Even after a recent surge, solar energy contributes less than 2% of electricity generation to the US electrical system. However, if we are to prevent the worst effects of climate change, the sun’s contribution will need to significantly increase. Where are all the solar panels going to be installed? You could envisage gigantic solar farms strewn across the Southwest’s sun-scorched desert terrain. However, two recent papers—one by researchers in Oregon and Utah and another by a team headquartered at the University of Arizona—indicate that a very different type of terrain makes the greatest sense for solar energy harvesting: land now occupied by agricultural farms.
That is because the technology that powers solar energy—silicon photovoltaic (PV) panels that convert light photons directly into electricity—performs optimally under a limited range of conditions. Of course, the most critical component of producing electricity is ample sunshine, which is why deserts offer attractive locations for solar energy generation. However, air temperature is critical as well. Above 78°F, the higher the temperature outdoors, the less efficient the photovoltaic panels are in converting sunlight to energy. That is why solar panels have certain difficulties in scorching deserts.
Where is the best place to build solar panels?
Solar works best in locations with abundant of sun and moderate temperatures, as well as gentle breezes and low humidity. Taking all of these factors into account, the Oregon/Utah authors discovered that locations now covered in crops and grass gave the most optimum sunlight conditions worldwide. Barren wasteland, the arid desert area that I associate with large solar arrays, came in fifth.
The authors observe that it seems logical that plants and solar panels thrive in comparable microclimates. “One may think of agriculture as a type of solar harvesting,” they write. “The sun’s energy is stored in the chemical bonds of the plant matter.” “And agricultural operations already occupy the most solar-harvestable areas on Earth.”
When I originally picked up the study, I assumed the authors were advocating for certain farmers to transition from agriculture to solar energy production. As it turns out, the two activities are compatible. Raising the panels off the ground creates space beneath them for humans and animals to move. Their shade effect can actually benefit some crops by alleviating heat stress; also, the cover conserves water in agriculture by minimizing evaporation.
Elnaz Adeh and Chad Higgins of Oregon State University are two of the paper’s authors. They co-authored a 2018 PLOS One study on grass production on a solar installation in a field in Corvallis, Oregon. Their discovery: Paneled areas created 90 percent more grass than previously, while consuming a quarter of the water. Similar programs in Germany and Massachusetts are yielding encouraging outcomes.
Greg Barron-Gafford of the University of Arizona heads a team evaluating the benefits of integrating solar panels and working farms in the desert southwest. They just published an article in Nature Sustainability summarizing the findings of a research in which tomatoes and chili peppers were grown beneath solar panels. The findings were conclusive. Through the sweltering Arizona summer, the shade aided crop growth—and conserved irrigation water; in turn, the plants aided in cooling the solar panels, enhancing their effectiveness.
Three crops were chosen for the study by the researchers. Chiltepin pepper production tripled under the panels compared to an unshaded control; jalapeo pepper production remained almost constant in both settings; and cherry tomato production quadrupled under PV cover. In terms of water retention, the soil under the panels held between 5% and 15% more moisture between irrigation episodes.
Additionally, the partnership helped the panels. Throughout the growing season, PV panels installed over crops were substantially cooler than those placed over bare ground, generating around 3% more power.
“The plants are not simply taking advantage of the shade provided by the solar panels,” Barron-Gafford explained. “Water evaporates each time they open their pores to allow carbon dioxide to enter for photosynthesis.” The process, known as transpiration, has a cooling effect, which is why restaurants use misters on patios on hot days.
“One of the reasons we were interested in this research was because we continued discovering that large-scale solar arrays produce this heat-island effect,” he explained—and that the increased temperatures lower the panels’ production. “We reasoned that part of the reason is probably because they always take away the vegetation—so why don’t we reintroduce some plants and restart the transpiration effect?”
Barron-Gafford said that it has been difficult to convince farmers to cultivate crops beneath solar panels. Covering vast swaths of agricultural land with sun filters continues to be paradoxical. However, as climate change progresses, irrigation water in the dry southwest will become more expensive, resulting in an increase in demand for carbon-free energy. Agrophotovoltaics—a term that refers to the overlapping of food and solar farms—can assist with both without jeopardizing agricultural output.