LEDs make greenhouses greener

Red and blue light has been shown to stimulate plant growth.  LED lighting delivers spectral composition control as well as energy efficiency to increase yields and reduce costs. Lance Hemmings, Global Product Group Manager at RS Components explains.

 

Academic research conducted over many years has demonstrated that plants are more sensitive to certain wavelengths of light.  Scientists have determined that red and blue lights are much more effective in stimulating photosynthesis and plant growth than white light alone.  In fact, the light absorption spectral peaks for chlorophyll absorption fall within the ranges of 400 to 500nm and 600 to 700nm.  Different spectral compositions have been found to promote different phases and aspects of plant growth, such as germination, root formation, leafing and fruiting.  Based on research and experimentation, horticulturists have determined how to accelerate plant growth, grow stronger plants and increase yields.

 

The red and blue spectral peaks for chlorophyll absorption

 

Greenhouse cultivation is becoming more important, not only as pressure grows to grow more food locally, reducing unnecessary transportation, but also with increased demand for specialist and out-of-season produce.  Greenhouse lighting is essential to extend the growing season in countries with long periods of darkness in winter.  Countries with high levels of urbanisation, and where good farming land is limited, are relying on greenhouses as well as indoor farms or plant factories, to boost domestic food production.

Meanwhile, technology advances have enabled LED lighting to become an attractive proposition in greenhouses for large and small-scale commercial growers worldwide.  The cost savings to be garnered from the use of energy-efficient LEDs are enough to convince many large-scale growers to switch from traditional technologies, such as HPS and fluorescent tubes.

But there are a host of additional benefits.  One of the most compelling is the ability to control and vary spectral composition of the light output. In this way, growers not only optimise production, but with healthier plants can also reduce, or even eliminate, the use of fertilisers, saving further costs and minimising the use of chemicals.

 

Less heat

Another major advantage of LED technology is high light output with little radiant heat.  This means that lights can be distributed more evenly throughout a greenhouse, and located closer to plants without heat damage.  New fixturing opportunities afforded by LEDs include strip lighting for shelving, ideal for seedlings, and allowing more shelves per greenhouse. Instead of overhead lights, planar or vertical intra-canopy configurations such as curtains can be installed to better suit the height and density of plants in their later growth phases.  Reduced radiant heat from the lighting system means that temperature management is easier, such that cooling and venting systems can be reduced and simplified.

Growers are beginning to experiment with vertical spaces, such as towers, that can be converted to provide multi-level growing areas.  Plant density per acre of land is dramatically increased, thanks to low profile and heat-free solid-state lighting systems.

LEDs are robust and reliable over a wide temperature range, and are easily integrated into waterproof housings. Long operational life, often quoted in the region of 10,000hrs, reduces maintenance requirements and saves costs too.

The easy programmability of solid state lighting systems enables zoning to suit successional sowing requirements, for example, or to accommodate different crops in the same greenhouse.  Sophisticated programmes can be set up and run automatically using brightening and dimming to simulate sunrise/sunset.

Very recent research indicates that bees are more active under solid-state lighting than under HPS sources, for example, thereby improving pollination rates.  Coincidentally, pests such as aphids are less active under LED lighting than HPS, possibly because there is less radiant heat.

 

Product choices

A broad spread of specialised components for horticultural applications are already available, from individual LEDs, lenses and fixtures, through modules and complete lighting systems in a variety of form factors.

The diodes themselves are available in specific wavelengths within the required red and blue spectral peaks.  High brightness, low profile devices can be installed in compact fixtures to replace overhead lamps or tubes, and in strings for shelving or curtains.  Dedicated lenses and secondary optics can be fitted to compact fixtures to provide specific beam patterns, according to height and brightness requirements.

Osram, for example, offers high efficiency red and blue LEDs suitable for horticultural applications from its Oslon SSL 80/150 and Golden Dragon Plus ranges, in wavelengths from 644 to 666nm and 449 to 461nm.  The red LEDs feature 37 to 49% efficiencies and typical power of 330mW at an operating current of 400mA.  The Golden Dragon Plus versions have a wide beam angle of 170o, while the Oslon SSL series has a smaller footprint and narrower, 80o beam angle, enabling them to be packed closer together for denser arrays with external optics.

Cree offers a comparable range of red and blue LEDs in its XP-E and XB-D ranges.  The red and blue XP-E devices, for example, feature wavelengths of 620 to 630nm and 465 to 485nm respectively, and have a viewing angle of 130o.  Cree’s XLamp XB-D colour series is claimed to deliver up to 40% higher maximum light output than the XP-E range, offering the potential for lower system cost.

 

Lens array

LEDiL Oy, meanwhile, has developed the Petunia lens array, designed specifically for lighting fixtures in greenhouses growing moderately tall plants.  This low profile LED array, measuring just 29.5 x 46.5 x 7.4mm, produces a medium beam angle (28 to 32o full width half maximum) to allow it to be installed a few metres above plant growth beds. This densely spaced 12-lens array is organised in a compact 3x4 pattern, comprising four red, one blue and five warm white LEDs.  Including white LEDs in the array is important for the comfort of personnel working in the greenhouses on a regular basis.

The lens array is manufactured from precision moulded, optical grade PMMA; a UL94HB rated material with an operating temperature range of -40 to 100oC.  It offers high light transmission efficiency, yet is robust, being scratch and abrasion resistant, and virtually impervious to damage by UV radiation.  Four Petunia versions are available, each based on a different manufacturer’s LEDs.  The primary difference between them is in the beam angle and luminous intensity.  Choosing between them may depend on the specific application, based on the height of the luminaire above the target, the preferred wavelength combination and the light output required.

The model based on Osram’s Oslon SSL 80/150 LH deep blue and hyper red LEDs provides the narrowest beam angle, at 28o FWHM and luminous intensity of 1123lm @ 250mA, giving a cd/lm figure of 2.3 according to the datasheet.  The Cree XP-E version, with a 29o beam angle, provides a luminous intensity of 862lm @250mA, deriving a cd/lm figure of 2.2.  The Cree XB-D model, also featuring a beam angle of 29o, has a lumen output of 1037lm @ 250mA, producing a cd/lm figure of 2.0.  Finally, the Petunia model based on Nichia NVSxx19A LEDs, delivers the widest beam angle of 32o FWHM, while lumen output is 1138lm@250mA, giving a cd/lm figure of 1.7.

 

Conclusion

Building your own plant lighting fixture from the diode upwards is perfectly feasible providing you know the light wavelength recipe you need for your application.  However, modules and fixtures, such as the Petunia lens array, can give you a head start, as they combine LEDs with secondary optics in a form factor designed specifically for greenhouse applications.