Green light for LEDs
10 April 2018
Rising population levels, climate change and competition for land use between sectors – food, fuel and animal feedstuffs – are all causing scientists to rethink how we approach agriculture at large. Horticultural lighting could be a solution to such issues.
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In photosynthesis, chlorophyll in the leaves of green plants uses energy from light to synthesise nutrients from carbon dioxide and water, before providing the starches required for the plant’s growth. An invaluable by-product of this process is the oxygen on which humans and other animals rely as one essential building block of their existence.
Pressure on water resources and land masses has led scientists to explore growing plants indoors, or in areas not previously devoted to horticultural use. Developments in lighting are also helping to drive forward these alternative approaches.
Artificial lighting has already been used for growing plants indoors, but it has previously proved costly to run in terms of its energy use. Another drawback has been that traditional incandescent lights produce heat: this not only wastes further energy, but also means that the lights cannot be placed too close to plants (for fear of damaging their leaves).
The advent of LEDs has changed the picture considerably. LEDs use 85% less energy than conventional lights, according to lighting specialist company, Philips. They are also available in a wide variety of different colours, making it easy to tailor the light that is supplied to each plant or group of plants. For instance, light in the red wavelength portion of the spectrum is vital to the fruiting and flowering of plants, while foliage requires blue light to grow.
Plants are highly sensitive to colours, and even different wavelengths within a colour band can produce varying effects. Cucumbers, for example, subjected to blue light at 455 nanometres (towards the low end of the blue wavelength, which runs from 450 to 495nm) will exhibit slow growth; those grown at 470nm, however, will produce more extensive foliage.
Tomatoes exposed to red light at 660nm (about one third of the way into the red wavelength band of 620 to 750nm) will fruit plentifully. For horticulturalists, it pays to know both your onions and your wavelengths!
Plants can be exposed to the most appropriate wavelength to allow them to photosynthesise and optimise their growth; moreover, using artificial light to control their growth also effectively enables the length of the day to be extended. Just as providing chickens with artificial light in winter – when daylight hours are short – encourages them to produce more eggs, so too can plants be helped to prosper by extending the number of ‘growing (daylight) hours’.
Because lighting levels are so crucial, it is important to choose devices from a manufacturer with a wide range of LED colour availability. Devices in the OSLON-SSLColors product range from OSRAM Opto Semiconductors are offered in eight different colours, covering the crucial wavelength band from 450 to 730nm.
The high performance LEDs are supplied in a robust ceramic package and have low thermal resistivity. They are supplemented by the availability of a wide range of products from Everlight Electronics designed specifically for the horticultural lighting sector.
Light according to need
One of the main advantages of using LEDs for horticultural lighting is that users can configure their own lighting strips according to preference. A combination of different colours can be incorporated to satisfy different growing requirements.
The small size of the LEDs means that the devices can be easily arranged in different formats and combinations. It also means that lights can be positioned between plants if necessary, rather than above them in a more conventional arrangement. This not only helps to reduce shading, but also enables users to cater for the differences in the plants’ growing heights.
As well as needing to be housed in racks, it is of course important that LEDs have the correct control and power management systems, enabling them to deliver the right levels of light. A variety of solutions are available from semiconductor manufacturers such as Infineon Technologies and Diodes. For single-colour topologies that operate above 100W, Infineon recommends the use of the ICL5101 AC-DC PFC (power factor correction) LED driver IC. This delivers high efficiency (94%), high power factor (0.95) and low harmonic distortion (<10%) throughout the entire load range.
The device can also be used with multi-string topologies, especially when combined with the CDM10V dimming IC. This cost-effective dedicated lighting interface IC is capable of transforming an analogue 0 to 10V input into a PWM (pulse wave modulation) – or other dimming input signal, as required by a lighting controller IC. It allows designers to replace many of the discrete components used in conventional 0 to 10V dimming schemes with a single device.
Another useful Infineon device is the ILD6150 DC-DC LED driver. This is a step-down (buck) converter that allows input operation up to the maximum safety electrical low voltage (SELV) of 60VDC. The 6150 is a switch mode device that has been tailored to drive high power LEDs and LED arrays and provides flexible dimming options. The driver IC handles currents up to 1.5A, allowing up to 16 to 18 LEDs to be stacked in series. As well as providing efficiency levels of up to 98%, the device has a thermal protection option, obviating the costs that would be incurred by the need to use a heat sink.
The 6150 can be used in conjunction with the XMC1300 microcontroller to provide simplified LED dimming and better colour control. High-frequency 12-bit pulse density modulation is used to provide automatic brightness control, which ensures dimming is completely flicker-free (even at dim levels of <0.1%).
Automatic exponential dimming and linear intensity changes make brightness or colour changes appear smooth and natural to the human eye, while a brightness and control unit (BCCU) simplifies digital LED dimming. It also makes for easy transitions in orthogonal colour space (for instance RGB) and provides automatic timed transitions independent of lamp brightness. The XMC1300 caters for DMX and DALI connectivity, plus sensory equipment can also be integrated.
LED lighting has provided a real boost to the sector of horticultural lighting. The development of specialised control systems will continue to add further dimensions to the area (for instance, in terms of better energy management) – helping it to grow and develop. For those involved in the business, it is important to go to a supplier who understands the area well and can offer a wide range, not only of LEDs themselves, but also all of the support systems and devices that may be required – such as heatsinks, connectors, cables, MOSFETs, diodes and microcontrollers.
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