- Energy Saving
LED lighting for horticultural applications
The arrival of commercially-viable light emitting diode (LED) lighting for horticultural applications has been promised for a number of years now, and recent developments by a number of manufacturers suggests that the promise is now close to reality.
On the other hand, commercially available units still seem difficult to find — they are expensive compared to conventional alternatives, and information about how to use them in horticultural applications is not readily available.
This article outlines the current status of the knowledge on LED lighting in horticulture, explains where they might be used now and highlights the development work that needs to be completed before traditional lighting solutions are replaced by LED.
Why use LEDs?
Using traditional lighting technologies in horticulture is a compromise solution. This is because many of the products available are adaptations of designs that have been produced for commercial and industrial applications; and this means that they are often not a perfect match to the requirements of plant growth and development.
Problems that occur in practice include:
- High electricity costs due to low efficiency of ‘plant appropriate’ light output.
- Uneven light distribution due to the ‘point source’ nature of the bulb and fitting.
- Excessive waste heat from the light, which makes achieving the optimum growing environment difficult. This is radiated heat, which means it is not normally possible to place the light close to the crop without causing overheating or scorching.
- The spectral output is not matched to the needs of the crop. Plants have different spectral requirements to humans, and because most traditional lights are adapted from designs that assist human vision, the end result is large parts of the light output are not absorbed by the plant.
- Expensive capital costs and short operating life.
The potential advantages for LEDs are:
- They are more energy efficient.
- They last longer.
- The spectral output can be matched to the particular application and only the parts of the light spectrum utilised by the plant can be produced.
- The heat output can be controlled more easily. There is very little radiated heat so the LED lights can be located close to the crop.
- A more even light distribution can be obtained. Many LEDs, each with a low light output, are combined together to make up a fitting and/or installation.
So are these advantages available to growers in commercially available products now and is using LEDs economically viable?
A word about LED development
Developing new lighting products for offices, retail, outdoor areas, homes etc., dominates the R&D activities of the lighting manufacturers. Consumers’ current appetite for energy saving products means the majority of the resources of the lighting industry are being devoted towards LED developments for these market areas right now. As a result LED technology is rapidly developing, with new and upgraded products constantly being introduced. However, such developments are concentrated on getting better performance from LEDs when they are used in a ‘human’ environment.
Whilst this is good news because it is improving LED technology and driving down the cost of the equipment, a word of caution should be noted for horticultural applications. Like all lighting applications specific developments are needed for horticultural applications of LEDs, and compared to work in the commercial sector, th
is is still relatively small.
Photoperiodic lighting – because of spectral output and cost considerations, tungsten light bulbs have remained the light source of choice for this application for many years. However, tungsten light bulbs were phased out by the UK Government on 1st September 2011 and growers are now seeking an alternative. Compact fluorescent lamps (CFLs) are an alternative, but their spectral make up is quite different to the red and far red outputs of tungsten bulbs. As a result, the performance of CFLs is not universally well suited to all species.
Because of the low light intensity and specific spectral requirement, designing an LED with the right light output for photoperiod applications that has good energy efficiency is possible and some commercial examples are now available. HDC research (PC 296 Examining the lighting requirements for daylength control so as to assess the suitability of energy saving bulbs) is currently ongoing to confirm the spectral requirements and performance of a range of lamps (including LEDs) for this
Supplementary lighting – this requires high intensity light to ‘make good’ the lack of sunlight that we suffer from in the UK during the late autumn, winter and early spring months. The current light source of choice is high pressure sodium (HPS), but it is only a compromise solution. Particular problems with HPS are a shortage of output in the blue and red parts of the spectrum together with a significant radiant heat output that is difficult to put to good use. Problems also occur with tall ‘vine crops’ such as tomato because it is difficult to get the light from HPS lamps mounted above the crop to penetrate into the canopy. This is why supplementary lighting applications in countries such as Finland have used inter-canopy lighting (using additional HPS lamps) to work alongside lamps positioned above the crop.
Two approaches can be taken to using LEDs for supplementary lighting:
- Completely replace the existing HPS with LED - no commercial LEDs are currently available that can compete with HPS for complete replacement. This is because the photosynthetic active radiation (PAR) efficiency of LEDs (measured in μmol/W) is still not as good as HPS. Leading horticultural lighting manufacturers are predicting that full replacement supplementary lighting solutions are still 5 years away from commercialisation.
- Part replace and/or supplementing HPS with LED – work here is concentrating on the development of hybrid lighting solutions that combine the efficiency of HPS with the advantages of LED. This involves using a combination of the two technologies for lighting vine crops such as tomato.
The potential advantages of this approach are:
- The growth potential of the leaves in the plant canopy can be exploited better with inter-lighting; and by using LEDs overheating of the parts of the plant that are closest to the lamp is less likely to be a problem.
- The intensity of the lights positioned above the crop can be reduced by moving some of the light down into the canopy. This reduces the ‘waste’ heat from the HPS lamps positioned in the greenhouse roofspace and reduces the need for ventilation and screen gapping.
- The LEDs used in the plant canopy can be designed to give the best spectral output to meet the needs of the crop. This is most likely to be blue and red light.
Several trials with this arrangement have reportedly been carried out, but the one with the highest public profile has been done by Green Q at their Improvement C
entre located in Bleiswijk, Netherlands. They have worked closely with Philips and Wageningen University.
As yet no definitive results from the work have been published, but verbal comments made by Green Q staff during a visit to the centre in September 2011 suggested that the technique still requires significant further developments. The results show that some limited yield advantages had been obtained so far, but problems have been experienced with too much ‘waste’ heat being emitted by the LEDs in
to the crop canopy. Also yield increases are insufficient to give acceptable paybacks on the capital cost of the LED installation.
The results of work to date suggest that, even with inter-lighting, the use of LEDs for supplementary lighting is still some way from widespread commercial uptake.
Tiered production systems – one of the most promising and talked about applications for LED is multi-layer production systems such as growing rooms, tiered benches etc. This is because these applications can exploit two of the major advantages of LED lighting systems, good light uniformity and the ability to put the light source close to the growing plant. Also, the light spectrum can be tailored to the specific needs of the crop, so any deficiencies from a total absence of solar radiation can be engineered into the equipment solution.
At the moment applications are being concentrated into two main areas:
- Replacing tubular fluorescent lighting in growing rooms.
- Extending space utilisation in greenhouses by providing under bench lighting etc.
Independent information on the lighting requirements and specific performance of systems is difficult to obtain and leading suppliers (e.g. Philips etc.) are claiming to be working with commercial growers to deliver solutions and operation ‘recipes’ that meet the needs of individual companies. In all cases the starting point for these recipes appears to be the delivery of μmol levels that are identical to those currently delivered by conventional equipment.
It is apparent that the commercial developments for LEDs will take place according to where the largest potential markets are seen to be. However, the HDC LED steering group have identified niche areas of interest beyond the impact of light spectrum on photosynthesis (and hence yield) and photoperiod that it may be necessary to pursue
outside of these commercial developments. The international scientific community are already undertaking studies that will contribute to our understanding of how these niche effects may be translated into commercial benefits. Areas of research already underway include:
- Spectral effects on insects (both pests and beneficial predators), which includes a new studentship (CP 88) at SAC.
- Spectral effects on plant growth responses e.g. plant height, production of secondary chemicals that may influence flavour/colour and on rooting.
The challenge will be to channel this work into useful outputs for growers and to avoid duplication of effort on topics where work is ongoing but unpublished. HDC are currently developing work to assist with moving these potential niche benefits through to useable outputs for levy payers and welcome any information or comments from growers.
There is considerable activity, both by researchers and manufacturers, to develop LEDs for horticultural applications. Whilst the technique undoubtedly offers significant potential, the current state of development is that the technology is still some way from wide commercial uptake.
Applications where commercial products are currently available are:
- For photoperiodic lighting as a replacement for tungsten light bulbs.
- For tiered bench applications (e.g. growing rooms) as a replacement for tubular fluorescent lamps.
In both of these applications care should be taken to ensure that the spectral output of the commercially available product matches the requirements of the species that are being grown. This needs to be determined by working closely with the equipment supplier and reference back to prior R&D results. To this end it is recommended that products should only be sourced from reputable suppliers.
For supplementary lighting applications, commercial uptake is still some way off. The most likely first use of LEDs for this application is going to be for inter-canopy lighting of vine crops such as tomato. However, current R&D results suggest that this is still two or three years away from successful commercialisation.