The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute published a new report that examined the energy usage and economic performance of LED horticultural luminaires. The study found that the economic benefits of using LED grow lights are not as clear as many in the industry believe. The LRC evaluated critical metrics for evaluating overall economic viability for using LED grow lights compared to high-pressure sodium (HPS) and metal halide lights. Photosynthetic photon flux density (PPFD) was the main criteria used to ensure a proper comparison.
They looked at power demand, overall life-cycle cost, luminaire intensity distribution, and luminaire shading. What made their study unique is that the LRC considered the effectiveness of the entire lighting system for particular controlled growing environments.
The LRC research group found that some of the LED grow lights that they tested saved energy compared to MH and HPS lights for growing in a particular area with a specific photosynthetic photon flux density (PPFD) requirement. However, surprisingly, energy savings was not always possible with the LED grow lights that they tested. They found considerable variation among products.
PPFD was Primary Criteria
The LRC looked at PPFD as the primary criteria for the evaluation. The researchers pointed out that for plants, the PPFD is analogous to photopic illuminance Flux per unit of area (lux) on a particular surface in an architectural application.
The researchers cautioned that looking at grow lighting applications is similar to looking at general lighting applications. A comparison is only legitimate (valid) if you examine the power densities of alternate lighting systems at equal illuminance levels on the specific work plane (lux for general lighting applications). Therefore, valid comparisons of LED grow lights require that they provide the same PPFD on the plant canopy. The LRC found that, on average, approximately three times as many LED horticultural luminaires would be needed to deliver the same PPFD as a typical 1000-watt HPS horticultural luminaire layout.
Actual PPFD Extremely Application Specific (Also Very Complex to Calculate)
While evaluating actual PPFD is hugely specific to each application, the LRC researchers devised a framework that ensures an “apples to apples” comparison. For this method of comparison, they looked at 11 luminaire-specific metrics and 5 application-specific metrics to give growers with the most accurate information for any given horticultural luminaire’s performance.
The LRC used this framework to assess 14 horticultural luminaires, including 10 LED products. In addition to photometric testing, the evaluation included application simulations and life-cycle cost analysis. Looking at luminaire efficiency alone can be misleading, according to the researchers. Instead, growers must look at the luminaire intensity distribution and layout of a growing area to reach a criterion PPFD necessary for a true life-cycle cost analysis. The LRC report provides a uniquely technology-neutral framework that stakeholders can use to evaluate lighting systems.
“Upon analyzing our data, we were intrigued by how intensity distribution and layout emerged as key factors in system performance,” said LRC Research Scientist Leora Radetsky, who authored the report.
LED Grow Lights Often Less Economical for Greenhouses Because of Shade
In greenhouses, sunlight exposure has to be considered as well. When selecting a grow lights for a greenhouse, growers should consider the size and number of luminaires necessary, because luminaires block the daylight from reaching the plants. The LRC shading analysis revealed an increase in shading from LED luminaires compared with HPS luminaires. This shading increase is due both to the luminaire sizes and the fact that more are needed to provide the same PPFD.
Importantly, the shading from LED luminaires reduced daylight in a greenhouse by up to 55% compared with just a 5% reduction in daylight from HPS luminaires. Therefore, in greenhouses especially, LED grow light systems often had to use more energy to get the same overall PPFD than the other grow lights in a setup with more daylight and less shading.
In a recent LRC survey, 75% of growers identified the cost of LED horticultural lighting to be a barrier to adoption. Therefore, it was essential to include an analysis of life-cycle cost in the report. The report card for the LED grow lights was not nearly as good as expected. The LRC found that just three of the tested LED-based horticultural lighting systems had lower life-cycle costs than either of the two tested 1000-watt HPS lighting systems. However, the remaining seven had higher life-cycle costs than either of the two tested 1000-watt HPS lighting systems.
“Energy use and life-cycle costs vary widely among LED and HPS lighting systems used in controlled environment horticulture,” said Radetsky. “It has been the standard approach for many years in the field of architectural lighting, and is becoming readily apparent in horticultural lighting, that we must conduct complete system energy and life-cycle cost analyses to generate an accurate picture of which technology would work best for each particular application.”
Lighting Energy Alliance and Natural Resources Canada funded the project.