The majority of the questions are:
- Are LEDs the right choice for my operation and how can I figure this out?
- There are so many LED suppliers, which shall I choose?
What are LEDs? LEDs are solid-state semiconductor devices that produce narrow spectrum light when voltage is applied. Different semiconductor materials can produce different light spectrum. Commercially available light spectra which are relevant for plant growth and development range from 215 to 910 nm which includes: ultraviolet, violet, blue, green, yellow, orange, red, far red, and near infrared.
LEDs emit little or no radiant heat, however high-output LEDs do generate significant amounts of heat from the diode junctions, which needs to be removed to prevent overheating and possible damage to the LED array.
Although the first LED was patented in the 1960s, application of this technology in plant research only began in the 1990s, mainly in NASA’s advanced life support research group. Since that time, many studies have been carried out and LED technologies are now in a period of rapid development with dramatic increases in energy efficiency and decreases in price.
Light and plant production: Light can control plant growth rates through driving photosynthesis, modify colour and morphology (e.g. internode length, leaf shape, size and thickness), and affect flowering etc.
Wavelengths between 400-700 nm are effective in driving photosynthesis. However, different wavelengths have different quantum efficiencies (i.e. a measure of the conversion of light, expressed in terms of number of photons incident on a leaf surface over a specified period of time, into photosynthate). In general, red (600-700 nm) has the highest efficiency, then blue (400-500 nm) which is about 25 per cent lower than that of red; green (500-600 nm) has the lowest efficiency.
Most plant morphologies are controlled by wavelengths in the near-ultraviolet (380-400 nm), blue (400-500 nm) or red and far-red regions (600-700 nm and 700-800 nm). Blue and near-ultraviolet are necessary for the development of chlorophyll and other pigments (e.g. anthocyanin and carotenoid); therefore, these wavelengths can make plants greener or be used to promote certain medicinal or beneficial second metabolites.
Blue light also stimulates stomatal opening, therefore increasing photosynthetic potential; and high blue light content light fixtures can keep internodes short. Excess of far-red can stretch internodes. Plants not only respond to different light spectrum and light intensity, but also respond to the ratios of different light spectra. For example, high far-red/red and/or red/blue ratios can induce plants to be stretched, and lowering these ratios can promote plants to be more compact.
Changing light environment can also promote or delay plant flowering. Some photomorphogenic responses may be induced by very low intensities (ex. <1.5 µmol·m-2·s-1 of specific spectra).
Different plant species respond to light differently. For example, research showed that using red light alone to provide long day conditions for chrysanthemum can keep plants vegetative; however, red light can increase the number of flowers and buds in begonia. Therefore it is important to investigate the published lighting research relevant to the crops you are growing.
Armed with the above information, it is easier to understand the advantages and disadvantages of using LEDs in comparison to using some of the conventional lamps.
ADVANTAGES OF LEDS
- Increasingly better energy efficiency (i.e. conversion of joules of energy to photons of photosynthetically active radiation). Research showed that LEDs were more efficient than incandescent and fluorescent lamps and on par with lamps such as high-pressure sodium (HPS) by as early as 2008. Before 2008, their efficiency was increasing 30 times per decade with a corresponding 10 time decrease in production cost (Bourget, 2008). This trend continues. Some researchers speculate that the electrical efficiency of red LEDs will be double that of HPS lamps by 2020. When comparing the electrical efficiencies of lighting technologies, it’s best to base comparisons on light quanta (the production of µmoles of photosynthetically active photons per joule of electricity input). It is important to have an understanding of the lighting distribution provided by different LED systems as well. Very specific instrumentation (ex. an integrating sphere) is needed to assess the total light output of a particular fixture. Total light output is, however, only one part of the equation when integrating any light fixture into a specific plant production scenario. Equally important are the interrelated factors of the: 1) light distribution pattern of a given fixture; 2) hang height; and 3) lateral positioning (i.e. light overlap) of adjacent fixtures. It is an oversimplification to assume that all of the light produced by a fixture will reach the crop. This is important to keep in mind when comparing LED technologies supplied by different companies, as well as comparing LED technologies to other lighting systems, such as HPS.
- LEDs can provide specific wavelengths (i.e. light spectrum). This is actually one of the most unique capabilities of LEDs. LED technology can provide lighting fixtures with different (sometimes customizable) spectra and different spectral combinations at different (sometimes user-adjustable) ratios. This can provide growers with endless opportunities for using light as a tool to “manage” their crops – something the conventional lighting systems simply cannot provide.
- LEDs are dimmable and can be rapidly cycled between on and off, and anywhere in between. This has the potential to give the greenhouse grower the power to integrate feedback control (based on ambient light levels) to modify the supplemental light intensity in real time, thus providing only the amount of light that a crop needs, ultimately resulting in further energy savings. While LED technology has this capability, it is not presently being widely used in commercial settings.
- LEDs can have more focused light. If you have narrow benches and wide walkways in the greenhouse, a more focused light can waste less light energy. However, less focused light can cover a wider floor space.
- LEDs emit a small fraction of the radiant heat that conventional lights do. Therefore we can keep the light very close to the plant canopy without the risk of harming the foliage. This is extremely useful for tissue culture and seedling propagation, especially for producing plants with multilevel indoor facilities, to save space. LEDs can also be used for inter-canopy lighting in greenhouse vegetable production.
- LEDs have much longer lifespans. Most LEDs can last up to 100,000 hours, and the HPS lamps have a lifetime ranging from 10,000 to 20,000 hours. In other words, good LEDs have more than five times longer lifetimes than HPS.
Compared with some of the conventional lighting systems, LEDs also have a few disadvantages.
- Expensive. Even though the price has been dropping rapidly in the past few years, horticultural LED technologies are still more expensive than some of the conventional lighting systems, such as metal halide and high-pressure sodium lamps.
- LEDs do not emit that much radiant heat. Conventional lighting systems, especially HPS, can produce heat to warm up and dry leaf surfaces faster. Radiant heat from the lighting can be an advantage in certain circumstances, such as in keeping leaf surfaces dry in order to reduce the occurrence of powdery mildew. This is one of the reasons that some greenhouses are using mixed LEDs and HPS in their production environments.
- Some LED fixtures have large footprints that tend to block the natural sunlight when used in a greenhouse as a supplemental lighting source.
- Some people do not feel comfortable working under certain light colours. One measure of the colour of a light source is the Color Rendering Index (CRI). A CRI of 80 or more is recommended for office and other workplaces. Similar to fluorescent lamps, some white LED lamps can provide a CRI of 70-90. However, most of the LEDs used as greenhouse supplemental lighting are predominantly red and blue. Under this type of lighting, the colour appears pink. Certain populations feel uncomfortable (e.g. dizzy) under this type of lighting environment. Further, it can be difficult to visually assess the health (including the “greenness,” prevalence of disease, etc.) of a crop grown under LED lights.
- Not enough research has been conducted for most plant species. We know that different plant species respond to changes in the lighting environment in different ways. To apply LEDs to specific crops, the grower needs to find out the best spectral combinations to fully utilize some of the advantages of LEDs.
ARE LEDS THE RIGHT CHOICE FOR MY OPERATION?
To address this question, let’s consider two separate scenarios of horticulture lighting applications: 1) sole source, and 2) supplemental lighting.
1. Indoor crop production with artificial light as the sole lighting source. If your operation is using artificial lighting as the sole lighting source, especially in multilayer growing systems, then LEDs are your best choice. LEDs can be arranged very close to the plants without damaging leaves from radiant heat; therefore the plant supporting layers can be placed closer to maximize your space use efficiency. The applications of LEDs in indoor plant factories have been demonstrated in a few countries in recent years, and some LED suppliers have accumulated valuable knowledge and experience to share with you when consulting on your particular application in your facility.
When using LEDs as the sole lighting source, it is very important to know the best light intensity and light spectra to use. Different plant species may respond to these parameters differently. When light spectra are not correct, or when there is an imbalance of certain wavelengths, plants may show abnormal growth and morphology, such as bumps on leaves (See photo on page 14). Also, light saturation points and optimum light levels vary among different crop species and lighting environments. By understanding these, you are able to increase the return on your lighting energy inputs.
2. Greenhouse production with artificial light as a supplement to natural lighting. LEDs can be used in greenhouse supplemental lighting scenarios for certain crops. For example, our research showed that red and blue LEDs can be used to replace HPS as supplemental lighting for cut gerbera production. Studies from the U.S. showed that LEDs can be used to replace HPS for producing some bedding plants and plugs. Research also demonstrated that LEDs can be used to provide inter-canopy lighting for fruit and vegetables, such as cucumber and tomato plants. However, when considering using LEDs to replace conventional lamps such as HPS, economics need to be taken into consideration. We will discuss this later.
LEDs are especially useful for producing nutraceutical and medicinal crops. As discussed earlier, certain wavelengths are able to enhance the production of certain secondary metabolites. For example, a study conducted at the University of Arizona showed that after 12 days of light quality treatment in baby lettuce, the concentration of anthocyanins increased by 11 per cent and 31 per cent with supplemental UV-A and B, respectively, and the concentration of phenolics increased by six per cent with supplemental red compared to those in the white light control. This advantage can be applied in both supplemental and sole source lighting scenarios.
Some secondary metabolites also have anti-microbial and insect deterrent properties; therefore, LEDs may be used to induce certain plants to be more resistant to pests.
CONSIDERATIONS WHEN CHOOSING LEDS
1. Economics: We know that some LEDs can be more energy efficient, have much longer lifetime, can be more focused, and can be used to save space. We also know LEDs are more expensive and that electricity rates vary according to geographic region and time of use. We also know that some regions have incentive programs for installing energy-saving lighting fixtures, and that there is a significant range of prices from different LED suppliers.
There are many factors to consider when doing cost-benefit comparison of LED lighting technologies, making it very difficult to say how economical LEDs are. Having said that, when considering whether to use LEDs or HPS as supplemental lighting in a greenhouse production operation, it would be advisable to ask lighting fixture suppliers to provide you with quotations (e.g. number of units needed, and the unit price) based on the average light (400-700 nm) intensity (µmol·m-2·s-1) you are trying to achieve at the crop’s canopy height. Based on the quotations and light specifications (e.g. electrical efficiency, electricity consumption per light unit, the price for electricity, etc.), you are able to estimate the actual costs for getting different lighting units installed, predict their electricity consumption, and ultimately assess the most economical solution for different periods of time (e.g. over five or 10 years, or over the predicted life of the fixture).
2. Not all LEDs are the same: As mentioned earlier, most of the LEDs are advertised as lasting up to 100,000 hours of use (number of years will depend on the daily photoperiod and the portion of the year that the fixtures are in use); however these lifetimes are based on low current and low temperature conditions. LED efficiency is inversely proportional to operating temperature.
For high-output LEDs used in plant production, the high current and high temperature can limit LEDs’ lifetime and efficiency if not cooled properly. To remove the heat from the diode junctions, different technologies are currently used, which includes: active (convection) cooling (ex. fans or water) or passive cooling (ex. by heat sinks connected to the back side of the LED arrays).
Moving parts are always a concern in the harsh greenhouse environment (ex. moisture, dust, large swings in ambient temperature). For example, if a cooling fan fails, the LED unit shuts down (assuming proper thermal protection) until the fan gets replaced. Overheating can also cause irreversible damage to the fixture. Therefore it is important to shop for LEDs with robust designs and reliable peripheral technologies.
Some LED fixtures have fixed spectra (e.g., certain blue and red ratio) while others have adjustable spectra. The latter may be more expensive, but they provide you with the potential to make full use of the advantages of LEDs mentioned earlier for your current and future production scenarios.
OUR EXPERIENCES WITH DOING LED RESEARCH
Since the mid 1990s, and especially in recent years, many studies have been carried out in different parts of the world investigating the applications of LEDs in horticulture and showing some very promising results. However there are still enormous knowledge gaps and a great need for further research and exploration.
For an example from my own lab, a few years ago (supported by the International Cut Flower Growers Association, LumiGrow and local greenhouse growers), we grew three cut gerbera cultivars under both LED and HPS supplemental lighting, at the same canopy-level light intensity (PAR). Results showed that cut gerbera yield and quality were equal or better under LEDs than under HPS. Extending from that research, we are now looking at determining the optimum canopy-level supplemental LED intensity for growing various cut flowers, including gerbera and snapdragons.
Additionally, with support from Ontario Ministry of Agriculture, Food and Rural Affairs, Heliospectra, Flowers Canada Ontario, and local greenhouse growers, we have just started a three-year project entitled: “Develop feedback control systems for optimizing the use of temporally variable, intelligent LED light systems to save energy and improve crop quality in greenhouse production.”
In addition to developing lighting system feedback control (based on ambient light level and set target Daily Light Integrals), we also will investigate using different light spectral combinations to manage ornamental crop quality, such as modifying R/FR ratio to control plant height, instead of using chemical growth regulators.