By: Laura Panjwan
Although solar energy is one of the cleanest and most abundant renewable energy sources available, the space required to install solar systems can be a barrier to its implementation.
In countries where farmland is limited, solar panel insulation just isn’t a realistic option, said Jan Ingenhoff, PhD, a research professor at the Institute of Advanced Technology, the University of Science & Technology of China (USTC).
“If you look at the solar panels that are typically installed on farmland areas, you can’t do much beneath them so farmers have basically had to give up part of their land for this solar panel insulation,” Ingenhoff, in an interview with R&D Magazine, said. “Some farmers are OK with it because they have very large land areas, but in countries like Israel or China, where you have a shortage of land, it is not good to sacrifice that land for regular solar panels.”
To solve this problem, Ingenhoff—along with his colleague Wen Liu, PhD and their team at USTC—has developed the Agriculture Solar Concentrator Photovoltaic, a new type of solar panel system that allows for simultaneous plant growth and solar energy generation on the same land.
The team currently has four working prototypes installed in China, a country in urgent need of maintaining and recovering any agricultural land due to drastic urbanization in the past.
The researchers have two more installations planned for 2019 and 2020, and are moving toward making their system commercially available.
The technology was a 2017 R&D 100 Award winner.
How it Works
The system is based on the concept that plants don’t actually need 100 percent of the light sources the sun provides, Ingenhoff explained.
“Plants need only about 10 percent of the light, some blue and some red light and that’s all. The rest you can leverage for solar energy generation,” he said. “The solar panels could be described as semi-transparent. Some light is going through them allowing the plants to grow, while the rest is used for solar energy.”
The system uses curved solar panels that are covered with a film created by several polymer layers staged together to form a dichroitic multilayer film. This allows the selective transmission of only the wavelengths necessary for photosynthesis and plant growth. All remaining sunlight is reflected and focused to concentrating solar cells for photovoltaic power generation. A dual tracking system ensures that the reflected wavelengths are focused on the concentrating solar cells throughout the day.
The system can also be adjusted for the seasons.
During winter, the generated electricity could be additionally used to supply light to the plants via red and blue LED, while during the summer, when sunlight is vastly available, the split-ratio could be such, that the sunlight can majorly be used to generate electricity, while there is no negative effect on the plant growth.
The way the light is split between solar energy generation and the plants can also be adjusted for specific plant needs, Ingenhoff said.
“We have studied what kind of light different plants need,” he said. “For example tomatoes need a little bit more red light, while lettuce may need a little bit more blue light. You can adjust this very specifically to these plants and what they need. That is a huge benefit for the farmers because some want to focus on strawberries, some want to focus on tomatoes and they can say, OK we need this kind of film system and we [the USTC team] can make that happen.”
Currently, the system can produce approximately 90 watts per square meter of energy, but the researchers are are working to optimize their film to transmit more precisely the amount of light the plants need to increase its energy generation. They expect to soon be able to produce 120 to 130 watts per square meter.
This is slightly short of the 150 watts per square meter of energy that conventional solar panels produce, Ingenhoff said.
“We will be indeed always be a little bit short compared to the regular solar panels because some of the light is used for the plants,” he added. “But the advantage is that you can establish this system on farmland and do both solar energy generation and plant growth so I believe it’s worth it.”
In addition to solar energy generation, the Agriculture Solar Concentrator Photovoltaic can also be used to improve plant growth, especially in drought-stricken regions.
“About five to ten years ago people were a little concerned about letting plants just grow by blue and red light, because your first gut feeling is that the plants need all the light,” said Ingenhoff. “But there have been many studies that plants grow very well and even indeed better with just blue and red light. One of the reason for this is that if you block these near infrared (NIR) and far infrared light (FIR) from reaching the plants, you protect them from the heat, and the water evaporation on the farm is reduced. You have a better growth potential for the plants with the growth system.”
Reducing water use is an especially important issue in countries in the southern hemispheres which face water shortages, as well as China, Ingenhoff noted.
The researchers grew lettuce, cucumber and water spinach with the film covering and without the film and found that the lettuce grew to 17.59 cm with the film and 12.83 cm without, the cucumber grew to 15.50 cm with film and 15 cm without, and the water spinach grew to 12.67 cm with film, and 11.160 cm without.
The team now plans to adjust the current systems in use based on farmer feedback. They also need to assure that the systems hold up long-term without deteriorating, and can be cleaned and maintained easily.
The team is currently working to expand the use of their system outside of Asia and are working speaking with potential clients in Spain as well as the National Renewable Energy Laboratory (NREL) in Colorado to set up joint actives and new prototypes.
Decreasing the cost of the system is also a goal of the research team, said Ingenhoff.
“We need to understand how the cost could be brought down when you install the system in larger areas. At the moment the cost is OK, but when we want to promote this product on a larger scale we will need to bring the cost down. We are working on making the film more cost effective so that more people can utilize it.”