Articles
Focused lighting: an energy-saving strategy for indoor production of young, widely spaced plants
Article number
1423_17
Pages
129 – 134
Language
English
Abstract
Procedures for growing leafy-green crops in vertical farms typically include periodic re-spacing of plants to avoid incidence of ‘full-coverage” photons on empty growth spaces before foliar canopies close.
In this study, we postulated that “focused lighting” could be an effective energy-saving strategy for widely spaced young plants, eliminating the need for some or all transplant events.
Secondary optics fitted around individual LED engines on a light fixture truncated conical light-beam spread from ±60 to ±30°. With light engines positioned directly above plant holders on a hydroponics lid, a control leaf-lettuce crop was grown at a light intensity of 200 μmol m‑2 s‑1 and a standard 40-cm vertical separation distance between plants and lights, from which distance even the ±30° circular photon beam irradiated empty spaces between growing plants for much of the 21-day cropping cycle.
For a more focused lighting treatment, the fixture with secondary optics was positioned initially only 20 cm above the cropping surface at a light intensity of 105 μmol m‑2 s‑1. The ±30° beam spread at closer separation wasted less light beyond the diameter of each plant during the first 12 days of growth, after which the LED fixture was raised to 40 cm, light intensity was increased to 200 μmol m‑2 s‑1, and the crop harvested on day 21. There were no significant differences in shoot fresh or dry biomass between treatments, but energy-use efficiency was 47 and 73% higher for focused lighting than for standard lighting.
Modeling light output from individual LEDs with secondary optics indicated a higher light intensity at the center of the circular beam for the same power input at 20 cm than at 40 cm.
Thus, the combination of light emitted mostly downward from each engine with closer separation and less photon waste between plants in an open crop stand encourages the design of adjustable-height lighting systems combining secondary optics and close-canopy lighting with much-improved energy-use efficiency.
In this study, we postulated that “focused lighting” could be an effective energy-saving strategy for widely spaced young plants, eliminating the need for some or all transplant events.
Secondary optics fitted around individual LED engines on a light fixture truncated conical light-beam spread from ±60 to ±30°. With light engines positioned directly above plant holders on a hydroponics lid, a control leaf-lettuce crop was grown at a light intensity of 200 μmol m‑2 s‑1 and a standard 40-cm vertical separation distance between plants and lights, from which distance even the ±30° circular photon beam irradiated empty spaces between growing plants for much of the 21-day cropping cycle.
For a more focused lighting treatment, the fixture with secondary optics was positioned initially only 20 cm above the cropping surface at a light intensity of 105 μmol m‑2 s‑1. The ±30° beam spread at closer separation wasted less light beyond the diameter of each plant during the first 12 days of growth, after which the LED fixture was raised to 40 cm, light intensity was increased to 200 μmol m‑2 s‑1, and the crop harvested on day 21. There were no significant differences in shoot fresh or dry biomass between treatments, but energy-use efficiency was 47 and 73% higher for focused lighting than for standard lighting.
Modeling light output from individual LEDs with secondary optics indicated a higher light intensity at the center of the circular beam for the same power input at 20 cm than at 40 cm.
Thus, the combination of light emitted mostly downward from each engine with closer separation and less photon waste between plants in an open crop stand encourages the design of adjustable-height lighting systems combining secondary optics and close-canopy lighting with much-improved energy-use efficiency.
Authors
F. Sheibani, M. Bourget, A.C. Mitchell
Keywords
canopy photon-capture efficiency, Lambertian light distribution, spatial irradiation pattern, focused lighting, targeted lighting
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