LED Grow Lights for Greenhouse Toplighting

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A greenhouse is a structure enclosed entirely by transparent materials such as glass, rigid plastic, or a thin plastic (polyethylene film) that protect plants from the effects of wind, cold, dehydration, and insects while still admitting the light they need to grow. Greenhouses allow for regulation of microclimatic parameters such as temperature and humidity, giving plants the most optimal environment in which to grow. However, the single most important variable with respect to plant growth and development—light—often becomes a limiting factor. The influence of light on the production of various plants, such as vegetables, fruits, flowers and herbs has been well documented. Plant cultivation in greenhouse heavily relies on sunlight, of which the availability depends strictly on weather conditions. Natural sunlight is not always attainable in sufficient quantities. Supplemental lighting is often used in permanent rigid plastic and glass greenhouse structures to extend daylight hours during winter seasons with short periods of solar radiation or to fill in the natural fluctuations in sunlight due to weather.

The use of artificial lighting in horticultural applications generally serve three purposes: transplant production and propagation of seedling that involve both photosynthetic and photomorphogenic lighting; supplemental lighting to enhance photosynthesis and thereby improving growth and quality of plants, especially during light-limited periods of the year; and photoperiodic lighting to control developmental phases, e.g., inducing early or out-of-season flowering. Photosynthesis, photomorphogenesis, and photoperiodism are the three basic processes related to interaction between optical radiation and plants. Photosynthesis is the process by which plants harvest energy from photons to synthesize sugars, for example glucose, from carbon dioxide and water. Photomorphogenesis is about the development of plant morphology (plant architecture) under the influence of light spectrum. Photoperiodism relates to the reaction of the plants to the light-dark cycle. At its most basic, supplemental greenhouse lighting should address the minimum requirements on daily light integral (DLI) from plants. DLI quantifies the total number of photosynthetically active photons that a plant receives over the 24 hours of a solar day. It's especially important to measure DLI in every greenhouse where plants are receiving varying amounts of sunlight. In addition to the seasonal fluxes in solar radiation and unpredictability of weather, light transmission inside a greenhouse is usually reduced by 35-50%. The amount of photosynthetically active radiation (PAR) received each day can have a profound effect on plant growth, development, yield, and quality. There is a direct correlation between DLI and shoot mass, root development, transplant propagation, as well as finish plant quality attributes such as stem thickness, plant branching and number of leaves and flowers.

High intensity discharge (HID) lamps, such as high pressure sodium (HPS) and metal halide (MH), have been the usual choice of light source for greenhouse lighting systems because of their ability to deliver adequate photosynthetically active radiation (PAR) with economic viability. An HPS lamp emits in the red-orange region of the visible light spectrum, which can power the flowering and fruiting processes. The blue-white emission of metal halide lamps may be better suited for vegetative growth and seedlings. However, the spectral power distribution (SPD) of both MH and HPS lamps do not exactly match chlorophyll absorption spectrum. About 20% to 30% of the light is emitted outside the PAR region. Radiant energy that is not absorbed by the plant photoreceptors is simply wasted. HID lighting requires considerable separation between light fixtures and plants to ensure uniform light distribution as well as to avoid thermal stress from infrared radiation. Achieving a very high photosynthetic photon flux density (PPFD) with HID lighting has always been limited by the distance that the lamps need to be kept from the plant canopy. Furthermore, limited dimming ability and fixed light spectrum of HID lamps deprive conventional greenhouse toplights of the flexibility to develop a customized light recipe that works for each stage of plant growth.

The advent of solid state lighting based on LED technology has sparked a revolution in horticulture lighting. Capable of delivering electromagnetic radiation that plants need in the exact spectrums across all stages of crop production from, seedling, cloning, all through vegetative to flowering, fruiting and harvest, the benefits of LED lighting in the cultivation of herbs, vegetables, fruit, flowers and other plants in greenhouses is nothing short of remarkable. While recent implementation of double-ended HPS lamps provides a significantly improved photosynthetic photon efficacy (PPE) (up to 1.7 µmol/J) over traditional mogul-based HPS lamps (1.02 µmol/J), the best-in-class PPE of LED grow lights has reached 3.8 µmol/J and continues to advance toward the practical potential of 4 µmol/J. Improved optical design thanks to the directionality and small source size of LEDs can allow more efficient delivery of light with the optimum optical distribution. In contrast, much of the light produced by an HID lamp is lost within the fixture due to the omni-directional emission of the light source. The light that exits the HID fixture is not uniformly distributed, thus further lowering the optical delivery efficiency of the entire lighting system. The estimated lighting power density (LPD) of incumbent HID fixtures for greenhouse lighting is 10.7 W/ft2. With LED lighting, a 40% reduction in electricity consumption per square feet of grow area can be achieved. An overwhelming advantage of LED lighting is the ability to optimize of the spectral power distribution for the function of the light. The LED packages can be spectrally engineered to target the active spectrum of photosynthetic, photomorphological, photoperiodic, and phototropic photoreceptors. The SPD of an LED grow light can be actively controllable, which allows to provide a spectrum tailored to the optimal needs of specific plant species over the course of a plant's life cycle. Last but not least, the digital nature of LED technology brings lighting, intelligence and networking together, allowing horticulture lighting systems to participate in smart farming enabled by the Internet of Things (IoT).

Greenhouse LED grow lights may be designed with light engines that emit light in a wide spectral variety to meet the needs of specific cultivation environments and applications. The selection of spectral composition for greenhouse applications will depend on the particular plant being grown, stage of growth, geographic location of greenhouse, availability of natural daylight, and desired plant growth or development. Broad spectrum lighting with electromagnetic radiation spanning a wide range of wavelengths is generally used to supplement daylight when DLI is a major factor limiting plant production. The spectrum typically plays an emphasis on red and blue light and may expand beyond the photosynthetically active range of wavelengths, into the UV and far-red regions. Narrow band lighting is designed to stimulate specific plant photoreceptors for desired plant responses. The LEDs of the light engine are selected to emit photon colors that match the absorbance peaks of the desired plant photoreceptors. Chlorophyll photoreceptors tend to absorb red and blue light most strongly to drive photosynthesis and growth. Photons with a wavelength of 660–680 nm in the red waveband yield the highest photosynthetic productivity. While chlorophylls have their photosynthetic peaks in the red part of the visible light spectrum, red light alone is insufficient to achieve maximal yield from photosynthesis. Plants grown under the spectral combination of red and blue show a higher photosynthetic activity than that under either monochromatic light. Accessory pigments such as carotenoids play an important role in photosynthesis as they can assist in capturing light that chlorophyll does not absorb. The absorbance range of carotenoids covers UV and blue and extends into the green region (400–550 nm, peaking at around 470 nm). Another well-characterized function of carotenoids is to protect chlorophylls from photooxidation in high irradiance environments.

The influence of the spectral composition of an LED grow light on plant is not only limited to growth. The development and physiology of plants are also strongly influenced by the particular spectrum of light they receive. Photomorphogenesis is controlled by phytochromes and cryptochromes. Phytochromes come in two isoforms which respond to 660 nm red and 735 nm infrared radiation respectively. The conversion between two isoforms is determined by the ratio of surrounding red and far-red light. Photomorphogenetic responses mediated by phytochromes are related to flowering induction, seed development, inhibition of hypocotyl elongation, apical hook straightening, leaf expansion, circadian rhythm entrainment, shade avoidance syndrome, and chlorophyll biosynthesis. The prevalence of one form or the other (R:FR spectral ratio) can stimulate or inhibit a number of developmental processes such as flowering induction, seed development, hypocotyl elongation, apical hook straightening, leaf expansion, shade avoidance syndrome, and chlorophyll biosynthesis. Phytochromes interact with and regulate cryptochromes, which most strongly absorb at 350 nm (UV-A) and 450 nm (blue). Cryptochromes are involved in the regulation of germination, hypocotyl elongation, pigment synthesis, photoperiodism (circadian rhythm entrainment), sensing of magnetic fields, normal chlorophyll and chloroplast development, and enzyme synthesis. Phototropins are also blue/UV-A absorbing pigments. These photoreceptors regulate stomatal aperture (opening and closing), phototropism (bending towards the light), chloroplast movement (photosynthetic organelles) within leaf cells, leaf flattening, and hypocotyl elongation.

A greenhouse toplighting LED luminaire is a high output system that can produce a photosynthetic photon flux (PPF) of up to 2500 μmol/s. It can take many forms, but often comes in a linear shape which minimizes blocking of sunlight. Modular design of linear luminaires allows for end-to-end installation or parallel configuration so that lighting can be easily adapted to any layout. The need for longevity from a lighting system runs parallel with the incentive to maximize energy efficiency in the horticulture lighting market. The longevity and efficiency of LEDs are achieved on the prerequisite that the LEDs are driven under the anticipated operating conditions. The high power architecture of greenhouse LED grow lights presents a litany of challenges to effectively mining the full potential of LED lighting. A holistic approach is necessary to ensure all subsystems of the luminaire are orchestrated to provide an optimal operating environment for the LEDs. At the same time, many of the performance variants that a high power LED grow light offer depend on the performance of the LEDs under operational and environmental stresses. For LEDs, their ability to perform under given conditions for a required period of time depends on the package platform they use. While chip-on-board (COB) and plastic-molded lead frame packages find their applications in high power LED systems, high power LED packages are currently the mainstream choice of light source for greenhouse toplighting applications. These ceramic-based semiconductor emitters have gained a reputation for excellent lumen maintenance and long-term reliability in high power density and high luminous intensity lighting applications. Chip-scale package (CSP) LEDs are finding their way into high power horticulture lighting applications. Built on a platform that removes as many of the packaging elements found in conventional LEDs as possible, these semiconductor packages have the lowest thermal resistance and this offer high drive current capability. Most LEDs, such as full spectrum white LEDs, and monochromatic green, cyan, blue and royal blue LEDs use indium gallium nitride (InGaN) chip technology. Red LEDs are currently fabricated from aluminum indium gallium phosphide (AlInGaP).

When the LEDs are installed in a luminaire, their performance metrics and service life values are only valid provided that their p-n junction temperatures are controlled below the limit above which there is irreversible damage. Notwithstanding the impressive improvement in energy efficiency over more traditional light sources, 50 to 80% of the power fed to the LEDs are converted into heat. The external quantum efficiencies of red AlInGaP LEDs are only about 25%. While InGaN LEDs have external quantum efficiencies that exceed 60%, a significant portion of radiant energy is converted to heat in full spectrum white LED packages during wavelength conversion (phosphor down-conversion). Unlike HID lamps that dissipate most of thermal energy through infrared radiation, LEDs do not radiate heat. The heat generated within the semiconductor package must be dissipated through conduction and convection via a physical thermal path. The LED junction temperature is a function of the drive current, thermal path, and ambient temperature. While it's necessary to exercise proper regulation of drive current, thermal management for LED grow lights revolves around minimizing the effective thermal resistance for every component along the thermal path. The high power LEDs are generally populated on a metal core printed circuit board (MCPCB) which incorporates a dielectric polymer layer with high thermal conductivity in its construction to reduce thermal resistance. Thermal interface resistance formed between the PCB and heat sink has taken into account. Thus, it is often required to use thermal interfacing material (TIM) that completely fills interfacial air gaps and voids and facilitates heat transfer between two mating surfaces. Most greenhouse LED lighting systems use passive heat sinks to dissipate heat, rather than fan-assisted or other active cooling systems. For a passive heat sink to handle the massive thermal load, its critical to maximize its maximizing material thermal conductivity and effective surface area. The heat sink is usually constructed from die cast, extruded, or cold forged aluminum. Different manufacturing processes lend aluminum heat sinks different thermal characteristics. The shape and geometry of a heat sink should be optimized as well to enhance convective thermal transfer.

An LED driver converts power supplied from an alternating current (AC) source to direct current (DC) power with forward voltage and current matched to the LED array. Typically, such transformation of AC electrical energy into DC electrical energy is accomplished by an intelligent, two-stage SMPS LED driver that implements an active PFC stage to provide a near unity power factor and a low total harmonic distortion (THD) across a wide input voltage range. A DC-DC converter then converts the pre-regulated DC power into a predetermined magnitude of DC power. This circuit configuration provides more efficient power conversion for high power LED systems and higher surge immunity than it is possible with a single-stage solution. The LED driver may be designed with multiple output channels which keep groups of LEDs on a circuit board operating independently of each other to stimulate specific plant photoreceptors. Protection features against overvoltage, overload, open- and short-load conditions, as well as intelligent overtemperature protection are also a part of the driver design. Lighting controls, which include various dimming and sensor technologies, may be used to operate the LEDs for further energy savings or advanced lighting capabilities.

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