InSPIRE/low impact/habitat/o m considerations
Low-Impact Development Strategies
Native Vegetation and Pollinator Habitat
O&M considerations related to low-impact development mainly involve managing vegetation on site, including 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.
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 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
“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.
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.
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).
- Best Practices in Photovoltaic System Operations and Maintenance: 2nd Edition, https://www.nrel.gov/docs/fy17osti/67553.pdf
- Huff, James. 2013. Solar Farm Grounds Management Vegetation Control. Blog. Abakus Solar, Chesterfield, VA (US). http://www.abakus-solar.us/blog/solar-farm-pv-power-plant-grounds-management-vegetation-control/
- GTM O&M Study 2015 http://splunk01.nrel.gov/store/GTM/GTM2015/Megawatt-Scale-PV-OM-Asset-Management-2015-2020-Report.pdf