Low-Impact Solar Development Strategies Primer
The InSPIRE project designed this primer to provide insights and best practices associated with low-impact solar development. It can assist solar developers, state agencies, and other interested parties in siting, designing, installing, and operating low-impact solar development projects.
Low-Impact Solar Development Basics
Low-impact solar development improves soil health, retains water, nurtures native species, produces food, and reduces energy costs for local communities. It's unlike traditional solar development, which uses land solely for energy production.
What Is Low-Impact Solar Development?
Low-impact solar development can mean a lot of different things, but some general principles are shown in the table below.
| Conventional Site Preparation: | Low-Impact Site Preparation: |
|---|---|
| Clearing and grubbing of soil and roots | Existing vegetation is left intact or is replaced with low-growing native vegetation species or crops |
| Topsoil stripping and stockpiling | Existing topsoil is left in place to allow for the successful growth of native vegetation and to promote soil health post-decommissioning of the solar project |
| Land grading and leveling utilizing heavy machinery | Natural contours of land are worked into the design and configuration of the solar project, with minimal if any land grading required |
| Soil compaction utilizing heavy machinery | Soil and vegetation are left intact to facilitate the growth of native vegetation, improved stormwater management through less runoff and erosion, and soil health |
| Land footprint for the foundations of vertical support structures, often including concrete | Lower land footprint for foundations of vertical support structures, often driven piles |
| Vegetation that supports habitat is discouraged and removed | Vegetation that supports habitat (e.g., pollinator species, other native fauna) is encouraged |
| O&M activities include herbicide spraying, mowing of weeds and other vegetation | Minimal O&M activities due to low-growing native vegetation species, could involve livestock grazing |
Low-Impact Solar Development Opportunities
There are many different configurations that can support low-development solar development:
Solar Centric
- Minimal changes to solar configuration
- Low-lying vegetation for ground cover and habitat
Solar-centric pollinator-friendly solar project in Wisconsin. Photo by Jordan Macknick
Vegetation Centric
- Minimal changes to vegetation design
- Large spacing in solar technologies
Photo by Jake Marley / Hyperion
Co-Location and Co-Optimization
- Solar and vegetation configurations are designed jointly for maximum dual output
Photo by Dennis Schroeder / NREL
Benefits of Low-Impact Solar Development
Potential Benefits to Land Owners
- Self-generation of electricity and reduced energy bills
- Additional income stream and increased revenue security
- Control of wind and soil erosion
- Compatible with grazing activities, provides shade and cover for livestock
- New market opportunities for shade tolerant crops
- Protection of natural habitat
- Safeguarding soil health
- Improved habitat for pollinator species
Potential Benefits to Solar Developers
- Reductions in site preparation and installation costs
- Reductions in O&M costs
- Reduced need for dust suppression
- Reduction in litigation vulnerability
- Decreased permitting time
- Increased solar energy production from cooler air zone created under modules
- Reduction in environmental mitigation investments
Each solar and agricultural project is unique, with benefits and trade-offs varying with each site.
Upfront Solar Site Screening and Selection Considerations
When siting a proper location for a solar facility, there are many factors to consider—from avoiding environmentally sensitive areas and working with local landowners to exploring previous land use.
Construction activities at most large-scale, ground-mounted installations of photovoltaic (PV) arrays are preceded by land clearing and re-grading to standardized slope and surface conditions to facilitate construction access, racking installation, and facility operations and maintenance (O&M). The impact to existing vegetation may be total eradication followed by installation of a gravel cover and subsequent application of herbicides. NREL cost modeling estimates that grading practices can account for 3-6% of total installed capital costs of utility-scale solar projects, and all conventional site preparation practices are expected to account for 20% of total utility-scale PV installed costs by 2020 (DOE, 2012).
A solar project sits on an active farm. Photo by Jordan Macknick
The first step in the solar design process is to pick a location for where to site a solar facility. Many factors will influence this decision, that include, but are not limited to: solar resource quality, distance to transmission lines, land ownership and rights, previous land-use, local workforce considerations, and environmental limitations. Picking the optimal location can reduce upfront costs to solar developers, ensure community acceptance of the new solar facility, and reduce environmental impacts. During the siting process, developers need to balance costs and risks (environmental, public opinion, cost, etc.) for successful project development.
Industry Identified best practices include:
- Avoid environmentally sensitive areas (wetlands, endangered species habitat, etc.) as much as possible.
- Work with local landowners/environmental groups to identify cultural sits and sites of local significance to identify areas to avoid.
Potential benefits of proper solar siting include:
- Cost savings from reduced grading and environmental mitigation steps
- Easier path to community acceptance
- Reduced chance of litigation during permit process
- Reduced project timelines from surveys to construction
- Reduced project risk during construction phase
Previous Land Use
Previous land use considerations can impact the solar siting process. Land types can be broken up into:
- Contaminated lands- land that has been classified contaminated from the previous use cycle. Examples include capped landfills, industrial sites and Superfund sites.
- Brownfield sites- sites that were or are currently being used for other processes. Examples include abandoned lots, parking lots, etc.
- Previously tilled agricultural land- sites that were used for farming in the past. The land may no longer be usable due to salt accumulation or left fallow for economic reasons.
- Greenfield sites- land that has not been used in the past for human use and that may or may not contain native vegetation
All of these different sites have benefits and drawbacks that are discussed below.
Contaminated Lands
Siting solar on contaminated land can have environmental and economic benefits. Photo from EPA
Siting solar on contaminated lands can result in environmental and economic benefits. Through reuse of these sites, communities see a property that has been vacant and or underutilized for many years turned into a facility that may help improve the local tax base, create jobs and turn blight into an economic opportunity.
This approach not only uses the land but transforms a potential liability into an asset that will serve the community. The economic benefits often mentioned for solar on contaminated lands are electricity cost savings, additional land lease revenue to the town or city site owner, and increased tax payments for the land and/or solar power system to the local municipality and/or state.
Cost savings can vary across installations, since they are determined by tax and solar energy incentives, renewable energy policies such as Renewable Portfolio Standards, local electricity rates, and power purchase agreement (PPA) terms. In some cases the site owner agrees to lease the land to the renewable energy developer at a low rate in exchange for reduced electric charges or for other terms. Other benefits associated with developing solar on contaminated lands include the use of local resources to construct these projects and the economic revitalization of an area.
Contaminated lands may also lead to risk and liability for the developer that may raise costs or impact project viability at the site. The permitting process for contaminated lands is often more time intensive due to involvement of many permitting agencies (federal, state, county, etc.). Often, previously contaminated lands will carry design and construction implications. For example, on capped landfills, the only stable soil may be 6-12” below the surface that may change the racking design for the site (i.e. a developer may have to use concrete blocks rather than driven posts). When disturbing soil, past environmental contaminants may pose health risks to construction contractors that may facilitate costly safety measures to reduce impact on human health.
Brownfield Sites
Siting a solar facility at a brownfield site can allow for use of land that is underutilized and provide a diversified revenue source for landowners through leasing payments or land sales. Purchasing and leasing brownfield lands can often be cheaper for developers than other land types which can improve the economics of a solar project. The environmental permitting process can also be expedited on previously disturbed lands due to a reduction in native vegetation and animal habitat on site. On top of that, use of brownfield sites can reduce the impact of PV on other types of land use or habitat such as farmland or prime species habitat. Siting solar on abandoned lots can provide a new revenue source for land owners and provide electricity close to urban populations, while reducing land change impacts on species. Also, previously developed sites may not require lengthy new interconnection lines or site access driveways and roads, potentially avoiding these associated costs and impacts.
Brownfield land may also present risks to developers. Previous use of the land can often leave behind unstable soil or remnants that may present permitting and construction challenges. Use of brownfield sites increases the need and specificity of geotechnical surveys and can lead to remediation efforts to make the land usable for PV. Also, procuring land rights for these sites may be difficult with multiple land rights claims.
Previously Tilled Agricultural Land
Previously tilled agricultural land can provide an asset to both landowners, farmers, and solar developers. From discussion with solar industry members, previously tilled agricultural lands present the least upfront risk from a land and siting perspective. There are often little soil and geotechnical issues and the slope of this land is often uniform enough to support all types of racking systems.
Farmers can gain numerous benefits from solar systems being sited on their land. Solar can be installed on the margins of the field that are not being used for farming; this allows farming activities and energy production at the same time. PV projects can also provide an additional revenue source for farmers that can increase the economics of farming land use (see Co-location section) and provide for diversification of revenue sources (beneficial in drought years). Also, farmers can offer salt contaminated or otherwise non-usable land to developers. Solar sites often have a life span of 20-25 years and can easily be removed at the end of its life that will allow for land recharge and future farming use of this land.
A solar project sits on an active farm. Photo by Jordan Macknick
Drawbacks of using previously tilled agricultural land can include the reduction of available farming area for landowners. Even though the land is currently uneconomic, market conditions may change in the future that may provide a better investment for the farmer. With solar, farmer will have to sign a long-term lease of the land and accept the fact that the site will be there for 20-25 years. Previously tilled agricultural land may often have latent seed banks that may increase the O&M considerations for managing vegetation that may not be suitable for growth under solar modules.
Greenfield Sites
Greenfield sites include land that have not been disturbed by human use in the past. Developers often find that land risks are reduced when using greenfield sites, but this land is often the most expensive during the land procurement and leasing process.
Some risks to developers include possible environmental delays. Greenfield lands may provide suitable habitat for sensitive and/or endangered species that can potentially derail a project or cause re-siting. Because this greenfield land could be used for other purposes, the environmental permitting and land procurement process may be highly visible and sensitive. Many people may feel that the land should be used for preservation or other less invasive uses that may slow down the development process. One example includes solar sites in the Mojave Desert that impacted Desert Tortoise habitat and migration. Environmental groups have taken legal actions for these contentious sites which has added 2-3 years of delay and caused other solar sites to change locations.
Permitting Process
Environmental permitting costs were found to vary by region and jurisdiction and could be drastically impacted by litigation occurrences during environmental review. Permitting time was found to take roughly 1 year but could take up to 2-3 years in some cases. Permitting, inspection, and interconnection (PII) costs for commercial PV projects are between 0.2-1.4 percent of total project capital costs. This represents a small portion of the overall total cost, but PII was identified as a potential market barrier by respondents. Thorough upfront site screening and avoidance of environmentally impactful sites was identified as a best practice from discussion with industries.
Storm Water Prevention Permits (SWPP) may also present a large upfront cost and delay. SWPP permits are needed anytime a constructor may change the natural runoff and erosion coefficients. Large retention ponds may be needed to mitigate large grading events that takes away available land for solar use.
Industry Feedback and Testimonials
‘Well suited solar sites found to reduce project costs and yield community acceptance sooner’
One industry interviewee described his company’s preference for siting projects on previously tilled agriculture sites: “You just come in, mow the vegetation, and commence construction.”
Resources and Further Information
Solar Site Preparation Considerations to Protect Native Vegetation and Pollinator Habitat
When considering low-impact solar development strategies, it's important to consider native vegetation and pollinator habitat during site preparation.
Construction activities at most large-scale, ground-mounted installations of photovoltaic (PV) arrays are preceded by land clearing and re-grading to standardized slope and surface conditions to facilitate construction access, racking installation, and facility operations and maintenance (O&M). The impact to existing vegetation may be total eradication followed by installation of a gravel cover and subsequent application of herbicides. NREL cost modeling estimates that grading practices can account for 3-6% of total installed capital costs of utility-scale solar projects, and all conventional site preparation practices are expected to account for 20% of total utility-scale PV installed costs by 2020 (DOE, 2012).
Site preparation activities on a solar construction site. Photo by Jordan Macknick
Low-Impact Strategies: Reduced Grading and Working with Existing Vegetation
Solar site preparation and construction developed around existing vegetation. Photo by Jordan Macknick
Mass grading can strip fertile soils from the site, preventing current and future agricultural use of the land. Grading on greenfield sites may also remove native vegetation and damage habitat for native flora and fauna. Mass grading can also disturb land and provide a pathway for weeds and other invasive species to grow. Application of herbicides to prevent weeds on graveled sites over the life of a solar installation can prevent any future use of the land for agricultural production and potentially contaminate nearby waterways and wetlands.
Grading requirements differ depending on whether a PV system is tracking or fixed. Single-axis tracking systems are increasingly common among large-scale PV installations, accounting for nearly 80% of the utility-scale PV capacity installed in 2017. Single-axis tracking systems, however, can only accommodate slopes of approximately 3-6%. Within this range, the slope must be consistent across the rows, which can be 300’ in length. Longer and variable pile lengths may be used to maintain a consistent array slope with more variable topography underneath the arrays; however, this incurs higher costs for the additional racking material.
New techniques and racking systems can reduce the grading needs and accommodate up to 15% slopes, but sites must still have a consistent slope to accommodate most tracking systems. The added costs of techniques to accommodate existing topography may be offset by reduced costs of weed control, erosion, and dust mitigation over the life of the project. Reduced grading maintains the surface layer of the soil intact and can prevent intrusion of weeds or other invasive species into the project site.
Fixed PV systems can accommodate significantly more variation in the topography. If a site is slightly rolling over a large area, variable racking post lengths may be able to eliminate the need for significant grading. However, if the variation in the topography occurs over a smaller area, more grading will be required to accommodate tracking systems and to establish suitable and safe terrain for workers and equipment for fixed systems.
Reduced grading can preserve natural vegetation where it exists, minimize weeds and erosion that can result after vegetation removal, and potentially expedite permitting. Selecting sites where minimal grading is needed, such as marginal or no longer productive tilled agricultural land, can help minimize the costs and impacts from grading.
According to Goodrich, James, and Woodhouse (2012), sites well-suited for solar installation and low impact design practices were found to reduce the overall cost of site preparation. Also, low-impact site preparation practices were found to increase the probability of a successful and timely environmental permit review process. Reductions in grading of the project area were found to reduce project costs and yield community acceptance more readily compared to more impactful site preparation practices.
While reduced grading can save civil engineering and construction costs, it may also lead to higher costs for racking as post lengths may increase to ensure sufficiently uniform PV array heights. Complicated racking systems needed to accommodate uneven topography and avoid mass grading can in turn increase construction time and labor costs.
Grading and the resulting soil disturbance can lead to additional weed abatement costs and may add more costs to re-establish ground cover. The exposed soils from grading add costs for storm water pollution prevention measures and add the risk of potential stormwater pollution violations.
Industry Feedback and Testimonials
In feedback gathered by NREL from 38 large-scale solar industry members with knowledge of the site development process, the most common method cited to reduce site preparation costs was minimizing grading and accommodating existing topography. Industry research found significant variation in the costs of site preparation, supporting the idea that this is an opportunity for significant cost savings. Land preparation practices in Goodrich, James, and Woodhouse (2012), including ground levelling, sediment control, hydrology, road construction, and vegetation clearing, cost between $5,000-65,000 per acre.
The grading that does need to occur for equipment pads, access roads, and tracking systems can stockpile topsoil for future use. Stockpiling more productive soils with higher organic material content allows these soils to be set aside and then spread over the graded surface to support re-establishment of a vegetative ground cover.
Industry members indicated the potential benefits of minimizing grading, particularly on greenfield sites, include:
- Reduced civil construction costs
- Reduced risk of cost overruns
- Minimized impacts on topsoil in prime agricultural areas
- Reduced permitting costs and delays
- Minimized soil stabilization, revegetation, and weed control costs
Resources and Further Information
Solar Site Ground Cover Considerations to Protect Native Vegetation and Pollinator Habitat
To protect native vegetation and pollinator habitats, and reduce future O&M costs, it's important to select the appropriate ground cover when designing a solar site.
Common ground cover types include: leaving the earth bare after construction; limiting vegetation growth, either through installation of an imperviable mat or applying pre-emergent herbicide to the site; covering the site with gravel; seeding with low growth or native vegetation; and some combination of the above.
Survey a pollinator test plot (planted) underneath the PV array as part of the Aurora Solar Project in Minnesota. Photo from NREL Image Gallery
The type of ground cover suitable for each site can be highly dependent on site-specific characteristics, such as previous land use and climate. For example, some solar sites in desert regions often employ either bare ground or gravel cover. The dry climate and remote locations lead to issues with obtaining water for establishing vegetation and there are less issues of invasive weeds due to the lack of water. However, the O&M considerations in the desert are different as dusty, dry climates can facilitate the need for washing panels often which can provide a water source for weeds and impact site performance. In other, more temperate climates (e.g. Minnesota, Oregon, etc.) issues arise with invasive species that move in after ground disturbance and O&M considerations include steps to prevent certain vegetation growth. Industry members indicated that permitting requirements and vegetation establishment costs are the primary drivers of the choice of ground cover for solar sites.
Benefits of Native and Low-Growth Vegetation
Native vegetation grows at the solar site. Photo by Rob Davis / Fresh Energy
Industry representatives have identified numerous benefits of establishing native and/or low-growth vegetation, including:
- Potential for reduced Stormwater Pollution Prevention (SWPP) permitting and land needed for storm water management. This can reduce construction costs and open otherwise unusable land for PV generation
- Protection of beneficial vegetation in the region that can provide benefits to local communities and species
- Provision of habitat for multiple species
- If pollinator-dependent crops are near the solar site, pollinator-friendly ground cover can create habitat and promote increased agricultural yields4
- Native vegetation may provide nutrient recharge for previous agricultural lands and lead to enhancement of soils for future agricultural uses
- Reduced O&M costs over time (after vegetation is established) from reduced mowing events and reduced herbicide application
- Reduced dust on modules and cleaning reductions by establishing ground cover
Low-Impact Strategies: Re-Vegetation using Native, Low-Growth, and Pollinator-friendly Vegetation
Following construction all waste, construction materials, and debris from construction activities will be removed from the site. Temporarily disturbed areas and any areas of bare ground should be re-vegetated with appropriate seed mixes identified for the site-specific area. Beneficial vegetation could include, depending on the location: short grasses or flowers, low-growing forbs, low-growing wetland seed mixes or some other low-growing perennial cover.
Site preparation for re-vegetation and seeding activities generally occur after completing construction cleanup of the solar project. Soil underneath the panels can be tilled with a disc, field cultivator, or chisel plow to prepare the soil. In some cases, mulch can be applied. Prior to seeding, the ground should be sufficiently soft to allow for seed penetration but still provide surface soil stability. The soil should be prepared as appropriate to the seed mix type and site conditions.
Researchers examine a pollinator test plot. Photo from NREL Image Gallery
Re-vegetation should be timed such that seeding/plantings are done at a time congruent with the area’s growing schedule/season, to the extent practicable with the construction schedule. If final construction occurs in winter months, reseeding augmentation may be necessary in the following spring, depending on local conditions.
Seeding of a site can often be successfully accomplished using broadcast seeding techniques. In small areas, seeds can be uniformly distributed by a mechanical, hand-operated seeder; or by hand. Larger areas can utilize tractors or other equipment that broadcasts seeds. Following seeding, the surface should be raked with tractor equipment, a cultipacker, or hand raked, as necessary.
A temporary cover-crop seed mix may be broadcast-seeded to provide temporary cover and reduce the potential for noxious weed invasion while native vegetation becomes established.
Low-Impact Strategies: Identifying Appropriate Seed Mixes
Native vegetation grows at the solar site. Photo by Rob Davis / Fresh Energy
Solar developers should work with regional vegetation professionals to develop a diverse and appropriate seeding plan for each site. Seed mixes can vary throughout the site underneath the solar array, depending on site conditions (e.g., parts of wetlands or differing soil types), aesthetics, or for experimental purposes. Seed mixes might also vary throughout an entire project site, with different mixes on the site perimeter, on the solar array perimeter, and underneath the solar arrays.
The preferred method for re-vegetation of disturbed areas within wetlands is reliance on re-vegetation by resident hydrophytic plant communities. In some situations, a disturbed area may be dominated by native wetland plant species with rhizomatous root systems. In these cases, these root systems may be able to recolonize small areas of disturbance rapidly.
Seed Mixes should be locally sourced, when possible, and purchased on a Pure Live Seed (“PLS”) basis for seeding re-vegetation areas. Seed tags should identify purity, germination, date tested, total weight, PLS weight, weed seed content, and the supplier’s name and contact information.
Seeds should be used within 12 months of testing. The seeds should also be certified by the supplier as being free of noxious weeds.
Low-Impact Strategies: Re-Vegetation using Native, Low-Growth, and Pollinator-friendly Vegetation
Different ground cover choices may impact solar site design. For example, if a developer chooses native vegetation ground cover the racking system may have to be raised to accommodate vegetation height. Leaving the ground bare or installing gravel may change the stormwater characteristics on the site requiring larger retention ponds to manage stormwater runoff. In contrast, establishing vegetation could reduce the water runoff coefficients and allow the developer to reduce stormwater mitigation efforts, which can reduce the installed cost and require the developer to purchase less land.
Equipment is used for groundcover establishment. Photo by Jordan Macknick
Ground Cover Impacts on O&M
When selecting ground cover for the solar site, future O&M measures should be balanced with the cost of ground cover establishment. Installing low growth grass will reduce the number of mowing events needed for the year compared to other vegetation types but may require a few years of management and water upfront to establish the grass. Installation of gravel on the site will inhibit vegetation growth but will require operators to use herbicide to control weeds on site. Mowing events on gravel installations can introduce risks of rocks being caught up in the mower and potentially flinging these rocks into solar modules, damaging equipment. Installation of pollinator friendly habitat may limit the time of year for mowing events and cause operators to reduce herbicide use that can lead to complications in managing vegetation. Gravel and bare ground sites may also increase dust issues and require the operator to perform costly module washing efforts more frequently. Several industry members noted the high expense of landscaping fabric designed to control weeds and the lack of a long term weed abatement solution it can provide.
Industry Identified Best Practices and Challenges
During conversations and informal survey industry members identified several best practices for ground cover. Numerous installers mentioned that establishment of low-growth vegetation to reduce O&M on the site. When establishing native and low-growth vegetation, industry members prioritized working with local botanists and other resources such as local BLM offices and agricultural extension offices to design native, low growth seed mix tailored to each site and soil type within the site. Working with local botanists and resource can increase the likelihood of vegetation establishment at the site. If possible and feasible, prioritize native pollinator-friendly vegetation to provide benefits to nearby agricultural crops and promote goodwill in the local community.
While revegetation was the most favored ground cover, many survey respondents noted significant challenges with reestablishing vegetation, with failures including:
- Reseeding before piles and racking were installed which prevented establishment
- Hydroseeding in extremely hot climates, which did not take and required ongoing maintenance from resulting erosion
- Rainfall not meeting predictions, which prevented seeding from establishing
- Lack of irrigation water to establish vegetation
- Difficulty re-establishing native vegetation after it is removed from a site due to competition from weeds and other invasive species
Applying gravel as a ground cover was widely identified by industry interviewees as expensive and problematic as it creates uneven work surfaces, changes runoff coefficients, and does not provide a long term weed abatement solution. Gravel applications were described as requiring either regular application of herbicides, which can be restricted by local regulations, or mechanical weed control, which can kick up rocks and damage modules. However, survey respondents mentioned that gravel may be more feasible at certain sites that are not beneficial for establishing vegetation.
“Revegetation is not only possible but can achieve ground cover sufficient to control erosion and to begin to restore wildlife habitat. Further, successful establishment of low-growing warm-season grasses also achieves the goal of retaining short vegetation to avoid interference with panels and minimize fuel load for potential wildfires.” (Beatty et al. 2017)
“Low growth or beneficial vegetation underneath modules reduces the need for mowing and can prevent erosion, dust, or reduce the amount of stormwater prevention measures needed for the entire site.”
“Not vegetating after construction increased project cost due to increased drainage/run-off. Roughly 20% increase in civil costs due to changing drainage and for controlling sedimentation and run-off.”
Solar Policies and Incentives to Protect Native Vegetation and Pollinator Habitat
Some states have policies and incentives designed to encourage low-impact solar development strategies like protecting native vegetation and establishing a pollinator-friendly habitat.
Common ground cover types include: leaving the earth bare after construction; limiting vegetation growth, either through installation of an imperviable mat or applying pre-emergent herbicide to the site; covering the site with gravel; seeding with low growth or native vegetation; and some combination of the above.
A pollinator-friendly solar groundcover sits in a bed of wildflowers at a solar site. Photo by Rob Davis / Fresh Energy
The type of ground cover suitable for each site can be highly dependent on site-specific characteristics, such as previous land use and climate. For example, some solar sites in desert regions often employ either bare ground or gravel cover. The dry climate and remote locations lead to issues with obtaining water for establishing vegetation and there are less issues of invasive weeds due to the lack of water. However, the O&M considerations in the desert are different as dusty, dry climates can facilitate the need for washing panels often which can provide a water source for weeds and impact site performance. In other, more temperate climates (e.g. Minnesota, Oregon, etc.) issues arise with invasive species that move in after ground disturbance and O&M considerations include steps to prevent certain vegetation growth. Industry members indicated that permitting requirements and vegetation establishment costs are the primary drivers of the choice of ground cover for solar sites.
Pollinator-Friendly Ground Cover Policies
Policies and incentives for low impact development include several state statutes that encourage establishing pollinator-friendly or other habitat-supporting ground cover in coordination with local agricultural extension services or other partners. Maryland enacted a 2017 state statute to establish a pollinator-friendly vegetation management standard in coordination with recommendations from the University of Maryland Bee Lab. Minnesota also established standards for pollinator-friendly ground cover at solar sites in 2017 and South Carolina enacted voluntary solar best-management practices to establish native vegetation and pollinator habitat in the Solar Habitat Act.
Stearns County, Minnesota, a state leader in solar development, requires solar farm ground cover to meet the standards established by the state statute in their Land Use and Zoning Ordinance. Linn County, Iowa moved ahead without state enabling legislation and amended the local Development Code to require that solar farms be planted with native grasses and wildflowers in addition to prohibiting the application of insecticides.
Avoiding Prime Agricultural Soils
The state of Oregon enacted a highly detailed approach to protecting high-value farmland and arable land from excessive encroachment from photovoltaic power generation facilities, while still allowing for development. The state’s approach is based in part on the Natural Resource Conservation Service (NRCS) Crop Productivity Index (CPI) that is frequently used to define agricultural soils in protection programs. Counties within Oregon are implementing the state land use requirements.
State Policies that Encourage Solar Development on Contaminated Lands
Several states have policies that encourage consideration or create preferences for developing contaminated lands with solar.
- Maryland has a community solar pilot program to create separate program capacity for small systems and systems built on brownfields, parking lots or industrial areas. This is a three-year community solar pilot program (started April 2017) which aims to increase the opportunity to invest in or contract for solar generating equipment for all Maryland ratepayers.
- Massachusetts has the SREC II (solar renewable energy certificates) program that includes specific incentives for renewable energy on landfills and brownfields. A specific market sector under SREC II is designated for eligible landfills and brownfields and assigns an SREC factor of 80%, which indicates the percentage of the production output from their PV system that is eligible to generate SREC IIs.
- New Jersey has the 2012 Solar Act which explicitly identifies solar-electric systems on brownfields, areas of historic fill and closed landfills as eligible to generate Solar Renewable Energy Certificates (SERCs).
- Vermont Act no. 99 relating to self-generation and net metering offers specific considerations to facilitate solar installations on landfills.
Resources and Further Information
Solar Site Design and Construction Considerations to Protect Native Vegetation and Pollinator Habitat
When thinking about a solar site's design and construction along with associated costs, consider low-impact strategies for removing vegetation, controlling erosion, and managing weeds, among others.
The upfront design of a solar facility can often impact the total installed cost along with needed O&M spending over the life of the project. Often the most important thing for reducing costs is minimizing grading before equipment installation. However, a less uniform slope can lead to changes in the racking design and may not allow for tracking systems. Cost savings from reduced grading can sometimes be partially offset by additional racking expenses as the length of many of the posts must increase to maintain a consistent height across varying topography. Steps can be taken during construction to minimize immediate impacts on the environment that could lead to costly issues later.
Construction workers build site of solar project. Photo by Jordan Macknick
Low-Impact Strategies: Racking and Construction Considerations
Construction using driven posts minimize impacts on groundcover. Photo by Jordan Macknick
Even though a best practice of site design and construction is a reduction in grading, the solar developer may incur higher costs trying to install the racking system at the same height for module installation if the land is uneven. There are several different racking systems that solar installers can use, including concrete sleeper and rammed posts.
With concrete sleepers, developers can place concrete blocks that act as foundations for the solar equipment. This option can preserve local vegetation, be fast to install, and allow for easy site remediation after project life. However, this option requires very level ground and may lead to increased grading. Also, only fixed tilt systems can be employed for this option.
Rammed posts, however, are more conducive to uneven terrain and can help facilitate the avoidance or reduction of grading. Rammed posts involve using a machine whereby solar installers can drive posts into the ground that provide the foundations for installing PV systems. This option has become an industry standard as it is cheap, fast, and can accommodate different grades. Also, rammed posts can be used for both fixed tilt and tracking systems. Installing rammed posts may also allow developers to preserve native vegetation and habitat and reduce grading. Soil and rock conditions (sometimes not surveyed upfront during site selection) may impact rammed posts and unideal conditions can lead to project delays.
Installers can minimize soil compaction during construction by avoiding construction activities during wet and rainy conditions, establishing set pathways for construction equipment, and minimizing the duration of construction equipment on-site.
Low-Impact Strategies: Vegetation Removal
Minimal vegetation removal and some grading could still be required in order to provide a relatively level surface for the solar arrays and racking system. Vegetation removal might also be needed for access roads and construction laydown areas. Brush, trees, and some herbaceous vegetation might need to be cleared to facilitate access for workers and equipment, as well as to meet safety standards. Vegetation removal can be accomplished with the use of handheld non-motorized equipment as well as with chainsaws, mowers, and hydraulic tree-cutting equipment, if necessary. To minimize disturbance, vegetation can be cut at or slightly above the ground surface. Rootstock or stumps can be ground down to the soil surface. Rare and unique natural resources should be given prioritization to remain or be transplanted.
Low Impact Strategies: Erosion Control
If necessary, the developer should obtain authorization under a National Pollutant Discharge Elimination System (NPDES) Permit. A Stormwater Pollution Prevention Plan (SWPPP) should be prepared in accordance with the NPDES prior to initiating any construction. During the construction phase, installers should implement appropriate measures to stabilize any recently graded and/or exposed soils. Soil replacement and/or soil amendments may be necessary in some areas.
Specific erosion control measures may include, but are not limited to:
- Minimizing vegetation removal, grading, and other ground disturbance
- Erosion control materials/mats/fabrics
- Sandbags
- Weed-free mulching
- Protective berms
- Silt Fences
Low Impact Strategies: Minimizing and Managing Weeds during Construction
To prevent the introduction of noxious weeds and invasive species (NWIS) on lands disturbed by construction activities, the following practices can be beneficial:
- Minimize soil disturbance and grading to the extent possible
- Check construction equipment for weeds
- Clean equipment prior to arrival onsite to prevent the introduction and spread of NWIS into the facilities from offsite locations
- Re-vegetate with intended plant species (e.g., low-growing perennials) as soon as practicable to discourage noxious weed growth
- Encourage early detection and eradication of patches of weeds through appropriate measures
- Locate and use weed-free staging areas if there is an issue at the site
- Mulch and wood chips used for soil stabilization should consist of certified weed-free material
Industry Feedback and Testimonials
Based on discussions with industry members, the following best practices were identified:
- Use and development of solar racking technologies that can handle greater tolerances for topography, thus requiring less site grading work
- Avoid sensitive areas, like wetlands or drainage features, that require special design, this leading to permitting delays and cost increases
- Keep inverters and other electrical equipment out of flood plains if possible as this can cause increased racking and insurance costs
- Driven posts are advantageous to reduce cost and minimize environmental impact. Driven posts can reduce the loss impact for wetlands, thereby avoiding extensive permitting requirements
One industry representative was quoted saying, “Measurable environmental costs probably don’t exceed $0.10/watt. However, the most extreme environmental-related costs are associated with the significant delays in project completion resulting from problems obtaining environmental permits.”
Solar Site O&M Considerations for Vegetation Management
O&M considerations related to low-impact solar development mainly involve managing vegetation on site. This includes desired vegetation seeded after construction as well as non-desired vegetation, such as invasive species that moved into disturbed areas on the site.
Vegetation management can often account for 3-8% of yearly O&M spending. Typical O&M activities to manage vegetation involve mowing, application of herbicide, and hand pulling weeds (at certain sites).
The long-term maintenance of the ground cover and drainage should be considered in the design, civil engineering, and construction phases of ground-mounted systems to reduce O&M risks and costs. In climates with high rainfall, for example, grass cutting and vegetation control costs can equal or exceed equipment O&M costs.
Native vegetation blooms at the solar site. Photo by Jordan Macknick
Upfront site selection, design, and ground cover can heavily influence the types of O&M practices needed on site. Siting a solar facility in the desert to take advantage of optimal solar resources may lead to increased spending to clean the dust and dirt off the modules. Installing gravel for ground cover may cause the operator to rely on herbicide application and costly hand pulling of weeds to prevent stray rocks from damaging equipment during mowing events. Often, more specialized O&M practices can lead to increased cost for the operator.
Mowing
Native vegetation blooms at the solar site. Photo by Jordan Macknick
Mowing grass and vegetation on solar sites can often be an effective measure in controlling the height of vegetation. Tall vegetation can lead to a reduction of solar generation and can cause degradation of PV cells that are shaded (electricity flows into the shaded region the results in high temperature in the cell and higher rates of degradation). Reduction of mowing events is preferred due to risk of projectiles, but often it may be the most effective control. Mowing can cost anywhere between $50-$100/acre per event. Tree removal may also be needed on solar sites to remove saplings. When mowing pollinator-friendly vegetation, it is important to time mowing events to not adversely affect seasonal blooming. The use of native vegetation and pollinator-friendly habitat can help reduce the need and frequency of mowing. Mowing efforts should occur in late spring and early fall to avoid impacts to potential ground nesting species. Material that is mowed in many cases should be bagged and removed from the site to prevent smothering of the vegetation.
Weed Management
“Chemical vegetation abatement may be more efficient and at times less costly, particularly in arid areas. However, several NREL industry survey respondents cited problems with soil stabilization after herbicides eliminated vegetation; the added risk to, and safety requirements of, those handling the chemicals; and, most often, local or state regulations that restrict herbicide use.” 5
Most industry contacts surveyed mentioned efforts to reduce herbicide use both from a cost and environmental impact perspective. Particularly resilient weeds may only be removed through herbicide use due to long root systems and tolerance to mowing. Herbicide can also be used to “beat back” weeds and allow native and or otherwise beneficial vegetation to thrive. Any herbicide use should be spot-sprayed to specifically target the NWIS colony and minimize killing non-targeted plants. Broadcast treatment of herbicides should not be utilized.
Ground cover choices may also lead to herbicide use. Vegetation on gravel covered sites can only be removed through herbicide or hand pulling due to risk of projectiles from mowing events. Herbicide application will need to be performed by specialized contractors due to safety concerns with the chemicals used and the need to avoid spraying herbicide on modules that will reduce generation. The use of native vegetation can help eliminate the need for herbicide when the native vegetation outcompetes weeds.
Animal Grazing
Sheep grazing can reduce mowing and herbicide use and provide another revenue stream for herders. Those with experience in this area report greater success when forage needs are taken into consideration in the design phase and determining the re-seeding mix. If a solar operator is considering using sheep grazing for vegetation management then modules, wires, and electrical boxes should be raised to prevent possible damage to equipment. Sheep that are raised for meat are often preferred as they tend to be more docile than those raised for wool production. Often a herder will require access to a water source on a solar site that needs to be provided by the operator. Insurance companies are comfortable insuring solar farms that use sheep for grazing as they have been used on numerous solar sites in the U.S. and in Europe.
Sheep can also be beneficial when coupled with native plant establishment, but timing of sheep grazing will need to be accounted for. Sheep can reduce weed cover and allow native vegetation to take over after sheep hooves bring up latent seeds. However, care should be taken to prevent the sheep from eating vulnerable, smaller native species. Also, if pollinator friendly habitat is being cultivated care around flowering schedules and sheep grazing should be exercised to allow for pollinators to take advantage of the flowering habitat.
Other animals have been proposed for grazing, such as cows, buffalo, and goats. Cows and buffalo are not preferred due to their large weight and tendencies to rub up against and damage equipment. Goats are not preferred as they are likely to chew on electrical wires and jump up on modules, possibly damaging modules and leading to shading.
Sheep graze at solar site. Photo by Rob Davis / Fresh Energy
Industry Feedback and Testimonials
Several survey interviewees cited significant problems with panels mounted too close to the ground to allow access under the panels by an arm of a mower, causing significant costs for more labor-intensive vegetation management approaches.
Initial design considerations that can significantly impact O&M costs for ground-mounted systems include ensuring that panels are mounted with sufficient and relatively uniform clearance from the ground and racking is spaced widely enough to allow access for efficiently sized mowing and cleaning equipment and to protect panels from damage from such equipment (EPRI 2010) (Brehaut 2015).
Resources and Further Information
Low-Impact Solar Development Partnerships and Outreach
The success of low-impact solar development can be facilitated through strategic partnerships and effective outreach and communication plans.
Partnerships can help encourage greater acceptance of a project from local stakeholders and can also help incorporate local expertise. Partners can also improve communication and outreach activities by providing an additional, and potentially more well known locally, outlet for stakeholders.
NREL researcher Jordan Macknick works with teams from UMass Clean Energy Extension and Hyperion on a photovoltaic dual-use research project. Photo from NREL Image Gallery
External Partnership Opportunities
A group of visitors explore a pollinator friendly solar array. Photo by Jordan Macknick
Solar project developers should consider a wide variety of partnerships for each solar project. Specific partners to target could include: state agencies, universities, environmental and conservation organizations, local K-12 schools, and agricultural organizations. In developing partnerships, there should be a clear purpose and mutual benefits associated with each partnership. When working with state agencies, partnerships could focus on sharing data on the success rate of low-impact solar development techniques and approaches. The project site could become a testbed for state agencies to evaluate potential regulations or incentives. University partnerships could be forged around conducting research on-site as well as for other educational purposes. Environmental and conservation partnerships could be formed around the demonstration of how solar projects can provide multiple ecosystem service benefits. K-12 school partnerships could focus on field trips and educational activities. Partnerships with agricultural organizations could focus on how solar projects can benefit agriculture through the use of pollinator-friendly vegetation as well as how the ground cover activities could serve to benefit soil quality.
Interactions with Surrounding Landowners
Healthy relationships with surrounding landowners and neighbors can be an important mechanism for ensuring timely project development and broad community support. Relationships with neighbors can have a higher likelihood of success if there is transparent and effective communication between the landowner and the developer. Personalized letters, in-person visits, and/or phone calls could help initiate conversations with neighbors. Clearly communicating project activities, including how the project could potentially provide benefits to surrounding farms, soil quality, education, research, and the local economy is important.
Research Opportunities
Researchers survey a pollinator-friendly solar site. Photo by Jordan Macknick
As the field of low-impact solar development is relatively new, it is likely that each solar project could incorporate some form of research activity to increase the state of knowledge and provide unique local insights. Research outcomes have the potential to not only benefit the individual site on which it occurs, but also to have broader impacts on knowledge in that area, at the state-level, and in other parts of the country (or world) with similar conditions. Research activities could be performed by trained staff from local universities, national laboratories, engineering/consulting companies, or by the solar developer. In some cases undertaking research (such as identifying pollinator-friendly species that are blooming) could be a team-building activity for staff. Some specific research opportunities for low-impact solar development include:
- Performance of different seed mixes underneath the solar array
- Effectiveness of alternative planting or vegetation management approaches, such as: utilizing a cover crop, planting in the fall vs. in the spring, site preparation variations, mowing and maintenance timing variations, irrigation variations, etc.
- Microclimate conditions underneath solar arrays with native vegetation or pollinator-friendly habitat
- Impacts of microclimate conditions on solar array output
- Soil moisture and quality tests
- Carbon sequestration of native vegetation and pollinator-friendly habitat
- Pollinator and beneficial insect populations
- Apiary and beekeeping opportunities
- Economic benefits and tradeoffs of low-impact solar
- Ecosystem services provided by low-impact solar
Communication and Outreach
Developing effective communication and outreach materials can help raise awareness regarding the benefits of low-impact solar development. Materials and approaches can take a variety of forms, including:
- Signs along the fence or perimeter of the project highlighting low-impact features of the project
- Informational brochures on various low-impact solar development features
- Public events at or near the project site for educational purposes
- Quarterly newsletters highlighting the success of the project and its low-impact features
- Targeted communications to local landowners or the community on project activities
- Press releases to local news outlets
Agricultural Co-Location Considerations for Solar Site Preparation
For site preparation, consider low-impact solar development strategies for agricultural co-location, such as minimizing grading and accommodating existing topography.
Construction activities at most large-scale, ground-mounted installations of photovoltaic (PV) arrays are preceded by land clearing and re-grading to standardized slope and surface conditions to facilitate construction access, racking installation, and facility operations and maintenance (O&M). The impact to existing vegetation may be total eradication followed by installation of a gravel cover and subsequent application of herbicides. NREL cost modeling estimates that grading practices can account for 3-6% of total installed capital costs of utility-scale solar projects, and all conventional site preparation practices are expected to account for 20% of total utility-scale PV installed costs by 2020 (DOE, 2012). When considering agricultural co-location, efforts should be made to minimize disturbance and compaction to the soil, which can have negative impacts on crop production outcomes.
Site preparation for planting at agriculture co-location site. Photo by Amy Garrett
Low-Impact Strategies: Reduced Grading and Working with Existing Contours
Mass grading can strip fertile soils from the site, preventing current and future agricultural use of the land. Grading on greenfield sites may also remove native vegetation and damage habitat for native flora and fauna. Mass grading can also disturb land and provide a pathway for weeds and other invasive species to grow. Application of herbicides to prevent weeds on graveled sites over the life of a solar installation can prevent any future use of the land for agricultural production and potentially contaminate nearby waterways and wetlands.
Grading requirements differ depending on whether a PV system is tracking or fixed. Single-axis tracking systems are increasingly common among large-scale PV installations, accounting for nearly 80% of the utility-scale PV capacity installed in 2017. Single-axis tracking systems, however, can only accommodate slopes of approximately 3-6%. Within this range, the slope must be consistent across the rows, which can be 300’ in length. Longer and variable pile lengths may be used to maintain a consistent array slope with more variable topography underneath the arrays; however, this incurs higher costs for the additional racking material.
New techniques and racking systems can reduce the grading needs and accommodate up to 15% slopes, but sites must still have a consistent slope to accommodate most tracking systems. The added costs of techniques to accommodate existing topography may be offset by reduced costs of weed control, erosion, and dust mitigation over the life of the project. Reduced grading maintains the surface layer of the soil intact and can prevent intrusion of weeds or other invasive species into the project site.
Fixed PV systems can accommodate significantly more variation in the topography. If a site is slightly rolling over a large area, variable racking post lengths may be able to eliminate the need for significant grading. However, if the variation in the topography occurs over a smaller area, more grading will be required to accommodate tracking systems and to establish suitable and safe terrain for workers and equipment for fixed systems.
Reduced grading can preserve natural vegetation where it exists, minimize weeds and erosion that can result after vegetation removal, and potentially expedite permitting. Selecting sites where minimal grading is needed, such as marginal or no longer productive tilled agricultural land, can help minimize the costs and impacts from grading.
According to Goodrich, James, and Woodhouse (2012), sites well-suited for solar installation and low impact design practices were found to reduce the overall cost of site preparation. Also, low-impact site preparation practices were found to increase the probability of a successful and timely environmental permit review process. Reductions in grading of the project area were found to reduce project costs and yield community acceptance more readily compared to more impactful site preparation practices.
While reduced grading can save civil engineering and construction costs, it may also lead to higher costs for racking as post lengths may increase to ensure sufficiently uniform PV array heights. Complicated racking systems needed to accommodate uneven topography and avoid mass grading can in turn increase construction time and labor costs.
Grading and the resulting soil disturbance can lead to additional weed abatement costs and may add more costs to re-establish ground cover. The exposed soils from grading add costs for stormwater pollution prevention measures and add the risk of potential stormwater pollution violations.
Industry Feedback and Testimonials
In feedback gathered by NREL from 38 large-scale solar industry members with knowledge of the site development process, the most common method cited to reduce site preparation costs was minimizing grading and accommodating existing topography. Industry research found significant variation in the costs of site preparation, supporting the idea that this is an opportunity for significant cost savings. Land preparation practices in Goodrich, James, and Woodhouse (2012), including ground leveling, sediment control, hydrology, road construction, and vegetation clearing, cost between $5,000-65,000 per acre.
The grading that does need to occur for equipment pads, access roads, and tracking systems can stockpile topsoil for future use. Stockpiling more productive soils with higher organic material content allows these soils to be set aside and then spread over the graded surface to support re-establishment of a vegetative ground cover.
Industry members indicated the potential benefits of minimizing grading, particularly on greenfield sites, include:
- Reduced civil construction costs
- Reduced risk of cost overruns
- Minimized impacts on topsoil in prime agricultural areas
- Reduced permitting costs and delays
- Minimized soil stabilization, revegetation, and weed control costs
Suitable Agricultural Activities for Low-Impact Solar Development
A wide variety of agricultural activities are suitable for integration with solar installations. Agriculture co-location can include growing crops, grazing livestock, supporting pollinator habitat, as well as hosting apiaries.
The success of the agricultural activity will be determined by local conditions as well as management practices. In general, most agricultural activities that are possible in a specific region will also be possible in an agriculture co-location context. Certain exceptions include large-scale agricultural activities that require massive equipment (e.g., corn, soybeans), crops that grow extremely tall, or incompatible livestock (e.g., goats, bison).
Site preparation for planting at agriculture co-location site. Photo by Amy Garrett
Agricultural Crops
Student researchers plant vegetable crops underneath solar arrays. Photo by Greg Barron-Gafford
Agricultural crops can thrive underneath the partial shade conditions of solar installations. Crops that are successful in a particular region are likely suitable in a co-location context. Crops can be grown directly underneath panels, in between rows of panels, and outside the perimeter of the solar installation. The partial shade conditions of solar installations can lead to changes in performance and maintenance. Some specific differences are highlighted below.
Sunlight The partial shade will likely reduce the amount of direct sunlight reaching the crops, while also changing microclimate and hydrological conditions.
Microclimate Below the panels the crops are likely to be cooler during the day, but warmer at night. This has the impact of reducing heat stress as well as reducing risks of frost damage. It can also extend growing seasons in multiple regions. Humidity and wind patterns are also likely to change.
Soil Moisture Conditions underneath the panels also will likely increase soil moisture levels, which can lead to a reduction in irrigation needs. This can lead to a challenge in wetlands or other extremely wet environments, if too much water is already a concern for agricultural crops.
Research is still ongoing to best understand the relative performance of crops in agriculture-solar co-location configurations compared with those in the open-air. In general, results indicate that there can be both benefits and tradeoffs of co-location, depending on crop selection and specific location. In some arid environments, for example, research highlights substantially higher yields with lower water requirements for many crops, indicating great successes. In more temperate environments, results are more nuanced, with panel spacing and specific summer conditions playing an important role in determining eventual yields and impacts.
Pasture and Livestock
Livestock can effectively manage vegetation underneath solar arrays, and can provide multiple benefits to solar providers and grazing entities. Sheep grazing can reduce mowing and herbicide use and provide another revenue stream for herders. Those with experience in this area report greater success when forage needs are taken into consideration in the design phase and determining the re-seeding mix. If a solar operator is considering using sheep grazing for vegetation management then modules, wires, and electrical boxes should be raised to prevent possible damage to equipment. Sheep that are raised for meat are often preferred as they tend to be more docile than those raised for wool production. Often a herder will require access to a water source on a solar site that needs to be provided by the operator. Insurance companies are comfortable insuring solar farms that use sheep for grazing as they have been used on numerous solar sites in the U.S. and in Europe.
Sheep can also be beneficial when coupled with native plant establishment, but timing of sheep grazing will need to be accounted for. Sheep can reduce weed cover and allow native vegetation to take over after sheep hooves bring up latent seeds. However, care should be taken to prevent the sheep from eating vulnerable, smaller native species. Also, if pollinator friendly habitat is being cultivated care around flowering schedules and sheep grazing should be exercised to allow for pollinators to take advantage of the flowering habitat.
Other animals have been proposed for grazing, such as cows, bison, and goats. Cows and bison are generally not preferred due to their large weight, tall height, and tendencies to rub up against and damage equipment. Goats are not preferred as they are likely to chew on electrical wires and jump up on modules, possibly damaging modules and leading to shading.
Other Vegetation
Other vegetation can be grown underneath and around solar installations that can benefit local agriculture. Specifically, growing native vegetation and/or pollinator habitat can provide multiple benefits to soil quality and agricultural yields. Growing native vegetation and pollinator habitat can help improve soils by retaining and building valuable nutrients, improving water infiltration, and reducing erosion. Pollinator habitat can improve local agricultural yields by providing habitat and forage for beneficial insects that can assist nearby farms.
Apiaries
Beekeeper inspects hives near a pollinator friendly PV array site. Photo from NREL Image Gallery
When growing pollinator-friendly vegetation underneath solar arrays, apiaries can often be a suitable accompanying agricultural activity. Pollinator-friendly solar arrays can provide forage for honeybees as well as serve as a wind block, when apiaries are located alongside solar installations. Apiaries can be both within or outside of the specific project perimeter; honeybees have a range extending a few miles, and are likely to visit multiple sites each day.
Policies and Incentives for the Co-Location of Solar and Agriculture
Very few policies and incentives exist currently for the co-location of solar and agriculture.
NREL researcher Jordan Macknick works with teams from University of Massachusetts (UMass) Clean Energy Extension and Hyperion on a photovoltaic dual-use research project. Photo from NREL Image Gallery
State-level Policies and Incentives
The Massachusetts Department of Energy Resources has established the Solar Massachusetts Renewable Target (SMART) program, which supports agricultural co-location through what it calls “dual-use.” In the state of Massachusetts, specific kinds of dual-use systems are known as “Agricultural Solar Tariff Generation Units,” and can qualify for financial incentives under the SMART program. In these systems, every square foot of land possible must be maintained in continuous agricultural production. The SMART regulation can be found on the MA DOER website. More detail from the UMass Clean Energy Extension: https://ag.umass.edu/clean-energy/fact-sheets/dual-use-agriculture-solar-photovoltaics.
Solar Site Configuration and Construction Considerations for Agricultural Co-Location
When configuring and constructing a solar facility, it is important to explore low-impact strategies for the co-location of agricultural activity.
The upfront design of an agriculture and solar co-location facility can often impact the total installed cost along with its associated O&M spending over the life of the project. Often the most important mechanism for reducing costs is minimizing grading before equipment installation. Cost savings from reduced grading can sometimes be partially offset by additional racking expenses as the length of many of the posts must increase to maintain a consistent height across varying topography, and many agriculture co-location projects have elevated and/or spaced panels, which can also lead to higher installation costs. Steps can be taken during construction to minimize immediate impacts on the environment that could lead to costly issues later.
Vegetables grow under PV arrays at a test plot as part of a project researching growing crops under PV arrays while simultaneously producing electricity from the panels. Photo from NREL Image Gallery
Agricultural Co-Location Site Configuration Options
Example of an agrivoltaics configuration. Photo by Greg Barron-Gafford
The configuration of the solar installation can have important implications on the type and the eventual success of agricultural activity on-site. Still, almost any ground-mounted solar configuration could enable some type of agricultural activity underneath or around the structure. When designing the solar installation, it is important to consider the type of agricultural activity and the expected needs. Other specific design considerations include the location of agricultural activity, access for workers and equipment, access to water, panel height, panel spacing, row spacing, as well as the type of tracking system. Each of these elements is addressed below.
Location of Agricultural Activity Co-location agricultural activities can occur underneath the solar infrastructure and/or surrounding the structures, including in between rows and surrounding the perimeter of the project. Under most configurations, sufficient direct and/or indirect sunlight will reach soil underneath solar arrays to support thriving vegetation. However, it is important to avoid having vegetation of any kind grow taller than the bottom edge of PV installations such that they would shade the PV panels or grow into the backside of the panels. This tradeoff could influence decisions on panel heights, vegetation/crop types, and vegetation/crop locations. In some cases, taller vegetation/crops could be grown in the rows between the panels, with other types of vegetation directly beneath panels to avoid shading or vegetation-equipment interactions. Elevated panels can avoid many of these conflicts and can support higher agricultural yields. The InSPIRE project is undertaking research where agricultural activities are located in a variety of different locations with different solar infrastructure configurations, including directly underneath panels, in between panel rows, and outside the perimeter of the solar project.
Access for Workers and Equipment Different types of agricultural activities and crops will require different levels of maintenance, different types of equipment, and different frequencies of visitation. The expected access requirements for both workers and equipment should be considered in the design of the co-location system. If all crops will be hand-harvested and infrequent visits are expected, then designs could differ from those that include crops that would be harvested with mechanized equipment or that might require more frequent maintenance visits. Designs should safely accommodate the type of access that is needed. In some cases, this could lead to a need for higher panels, larger spaces in between rows, or spaces in between panels. Operators can also implement safety practices on-site. One example safety practice with a tracking array could be positioning the panels to be horizontal any time workers are performing maintenance underneath the arrays.
Access to Water If irrigation is to be utilized for the agriculture activity, providing access to water could affect solar design configuration. In some cases, water might be hand-delivered. In other cases, drip or sprinkler irrigation could be utilized. Drip irrigation could involve piping along the ground or piping that follows the underside of the panels. Sprinkler irrigation could involve sprinklers in the ground, sprinklers attached to the underside of the panels, or sprinklers on top of the panels that wash panels and then the runoff feeds the crops. In all cases, it is important to avoid water contact with electrical or other sensitive components of the solar installation.
Panel Height One of the more effective approaches to increasing agricultural viability and yields is to increase the height of panels. The InSPIRE project is currently evaluating solar PV agricultural projects where panel heights range from four feet to ten feet. Higher heights allow for greater penetration of direct sunlight on agricultural crops. Higher panel heights can also lead to greater spacing between rows to avoid panels shading each other during early morning and late afternoon times. Higher panel heights can also lead to higher installation costs due to the need for additional racking materials (both above and below ground) as well as to ensure that the installation meets local wind-loading requirements.
Row Spacing Increasing the spacing in between rows of panels can increase the amount of direct sunlight in between rows as well as underneath panels. Row spacing also must be increased when raising panel heights. Increasing row spacing will lead to larger land area requirements to achieve similar energy generation output. Increasing row spacing can facilitate safer access for workers and equipment performing agricultural activities. If rows are increased past a certain threshold, then then the beneficial or negative impacts of the partial shading that co-location provides will be reduced. Specific crops will respond differently to increased row spacing, depending on the local site conditions and management techniques.
Panel Spacing One strategy for increasing the amount of direct sunlight directly underneath solar installations is to incorporate spaces in between individual panels along rows. The InSPIRE project is currently evaluating crop performance under multiple panel spacing configurations, ranging from one foot to four feet in between panels. Increasing panel spacing can be beneficial for some crops in certain conditions, whereas it can also reduce yields for other shade-tolerant crops, depending on local conditions. Increasing panel spacing can also lead to greater penetration of precipitation and precipitation runoff directly beneath solar infrastructure.
Fixed and Tracking Systems Co-location activities can be suitable for fixed as well as tracking systems. Fixed and tracking systems will allow for differing levels of direct sunlight to penetrate the ground, and can also impact the location of precipitation runoff depending the timing of the precipitation and the positioning of the panels. Fixed and tracking systems also have tradeoffs as it relates to worker safety and access. Fixed systems cannot move to accommodate worker or equipment, whereas tracking systems move throughout the day and might have to be moved into alternative positions for safety considerations.
Low-Impact Strategies: Racking and Construction Considerations
An agrivoltaics design with elevated panels. Photo by Jordan Macknick
Even though a best practice of site design and construction is a reduction in grading, the solar developer may incur higher costs trying to install the racking system at the same height for module installation if the land is uneven. There are several different racking systems that solar installers can use, including concrete sleeper and rammed posts.
With concrete sleepers, developers can place concrete blocks that act as foundations for the solar equipment. This option can preserve local vegetation, be fast to install, and allow for easy site remediation after project life. However, this option requires very level ground and may lead to increased grading. Also, only fixed tilt systems can be employed for this option. This approach may not be appropriate for agriculture co-location.
Rammed posts, however, are more conducive to uneven terrain and can help facilitate the avoidance or reduction of grading. Rammed posts involve using a machine whereby solar installers can drive posts into the ground that provide the foundations for installing PV systems. This option has become an industry standard as it is cheap, fast, and can accommodate different grades. Also, rammed posts can be used for both fixed tilt and tracking systems. Installing rammed posts may also allow developers to reduce grading. Soil and rock conditions (sometimes not surveyed upfront during site selection) may impact rammed posts and unideal conditions can lead to project delays.
Installers can minimize soil compaction during construction by avoiding construction activities during wet and rainy conditions, establishing set pathways for construction equipment, and minimizing the duration of construction equipment on-site.
Low-Impact Strategies: Vegetation Removal
Minimal vegetation removal and some grading could still be required in order to provide a relatively level surface for the solar arrays and racking system, especially on sites that were not previously farmed. Vegetation removal might also be needed for access roads and construction laydown areas. Brush, trees, and some herbaceous vegetation might need to be cleared to facilitate access for workers and equipment, as well as to meet safety standards. Vegetation removal can be accomplished with the use of handheld non-motorized equipment as well as with chainsaws, mowers, and hydraulic tree-cutting equipment, if necessary. To minimize disturbance, vegetation can be cut at or slightly above the ground surface. Rootstock or stumps can be ground down to the soil surface. Rare and unique natural resources should be given prioritization to remain or be transplanted.
Low-Impact Strategies: Erosion Control
If necessary, the developer should obtain authorization under a National Pollutant Discharge Elimination System (NPDES) Permit. A Stormwater Pollution Prevention Plan (SWPPP) should be prepared in accordance with the NPDES prior to initiating any construction. During the construction phase, installers should implement appropriate measures to stabilize any recently graded and/or exposed soils. Soil replacement and/or soil amendments may be necessary in some areas.
Specific erosion control measures may include, but are not limited to:
- Minimizing vegetation removal, grading, and other ground disturbance
- Erosion control materials/mats/fabric
- Sandbags
- Weed-free mulching
- Protective berms
- Silt fences
Low-Impact Strategies: Minimizing and Managing Weeds during Construction
To prevent the introduction of noxious weeds and invasive species (NWIS) on lands disturbed by construction activities, the following practices can be beneficial:
- Minimize soil disturbance and grading to the extent possible
- Check construction equipment for weeds
- Clean equipment prior to arrival onsite to prevent the introduction and spread of NWIS into the facilities from offsite locations
- Re-vegetate with intended plant species (e.g., low-growing perennials) as soon as practicable to discourage noxious weed growth
- Encourage early detection and eradication of patches of weeds through appropriate measures
- Locate and use weed-free staging areas if there is an issue at the site
- Mulch and wood chips used for soil stabilization should consist of certified weed-free material
Industry Feedback and Testimonials
Based on discussions with industry members, the following best practices were identified:
- Use and development of solar racking technologies that can handle greater tolerances for topography, thus requiring less site grading work
- Avoid sensitive areas, like wetlands or drainage features, that require special design, thus leading to permitting delays and cost increases
- Keep inverters and other electrical equipment out of flood plains if possible as this can cause increased racking and insurance costs
- Driven posts are advantageous to reduce cost and minimize environmental impact. Driven posts can reduce the loss impact for wetlands, thereby avoiding extensive permitting requirements
- Encourage early detection and eradication of patches of weeds through appropriate measures
- Locate and use weed-free staging areas if there is an issue at the site
- Mulch and wood chips used for soil stabilization should consist of certified weed-free material
One industry representative was quoted saying, “Measurable environmental costs probably don’t exceed $0.10/watt. However, the most extreme environmental-related costs are associated with the significant delays in project completion resulting from problems obtaining environmental permits.”
Solar Site O&M Considerations for Agricultural Co-Location
For solar sites, several factors, such as maintenance and irrigation, can influence the types of O&M practices needed when considering agricultural co-location.
Maintaining agricultural activities can be slightly different in the co-location context. For agricultural crops, there can be differences in the approach used for planting, weeding, watering, and harvesting. Hosting livestock can be different as rotational grazing might be utilized underneath the solar arrays, and the inclusion of livestock at solar arrays will require temporary fencing suitable for both solar installations and the specific livestock.
UMass grad student Kristin Oleskwwicz and professor Stephen Herbert harvest kale growing under PV arrays at a test plot at the UMass Crop Animal Research and Education Center in South Deerfield, MA. Photo from NREL Image Gallery
Irrigation
Researcher configures irrigation lines on agrivoltaics site. Photo by Stephen Herbert
If irrigation is to be utilized for the agriculture activity, providing access to water could affect solar design configuration. In some cases, water might be hand-delivered. In other cases, drip or sprinkler irrigation could be utilized. Drip irrigation could involve piping along the ground or piping that follows the underside of the panels. Sprinkler irrigation could involve sprinklers in the ground, sprinklers attached to the underside of the panels, or sprinklers on top of the panels that wash panels and then the runoff feeds the crops. In all cases, it is important to avoid water contact with electrical or other sensitive components of the solar installation.
Access for Planting, Maintenance, and Harvesting
Different types of agricultural activities and crops will require different levels of maintenance, different types of equipment, and different frequencies of visitation. The expected access requirements for both workers and equipment should be considered in the design of the co-location system. If all crops will be hand-harvested and infrequent visits are expected, then designs could differ from those that include crops that would be harvested with mechanized equipment or that might require more frequent maintenance visits. Designs should safely accommodate the type of access that is needed. In some cases, this could lead to a need for higher panels, larger spaces in between rows, or spaces in between panels. Operators can also implement safety practices on-site. One example safety practice with a tracking array could be positioning the panels to be horizontal any time workers are performing maintenance underneath the arrays.
Low-Impact Solar Development Partnerships and Outreach
The success of low-impact solar development can be facilitated through strategic partnerships and effective outreach and communication plans.
Partnerships can help encourage greater acceptance of a project from local stakeholders and can also help incorporate local expertise. Partners can also improve communication and outreach activities by providing an additional, and potentially more well known locally, outlet for stakeholders.
NREL researcher Jordan Macknick works with teams from UMass Clean Energy Extension and Hyperion on a photovoltaic dual-use research project. Photo from NREL Image Gallery
External Partnership Opportunities
A group of visitors explore a pollinator friendly solar array. Photo by Jordan Macknick
Solar project developers should consider a wide variety of partnerships for each solar project. Specific partners to target could include: state agencies, universities, environmental and conservation organizations, local K-12 schools, and agricultural organizations. In developing partnerships, there should be a clear purpose and mutual benefits associated with each partnership. When working with state agencies, partnerships could focus on sharing data on the success rate of low-impact solar development techniques and approaches. The project site could become a testbed for state agencies to evaluate potential regulations or incentives. University partnerships could be forged around conducting research on-site as well as for other educational purposes. Environmental and conservation partnerships could be formed around the demonstration of how solar projects can provide multiple ecosystem service benefits. K-12 school partnerships could focus on field trips and educational activities. Partnerships with agricultural organizations could focus on how solar projects can benefit agriculture through the use of pollinator-friendly vegetation as well as how the ground cover activities could serve to benefit soil quality.
Interactions with Surrounding Landowners
Healthy relationships with surrounding landowners and neighbors can be an important mechanism for ensuring timely project development and broad community support. Relationships with neighbors can have a higher likelihood of success if there is transparent and effective communication between the landowner and the developer. Personalized letters, in-person visits, and/or phone calls could help initiate conversations with neighbors. Clearly communicating project activities, including how the project could potentially provide benefits to surrounding farms, soil quality, education, research, and the local economy is important.
Research Opportunities
As the field of low-impact solar development is relatively new, it is likely that each solar project could incorporate some form of research activity to increase the state of knowledge and provide unique local insights. Research outcomes have the potential to not only benefit the individual site on which it occurs, but also to have broader impacts on knowledge in that area, at the state-level, and in other parts of the country (or world) with similar conditions. Research activities could be performed by trained staff from local universities, national laboratories, engineering/consulting companies, or by the solar developer. In some cases undertaking research (such as identifying pollinator-friendly species that are blooming) could be a team-building activity for staff. Some specific research opportunities for low-impact solar development include:
Researchers survey a pollinator-friendly solar site. Photo by Jordan Macknick
- Performance of different seed mixes underneath the solar array
- Effectiveness of alternative planting or vegetation management approaches, such as: utilizing a cover crop, planting in the fall vs. in the spring, site preparation variations, mowing and maintenance timing variations, irrigation variations, etc.
- Microclimate conditions underneath solar arrays with native vegetation or pollinator-friendly habitat
- Impacts of microclimate conditions on solar array output
- Soil moisture and quality tests
- Carbon sequestration of native vegetation and pollinator-friendly habitat
- Pollinator and beneficial insect populations
- Apiary and beekeeping opportunities
- Economic benefits and tradeoffs of low-impact solar
- Ecosystem services provided by low-impact solar
Communication and Outreach
Developing effective communication and outreach materials can help raise awareness regarding the benefits of low-impact solar development. Materials and approaches can take a variety of forms, including:
- Signs along the fence or perimeter of the project highlighting low-impact features of the project
- Informational brochures on various low-impact solar development features
- Public events at or near the project site for educational purposes
- Quarterly newsletters highlighting the success of the project and its low-impact features
- Targeted communications to local landowners or the community on project activities
- Press releases to local news outlets
