Greenhouse drawing

A model developed by researchers at North Carolina State University, USA, suggests that many greenhouses could become energy neutral, or even capture and sell excess energy, by using see-through solar panels to harvest energy. [Image: Brendan O'Connor, NC State University]

Anyone who enjoys a juicy, ripe tomato in the dead of winter can probably thank the greenhouse agriculture industry—which also produces lettuce, cucumbers and a variety of other vegetables year-round. In fact, greenhouse-based agriculture has many advantages compared with conventional agriculture, including land-use efficiency, water conservation and reduced need for pesticides.

At commercial greenhouses, though, the benefits come at a great cost in energy consumption. Energy costs are a major limiting factor for such greenhouses to become a sustainable agriculture solution, according the authors of a new study that used a computer model to demonstrate the viability of semitransparent organic solar cells (OSC) to achieve net-zero-energy greenhouses (Joule, doi: 10.1016/j.joule.2019.12.018 ).

Less energy to make food

“If we can use less energy to make food, that’s a good thing,” says study co-author Carole Saravitz, a professor of plant and microbial biology and director of the Phytotron plant-science facility at North Carolina State University (NCSU), Raleigh, USA. Heating and cooling of greenhouse facilities is where growers are looking to save energy, she says—and semitransparent solar cells, as the multidisciplinary study suggests, could be a good way to do that.

Saravitz says the study of OSCs complements other work at the NCSU Phytotron. Work at the plant-science facility, she says, shows that “by playing with the light you can really change the plants,” and even, perhaps, produce better agricultural products by filtering the spectrum of light they receive.

She cites studies, for example, that show that different colors of lettuce can be produced by exposing the plants to different colors of LED light. Follow-on studies with solar cells, according to Saravitz, could help determine which films provide the most benefit to plants while reducing energy costs.

Model greenhouses

The net-zero-energy greenhouse study relied on computer modeling rather than real-life greenhouse experimentation, though such studies are coming, says co-author Brendan T. O’Connor, a professor of mechanical engineering at NCSU. The models showed that OSCs, with their tunable absorption spectra, can convert wavelengths of light not used by plants for photosynthesis into energy required to power greenhouse operations (mainly heating and cooling).

The researchers—who included NCSU physicists, chemists, plant scientists and engineers—chose tomatoes as their model crop, O’Connor notes, because they are the largest greenhouse- grown crop in the world. Indeed, close to 40% of tomatoes in grocery stores are grown in greenhouses, he says.

Location, location, location

Another study parameter was location. The authors chose Arizona, Wisconsin and North Carolina as the sites of their model greenhouses because those choices correspond to real-world commercial-scale greenhouse operations. Those states also represent three different climate areas—hot-dry, mixed-humid and cold—in which the outside environment would have differing impacts on greenhouse heating and cooling needs at different times of the year.

The computer model showed, according to the authors, that if OSCs are used, net-zero-energy greenhouses are possible in hot and mixed climates—and that even in cold climates, significant reductions in energy used for heating and cooling can be achieved throughout the year. The studied modeled two types of OSCs with slightly different chemistries; both delivered similar model results in terms of impact on greenhouse thermal control and heating and cooling needs.

Tunable spectrum

The OSCs filter sunlight, O’Connor says. This means, for example, that the solar panels could reduce thermal gains during the day by lowering infrared light coming into the greenhouse, while using light in wavelengths not needed for photosynthesis to generate photovoltaic electric power for ventilation. The model showed that the panels would generate excess power in hot and mixed climates throughout the year, which could be sold back to the grid or used for other greenhouse needs such as lighting and watering systems.

In hot places like Arizona, less water would be needed for evaporative cooling, the researchers say. Also, greenhouses in hot and mixed climates typically use shading cloths to mitigate overheating from afternoon sunlight. With OSCs filtering the sunlight, shade cloths wouldn’t be necessary, the authors say. And the OSCs would achieve overall better temperature control not only during the day but into the night as well.

Balanced lighting

In terms of light for photosynthesis, O’Connor says, the attenuation of the spectrum by the OSCs was much less than he expected (it can be less than 20%). That’s partly due to the elimination of shade cloths, but location and design of the solar cells are factors, too. The model shows, he says, that sunlight entering from sidewalls of the greenhouse balances out light for photosynthesis being absorbed by the solar cells.

From an optics perspective, O’Connor says, studying the use of OSCs for greenhouse facilities was an especially good fit. There have not been a lot of studies on energy inputs for greenhouses, and OSCs could be easily adapted for use on the roof of these structures—no separate mount or facilities needed.

While the technology is not yet readily available commercially, he says, demand from agricultural operations could bring OSCs to market with an application no other type of solar cell could match in performance, providing sustainable energy to grow crops.