How can controlled-environment growers improve energy efficiency and grow light optimization?

Are you using or thinking about using grow lights to produce your controlled environment crops? If so, would you be interested in how to reduce the amount of electricity needed to operate those lights?

Researchers with the Greenhouse Lighting and Systems Engineering (GLASE) consortium at Cornell University in Ithaca, N.Y., are studying the use of control algorithms to optimize the light used on controlled-environment crops while reducing the amount of electricity used to operate grow lights.

“These control algorithms that deal with lighting were developed over 20 years ago,” said research associate Dr. Kale Harbick. “Unfortunately, this technology, which was under patent until about four years ago, never achieved large scale commercial use.

“The algorithms are related to delivering a constant amount of light to the plants every day, which is called daily light integral (DLI). Controlled-environment crops like lettuce prefer to receive a constant amount of light every day. This enables the plants to maximize their growth and helps to avoid problems with tipburn, which can make the crops unsalable.”

Even though this technology is off patent, Harbick said some growers are still hesitant to incorporate it into their environmental control systems.

“As part of the GLASE research program we are putting this control technology into two commercial pilot facilities in New York,” he said. “We are going to run multi-year experiments so that the growers in those facilities can compare the performance under our control system with what they are currently using. Incorporating this technology, we are looking to demonstrate a large amount of energy savings for the crop yields produced.

“We’re specifically focused on lettuce, tomato and strawberry. The first two pilot facilities are growing leafy greens, primarily lettuce. Lettuce is the crop the Cornell controlled-environment agriculture group has studied the longest and we understand the best. Tomato and strawberry are newer crops to us so we are doing a lot of greenhouse experiments right now with these plants to analyze the relationship between light, carbon dioxide and growth. We have the information well established for lettuce, but it’s not as well established for fruiting crops. We would eventually like to roll out pilot programs for tomatoes as well. We have some tomato growers in the state who have expressed interest in using the technology.”

GLASE researcher Dr. Kale Harbick is studying the use of control algorithms to optimize light while reducing the amount of electricity used to operate grow lights.
Photos courtesy of Kale Harbick, Cornell Univ.
Harbick said New York State Energy Research and Development Authority (NYSERDA), which is financially supporting GLASE, is interested in trying to meet greenhouse gas emission production targets. CEA has the potential to use a lot of energy. Any energy savings that the GLASE research can realize has a corresponding large reduction in greenhouse gas emissions.

Lack of light uniformity

One of the problems that Harbick often sees with grow lights is growers don’t install enough fixtures.

“The most basic problem is light intensity,” he said. “We often see greenhouses where growers are trying to produce lettuce with grow lights, but they only have installed half the number of fixtures they should have in order to grow lettuce optimally. What that means is even if they ran those lights 24/7 they wouldn’t be able to achieve the DLI needed to reach the target amount.”

Harbick said the issue of not installing enough lights is related to measuring light.

“Trying to measure the light is not a trivial thing to assess,” he said. “If there is an array of lights that are regularly spaced in the same plane, the problem is the light is not uniform on the crop. It kind of has a bullseye effect where there is a lot of light in the center and not very much on the edges. Lighting manufacturers and designers often provide growers with designs that show the light intensity at the center of the space, which is not representative of the light that is received in the rest of the space. The lights might be sized according to that center spot which is just fine, but everything else is undersized. We hear regularly from growers who’ve spent a lot of money on a lighting system and then realize later that it was undersized.”

Harbick has done a lot of work on lighting uniformity to try to address this issue of undersizing.

“We’ve looked at techniques to change the positon of the lights to make it more uniform,” he said. “We’ve looked at changing the brightness of the lights depending on where they are in the space to make the light more uniform. We have a couple of greenhouse spaces at Cornell that have this uniformity optimization. These are the only greenhouses on campus to have uniform lighting.”

One of the common issues encountered by growers who have installed grow lights is the lack of light uniformity which can be caused by undersizing the number of fixtures.

Harbick said this lack of light uniformity is not often noticed because of the way humans see light distribution.

“We could look out at a crop and the lighting looks uniform to our eyes because our eyes are so good at attenuating brightness levels,” he said. We don’t notice these differences very easily. The brightness level differences have to be measured.”

Although the lack of light uniformity can be related to the number of light fixtures installed, Harbick said there could be other factors involved.

“In the research greenhouse that we have optimized light uniformity, initially we received a design from the manufacturer that called for 20 LED fixtures,” he said. “I modified the design using a computer program that I wrote that was able to reduce the number of fixtures to 16, but improved the light uniformity. The light is much more uniform and we were able to do it with fewer fixtures.

“It’s not always a case of installing more fixtures to increase light uniformity. It can also be things like changing the brightness of the fixtures or changing the position of the fixtures. There are other possibilities to improve uniformity besides the number of lights.”

Loss of crop yields

One of the reasons that there continues to be light uniformity issues is how the fixtures are spaced in greenhouses and indoor farms.

“It’s a status quo thing,” Harbick said. “Lights have always been placed in a plane and regularly spaced out. This is how it has always been done. Until recently most grow lights have had a fixed intensity. Under those conditions it is difficult to overcome this light uniformity problem.

“Lack of light uniformity is less of problem in large greenhouses because there is a lot of interior space and not much edge space. However, this is a major problem in indoor farms where there are long rows or shelves. These indoor farms don’t have the background natural light like greenhouses to mitigate some of these effects. I have seen warehouses where they were growing a high DLI crop and the plants in the center of the room were a foot taller than the plants on the edge. It was simply because the plants in the center were receiving more light. In indoor farms, whether it’s lettuce, tomatoes or other high DLI crops, that is where the payoff really comes for optimizing light uniformity. For indoor farms light uniformity is something that people aren’t looking at yet. They just kind of live with it. They just move plants around to try and even out the growth. But that creates a lot of extra labor and logistic issues. They’re not optimizing the total yield of the crops. Every plant on the edges that is shorter than the ones in the center of the room is lost sales.”

GLASE researchers are conducting greenhouse experiments on tomato and strawberry to analyze the relationship between light, carbon dioxide and plant growth.

Harbick said that the issues with light uniformity may improve as changes are made to lighting fixtures.

“As the lights become more capable in terms of adjusting brightness and spectrum, there are opportunities for GLASE researchers to share some of that optimization technology with lighting manufacturers to try to get these improvements out to the industry. But it’s going to take some time. We’re still doing a lot of the research ourselves. We don’t have an active grant in this area right now so this is something that I have been doing on my own. It’s definitely an opportunity for future study.”

For more: Kale Harbick, Cornell University, School of lntegrative Plant Science, Horticulture Section, Ithaca, NY 14853; kh526@cornell.edu.

David Kuack is a freelance technical writer in Fort Worth, Texas; dkuack@gmail.com.