Artificial illumination

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Artificial illumination is used in northern Europe and Canada to supplement total daily light in winter due to the significantly lower light intensity and shorter day length compared to mid-summer. In these regions, supplementary light can improve yield and product quality, allow for year-round production, and enable day-length control for flowering. Thus, apart from facilitation of day-length control, supplementary lighting systems are not a wise-choice in terms of energy-use as well as economically, in regions receiving high intensity of sun radiation.

HPS v/s LED Lamps:

The two main kinds of artificial lighting systems installed in grenhouses are: (i) conventionally used high pressure sodium (HPS) lamps, and (ii) the more recently emerging LED lamps. HPS lamps are high-intensity discharge lamps, and produce a lot of heat in addition to light. Meanwhile, LED lamps produce ‘cold’ light, decoupled from heating. Additionally, LEDs offer the possibility for both low-intensity lighting (for inter-lighting or day length control) and high-intensity lighting (for light assimilation by the crops).

The choice between HPS and LED lamps depends on a few factors such as:

• Cost : LEDs are much more expensive than HPS lamps

• Importance of light spectrum for the crop : HPS lamps have a fixed spectrum with a high component of yellow-red light, whereas LEDs offer the option to tailor the spectrum of emitted light to improve the photosynthetic efficiency, or secondary metabolite production).

• Need for inter-lighting : ‘cold’ LEDs are suitable for inter-lighting, whereas heat-emitting HPS lamps need to be placed at least 1.2 m away from the crop vertically and are not suitable for inter-lighting in compact growing systems where light is close to the crops.

Hybrid lighting possibilities can also be considered to combine the advantages of both kinds of light sources.

Contribution to energy balance and resource use of greenhouses:

Both HPS and LED lamps will add additional energy (heat) to the greenhouse and raise its temperature, though the heating effect is significantly lower for LEDs than for HPS lamps. Depending on the climate conditions and the time of the year, the additional heat maybe advantageous or disadvantageous.

Both HPS and LEDs consume electricity and convert it to light with different efficiencies: the electricity-to-PAR-light conversion is lower for HPS lamps (up to 1.85 µmol of light per joule of electricity input, dropping to 1.78 µmol/J over time) than for LEDs (up to 2.7 or even 3 µmol of light per joule of electricity, lasting for a much longer lifetime). Despite LEDs being more energy efficient relative to HPS lamps, their overall conversion efficiency is still limited to 50%.

Possible steps towards sustainable, fossil-free greenhouses

Careful tailoring of the lighting strategy based on crop/climate requirements, accounting for the heating effects of lamps in the climate control, developing/using more energy-efficient lighting, and switching to renewable/sustainable sources for the energy input for the lighting can improve the energy-efficiency of artificially-illuminated greenhouses. Apart from technical developments of light sources, energy savings could also be realized by adaptations of the technical layout and positioning of lights, such as combining top-lighting and inter-lighting.