Image credit: Chuyu

Mounting evidence suggests allowing plants to grow on solar farms can be more cost efficient than the common US PV sector practice of eliminating growth with gravel.

“There could be lower costs and less environmental litigation,” said Jordan Macknick, an energy and environmental analyst in the Energy Forecasting and Modeling Group at the US National Energy Renewable Laboratory (NREL).

NREL launched in October 2015 a three-year project called INSPIRE (Innovative Site Preparation and Impact Reductions on the Environment), to test how different vegetation regimes compare to gravel on cost reduction potential.

Current practice across much of the US is to stop vegetation growth by laying gravel under solar panels.

This is intended to reduce the operations and maintenance (O&M) cost but may not be as effective as previously thought, since the substrate still needs to be sprayed with herbicide and tends to heat up under the sun, reducing the efficiency of the PV plant.

In Europe, nurturing appropriate vegetation under and around the solar panels is more commonplace.

German developer Belectric, for example, focuses on ground-based systems which allow natural vegetation growth. Belectric has installed some 1.5 GW of solar power projects, in Europe, US, Middle East and Asia.

"Only a fraction of the area occupied is sealed. New habitat for animals and plants is created underneath and between the modules," according to documentation from the company.

In the US, other operators are also switching towards wilder vegetation plans.

At a 7.3 MW plant in Maryland, for example, developer and O&M specialist Abakus Solar USA abandoned gravel in favor of a low-growing grass mix in order to satisfy landscaping requirements for approval of the project.

The Maryland experience led Abakus to favor controlled vegetation growth.

Efficiency gains

“Plants are more effective at making sure the ground stays cool,” said Abakus chief executive James Huff. “When you put gravel down, it’s just stone. When stone gets hot it radiates [heat] upwards. What you end up doing is creating an oven effect.”

Although the exact impact on output has not been measured, “if you have a heated surface under a solar module you are definitely going to lose efficiency,” said Huff.

Another advantage of vegetation over gravel is that it is cheaper in terms of capital cost. Costs for gravel vary widely but Huff confirmed the minimum is likely to be in the region of $2,750 per acre for materials alone. There are additional labor costs for the spreading and leveling of gravel.

Meanwhile the materials cost for grass is around $1,000 per acre, Huff said. Preparing the soil and spreading seeds can be done with limited equipment and a low number of workers.

Low-growing grass needs to be weeded for the first two years of growth, according to Huff, but this does not represent a significant cost. Thereafter the vegetation is more or less maintenance free, Huff said.

As well as costing less, grass is better than gravel at reducing dust. Gravel needs to be kept moist to prevent dust from forming and soiling solar panels. This adds to the O&M effort and cost, and can be a problem in locations where water is scarce.

Vegetation, on the other hand, traps dust and does not always require watering, since plants can often survive off the runoff from water that condenses on solar panels overnight.

                         Comparison of land management strategies 

Source: Abakus Solar USA.

Additionally, allowing native plants to re-grow under solar panels may help with environmental approval or avoid the need for compensatory land mitigation purchases.

The nature of the native flora must be considered, as it may be necessary to cut the vegetation in order to prevent it from shading panels.

While low growing or native grass may present a cost efficient O&M program, a popular alternative is to use the space under panels for agricultural purposes.

Agricultural use has the added advantage of providing an extra revenue stream for the plant owner, as well as further facilitating community acceptance of projects. In Europe, grass on solar plants is frequently given over to sheep for grazing.

Provided the solar panels have a minimum clearance of 80 cm off the ground, the sheep keep the grass below the level of the panels without the need for any other O&M effort, and without compromising the output of the plant.

Other grazing animals are less suited to coexistence with solar panels.

“Cattle, horses, pigs and goats are likely to be too ‘physical’ with the solar arrays but sheep, chickens or geese should be acceptable,” according to planning guidance issued by BRE, a UK-based environmental build consultancy and certification agency.

Harvesting profits

On smaller plants it might be preferable to give the land over to cultivation of low-growing crops, particularly in land areas where solar projects compete with food production.

NREL’s INSPIRE project will seek to discover which strategies work best for different regions in the US, with different plant configurations.

“We’re going to develop guidebooks for this that will talk about what are some best practices, what are some easy as well as medium-level ways in which you can reduce the impact on vegetation and potentially increase your income,” Macknick said.

To achieve the highest gains, O&M providers will need to apply the land management practices most appropriate to the project site.

“Depending on contract requirements, we have sites that use riding mowers; some use hand-held equipment to maintain vegetation and in some we use various spraying techniques,” Joe Brotherton, president of the US solar O&M provider MaxGen Services, said.

By Jason Deign