Targeting plants with red and blue LEDs provides energy-efficient lighting in restricted environments, a Purdue University study shows. The finding could further the development of crop-growth modules for space exploration.

Research found that leaf lettuce grew well under a 95-to-5 ratio of red and blue light-emitting diodes, or LEDs, placed close to the plant canopy. Thhis targeted LED lighting consumed about 90 percent less electrical power per growing area than conventional lighting and an additional 50 percent less energy than full-coverage LED lighting.

Suder leader Cary Mitchell, professor of horticulture, said:

“Everything on Earth is ultimately driven by sunlight and photosynthesis. The question is how we can replicate that in space. If you have to generate your own light with limited energy resources, targeted LED lighting is your best option. We’re no longer stuck in the era of high-power lighting and large, hot, fragile lamps.”

The main challenge to creating a crop-growth module for space travel has been the overwhelming energy cost of the 600- to 1,000-watt conventional high-pressure sodium lamps traditionally used to simulate sunlight and stimulate plant photosynthesis in contained environments. The lamps also scorch plants if placed too close and require a filtration system to absorb the excess heat they create.

“Lighting was taking about 90 percent of the energy demand,” study co-leader Lucie Poulet said. “You’d need a nuclear reactor to feed a crew of four people on a regular basis with plants grown under traditional electric lights.”

Poulet and Mitchell turned to high-intensity LEDs to design a more efficient system. The LEDs only require about 1 watt each, plus are much smaller and longer lasting than traditional lights. Because they emit no radiant heat, LEDs are also cool enough to be positioned close to the plant canopy and at strategic positions to maximize the amount of light that reaches the leaves.

“Instead of the minimum 4-foot separation we had between conventional lamps and lettuce, we could get LEDs as close as 4 centimeters away from the leaves,” Mitchell said.

Mitchell said targeted LEDs could also help make controlled-environment agriculture on Earth more economically viable by reducing lighting costs.

The next phase of research, he said, will be to fine-tune when to increase and decrease lighting according to plant growth stage to optimize growing conditions and save energy.

L. Poulet, G.D. Massa, R.C. Morrow, C.M. Bourget, R.M. Wheelerb, C.A. Mitchell Significant reduction in energy for plant-growth lighting in space using targeted LED lighting and spectral manipulation Life Sciences in Space Research Volume 2, July 2014, Pages 43–53 doi:10.1016/j.lssr.2014.06.002

Photo: Targeting hydroponically-grown leaf lettuce with red and blue LEDs saves a significant amount of energy compared with traditional lighting. Credit: Purdue University/Lucie Poulet

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