Demystifying the Science | Solar Heat Islands

Every week, our journalist answers readers’ scientific questions.

Yes, they create heat islands, but this effect dissipates over less than 30 meters. And there is no increase in temperature if we leave plants under the panels.

“We got to the bottom of this question at the request of a city council in Arizona,” says biologist Greg Barron-Gafford of the University of Arizona, who is the lead author of a 2016 study on the issue in the journal Scientific Reports. “We have realized that this is absolutely not a problem. A new type of agriculture, agrovoltaics, has even been born from this concern.”

When vegetation is removed under solar panels, the temperature there increases by 1 to 3 °C. The effect dissipates after a distance of 30 meters from the solar panels.

If vegetation is left under the solar panels, however, the plants’ “transpiration” – the evaporation of water from their leaves – reduces the temperature by several degrees. “That completely offsets the temperature increase under the panels,” Barron-Gafford says.

Transpiration serves to cool plants, stimulate sap circulation and release communication hormones.

The investigation was conducted after a resident of a small Arizona town raised concerns at the city council that a solar farm planned near his home would cause temperatures to spike in the neighborhood. Researchers at the University of Arizona were tasked with getting to the bottom of the issue.

“At the time, there was no rigorous field study. And physical models predicted either increases or decreases.”

A new type of culture: agrovoltaics

As he investigated further, Barron-Gafford found that vegetables sweat even more than grass. “Sometimes they cool the temperature under the panels by 10 degrees Celsius,” he says. He tried tomatoes, carrots, peppers, beans, broccoli and more.

This discovery launched a new type of cultivation: agrovoltaics. Vegetables are grown under solar panels, which are raised two meters to be able to work.

The productivity of the vegetable garden is reduced by less than 10% due to obstacles to the market gardener’s work, but this drop in income is offset by income from solar energy.

Mr. Barron-Gafford has agrivoltaic projects throughout Africa, Mexico and the United States.

How can vegetables grow under the shade of the panels? “This is the question that led us to wait five years for federal grants. Officials said, ‘Vegetables don’t grow in the shade.’ That’s a big misunderstanding. Plants only use a small fraction of the light they get.”

Isn’t the shade from solar panels harmful at higher latitudes, for example in Quebec, where we generally ensure that vegetable gardens are not in the shade of trees?

“If you are far enough north, you can make solar panel parks in a row, to minimize the shade. You can use them as fences. There are such installations in Germany, even in Norway.”

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