InSPIRE/low impact/agricultural/site configuration and construction
Low-Impact Development Strategies
Site Configuration and Construction
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.
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.
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.
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.
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
- Weed-free mulching
- Protective berms
- Silt fences
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
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
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.”