Duncan Clark looks at producing solar power glazing powering heating, cooling and irrigation systems
According to experts, growers of greenhouse crop in the US spend approximately $600 million per year on electricity needed to power lighting, heating, cooling and irrigation systems. This high energy consumption and the increased awareness of the consequences of burning fossil fuels have triggered a surge in demand for solar energy as a green and sustainable alternative.
The global commercial greenhouse market is projected to reach $50.6 billion by 2025, an increase from $29.6 billion in 2020. This growth is attributed to the rising demand for food from a growing population and a rapidly changing climate, which affects crop yield.
Greenhouse cultivation allows growers to manage a permanent favourable climate, while keeping operational costs low.
For example, most hot-season vegetables require temperatures of 20 to 30˚C during the day and 14 to 18˚C at night to grow. These temperatures are maintained using energy-intensive heating and cooling systems, which run on fossil fuels, or by using electric heaters with larger energy consumption.
In a Japanese study looking at the environmental burdens of greenhouse cultivation, it was reported that common kerosene greenhouse heaters had energy consumptions of 0.42 to 0.76 megajoules per square metre. Carbon emissions were also high, at 31 to 55 grams per square metre.
The biggest challenge for agricultural greenhouses now is finding ways of improving energy efficiency, as well as reducing carbon emissions. So, what are our options?
Using solar energy in greenhouses
As growers are paying more attention to their impact on the environment, sustainable greenhouse crop production is becoming an urgent puzzle to solve. So far, solar power is the main renewable energy source used in agriculture, with greenhouses now being constructed with photovoltaic elements.
In January 2020, experts at the North Carolina State University were able to manipulate the wavelengths of light passing through a greenhouse roof using semi-transparent organic solar cells. They found no significant differences in the fresh weight and chlorophyll content of the lettuce grown under the organic cell filters across three harvest cycles, suggesting that OVPs are as efficient as traditional solar cells. These results provide an opportunity for further light and thermal management of the greenhouse through semi-transparent OPVs.
The organic solar cells can thus impact plant growth, power generation, and thermal load of the greenhouse.
Traditional solar cells are an attractive alternative to using energy from non-renewable sources, but they still have their drawbacks.
For example, photovoltaics is composed using inorganic materials prepared in carcinogenic solvents and strong acids, such as chlorobenzene and hydrochloric acid. What’s more, toxin-heavy metals, like lead perovskites, are used for their photoactive properties.
Lead can be absorbed easily by plants from greenhouse solar cells, and therefore has means of entering the food chain and affecting kidney function in animals. If we’re to achieve a truly sustainable future for agriculture, research into safer, greener photovoltaic devices is needed.
Greener practice with PolyPower
Farmers are starting to turn their attention to organic photovoltaic (OPV) technology, like NextGen Nano’s PolyPower, which have earth-friendly compositions, superior visible transparency and power conversion efficiency. This is because they have larger optical bandgaps, meaning they can absorb higher energy photons without much energy loss.
Developed at a nano level using lightweight, flexible and affordable organic semiconductors, OPVs could be applied as a semi-transparent thin glazing on the surface of a large-scale agricultural greenhouse.
The solar energy absorbed can then be used in heating, cooling, ventilation, lighting and irrigation to maintain optimal crop growing conditions.
Using ‘green solvents’ to prepare materials, the manufacturing and operating of OPVs has less of an adverse environmental impact and can be potentially ‘disposable’, unlike traditional solar cells. Further investigations into organic photovoltaics will cut growers’ energy costs, as well as helping cultivators to adapt to more sustainable agricultural practice.
Duncan Clark is operations director at organic photovoltaic devices innovator, NextGen Nano.