ludovicianus State: species of special concern 
Long-billed curlew Numenius americanus  
Mourning dove Zenaida macroura  
Northern harrier Circus cyaneus  
Prairie falcon Falco mexicanus State: watch list (nesting) 
Red-tailed hawk (nesting) Buteo jamaicensis  
Savannah sparrow Passerculus sandwichensis  
Say’s phoebe Sayornis saya  
Song sparrow Melospiza melodia  
Turkey vulture Cathartes aura  
White-crowned sparrow Zonotrichia leucophrys  
Western bluebird Sialia mexicana  
Western kingbird Tyrannus verticalis  
Western meadowlark Sturnella neglecta  
Mammals 
American badger Taxidea taxus State: species of special concern 
Black-tailed jackrabbit Lepus californicus  
Bobcat Lynx rufus  
Botta’s pocket gopher Thomomys bottae  
California ground squirrel Otospermophilus beecheyi  
Coyote Canis latrans  
Pronghorn Antilocapra americana  
San Joaquin kit fox Vulpes macrotis mutica Federal: endangered; state: threatened 
Reptiles 
Coast Range fence lizard Sceloporus occidentalis bocourtii  
Common side-blotched lizard Uta stansburiana  
Gopher snake Pituophis catenifer catenifer  
Northern Pacific rattlesnake Crotalus oreganus oreganus  
Common nameScientific nameConservation status
Avifauna 
American crow Corvus brachyrhynchos  
American kestrel Falco sparverius  
Ash-throated flycatcher Myiarchus cinerascens  
Black phoebe Sayornis nigricans  
Brewer’s blackbird Euphagus cyanocephalus  
California horned lark Eremophila alpestris actia State: watch list (nesting) 
Common raven (nesting) Corvus corax  
Dark-eyed junco Junco hyemalis  
European starling (nesting) Sturnus vulgaris  
Ferruginous hawk Buteo regalis State: watch list (wintering) 
House finch Haemorhous mexicanus  
Lark sparrow Chondestes grammacus  
Lesser goldfinch Spinus psaltria  
Loggerhead shrike (nesting) Lanius ludovicianus State: species of special concern 
Long-billed curlew Numenius americanus  
Mourning dove Zenaida macroura  
Northern harrier Circus cyaneus  
Prairie falcon Falco mexicanus State: watch list (nesting) 
Red-tailed hawk (nesting) Buteo jamaicensis  
Savannah sparrow Passerculus sandwichensis  
Say’s phoebe Sayornis saya  
Song sparrow Melospiza melodia  
Turkey vulture Cathartes aura  
White-crowned sparrow Zonotrichia leucophrys  
Western bluebird Sialia mexicana  
Western kingbird Tyrannus verticalis  
Western meadowlark Sturnella neglecta  
Mammals 
American badger Taxidea taxus State: species of special concern 
Black-tailed jackrabbit Lepus californicus  
Bobcat Lynx rufus  
Botta’s pocket gopher Thomomys bottae  
California ground squirrel Otospermophilus beecheyi  
Coyote Canis latrans  
Pronghorn Antilocapra americana  
San Joaquin kit fox Vulpes macrotis mutica Federal: endangered; state: threatened 
Reptiles 
Coast Range fence lizard Sceloporus occidentalis bocourtii  
Common side-blotched lizard Uta stansburiana  
Gopher snake Pituophis catenifer catenifer  
Northern Pacific rattlesnake Crotalus oreganus oreganus  

As part of monitoring botanical and wildlife species within the solar plant and surrounding Stewardship Lands, invasive species are also evaluated. Array fields exhibit a low contribution (less than 10%) of species considered invasive for the Topaz project’s habitat management goals: a) low fuel height for fire management; b) low plant density and height for optimal San Joaquin kit fox habitat; c) sufficient annual seed production to support rodent prey base for San Joaquin kit fox and burrowing owl, the target species for the Project; and d) manage stewardship lands for wildlife passage and to support habitat for San Joaquin kit fox, pronghorn antelope, burrowing owl, badger, tule elk, and common wildlife.

For the Topaz project, invasive species are defined to be managed or eradicated based on California Invasive Plant Council rating of “High” and California Department of Food and Agriculture (CDFA) List A or B. Naturalized grasses that are dominant components of surrounding grassland are not considered invasive for this project. The Topaz project area was historically dryland farmed for over 100 years. Thousands of acres of the northern Carrizo Plain are still farmed, and additional thousands of acres have been allowed to return to grassland habitat, partly now owned by the California Department of Fish and Wildlife (CDFW). The grassland habitat is dominated by Mediterranean grasses.

Within the arrays, there are five species on site considered invasive for this project. For red brome (Bromus madritensis ssp. rubens), cheat grass (Bromus tectorum) and yellow star-thistle (Centaurea solstitialis), the management strategy is to reduce their abundance by grazing management (timing and intensity) and spot-spraying for yellow star-thistle in areas with the highest concentration. For skeleton weed (Chondrilla juncea) and Russian thistle (Salsola tragus), targeted herbicide application is being applied to eradicate them from the project site. Field bindweed (Convolvulus arvensis) is on the CDFA list of noxious weeds. For this project, we do not consider it invasive because it provides important forb forage for pronghorn antelope on the Stewardship lands where they frequently graze. Historically, this species had been reduced or eliminated by herbicide application in the grain fields.

With regards to the Stewardship Lands that serve as a reference site for biological monitoring, these lands were historically part of the dryland grain production and were left as fallow fields after farming stopped in 2011. Managed prescribed livestock grazing was implemented in 2015. These areas are managed for wildlife passage and provide habitat for federally listed San Joaquin kit fox and species of concern such as burrowing owls and American badgers, foraging habitat for eagles and hawks, grazing and reproductive habitat for pronghorn antelope (priority management species for CDFW), and foraging habitat for tule elk (another priority large animal managed by CDFW). Grazing by cattle and sheep is recommended when vegetation height and density exceeds levels for San Joaquin kit fox habitat.

The reason forage production is so low on the Stewardship lands is because of more xeric conditions, especially apparent during drought years such as 2015. The array fields provide a shading effect similar to an orchard allowing the growing season to be extended compared to the Stewardship lands.

Best Practices

In 2012, the World Wildlife Fund (WWF) published their solar atlas on the land-energy nexus related to solar PV [20]. The primary focus of the atlas was to evaluate country-specific land area requirements for achieving 100% renewable electricity with solar PV by 2050. Given that there is currently no international standard for responsible utility-scale PV development, the atlas also included best practices in responsible land use in the categories of community, biology, water, design and construction, and end of life. The best practices were included to provide guidelines for factors that can arise and should be addressed when developing a solar project. Table 4 documents how the Topaz project compares to the best practices outlined by WWF.

TABLE 4.

Environmental impact categories and guidelines for utility-scale solar PV [20] and project-specific impacts for Topaz project

CategorySub-categoryLow scoreHigh scoreTopaz project
Community Dust Little regard for dust generation, no control efforts High regard for dust generation, worker education, control methods (palliatives, focused water use) High score 
Visual Arrays adjacent to property lines or high traffic roadways; no screening or landscaping; night illumination Completely out of sight from roads and neighbors Medium high score 
Noise Equipment backup alarms, post driving, heavy equipment, close to property lines or receptors; night work Significant distance buffer; equipment selection; equipment noise shielding; weekday/daylight hours work only High score 
Stakeholder engagement Little to no engagement Active local engagement through community organizations and governments; local educational or college programs tours; public outreach activities (meetings, tours) High score 
Labor Non-local workers; minimum wage; minimum safety requirements All local workers; prevailing wage; full personal protective equipment and extensive safety training and oversight; maximizing local economic development and job creation; focus on aboriginal and indigenous engagement and employment High score 
Biology Species, plants, etc. Design and construction with no regard to local biodiversity Detailed surveys conducted; special interests and other stakeholders consulted; design and construction with high regard for biodiversity; appropriate mitigation measures; ongoing monitoring of impacts; maximizing buffer areas around the active site, providing improved habitat potential, visual buffer, etc. High score 
Environmental impact studies Not performed; no awareness of any environmental issues Environmental Impact Study/Assessment conducted, mitigation plan developed with stakeholder involvement High score 
Soil protection Little regard for protecting the grassland or site soils Rigorous fire protection plans; topsoil conserved or replaced; adequate seeding of native grasses; compaction and permeable surfaces support growth High score 
Water Usage Little regard for water use Usage measured and reported; ambitious water reduction goals set; construction methods implemented to minimize water use High score 
 Storm water Little regard for storm water or runoff onto neighboring properties Appropriately sized and protected protection and conveyance measures (retention ponds; rip rap; silt fencing; etc.), effective measures to counter stormwater flow and runoff are in place; postevent performance and condition assessment High score 
Design and construction Site selection Prime agricultural, biological, or cultural land used Use of disturbed or previously used sites; superimposed on existing structures (roofs, landfills, parking lots, etc.); greenfield or prime agricultural land avoided, worn agricultural or contaminated land used to restore biodiversity; consider potential for “dual use” of sites (e.g., agricultural/grazing) – this will depend on local climate and farm practices High Score 
Grading Heavy cut and fill; stripped topsoil; invasive seeds introduced; long-term drainage or dust issues Minimizing grading, installation follows existing topography, minimizing built roads/gravel, minimizing trenching; topsoil retained or restored; no standing water or dust areas High score 
Footprint/layout Inefficient use of space Minimize project footprint with careful balance of ground coverage ratio, row spacing, panel height, etc. High score 
Decommissioning/end-of-life (EOL) Site restoration No consideration of land restoration after project life Ensure that a site can be restored to its original state (or better) at the end of project’s useful life High score 
Recycling No take-back and recycling at panel EOL offered Take-back and recycling of EOL panels and Balance of System products considered and addressed as part of project development and permitting phase High score 
CategorySub-categoryLow scoreHigh scoreTopaz project
Community Dust Little regard for dust generation, no control efforts High regard for dust generation, worker education, control methods (palliatives, focused water use) High score 
Visual Arrays adjacent to property lines or high traffic roadways; no screening or landscaping; night illumination Completely out of sight from roads and neighbors Medium high score 
Noise Equipment backup alarms, post driving, heavy equipment, close to property lines or receptors; night work Significant distance buffer; equipment selection; equipment noise shielding; weekday/daylight hours work only High score 
Stakeholder engagement Little to no engagement Active local engagement through community organizations and governments; local educational or college programs tours; public outreach activities (meetings, tours) High score 
Labor Non-local workers; minimum wage; minimum safety requirements All local workers; prevailing wage; full personal protective equipment and extensive safety training and oversight; maximizing local economic development and job creation; focus on aboriginal and indigenous engagement and employment High score 
Biology Species, plants, etc. Design and construction with no regard to local biodiversity Detailed surveys conducted; special interests and other stakeholders consulted; design and construction with high regard for biodiversity; appropriate mitigation measures; ongoing monitoring of impacts; maximizing buffer areas around the active site, providing improved habitat potential, visual buffer, etc. High score 
Environmental impact studies Not performed; no awareness of any environmental issues Environmental Impact Study/Assessment conducted, mitigation plan developed with stakeholder involvement High score 
Soil protection Little regard for protecting the grassland or site soils Rigorous fire protection plans; topsoil conserved or replaced; adequate seeding of native grasses; compaction and permeable surfaces support growth High score 
Water Usage Little regard for water use Usage measured and reported; ambitious water reduction goals set; construction methods implemented to minimize water use High score 
 Storm water Little regard for storm water or runoff onto neighboring properties Appropriately sized and protected protection and conveyance measures (retention ponds; rip rap; silt fencing; etc.), effective measures to counter stormwater flow and runoff are in place; postevent performance and condition assessment High score 
Design and construction Site selection Prime agricultural, biological, or cultural land used Use of disturbed or previously used sites; superimposed on existing structures (roofs, landfills, parking lots, etc.); greenfield or prime agricultural land avoided, worn agricultural or contaminated land used to restore biodiversity; consider potential for “dual use” of sites (e.g., agricultural/grazing) – this will depend on local climate and farm practices High Score 
Grading Heavy cut and fill; stripped topsoil; invasive seeds introduced; long-term drainage or dust issues Minimizing grading, installation follows existing topography, minimizing built roads/gravel, minimizing trenching; topsoil retained or restored; no standing water or dust areas High score 
Footprint/layout Inefficient use of space Minimize project footprint with careful balance of ground coverage ratio, row spacing, panel height, etc. High score 
Decommissioning/end-of-life (EOL) Site restoration No consideration of land restoration after project life Ensure that a site can be restored to its original state (or better) at the end of project’s useful life High score 
Recycling No take-back and recycling at panel EOL offered Take-back and recycling of EOL panels and Balance of System products considered and addressed as part of project development and permitting phase High score 

Working closely with San Luis Obispo County, state and federal resource agencies, and national and local environmental groups, Topaz was designed to avoid sensitive areas, preserve wildlife habitat, and minimize site disturbance. A conservation corridor was preserved to enable kit foxes, pronghorn antelope, and tule elk to freely pass between the various project blocks (array fields).

Prior to beginning work on the site, workers received comprehensive training and information about the biology, habitat needs, status of endangered species, and measures to protect threatened or endangered species that may be found in the project area. Daily biological monitoring was conducted throughout construction to protect species and habitats and identify exclusion areas (e.g., active bird nests and protected mammal dens). Regular monitoring with annual scat surveys and GPS collars helped determine the best species conservation strategies at the Topaz site.

The entire Topaz site was reseeded with native flora species to allow vegetation to grow beneath the solar panels, creating new habitats, providing sources of food for various wildlife species, and providing dust control. Specifically, locally sourced native grasses and forbs in the seed mix were collected from the Carrizo Plains and propagated for multiple years to produce sufficient seed for the solar array fields. Array fields were drill seeded with the mix shown in Table 5.

TABLE 5.

Native seed mix for PV array fields

Scientific nameCommon nameCollected within 25 miles of Carrizo Plain
Vulpia microstachys Annual fescue Yes 
Nassella cernua Nodding needlegrass Yes 
Vulpia myuros Rattail fescue (naturalized) Not applicable 
Bromus hordeaceus Soft chess brome (naturalized) Not applicable 
Poa secunda One-sided bluegrass Yes 
Lasthenia gracilia Goldfields Yes 
Scientific nameCommon nameCollected within 25 miles of Carrizo Plain
Vulpia microstachys Annual fescue Yes 
Nassella cernua Nodding needlegrass Yes 
Vulpia myuros Rattail fescue (naturalized) Not applicable 
Bromus hordeaceus Soft chess brome (naturalized) Not applicable 
Poa secunda One-sided bluegrass Yes 
Lasthenia gracilia Goldfields Yes 

On-site vegetation is maintained by periodic pulse-grazing with several thousand sheep, demonstrating how PV power plants can accommodate agriculture (Figure 4). Targeted grazing allows for regeneration of preferred species (soft chess brome, annual fescues, perennial needlegrass, and native forbs) and reduction of less desirable species. Although there is the potential for positive interactions between grazing and invasive species [21], invasive species are tracked and managed as described above, and sheep have not been found to introduce or enhance invasive species.

FIGURE 4.

Vegetation management by grazing sheep.

FIGURE 4.

Vegetation management by grazing sheep.

Grazing and periodic herbicide application provides ongoing invasive weed control. Operational objectives during maintenance activities at utility-scale solar facilities include managing vegetation height to avoid interference with solar panels and limiting fuel load for potential wildfires. At the same time, adequate vegetation cover is needed to limit the emergence of noxious weeds. Optimizing these objectives can be a matter of timing (e.g., allowing vegetation to go to seed before initiating grazing).

Fencing was designed to be friendly to a federally listed endangered species (San Joaquin kit fox) and exclude their primary predator (coyote), with the relative absence on-site of top predators enabling challenged species to thrive. General monitoring and documentation of the status of various species of concern (Table 3) helps inform ongoing species management practices. For the high-voltage components and transmission lines emanating from the PV plants, installation of avian collision and perching avoidance features, fence reflectors, and other devices mitigate against adverse avian contact and collision.

In addition to on-site measures, a portion of the mitigation measures for the Topaz project includes acquiring mitigation land to offset project impacts. The Topaz project is contributing to the perpetual conservation of over 17,000 acres of land. After decommissioning, the total area will be restored and added to the footprint of the mitigation land, resulting in ~22,000 acres planned for protection in perpetuity. Land restoration after development activities can be challenging, particularly in arid lands [22, 23]. However, the Topaz project site is already providing habitat value for listed species such as the San Joaquin kit fox [17] by converting farmed grassland historic habitat into a more stable land use. If managed appropriately, the site can continue to provide habitat value after decommissioning.

Opportunities for Improvement

Future innovations in solar project development relate to reduced ground disturbance from project construction. Ground disturbance can be reduced using the disk-and-roll site preparation method (Figure 5), which contours the land without changing the macro-level topography and existing drainage patterns. When possible based on vegetation type, density, and relative absence of residual tripping hazards, site grading can also be largely eliminated and replaced simply by vegetation mowing. Both of these methods help maintain the native seed and root structures in the soil, minimizing wind and water erosion risks and increasing the likelihood of natural rehabilitation. These reduced grading site preparation techniques have the potential to both reduce impacts on biodiversity and provide benefits to the solar energy project and surrounding communities, as described below, thereby increasing the likelihood of their adoption. These benefits include lower construction costs from reduced heavy machinery and labor, and potentially improved energy yield from microclimatic cooling effects from vegetation underneath arrays [24]. Less intensive site grading results in reduced emissions from heavy equipment, lower likelihood of fugitive dust emissions, and significantly lower water consumption dedicated to dust suppression. Multi-year monitoring also indicates that postconstruction dust concentrations downwind of large-scale solar facilities are reduced relative to background as a result of a wind-shielding effect of the PV arrays [25]. The ability to reduce ground disturbance and accommodate existing land contours is also facilitated by advancements in PV tracking technology with higher range-of-motion trackers providing a greater ability to accommodate existing land slopes.

FIGURE 5.

Disk and roll site preparation technique.

FIGURE 5.

Disk and roll site preparation technique.

An additional innovation that reduces ground disturbance is the shift from below ground trenching of electrical cables to above-ground housing of cables in cable trays. Avoidance of trenching has a number of benefits such as reduction in the potential for disturbing ground-dwelling fauna, a reduction in fugitive dust emissions, reduction of water consumption to suppress fugitive dust, avoidance of emissions from heavy equipment, avoidance of impacts on potential buried cultural artifacts, reduction of potential species entrapment risks, and reduced risks to workers posed by exposed trenches and soil-dwelling fungal pathogens such as Coccidioides immitis/posadasii. In addition to construction-related benefits, above-ground cabling provides benefits at the project end-of-life where scrap metal revenues have been shown to exceed decommissioning costs with above-ground cabling, which facilitates high copper recycling rates [26].

CONCLUSION

Solar projects are often surrounded by agricultural, grazing, rural residential, outdoor recreational, urban, and other land uses that present ongoing impacts and threats to flora and fauna. After the short-term solar project construction disturbance period, the vegetation within the project fencing can return to its native origins accompanied by the return of associated fauna. As a result, the acreage inside the project fence can become a refuge for species from the continuous ground disturbance and predation that occurs outside the project fence, as evidenced by biological monitoring data at the Topaz project. Best practice guidelines for land use at solar facilities outlined by the World Wildlife Federation can be applied to enhance biodiversity at other facilities.

CASE STUDY QUESTIONS

  1. 1.

    During the public comment period for utility-scale solar project permitting, which categories of stakeholders may comment and what example issues may be raised? Which of these issues are common to large-scale construction projects and which issues may be unique to utility-scale solar?

  2. 2.

    Consider a public hearing to explain a proposed utility-scale solar project to a local planning commission and/or community. How would a project developer prepare for such a hearing? What background activities and materials would be useful and informative?

  3. 3.

    What are some of the economic, technical, and environmental tradeoffs with developing utility-scale solar projects compared with distributed rooftop solar projects?

  4. 4.

    Of the land use guidelines outlined in the WWF Solar Atlas (Table 4), which are most critical to enhancing biodiversity at large-scale solar facilities? How might a developer make a business case for implementing the land use guidelines?

AUTHOR CONTRIBUTIONS

Parikhit Sinha contributed to original draft preparation, reviewing, and editing. Beth Hoffman contributed to case study best practices. John Sakers contributed to case study opportunities for improvement. LynneDee Althouse contributed to case study biological monitoring.

The authors would like to acknowledge Dan Meade (A&M conservation lead on the Topaz project), Katie Brown (A&M Plant and Range Scientist, field lead scientist), and Royce Larsen (University of California Cooperative Extension Rangeland and Watershed Specialist that cooperated in study).

FUNDING

The research for this case study was supported by First Solar.

COMPETING INTERESTS

The authors have declared that no competing interests exist, beyond that authors Parikhit Sinha, Beth Hoffman, and John Sakers are paid employees of First Solar, a commercial solar energy company which funded the research, as is clearly noted in their affiliations.

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