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How to Optimize LED Lighting Designs for Indoor Growing

By Barry Manz

Contributed By Digi-Key's North American Editors

The indoor farming industry is increasingly exploiting the many benefits of LED lighting, and for good reason. LEDs are very small and lightweight, operate at least 10 times longer than any other light source, draw minimal current, are very efficient, can produce different spectral wavelengths, and are compatible with digital control systems. However, designing and optimizing the performance of an LED lighting system is a complex endeavor and requires attention to many more metrics than comparatively simple predecessors, such as high-pressure sodium (HPS) lamps.

This article describes the role of LEDs for indoor farming, discusses the challenges they pose, and gives recommendations for their use. Along the way it provides examples of LEDs and associated components for indoor farming applications from companies such as OSRAM, Luminous Devices, Würth Elektronik, ams, RayVio, and Microchip Technology. It ends by addressing recent developments in the utilization of the UV spectrum as well as other requirements for optimizing LED lighting systems.

The growing LED farming ecosystem

The transition from HPS and other light sources to LEDs for indoor growing has been made possible by the massive scale of the consumer lighting market that has provided the incentive to more rapidly advance the state of the art. As a result, the variety, performance, reliability, and cost of LEDs have dramatically improved in recent years. For example, the OSRAM Model GH CS8PM1.24-4T2U-1 is centered on the spectrum from 646 to 666 nanometers (nm) (red), has radiant power of 425 milliwatts (mW) with efficiency of 59%, and a radiation angle of 80˚.

The Luminous Devices Model SST-10-B is centered on a wavelength of 450 nm (blue) and delivers a minimum radiant power of 510 mW with efficiency of 57%. Radiant angle can be specified as either 90° or 130°. Würth Elektronik’s horticultural LEDs include the Model 150353GS74500 525 nm (green) device that has a viewing angle of 125˚. These manufacturers and others also offer LEDs at other wavelengths for indoor farms that cover the entire spectrum required for growing (Figure 1).

Graph of absorption spectra of the pigments used for photosynthesisFigure 1: The absorption spectra of the pigments used for photosynthesis is spread widely throughout the visible spectrum from about 400 to 700 nm. (Image source: Würth Elektronik)

Growing plants indoors spans multiple scientific disciplines, from botany to plant and soil science, crop management, and now electronic monitoring and control systems. Injecting a new light source into this environment is both challenging and rewarding as new discoveries are being made at a rapid pace. Under the optimal conditions achieved indoors using LED lighting, truly amazing results can be obtained.

A widely cited example is the Mirai vertical lettuce farm in the Japanese city of Tagajo (Figure 2). This 25,000 ft.2 facility, located in the clean rooms of a former Sony fabrication building, has been harvesting thousands of heads of lettuce, and other plants as well, per day since 2015. It achieves this using 17,500 LEDs, without the use of pesticides, using 1/50 of the water, and 40% less food waste, in a bacteria-free environment.

Image of Mirai vertical farmFigure 2: The Mirai vertical farm is the second largest in the world and one of the first to become operational. (Image source: National Geographic)

With versatility comes challenges

Ironically perhaps, the LED’s versatility, one of its unique and primary benefits for indoor growing, also makes implementing an LED-based indoor farm more complicated. For example, they’re dimmable so their drivers must include this capability. Also, achieving plant-specific wavelengths requires knowledge of the LED’s more complex specifications.

As solid-state devices, LEDs require attention to factors not required with a “bulb”, such as reliable and fast acting overload protection and precise matching of the diode to the control circuit, among others. Fortunately, the rapid growth of horticulture, especially vertical farming, has provided manufacturers of lighting components with the incentive to develop entire ecosystems devoted to this application, including reference designs, evaluation boards, and technical literature from basic to advanced that make the designer’s job much easier.

A common misconception among some growers is that LEDs produce less heat than HPS fixtures, but this is only true if the LED fixture is driven at a lower wattage. That is, a 600 watt LED fixture and a 600 watt HPS light source will produce about the same amount of heat. The difference between the two is how much light energy is produced and how the heat is radiated from the fixture.

Heat from HPS light sources can reach 800°F and radiates toward the crop, while the heat in LEDs resides where the diode and its electronics are mounted on the PC board and is not focused on the plant. This is a primary reason why LEDs are far superior to HPS for vertical farming as they can be placed very close to the plants without causing damage.

The logical choice based on the discussion above would be to choose lower power LEDs, and for close-in, multilayer applications this is usually the case. However, most low-power LEDs have a fixed radiation angle, while high-power LEDs are available with radiation angle increments from 80 through 150 degrees. In addition, many more low-power LEDs will be needed to match the performance of a high-power LED. High-power LEDs are often best suited for canopy applications in which their greater output can provide broad area coverage from a distance.

Nevertheless, the heat generated by the LED fixture is still present and must be removed quickly from the board through a thermal management system, or the longevity of the LEDs will decrease significantly, and complete failure is not uncommon. The primary cooling methods are passive fixtures with heatsinks and actively cooled fixtures that use fans or water. The latter types consume energy and as mechanical devices they can fail, resulting in LED overheating.

Optimizing operating lifetime

LEDs typically have a manufacturer specified operating life of at least 20,000 hours and often up to 50,000 hours, with end of life defined as a 70% reduction in brightness from its original value. The goal of the LED lighting system designer is to ensure LEDs achieve their rated life while also retaining the greatest output over time by stabilizing their input voltage and current. This is the task of the power supply, in particular the LED driver that acquires data continuously from temperature sensors and performs adjustments to maintain optimum performance. To supplement these capabilities, it is desirable to measure the brightness of the light sources in real-time, feeding the information once again to the driver. Spectral sensors are the most cost-effective and least complex means for accomplishing this.

For example, ams provides a family of spectral sensors that measures the actual spectral profile of LEDs in real-time and directly controls an LED driver to adjust the output until it matches the specified target values for chromaticity and intensity. The Model AS7263-BLGT has six independent optical filters whose spectral response is tailored to a range from 600 to 870 nm (Figure 3), while the AS7262-BLGT covers 450 to 650 nm. Together, they provide the ability to precisely monitor individual LEDs either within a fixture or directly at plant level. Communication is provided with text-based messages via UART or via I²C. Collectivity, these sensors, along with other capabilities, allow LED lifetimes to be optimized while enabling trend analysis and other analytics.

Diagram of ams AS7263-BLGT light sensorFigure 3: The AS7263-BLGT light sensor is sensitive to wavelengths between 450 and 650 nm. It is one of a family of spectral sensors that measures the spectral profile of LEDs in real time and directly controls an LED driver to adjust the output until it matches the specified target values for chromaticity and intensity. (Image source: ams)

Circuit protection

Most applications require that LED strings be fed with a constant current power supply, and designing for this in long strings can be challenging. Circuit protection relies on multiple components within the control system because the entire control circuit from the LED to the passive and active components must be protected from transients. The primary overvoltage protection device is a metal oxide varistor (MOV) located on the AC input that provides a high level of transient voltage suppression, as well as reducing stress caused by ring-wave effects. It will absorb potentially destructive energy and dissipate it as heat, helping to protect components. An LED string driver circuit generally also includes a positive temperature coefficient (PTC) resistor that protects LEDs from overcurrent and overtemperature, and a parallel transient voltage suppression (TVS) diode for overvoltage protection. The line rectifier circuit should include a high voltage DC fuse on the output for secondary protection. Adding a resettable fuse in series with the LED is also recommended to prevent thermal runaway.

Another consideration is that indoor farming typically requires relatively high ambient temperatures and high humidity to foster plant growth, so the lighting system must be capable of operating in this environment. In addition, unlike fixtures used in other applications that remain in one place throughout their operating life, in vertical farms they are designed to be raised, lowered, or repositioned to optimize plant growth. This impacts their wiring requirements which are detailed in UL 8000.

Driver considerations

There are two main types of drivers, those that use low voltage DC input power and those using high voltage AC power. For example, the Microchip Technology CL88030-E/MF is designed to drive a long string of low current LEDs directly from 120, 230, or 277 VAC. A typical application includes the driver IC, four power FETs, four resistors, two capacitors, and a bridge rectifier. Over-temperature protection is provided to gradually reduce light output with increases in temperature, along with line regulation. Additional over-temperature protection can be implemented with an NTC thermistor (Figure 4).

Diagram of Microchip Technology Model CL88030-E/MF sequential linear driverFigure 4: An application circuit for the Microchip Technology Model CL88030-E/MF sequential linear driver shows the device along with a protection circuit using an MOV. (Image source: Microchip Technology)

The number of LEDs that can be placed in series depends on the driver, input voltage, and electrical codes and safety standards. Placing LEDs in a single series chain benefits from the need to use just a single driver along with equal current flow through each LED. However, it results in a high output voltage and thus larger circuit components, and possibly the need to address additional safety standards.

A series-parallel array has a lower input voltage and reduces the chance of electric shock. If one LED branch fails the other branches will continue to operate, and failure of one LED will not disable the entire array. That said, the driver is a constant current source, so it will force more current into the operational devices with possible overheating. The series-parallel array also does not allow the LEDs to share drive current equally unless the LED forward voltages are very similar.

One answer to some of these issues is to use drivers for each LED string, which offers the highest reliability but adds cost and increases size. This approach allows some light output to be realized even if more than one LED string fails.

The question of UV lighting

There continues to be considerable discussion within academia and industry about the potential for using LEDs in the ultraviolet “B” (UV-B) non-visible portion of the spectrum between 280 to 385 nm for growing plants. UV light has generally been considered of less interest for indoor farming because it is outside of photosynthetically active wavelengths. Consequently, until about 15 years ago, minimal research was conducted on the subject.

Another factor limiting interest in this spectral region is safety: UV-B photons are well known for causing cell damage in humans and plants. In fact, lighting manufacturers take extensive measures to dramatically reduce UV light emitted by their devices. As such, employing UV in indoor farming would require extensive protection measures for everyone working within the enclosure.

What’s caught the interest of the vertical farming industry as well as agriculture in general is plants’ reaction to UV-B light, which causes the plant to activate its defense mechanisms to protect it from these wavelengths. Studies show that some plants can produce 15 different defense proteins when exposed to UV-B. Some of these proteins affect a plant’s smell, color, taste, and resistance to disease that are not produced by other wavelengths.

A bright light on this controversial subject appeared when the UV-B-specific photoreceptor (UVR8) was discovered in the early 2000s and characterized in 2011. The mechanisms by which UVR8 regulates gene expression are not clearly understood, nor is how the UVR8 pathway functions and how it interacts with other pathways under the control of other photoreceptors.

Nevertheless, potential benefits from UV-B light have been noted in literature, ranging from reduced extension growth, increased leaf thickness and waxiness, greater leaf coloration in red leaf lettuce and some other plants, high resistance to pathogens and insects, doubling of shelf life, increased production of beneficial antioxidants and flavonoids, and improved nutritional value of fruits and vegetables.

A great deal of research remains to determine whether the blizzard of claimed benefits are real and whether the use of UV-B lighting for indoor growing is worth the considerable investment in time, equipment, and training to ensure safety is maintained. In the interim, UV LEDs targeted for other applications are available, such as the RayVio Model RVXR-280-SB-073105 UV LED starboard with a spectral wavelength of 280 nm.

Conclusion

The flexibility LEDs provide comes with challenges well beyond those of facilities using comparatively simple light sources like HPS. Nevertheless, the ability to grow more plants in less space with no need for chemicals and far less soil (or none), while increasing the nutritional value of vegetables and improving the flowering of plants is extremely attractive. As a result, the lighting and semiconductor components industry are simplifying the application of LED lighting with well supported solutions, while simultaneously improving the technology.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Barry Manz

Founder of Manz Communications, Barry Manz has been writing about electronics for more than 27 years. He provides articles and all other types of editorial to generate visibility for companies with a highly technical message to convey. Services include technical, product-related opinion and application-type articles, data sheets, brochures, and other collateral, as well as catalogs.

About this publisher

Digi-Key's North American Editors