A team of engineers has designed a solar panel that can absorb energy from sunlight without blocking the light, making it ideal for placing on the roof of a greenhouse. Tests revealed that plants grown under the new panels actually outperformed those in a conventional greenhouse.
Solar panels can take up significant space and are often difficult to scale. This has led to the rise of the new field of agrivoltaics, which focuses on the simultaneous use of land for both solar power generation and agriculture. For example, replacing the glass in greenhouses with solar panels could power the lamps and water controls in the greenhouse, or even the whole farm. However, conventional solar panels block sunlight, making them unsuitable for an application such as this.
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To solve this problem, UCLA Samueli School of Engineering’s materials scientist Yang Yang and his team developed a strategy for augmenting semi-transparent organic solar cells. These cells rely upon carbon-based materials, as opposed to the inorganic substances in conventional devices. The investigators incorporated a layer of a naturally occurring chemical called L-glutathione, which is sold as an over-the-counter antioxidant dietary supplement, and found that the addition extended the solar cells’ lifetime, improved their efficiency and still allowed adequate sunlight to reach plants in a greenhouse prototype about the size of a small dollhouse.
‘Organic materials are uniquely suitable for agrivoltaics because of their light-absorption selectivity,’ said Yang, who also holds UCLA’s Carol and Lawrence E Tannas Jr. Chair in Engineering. ‘The main drawback that has prevented their widespread use up to now is their lack of stability.’
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Organic solar cells tend to degrade more quickly than their inorganic counterparts because sunlight can cause organic materials to oxidise and thus lose electrons. The researchers found that adding the layer L-glutathione prevented the other materials in the solar cell from oxidising, which resulted in the organic cells maintaining more than 80 per cent efficiency after 1,000 hours of continuous use – as opposed to less than 20 per cent without the added layer.
The research team also compared the growth of common crops, including wheat, mung beans and broccoli, under a transparent glass roof with segments of inorganic solar cells with that of crops grown under a roof made entirely from semi-transparent organic solar cells. They found that the latter outperformed the former. The scientists believe this is because the L-glutathione layer blocked ultraviolet rays, which can inhibit plant growth, and infrared rays, which can cause greenhouses to overheat and plants inside to require more water.
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‘We didn’t expect the organic solar cells to outperform a conventional glass-roof greenhouse,’ said Yepin Zhao, a postdoctoral scholar in Yang’s lab. ‘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.’
On the heels of these findings, the team has established a startup at UCLA that aims to scale up production of the organic solar cells for industrial use. The researchers said they hope to make environmentally friendly greenhouses that incorporate the organic solar cells commercially available in the future.
The research has been published in Nature Sustainability.