Solar Farm Grounds Management Vegetation Control
While operation and maintenance (O&M) are vital to the long term success of the PV industry, one of the aspects of maintaining the investor’s energy assets is often overlooked in both the design and implementation until the racking is up, the modules are on, and the grass begins to grow. But rather than jumping straight into an article about vegetation control for PV power plants, I think that it is important to tell the story that brought me to the keyboard on this issue.
I recently saw an article about a solar power plant in Germany, which was gleefully given great attention by groups that opposed the solar feed in tariff. The issue was not the tariff (although it was certainly used as a stick against the tariff, which is not a government subsidy, although mistakenly or purposefully painted as such), the issue was that the project was neglected by the investor. What interested me about the project itself was that the design seen in the photo is actually appropriate for the region and type of project, but the planning for O&M seemed to fall through the cracks. It is important to remember that the photo is a direct result of neglect and has nothing to do with the solar industry.
As abakus solar is active in building solar power plants around the world, it seems important to discuss the types of methods that can be implemented during the planning and construction phase of a project that will both reduce cost of O&M over the many years of a solar project’s life and prevent the type of damage that overgrown vegetation can cause. A google search for solutions to solar farm grounds management produces no comprehensive data, and other keywords like PV power plant grounds management or solar farm vegetation control have the same result. So it seems that the best way to correct this deficit of information is to place it here for all to read. As a thought leadership exercise for my own staff, I called a brainstorming meeting to develop a list of potential solutions, both in initial design and actual implementation.
My staff and I identified a set of seven solutions that are organized in order of feasibility through cost, risk, and on-going labor (not numbered according to recommendation since no two projects are the same). The design elements, size, and type of system will also determine the type of solution needed for managing vegetation.
The reason that sheep are #1 on the list is that they are ideal for large scale solar farms. Sheep provide the solar power plant investors the profitable opportunity to incorporate a marriage of agri-business with the production of energy. In order to accommodate sheep, a system must be designed with a clearance of no less than 80cm or 31.5 inches. This is to ensure that the animals can comfortably walk underneath the installation to seek shade and eat the grass under and around the project. A required design feature is to ensure that wiring is protected from the sheep and that the sheep are protected from the wiring, this is typically accomplished by a simple conduit. Sheep are not recommended for untrained layman, and a professional shepherd is required to care for them, although the sheep will primarily care for themselves and very little time and money is spent in caring for them.
One of the greatest advantages of sheep is that they reproduce at a rate of two lambs per yew (female sheep) per year. They are hardy animals and very low in cost. Sheep can be rescued from slaughter for less than $100 per head, although a breeding ram is slightly more expensive. So the solar project can get its biological lawn mowing system in place for less than the cost of a single mow using a crew and machines. It is a good idea to stick with sheep bred for meat rather than wool, as the wool is labor intensive and often will cost more to harvest than it is worth. After the sheep reach a population that is ideal for the acreage of the system, the excess flock can be sold at a profit.
As a note, other animals like goats are not recommended. Goats will destroy an installation because they jump, climb, break and eat everything (including wires). Bovine and horses will use the system as a scratching post or worse. And a funny anecdote from a colleague of mine details the attempts of a bull moose to mate with an array.
The sheep maintain the grass, and the PV installation provides shade in a sort of symbiotic relationship. Economically, a solar/sheep farm provides the investors with a multiple-use investment property that will not interfere with the agricultural zoning of a property and provides a secondary income stream.
Solution #2: Low Growing Grass
The idea that it is possible to seed certain types of grasses under and around the solar installation and leave it alone is extremely appealing. As long as the grasses do not grow above the clearance of the modules, they pose no threat to the installation. In fact, having grass present under the modules assists in cooling the system and improving long term efficiency. Having grass under the system prevents the ground from radiating heat upward under the modules, helps the soil retain moisture, and prevents erosion. As long as the grass never requires mowing, the risk of a lawn mower throwing projectiles into modules is eliminated. The initial cost is relatively low to seed the grounds compared to other methods and the cost of grounds maintenance is greatly diminished. Overall, this is the lowest risk choice for vegetation control with the least amount of specific design requirements. The design should have a clearance of about 18 inches or greater. The grass will not produce a secondary income stream for the project like sheep farming, but it is an effective hedge against the costs and risks of mowing or using chemicals.
Solution #3: Decorative Plants
Some plants grow well in shaded areas, and they tend to strangle out other plants like weeds. In the case of small ground mounted solar projects in a residential owner’s yard, this type of decorative ground cover may be ideal. It allows the owner to celebrate the solar installation as well as giving all the benefits of low growing vegetation. There is the need to maintain the plants, such as weeding and watering, so the design of the system must at least allow for work under the modules in a bending position. The cost of decorative planting under modules is high initially, and there are some types of plants that will thrive better than others, but the aesthetic reward for the owner can be great.
Solution #4: Low Light Crops
Some types of vegetables will grow in areas of shade (diffuse sunlight, indirect light, and morning/evening light) although slower than in direct light, where some others will not. In the case of large projects, studies need to be completed that show which types of crops can be effectively married to solar projects. But in the case of small projects and home gardens, plants like carrots, lettuce, strawberries, and broccoli are possible, where peppers and tomatoes are probably not going to work. Also, plants that are vines and grow around objects upward like peas are probably a bad idea. Incorporating a garden underneath a solar project can be expensive in terms of setting up the garden, but the expense shouldn’t be more than the cost of a standard garden. In terms of project design, the clearance under the modules should be high enough for a person to work underneath in a bending position. It is also possible to directly incorporate an irrigation system or screening to the mounting structure of the project to nourish and protect the garden. Ongoing expense is primarily expected to be in sweat equity, as time will be required to tend the garden. But many owners may find a great deal of enjoyment and satisfaction in the gardening itself.
Solution #5: Mowing
Mowing the vegetation under large solar farms is the most common method of vegetation control, but both the design of the system and the equipment to be used must be considered. The system design must allow for machinery to fit between rows of modules and the clearance must allow for the machinery to fit underneath. Mowing is usually a two-step process. First the mower or bush hog gets the large areas, then trimmers are used to cut around the structural elements and wherever the large machine cannot cut (the step neglected in the project which sparked this conversation). The risk of projectiles from the equipment damaging modules is ever present and the vegetation and pollen that is thrown can stick to modules, creating a shading effect and potentially a hot spot. Mowing should be followed by cleaning, which is also a cost. During months where mowing is required it could cost thousands of dollars in fuel, equipment, and labor for each cutting for a large project. This must be budgeted during the planning phase.
Solution #6: Chemicals
Spraying herbicide to prevent vegetation growth is a possible solution, but there are some drawbacks… In terms of environmental solutions, there are certain chemicals which are selective or become inert, but the idea of using these types of chemicals with a green technology seems counter-intuitive. If a chemical is used which kills most kinds of vegetation, the risk of land erosion is a problem, as the structural integrity of the mounting system depends on stable soil. Application of herbicides is not particularly expensive, but it must be repeated over time. Barren soil also allows the wind to carry dust onto modules, creating a shading issue. While the absence of vegetation allows some upfront savings to installation cost making the IRR (Internal Rate of Return) seem more attractive, bare earth dries out quickly and radiates the absorbed heat upward under the modules, which will reduce the ability for the system to shed heat and have a slight effect on the efficiency of modules. The loss in efficiency will not be greater than the savings to the project in the short or medium term, but dust and erosion could be catastrophic.
Solution #7: Vegetation Barriers
The number and variation of vegetation barriers available in the market is impressive, but in the case of a solar farm it may be costly and labor intensive to install. It must be placed properly and covered to prevent the wind from picking it up. It can be covered with rocks or soil to hold it down, and the soil seeded with low growing grass, but the barrier may be a redundant addition in this case. If covered with gravel, there is no mitigation to heat radiating upward but the efficiency losses would not be detrimental to the project. Also, mowing around or near the gravel could present a projectile risk. Ongoing expenses would be minimal, however. And since the project must not have a great deal of clearance, some money can saved on the mounting system initially.
While mowing may be the most common method of vegetation control, other methods such as sheep or low growing (no-mow) grass are ideal. For smaller projects in a residential setting, decorative ground cover or gardening may be attractive to some owners. Many factors must be taken into account including system size, climate, and native vegetation. But there is no problem that design, engineering, and planning cannot overcome.
CEO, abakus solar USA inc.