Simulating the sun: Considerations for grow light design
Building a lighting system for indoor farming is far more complex than simply plugging in a grow light, and is especially true at the industrial scale, where even a slight bump up in LED efficiency or a small savings in energy consumption can result in a significant boost in profitability.
To gain insights into the design considerations and help engineers develop better horticultural lighting systems, we spoke with Arvidas Jarasius, business development manager for lighting at Avnet.
Q: When engineers are designing a horticulture lighting system, what should they consider when selecting the LEDs?
A: The species and variety of plants to be grown, the type of facility and its configuration will dictate the lighting selection.
Short plants that reach heights up to 15 inches tall such as lettuce and herbs are grown in greenhouses that are set up for vertical farming. Taller plants, such as cannabis or tomatoes are grown in greenhouses that feature what’s called top or high bay lighting. Top lighting is mounted up on the ceiling and needs to cover large areas.
In vertical farming, plants are grown on stacked shelves. Because each shelf requires lighting, high-power LEDs are not recommended because they concentrate the heat they generate. When space is tight, mid-power LEDs will do a better job to help spread the heat.
The next consideration is the tuning of the light spectrum to the type of plant grown, which requires an understanding of how much light is needed. A designer should also determine the level of efficiency that is acceptable for a particular installation
The overall goal is not to merely mimic the sun, but to outperform it. Through the use of LEDs, a designer can add color spectrums that enhance plant growth.
A large component of LED white light, 450nm blue light has the most overall influence on plant growth, while 660nm red light speeds seed germination and the flowering stage. By tuning the light spectrum just right, a grower can produce a crop of lettuce, for example, in three weeks, as compared to just one or two crops a year under normal sunlight.
Q: Do different plants species respond to different light spectrums?
A: Not only does each plant respond to light in a different way, but also so do the varieties within a species of plant. Different lettuce varieties, for example, require vastly different light spectrums. Through an understanding of what a particular variety of plant needs, a grower can be able to get into the range of 99% plant grow efficiency, which is measured in pounds of product per system cost. Typical grow lights are basically white light with a little red, which is adequate for home use.
If you want to start experimenting in order to enhance each growth phase, then it’s best to use separate blue, white, green, 660 nanometer red, and 730 nanometer red LEDs. This allows for mixing colors and more precisely matching the spectrum to the specific variety of plant. This approach is more expensive, but the upside is that it allows for more control over the process.
Take cannabis, for example. It’s been shown that by adding ultraviolet light for the final two weeks before harvest a 10-12% increase in THC can be achieved. The question is whether a 10% increase offsets the cost of adding a bunch of LEDs that will be used only for two weeks of a three to four month growth cycle. Well, that's the call.
Q: What spectrum suits the widest range of plant species?
A: The most universal spectrum would probably be the cool white, because it has the high 450nm blue content.
Some unique technology is currently under development. ams OSRAM, for example, has developed an LED they call horticulture white. It's a cool white, but engineers have shifted the spectrum to provide more blue and less red. This horticulture white would be used for blue and green and a 660m, hype-red LED for the red. Since white isn’t an efficient red source, the advantage is that you get the most cost-effective light by using white for the blue and red for the red.
Q: How can one calculate the number of LEDs required for a given area?
A: That's where optics could come in. Whether you're using high bay lighting or whether you're growing on shelves, the light should be focused on the plant, reducing the number of LEDs needed.
To grow, plants require a certain PFD (photon flux density), which is literally the number of mostly blue and red photons hitting the plant. You need to calculate PFD based on the distance from the plant to the LEDs and any focusing optics in the application, which translates into the amount of light you need. By factoring in the LED type and type of light, you can calculate the number of LEDs required.
Q: How does the power environment affect LED selection?
A: If the installation consists of a lot of standard LED lights and each one has a power supply, all of those devices are adding heat to the environment. You will need to account for that extra heat in when designing the HVAC system for the environment.
Another option that's becoming popular is to remote the power supplies by putting them outside the room. So now you've removed the heat, you may be saving on air conditioning costs especially in places like Arizona and Southern California.
Q: How do commercial and home systems differ?
A: Commercial and home systems differ in both size and power requirements. A large greenhouse for a commercial requires kilowatts to megawatts of power for a large number of lights. With a large commercial operation, you may also want to be flexible enough to change the lighting requirements season to season or as the crop needs change.
For a home grow, you can buy a 4 by 4-foot greenhouse with a single light inside. Or you can buy tabletop grow trays if you only have a small area, these are low-power users. Because you’re not looking for an ROI on your growth, you can use white lights, maybe a little extra red, and not worry about efficiency or power consumption. In commercial grow houses, every penny counts.
Q: What ingress protection (IP) levels are appropriate for these systems?
A: IP67 for a lighting fixture is usually adequate because you're not going to be directly spraying the lights with the water hose. These systems do need protection against humidity.
You will also need waterproof and dustproof connectors for the cabling, These will be for power and sensors that are in the dirt between the plants.
Q: What thermal management options are there?
A: The number one enemy of an LED is heat. Especially for vertical farming, it's critical because the light sits underneath a row of plants. You don't want that light to be too hot, because now you have to factor that heat into the plant’s environment, which will impact the heat sink design and HVAC costs.
If you're using high-power LEDs and driving the LEDs hard, then you may need to add active cooling. You might need a fan that blows the heat through the heat sink the entire length of the shelf.
While high bay lighting is up in the ceiling and there's significant air exposure, you’ll still need a very efficient heat sink to cool it. You're probably not going to need a fan because there's plenty of space for a good heat sink to dissipate the heat.
Q: What benefits do connected monitoring and control systems provide?
A: You won't really know what the proper lighting spectrum should be and the specific moisture and environmental conditions are until you've grown one crop, experimented with a second crop, and analyzed the differences. That's why when it comes to horticulture it's taking so long to develop this magic formula of lighting and environment.
When you add controls such as CO2 sensors, soil moisture sensors and PAR light sensors, you can develop an idea of what is the best environment for a particular variety of plant. That's what will get you to that last 5% efficiency advantage over other growers.
When you’re growing crops outside, you have no control over a cloudy day or a sunny day—you just get what you get. But inside you can make each day a perfect sunny day with the amount of sunlight that the plant needs.
Q: Is information on ideal environmental conditions freely available?
A: That's the secret sauce. There's been a lot of discussion on, “OK, how can I protect my secret recipe?”
Universities do analysis on various crops and they publish their findings. For example, research shows that UV-C will eliminate fungus and mold problems with plants when they are lit up at night for five or ten minutes.
But private growers? They don’t want to share their secret sauce as it’s their competitive advantage.
With connected systems and sensors, growers can do their own testing and develop their own recipe for growing crops.
Q: What kinds of sensors are available?
A: There is are a number of standard sensors, such as CO2 and soil moisture sensors. CO2 is critical for plant growth.
Light sensors are also important. PAR sensors detect the light spectrum that reaches the plant.
You also have humidity and temperature sensors to monitor conditions within the room.
With a hydroponic system, you might also utilize flow sensors and water pressure sensors.
Q: What are the advantages of a remotely monitored control system?
A: The advantage of a remote system is that you can monitor your greenhouse from anywhere you have an Internet connection. Plus, you can monitor several facilities from one central point.
If your grow system is sophisticated enough, you have control of all the equipment--fans, the water valves and the lighting. You can adjust remotely.
The other advantage of a remote system is that when you are operating multiple facilities, you can easily compare results among them, which is particularly valuable when you're growing the same plant in different environments.
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Cameron Coward is a senior technology writer at Avnet. Before transitioning to a writing career, he was a mechanical designer/drafter with experience in the biomedical, automotive and security industries. He is the author of "Idiot’s Guides: 3D Printing" and "A Beginner’s Guide to 3D Modeling: A Guide to Autodesk Fusion 360."