Felix Corbett, TTI Europe
The horticultural lighting industry is in the middle of a disruptive change – growers are now getting to grips with the LED ‘solid-state’ artificial light alternative, traditionally provided by high-pressure sodium lamps (HPS). HPS lamps are relatively efficient at 100-150 lumens per watt, actually similar to high-brightness LEDs, but their light is not easily focused and directed where needed, resulting in higher power than necessary being used. This results in wasted energy, light pollution and heat that can damage sensitive plants. HPS lamps are most efficient at around 600W output so typical installations will be luminaires, top-lighting plants, with a large separation to avoid damage from the concentrated heat source. The light from HPS lamps is a warm yellow which is acceptable for general plant growing but cannot be changed except in a minor way in manufacturing.
LEDs on the other hand can be distributed throughout a growing area, even shining on individual plants with much less localized heating although there is a market for single 600W LED luminaires that can directly replace HPS types. The LED color can be chosen for optimum growth or flowering for specific plants. For example, deep blue and ‘hyper red’ are optimum for photosynthesis and ‘far red’ controls germination, vegetative growth and flowering. Intensity can also be varied easily and remotely without affecting color temperature, unlike HPS lamps. A measure for the relative amount of light plants use for photosynthesis in the range 400nm to 700nm is Photosynthetically Active Radiation or ‘PAR’. Figure 1 shows the average plant response to PAR which an LED fixture might try to emulate.
The benefits of LED horticultural lighting are enablers for the growth in increasing plant production, driven by population growth, limited availability of agricultural land and increasing unreliability of weather conditions. Other factors coming into play are demand for higher quality products, government subsidies and the legalization of cannabis for medicinal and recreational use. Acquisition costs for LED lighting systems are today higher than for HPS systems but the difference is narrowing and when installation, operating costs and plant yield improvements are considered, there can be a quick payback for LEDs which anyway have much longer life.
All this results in a horticultural lighting market which was valued by research company ‘MarketsandMarkets’ at USD 2.08 billion in 2017 and which is expected to reach USD 6.21 billion by 2023, at a CAGR of 20.61% from 2018 to 2023.
LEDs in the horticultural environment
It’s well known that LEDs should be driven with a constant current for their specified performance in terms of light output and colour temperature. There are some practical considerations as well though; the growing environment is often harsh with high humidity levels and the LED chip temperature needs to be kept within bounds by appropriate cooling to maintain lifespan. Running an LED at excessively high temperatures can easily bring its useful life down to figures that are little better than incandescents or HPS lamps, negating one of the prime LED advantages. Thermal management therefore of the environment around the LEDs is important, with heating from the LEDs, their drives, any adjacent power converters and ambient solar effects all needing to be factored in.
Traditional greenhouses are perhaps the most difficult to control thermally but modern indoor farming installations in sealed warehouses or even basements can be purpose-built to maintain tight temperature and lighting control with the added benefit of the exclusion of pests. Placement of AC-DC converters for the LED drives can also be chosen for optimum effect – outside the growing area as a more benign operating environment or inside, to contribute to heating necessary for the more tropical style plants.
Power architecture options
The fixed current that high brightness LEDs require is typically around one amp, dropping about three volts DC, but power for lighting in all cases will be derived from AC mains, either single- or three-phase for large installations. The AC-DC conversion would rarely be made for a single LED in commercial horticultural applications – the costs would be impossibly high and wiring unwieldy – so LEDs are typically driven in series strings at a controlled constant current from a higher source voltage.
An easy upgrade route from 600W HPS luminaires is to replace each with a self-contained LED version with its own AC power supply. Existing wiring can be used for power but if the dimming capability of LEDs is desired then extra signal cabling to each luminaire will be needed. The integrated AC-DC supply will dissipate some heat itself so this must be factored into energy/cooling calculations. For small installations this approach can be cost effective but with many luminaires, a centralised power arrangement can be better.
Having centralised power with distributed lighting puts the power dissipation from the AC-DC converter outside the growing area, potentially reducing cooling costs around the LEDs. Unlike in the first approach, the power converter need not necessarily be environmentally sealed and luminaires can be connected in strings with constant current from a high DC voltage, typically 275VDC. One high power converter replacing many smaller ones also generally gives a lower cost per watt of overall supplied power. Also, dimming can be achieved at the AC-DC power converter without additional wiring to luminaires, although all luminaires will dim together and a fault in one could shut down all others in that loop.
LED luminaires can be powered in parallel from a remote DC power supply if each luminaire has simple internal constant current control. Now, individual luminaires can be dimmed, either by hard wiring to a simple analog 0-5V or 0-10V control or by connection to a network or perhaps even wirelessly. Additional benefits are that a fault in one luminaire does not now shut down any others and high voltage DC distribution can be used reducing current and cable size for a given power level.
The three arrangements are shown in Figure 2 with example LCC600 and iHP power converters from Artesyn Embedded technologies.
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Figure 2: Possible power and luminaire arrangements in horticultural lighting Power converter product options
AC-DC power converters in each luminaire for the first architecture option are in a relatively harsh environment with local heating and sometimes high humidity, requiring a sealed unit, typically to IP65. Maintenance is not easy, so conduction- rather than fan-cooling is preferred. The Artesyn LCC600 (Figure 3) is a suitable converter for the application which is a high-efficiency design rated at 600W in a compact enclosed package. Output can be controlled by resistor setting, 0-10V analog input or digitally through an I2C PMBusinput. Units can be paralleled for active power sharing for higher load ratings and, for less severe environments, a fan cooled version is available at lower cost.
For centralized power architectures, the Artesyn iHP series is a good choice (Figure 3), in a 19” rack configuration, scalable in 3kW increments to 24kW in a large rack. With multiple racks in cabinets and with optional three-phase AC inputs, megawatt levels can be reached. Output voltage can be selected from 12-1000VDC to suit the luminaires chosen and for minimum wiring costs. Like the LCC600 part, the iHP series can be controlled by analog and digital inputs with the addition of a cloud-based GUI allowing the user to set lighting schedules and profiles remotely through the internet.
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Figure 3: Artesyn LCC600 (left) and iHP power converters (right) for horticultural LED lighting
The future of horticultural lighting
With an additional 82 million people to feed each year  and climate change uncertainty, agricultural production must become more efficient and reliable. Lighting is key to this with better use of available growing space. ‘Vertical’ farming is attracting interest with side illumination and hydroponics stacked in sealed environments with no soil or natural lighting is seen as a way to eliminate the effects of pests and reduce product air-miles and carbon footprint, bringing mass food production into cities.
LED lighting enables these developments and potentially makes food production more efficient in terms of energy and space utilization and although up-front costs are higher compared with older technologies, benefits do follow. Artesyn Embedded Technologies makes the transition to LED lighting easier with their range of power conversion products, tailored for the application.