Saturday read: Solar’s flexibility can be agriculture’s gain

From pv magazine 02/2020

Solar’s voracious appetite for land means that as installations continue to grow, the industry will increasingly come into conflict with other important concerns, in particular agriculture. Coupled with warnings that a holistic approach to the global food-energy-water nexus is needed to prevent shortages of any of the three, finding a way to play nicely with farmers is a growing concern for many in the solar industry.

And it is a concern now being acknowledged at the highest level, underlined last month by the signing of a memorandum of understanding between the Food and Agriculture Organization of the United Nations (FAO) and the International Renewable Energy Agency that promises broader collaboration and knowledge exchange between the two sectors.

“Renewable energy is essential for agri-food systems transformation, climate resilience and net-zero strategies,” said Qu Dongyu, FAO director-general, announcing the collaboration in January. “Through our collaboration, we aim to generate and share knowledge, innovative products and technologies, as well as data and information. This agreement will allow us to strengthen the role of renewable energy within FAO’s initiatives.”

Agrivoltaics (AV), where solar generation is integrated into land already in use for agriculture, not only promises to address worries around land scarcity, but is also beginning to demonstrate useful synergies and potential benefits to both food and energy production. For no small number of projects, simply allowing sheep to graze the grass below a PV installation has proved an effective collaboration, with the panel structures offering shade for the animals and the sheep providing vegetation management at no extra cost.

Combining solar with crop farming, however, is an altogether more complex affair. Though successful projects are already underway – such as Doral Energy’s “orchardvoltaics” concept for solar and avocado farming in Israel, featured in pv magazine January 2021 – there is still a lot of work to do to gain a full understanding of the possibilities, benefits and limitations here.

For now, the focus of research is on concerns coming from the agriculture side. Understanding how the presence of a PV installation will affect crop growth, watering and other working practices, and ultimately the crop yield, is essential. Given that this will be very different for every crop sown and every climate region; and will need to be tested for multiple PV system layouts, gathering the data to gain this understanding is no small task.

Crop suitability

Deciding which crops to integrate with PV is among the first concerns to address. Much of the research and the installations seen so far have focused on plant species known to have good shade tolerance. Max Trommsdorff, head of the agrivoltaics group at German research institute Fraunhofer ISE, cites blueberries and other perennial plants as a good example of crops that would be relatively uncomplicated to grow on land shared with a solar PV installation.

For many other crops though, modelling all of the factors that influence plant growth – insolation, temperature, wind, soil content, water supply and more – then adding the influence of a PV installation to this model will be required. And for many plant types, such growth models don’t exist in the first place, creating even more work for researchers in agrivoltaics. “For crops which are very widely used, like wheat or rapeseed for example, there are some pretty valid plant growth models available,” said Trommsdorff. “But specialty crops like coffee, hops, or asparagus, which are usually the types of application we are interested in, play a more minor role in agricultural science, meaning you don’t find many suitable growth models.”

Planting and harvesting methods for the crops are also important to consider: PV installations typically come in rows, which creates a synergy with crops also cultivated in rows. And if large machinery is used, this could mean less land available to build a PV installation without a mounting structure blocking access for the machinery. Trommsdorff noted, however, that in some cases this can be incorporated into a project design, with green areas in between rows serving as ‘biodiversity strips.’

French developer Sun’R is working on 91.2 MW of agrivoltaics projects won in two rounds of France’s special tenders for innovative PV technologies, and this year is beginning research and pilot installations in combination with pears, apricots and peaches – already finding very different results for each crop. The company’s researchers operate three experimental AV units of a few hundred square meters each. At these units, sensors gather granular data on the crop’s progress, monitoring parameters such as branch diameter and leaf temperature, comparing data from plots with and without solar. They are now working to refine their approach and learn which are the most important parameters to monitor.

Working in the south of France, Sun’R’s focus up to now has been on grapevines. Here it has found several potential advantages for growers, using a PV installation to have some degree of control over the amount of light hitting the plants.

“For grape vines, it’s very important to know the sugar, the alcohol, and the acid content. These factors will modify the quality and taste of the wine,” explained Jean Garcin, a Sun’R modelling engineer. “We have shown that additional shade actually enhances the quality of the grapevine, particularly in the further south regions of France, and elsewhere in southern Europe where the vine is usually rather strong in alcohol.”

Working with apples, meanwhile, Sun’R has found that while there is a period in the year where some apples fall off the tree, too much shade leads to too many apples falling and a reduced crop yield. And with cherries, heavy rain can cause serious damage to the crop, so larger panels and more coverage is desirable. “There are a lot of these specificities that you have for one crop but not another,” said Garcin. “This is why we have to adapt PV installations to each individual plant type.”

System design

Given the very different growth profiles between crops, PV will need to continue to be flexible in terms of system design for agrivoltaics to see growth. For the components themselves, no radical changes are required. Modules are mounted on structures usually three to five meters off the ground, but the combined use of land is generally enough to offset the extra steel cost this entails. And for bifacial modules, an increased installation height offers the benefit of less rear-side shading.

Industry-specific solutions, such as floating PV systems designed for fisheries or PV integrated greenhouses, are in various stages of development. Another system type also gaining ground relies on vertically installed, east/west-oriented bifacial modules, which entirely cuts out PV’s interference with crop growing. Further into the future, semi-transparent or even spectrally selective PV cells are a research area that could provide greatly increased possibilities for agrivoltaics. For the moment, researchers and developers are largely working on adapting existing mainstream PV technologies into the agricultural landscape, with significant differences from a standard ground-mount installation appearing at system level, in the height and spacing of modules, as well as in the operation of trackers.

With a standard, south-facing array, non-uniform shade cast by the panels can cause problems for the crop growing as well. “With a tilted, south-facing agrivoltaic array, shade cast on the ground is pretty heterogeneous,” explained Fraunhofer ISE’s Trommsdorff. “So it’s less appropriate for a homogeneous canopy of crops.” Planting rows of different crops with varying light demands could be one solution to this, but in most cases it proves more practical to make adaptations on the PV, rather than the agriculture side.

East-west orientation is one way to mitigate this shading issue, and making use of trackers offers further possibilities to alter the angle of the panels, according to both long- and short-term requirements of the crop. Sun’R is developing a software algorithm, AV Studio, that can simulate both electricity production and the microclimate beneath the panels, and interact with plant growth models to compute the ideal tilt angle for the crop. The company has developed three base strategies for panel steering, which can be applied to the needs of a given crop. The first, which it calls “strategic control,” is measured over a year and based on requirements of the crop at different stages in its life cycle. Second is “tactical control,” a day-long scenario led by the sun’s path and weather forecast data. And finally, “operational control” allows the system to react to weather events such as frost or heavy rain.

The expected lifetime of a PV system can also pose challenges to agrivoltaics. Crop rotation cycles are vital in agriculture to maintain healthy soil, and strategies for this also vary greatly in different regions. However, if a PV system designed to stay put for 20 years or more is built to the requirements of a specific crop, then farmers may be limited in what they can plant beneath it. Sun’R’s AV Studio would offer one solution here – allowing engineers to simply reprogram trackers to match different requirements. Trommsdorff also mentioned the possibility of mobile PV systems, that could be taken apart and moved to follow the crop each year, as a solution that might also simplify the permissions process.

Regional climates

The biggest benefits for AV are found in warm, dry climates. Sun’R’s work in the south of France provides one example of this, Italy and other areas of southern Europe have also seen successful projects. In general, more sunshine means more output from PV and more likelihood that plants will benefit from shade in the hottest parts of the day. In such regions, coverage by an AV installation can also help to prevent evaporation and reduce requirements for water.

Agriculture in colder, wetter regions may also be able to benefit from integration with PV. Fraunhofer ISE is trialling agrivoltaics in further combination with rainwater harvesting, and the panels can also help to prevent frost, potentially lengthening the growing season in multiple regions. Trommsdorff warned, however, that there are two different mechanisms by which frost forms, and the wrong approach to using solar PV could make things worse. “There are two different kinds of frost – one where you have ‘lakes’ of cold air close to the ground, and one where you simply have heat being reflected away from the ground,” he explained. “In the second case, PV panels would help keep the warmth in and prevent frost. But in the first they can reduce the air circulation and actually cause even more frost damage.”

While several gigawatts of agrivoltaics have already been deployed around the world, those working in the field are keen to point out that the concept is still far from realising its full potential. It is still too early to rely on private sector investment. And policies to support development, such as France’s tenders for innovative PV technology, are very much needed to keep developing our understanding of how agriculture and solar PV affect each other when sharing the same patch of land, and the new technologies that will come with this.