Semi-transparent Solar Cells Boost Growth Of Greenhouse Plants

Solar farms and conventional agriculture are competing for land, and that conflict will likely only get worse as our need for both grows. Scientists at UCLA have now tested a way to combine the two by placing semi-transparent solar cells on the glass roofs of greenhouses, finding that they can actually improve the growth of plants inside.

Engineers at the University of California have designed solar roofs that can harvest energy for greenhouses to power their lamps and water controls while still allowing sunlight to reach the plants.

Greenhouses and solar energy both need lots of sunlight, so it’s no surprise that engineers have experimented with building them into the same structures. Experiments have shown that it’s basically a win-win situation – semi-transparent solar panels can produce a decent amount of electricity, while also not starving the plants of their vital sunshine. In some cases the plants even thrived under there.

For the new study, the UCLA team investigated a few tweaks to an existing recipe. They started with organic solar cells, which are made from carbon materials and can be crafted into transparent, flexible solar cells. That sounds perfect for greenhouse use, but the downside is that these organic materials degrade quickly in sunlight.

So the team added a new ingredient – a layer of a chemical called L-glutathione, to prevent the organic materials from oxidizing and breaking down. In tests, organic solar cells both with and without this protective layer were placed under solar irradiation for 1,000 hours of continuous use. And sure enough, those with the extra layer retained over 84% of their original efficiency, while those without it dropped to under 20% in that time.

“Organic materials are uniquely suitable for agrivoltaics because of their light-absorption selectivity,” explained researcher Yang Yang. “The main drawback that has prevented their widespread use up to now is their lack of stability.”

Next, the researchers put the solar cells to work in model greenhouses, growing wheat, mung beans and broccoli. Each crop was grown in one of two greenhouses – one with a clear glass roof dotted with segments of inorganic solar cells, and the other with a roof entirely consisting of semi-transparent organic solar cells.

The organic cells demonstrated a power conversion efficiency of 13.5%, and allowed 21.5% of visible light to pass through. But that was enough, it seems – the plants inside these greenhouses grew, surprisingly, even better than those in the traditional ones. The team suggests this is because the L-glutathione layer blocked ultraviolet rays, which can damage plants, and infrared rays that can heat up a greenhouse too much.

“We didn’t expect the organic solar cells to outperform a conventional glass-roof greenhouse,” said Yepin Zhao, lead author of the study. “But we repeated the experiments multiple times with the same results and after further research and analysis, we discovered that plants don’t need as much sunlight to grow as we’d originally thought. In fact, too much sun exposure can do more harm than good, especially in climates such as California’s, where sunlight is more abundant.”

The team has now set up a startup to scale up production of these organic solar cells commercially.

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