Use sun to produce green plants, energy

Higher temperatures caused by climate change threaten crop yields and ranch animal health, but a team of researchers from the U.S. Department of Agriculture's Agricultural Research Service has a plan that will not only save crops and livestock but also help power farms and the nation.

The plan involves placing solar panels in Southwestern U.S. farm fields and ranches. In addition to producing electricity, shade from the panels will provide cover that can prevent plants and animals from overheating. The results will be both green crops and green energy – energy that can also produce revenue for farmers and ranchers.

According to Derek Whitelock, agricultural engineer at the USDA Agricultural Research Service Cotton Ginning Research Laboratory in Las Cruces, New Mexico, the United States needs more solar panels to produce renewable energy. Because a lot of ranch and farmland is flat and sunny, the idea of merging the two in one location makes sense.

“There’s an idea that uses land for both growing food and making energy from the sun, called ‘agrivoltaics’ – agriculture + photovoltaics,” he said. “We’re working on research to explore agrivoltaics in western states, like New Mexico, where there’s lots of sunshine and not much water.”

One of the key elements of the plan, in addition to capturing sunlight with the solar panels, is to provide shade for crops to give them respite from the heat.

“During the summer, in the middle of the afternoon, the sun is really bright and hot,” Whitelock said. “The solar panels will provide shade for crops during the hottest times of the day so they will be cooler, grow more, and need less water.”

Livestock can also graze under elevated panel arrays for relief from the heat.

The research team is working with several types of agrivoltaic panel arrays that are mounted in fields on steel supports. Some arrays are fixed to face the southern sky, while others have motors that track the sun so the panels get maximum sunlight throughout the day. The panels are set far enough apart to allow tractors and other farm equipment to move between them, and some are set eight to 10 feet high to allow livestock to graze under them.

Each solar array contains 60 photovoltaic cells that absorb light energy from the sun. Solar energy forces electrons within the semiconductor material to create a voltage potential, which causes electric current to flow. The electricity generated by the photovoltaic panels on farms can be used on the farm to power irrigation pumps, refrigeration and other processing equipment; stored in batteries for later use; or sold to the local electric grid.

Paul Funk, a retired USDA Agricultural Research Service scientist who continues to support the project, said, “Agrivoltaics has been a research subject in Europe, Japan and Korea for about 10 years. Our task is to build on existing research, particularly by looking at crop responses to partial shade and to shade at various times of day.”

Funk said that every plant has its own “ideal” light requirement, called the light saturation point, which is typically one half or less than the amount of light available on a clear day.

“Excess light can heat a plant; plants evaporate water to keep cool. When a plant can no longer take up enough water to keep cool, the pores in the leaves close and photosynthesis stops,” Funk said. “On a hot afternoon a plant in full sun may not be growing at all!”

“These situations can be mitigated by partial shade, explaining why, in many agrivoltaic experiments, plant growth and crop yield are higher under agrivoltaics than in the unshaded control plots,” Funk said. “There is also potential for the plants to help the solar panels produce more energy. Plants evaporate water, which lowers the air temperature around them. This cools the photovoltaic panels from beneath, and photovoltaic panels produce more electricity at lower temperatures.”