Making sunlight better: Using films to manage sunlight for crop productivity

Making sunlight better: Using films to manage sunlight for crop productivity

Today’s protected cropping systems have become astonishingly effective at extracting optimum potential from nutrients, water and plant genetics, while minimising harm from farming’s ancient adversaries, like insect pests.

There is a variable still to be fully optimised, though: light and energy from the sun. Australian researchers are developing films that can be fitted to greenhouses and glasshouses to modify sunlight in ways that aid energy efficiency and plant growth.

Sunlight – its intensity, duration and spectrum – exerts a primary influence on plant productivity, and on the profitability of glasshouse operations. The characteristics of sunlight are influenced by location.

Other countries are more advanced in this field, but Australia has specific climatic conditions best addressed by locally-developed technology.

Increasing climate variability and the Russia-Ukraine war are contributing urgency to the quest. In Europe, soaring energy costs have, in some cases, tripled the input cost of glasshouse vegetables, forcing some growers out of business.

In the Netherlands, which has used protected cropping technologies to build the world’s second-largest agricultural export sector in a country less than half the size of Tasmania, the challenges are limited sunlight and cold. Here in Australia, the challenges are reversed. In an Australian summer, there is more sunlight than plants can use, and in glasshouses it produces a surplus of heat that is even more costly to mitigate than it is to warm a structure against cold.

“If we are serious about feeding a growing global population without further exacerbating climate change, then I think we need to get very good at protected cropping.

Professor David Tissue

With exceptions like California, the need to deal with an over-abundance of sunlight in protected cropping systems is relatively rare. Which is why Professor David Tissue is among the advocates for a DIY mindset when it comes to solar modification research.

Tissue is the Scientific Research Director for Western Sydney University’s National Vegetable Protected Cropping Centre, and leads the Future Foods System CRC project. It is investigating the use of films to reduce glasshouse energy use, and boost plant productivity through spectrum modification.

The scope of the challenge is understood, Tissue says.

“We know which wavelengths of light contribute heat and which ones contribute biologically. We want to develop films that produce a square wave – that is, they let in all the light from about 380 nanometers to about 740 nanometers, and then they cut it off.

“If we can design film that’s able to do that, then we’ll get the maximum light for photosynthesis and production while minimising heat generation.”

For David Tissue, there is an existential element to the wonkish work of developing protected cropping films. Trained as an ecologist and biochemist, Tissue has, over the 40 years of his career, built on a reputation as an international expert on the effects of climate change on ecosystems.

Professor david tissue
Professor David Tissue

Protected cropping is a long way from a natural ecosystem, but for Tissue, the field offers energy savings and other efficiencies important if anthropogenic climate change is to be curbed.

“With protected cropping, you can locate your growing capacity near urban areas, so there is less transport in getting product to market and they can be eaten fresh right after harvest,” he says. “The plants use less nutrients, and require less chemical products to protect them against pests. Crops are largely protected against events like storms and floods.

“If we are serious about feeding a growing global population without further exacerbating climate change, then I think we need to get very good at protected cropping.

“Protected cropping provides a lot of benefits now, but this work on films is a gamechanger. There’s potential for Australia to get a lot better at protected cropping over the next 5–10 years.”

Currently, work on the photosynthesis and energy components is split. Nano-fabrication specialist Innofocus and photonics experts from RMIT are contributing to the development of energy-efficient “smart glass”, and University of NSW spinoff LLEAF (acronym for Luminescent Light-Emitting Agricultural Film) is manufacturing spectrum-shifting films. Industry R&D company Hort Innovation is a partner in the CRC project.

Heat-reducing films are an established technology. The windows of all modern skyscrapers are tinted with the distinctive bluish hues emitted by heat-reducing films coating the window glass. It would seem that the science of filtering heat from sunlight should be there for the taking.

Not so, Tissue says. “Window films reduce the amount of light that the crop sees by about 15 per cent to 20 per cent, and light is critical for photosynthesis. They also have a tendency to cut out important wavelengths of light, like red, which is a particularly important part of the spectrum for promoting flowering.

“The energy savings from using these films doesn’t compensate for the loss of crop productivity.”

While too much energy in a glasshouse is expensive, energy has value. The optimum film – and thus the optimum result from the CRC project or its heirs – would filter out excess solar energy from entering a glasshouse, but capture that excess in solar cells that convert it to electricity useable in glasshouse production.

Luminescent light-emitting agricultural film installed above a trial lettuce plot. It shifts sunlight's spectrum towards red wavelengths
Luminescent Light-Emitting Agricultural Film (LLEAF)-Red (pink) and SG ULR-80 (blue) installed above a trial lettuce plot. Credit: LLEAF Pty Ltd

Not all light is equal to plants, and so ideally, the energy directed to solar cells would come from parts of the light spectrum that seem to have little biological value. Yellow and green seem to play only a modest role in photosynthesis; red a very large role.

Sunlight spectrum modification is a speciality of LLEAF, led by polymer specialist Dr Alex Soeriyadi.

LLEAF’s red light-enhancing films were the first on the local market, and the company now sells to six other countries. It has some powerful data behind its product. In lettuces, Soeriyadi says that using LLEAF’s luminescent films – created using dyes rather than the nano-fabrication employed by Innofocus – to shift sunlight’s spectrum towards red wavelengths has produced productivity increases of up to 25 per cent.

Gains in agricultural productivity tend to be measured in fractions of a per cent, or single figures at best. Soeriyadi admits he had trouble persuading growers that LLEAF’s double-figure improvement claims were legitimate. That’s when he started seeking independent verification by working with WSU’s National Vegetable Protected Cropping Centre.

Now, as part of the CRC project, LLEAF has begun exploring other aspects of spectrum modification.

Many popular fruits and berries come from latitudes where there is a distinct summer and winter light cycle.

“Improving productivity by increasing red light is the most obvious thing to do,” says Soeriyadi. “We’re also moving into more proprietary areas, including the nutritional aspects. We are doing trials on changing the far red and far blue parts of the spectrum to see if we can influence antioxidant levels in berries, or whether we can increase capsaicin levels in red capsicum.”

The ability to manage sunlight also flows through to the practical aspects of protected cropping. Many popular fruits and berries come from latitudes where there is a distinct summer and winter light cycle.

“In a country like Indonesia, which doesn’t have those cycles, can we use films to trick these species into fruiting?“ Soeriyadi ponders.

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The research frontier of spectrum-changing film lies in the ability to change a film’s capabilities in-situ to address different parts of the crop cycle, much as the hue of wifi-connected lightbulbs can be changed from a smartphone app.

“There is some technology out there that indicates that it could work in principle,” Soeriyadi says. “But it would come with a cost, and keeping the cost of food production down is very important.”

LLEAF’s initial product, red-enhancing polycarbonate sheets, are estimated by Soeriyadi to have a two-year payback on purchase costs, based on a 10 per cent lift in production. The sheets have a minimum life expectancy of 10 years. Soeriyadi knows that future developments will need to have similar cost efficiencies if they are to evolve beyond the lab.

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