Energy management in protected cropping: horticultural lighting

In this factsheet, Farm Energy Centre's Chris Plackett, and Steve Adams and Allen Langton from Warwick Horticultural Research International (HRI) look at the energy implications of using horticultural lighting, and consider how energy inputs can be minimised whilst still maintaining plant yield and quality

Summary points:

• Supplementary lighting can account for 15% or more of a grower’s total ‘delivered’ energy. PAR (photosynthetically active radiation) determines growth and yield and this should be measured as W/m2 or μmol/m2/s.
• The high pressure sodium (HPS) lamp is still first choice for supplementary lighting. 600 W and 1000 W lamps are more energy-efficient than 400 W lamps, but typically need to be mounted higher. Routinely replace lamps after about 10,000 hours of operation. Reflectors should be chosen in relation to lamp wattage and mounting height, and should be regularly cleaned.
• Crop shading needs to be minimised. Electronic ballasts are more compact than older types and cause less shading. If it is practical, remove lamps and reflectors in summer. Avoid obstructions between the lamps and the plants and regularly clean the greenhouse glass, inside and out.
• Supplementary lighting is most beneficial when natural light levels are low. Operating times should, whenever possible, be chosen so as to give a long daily photoperiod. Settings should be used to turn the lighting off when outside light levels are high.
• Photoperiod lighting to control flowering is usually most cost-effectively given as a night-break (NB) rather than as a day-extension (DE). However, photoperiod lighting can also enhance dry weight and, in this case, DE lighting is more beneficial.
   

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