By: Ethan Beaulieu, Energy Policy Research Assistant
Agrivoltaics can be broadly understood as the co-opting of agricultural land and practices, with solar photovoltaics (PV). Agrivoltaics can appear in a variety of structural types, each intended to create conditions in which dual use of land with solar PV arrays is viable. Agrivoltaics demonstrates that the relationship between agriculture and solar PV around land use, need not be exclusively competitive. In many cases, agrivoltaics is mutually beneficial for both those wishing to protect or restore traditionally agricultural land, and those satisfying the demand for solar PV systems.
What Solar Agrivoltaics Look Like
Solar agrivoltaics is not exclusive to large-scale utility solar PV systems. Due to the flexibility in which arrays can be installed, solar agrivoltaics is attainable for both commercial use and large-scale utility projects.
The structure type that is first brought to mind for many people is raised structures. This technique includes the installation of solar PV arrays on stilts a number of feet above the ground. Crops are then cultivated underneath the panels. The solar PV arrays have the potential to have sun tracking systems for increased efficiency, though for temporary systems, the investment may not be worth it. This structure type appears to be the most popular and occupies the majority of research on agrivoltaics. However, the raised structures are not without drawbacks. Increased initial investment is required compared to traditional solar PV arrays, due to additional material and labor. The structures are by nature larger than traditional solar PV due to their elevation which, in turn, needs to be reinforced against wind.
The second structure type, spaced, is similar to traditional ground-mounted PV systems. In this arrangement, solar arrays are mounted low to the ground and set in alternating rows with crops. These systems have significantly decreased density of solar arrays compared to other systems. However, they allow for the possibility of mechanized agriculture where the machines are able to hurdle the arrays. Such systems also are a lower initial investment compared to raised structures.
How Agrivoltaics Affect Soil Quality and Conditions
It should first be noted that there is a great deal of variety in soil types, topography, and climate conditions throughout the United States. Therefore, some issues or benefits may be more or less exaggerated depending on the environmental factors.
The first issue that arises with agricultural land that intends to be used for normal operation in the future is soil compaction during construction and decommissioning. During both of these phases, heavy machinery and equipment will be needed to install or remove arrays on the land. The weight of the machines has the possibility of compacting soil, which can reduce the productivity of the field. To minimize compaction and maintain soil quality, the following practices are followed:
Reducing the number of roads on the site and placing them on less productive land
Avoiding the use of gravel
Not removing the layer of topsoil during construction
Leaving as much vegetation as possible during installation to avoid erosion
Using pallets, wooden planks, or other means to distribute weight of machinery on roads
Drilling in or ramming steel posts rather than using large machinery during installation
It should be mentioned that while it is possible to mitigate soil compaction, certain activities have a greater likelihood of compaction. For raised systems, the installation of the structure requires that steel beams be placed at a substantial depth within the ground. The taller the structure, the farther down it must go. The associated labor and weight of material contributes to the level of compaction. In cases of severe soil compaction, tools like tractor mounted chisels, or vibrators that can break up affected soil.
Compared to raised structures, spaced ones see a lesser degree of compaction. One of the benefits of having a spaced agrivoltaic layout is that in high wind conditions, the solar arrays act as a buffer to deter erosion.
What Types of Plants Are Best Suited to Agrivoltaics?
The biggest determining factor in crop choice is shade tolerance. Sufficient light must be passing through to the ground for the crop to flourish. In the case of raised structures, this is affected by the height of the structure. Crops that require large amounts of light and suffer significantly from its reduction are not good for agrivoltaics. This would include barley, corn, wheat, and fruits. However, many leaf vegetables and berries benefit from less solar radiation. These include spinach, hops, onions, cucumber, zucchini, and a variety of other crops. As a general rule, crops that are grown in rows and do not require mechanized agriculture have been seen to perform the best economically, since the panel height is less likely to interfere with farming activities.
A number of studies found that compared to control groups, agrivoltaic systems helped the soil retain moisture, improved the production of some crops, and reduced ground temperature during the day and made it warmer at night. For many, there may be a concern that heat from the solar panels will negatively affect the crops beneath or around them. Traditional solar PV arrays with gravel ground cover may have a heat feedback loop or “heat island” effect, which exists near the array. However, the heat feedback loop created by the gravel and panel’s greater ability to absorb temperature than normal land, can be countered by using ground cover vegetation and strategic planting. Studies found that the temperature of solar panels in agrivoltaic systems were lower than that of traditional PV systems using gravel ground cover. Lower panel temperature also allows the panels to perform better.
The Bigger Picture
There are still many roadblocks to the widespread use of agrivoltaic systems, namely issues with zoning and land use. Since authority over land use is delegated to state governments and then to municipalities, there is a great deal of inconsistency in zoning law regarding agrivoltaics. While such systems still perform their agricultural function, they are subject to the same permitting and regulatory process that a conventional solar PV installation would be. Only the state of Massachusetts has a policy program intended to promote and incentivize agrivoltaic development.
Agrivoltaics may not be the ideal solution for all utility-scale PV systems, but for farmers looking to capitalize on unused crop land, those who would like to revitalize topsoil while still obtaining revenue, or would simply like to preserve traditionally agricultural land, agrivoltaics presents a valuable opportunity. More broadly, it represents the combination of what are often conflicting interests, and how co-operation can be mutually beneficial.