Putting solar panels on a roof is a good idea. Growing food in a greenhouse is also a good idea. But putting those ideas together usually doesn’t work because the solar panels block out the sunlight the plants inside need to flourish. Researchers at the University of California, Santa Cruz, have created new solar panel technology they call Wavelength Selective Photovoltaic Systems. It uses part of the visible light spectrum to generate electricity while letting the rest of the light through to the plants growing inside. The results have been encouraging.
Wavelength Selective Photovoltaic Systems (WSPV) uses transparent roof panels embedded with a magenta luminescent dye to absorb some of the blue and green wavelengths of sunlight and transfer it to narrow photovoltaic strips that generate electricity. The other wavelengths pass through to the interior of the greenhouse. The result? Solar power that costs less per watt than conventional rooftop systems, and an increase in growth for many of the fruits and vegetables inside.
Inside two experimental greenhouses — one on campus and the other in Watsonville, California — the team raised 20 varieties of tomatoes, cucumbers, lemons, limes, peppers, strawberries, and basil. “80% of the plants weren’t affected, while 20% actually grew better under the magenta windows,” reports Michael Loik, professor of environmental studies at the University of California, Santa Cruz.
He is the lead author of a paper about the program published in the current issue of the American Geophysical Union’s journal Earth’s Future. “We have demonstrated that ‘smart greenhouses’ can capture solar energy for electricity without reducing plant growth, which is pretty exciting.” There’s more good news. The plants in the WSPV-equipped greenhouses used 5% less water than those raised in conventional facilities.
“I thought the plants would grow more slowly, because it’s darker under these pink panels. The color of the light makes it like being on the Red Planet,” says Loik. “Plants are sensitive not just to the intensity of light but also to color. But it turns out the plants grow just as well.”
Using greenhouses to grow fruits and vegetables around the world has increased 600% over the past 20 years, and greenhouses now cover the equivalent of 9 million acres — twice the size of New Jersey. “It’s big and getting bigger,” Loik says. “Canada relies heavily on greenhouses for vegetable production and their use is growing in China, too.” Tomatoes and cucumbers are among the top greenhouse-produced crops worldwide. Plastic greenhouses are also becoming popular for small-scale commercial farming and household food production, he adds.
Greenhouses consume a lot of electricity to run the fans, lights, pipes, and monitoring systems inside. “This technology has the potential to take greenhouses offline,” says Loik, who specializes in climate change, plant physiology, water resources, and sustainable technologies. The cost per watt of the WSPV system is 65 cents per watt — about 40% less than the per-watt cost of traditional silicon-based photovoltaic cells. “If greenhouses generate electricity on site, that reduces the need for an outside source, which helps lower greenhouse gas emissions even more,” he adds. “We’re moving toward self-sustaining greenhouses.”
This is yet more good news about the marriage of solar power and agriculture. The Fraunhofer Institute For Solar Energy Systems in Germany is reporting that solar panels mounted high above farmland can increase the efficiency of each acre of land by as much as 60%. Global warming is already changing traditional farming. Greenhouses may be vital to feeding the planet in years to come as climate change makes some agricultural areas less productive, reducing yields per acre.