Sunglasses for plants, and sustainable agriculture

A multilayer film that reflects heat while letting through light needed for photosynthesis could make greenhouse agriculture more energy- and water-efficient. Such a film has been developed by engineers at the University of California, Davis, and is described in a recent paper in Advanced Energy & Sustainability Research. 

Greenhouses enable higher yields of fruits and vegetables while conserving land,water, and fertilizers. But in warm climates, such as California’s Central Valley, the Mediterranean, or the Middle East, they can become extremely hot, which damages crops. Cooling greenhouses usually involves mechanical ventilation (fans) or evaporative cooling, which can use large amounts of energy and water. 

Shade screening is commonly practiced but this also blocks the important photosynthetically active radiation (PAR), which is critical for plant growth. There is therefore a lot of interest in shades or coatings that can reduce the infrared radiation which causes heating, while admitting optimal PAR for higher end quality products. 

Postdoctoral researcher Amrit Kumar Thakur, working with T. M. Abir Ahsan, Professor M. Saif Islam (Department of Electrical and Computer Engineering), and Assistant Professor Md Shamim Ahamed (Department of Biological and Agricultural Engineering, has developed a film (Cool-Cover) to do this and modeled its properties. 

The film is made up of stacked layers of materials, including zinc oxide, calcium fluoride, and silver. 

The team used simulation software to design the multilayer film for optimal performance. The results predict that glass coated with the film would reduce near-infrared light passing through by almost 90 percent, while allowing through about 51 percent of  the PAR optimal for photosynthesis. 

The researchers also used an established model of a standard greenhouse to predict how the film would perform in different environments with hot, dry summers and milder winters. The models show that the coating could dramatically reduce water use and energy consumption in greenhouses with only a small reduction in crop yield. 

The tradeoff of water and power savings against yield is critical to consider, the authors wrote, especially in regions with high solar radiation and water scarcity, such as Fresno. 

The researchers are currently working with Lawrence Berkeley National Laboratory to produce a version of the multilayer coating. A provisional patent has been filed.