Proponents of LED horticultural luminaires have claimed significant energy savings from their use. The LRCâs study shows that the method of calculating energy savings is very important to the outcome.First, the luminaires need to be compared on a consistent and meaningful basis. This study compared HID and LED lighting systems for a constant PPFD on the plant canopy. PPFD for plants is analogous to photopic illuminance on a work surface in an architectural application. Just as it is only valid to compare the power densities of alternate lighting systems at equal illuminance levels on the work plane, the power densities of alternate horticultural luminaires should only be compared when they provide the same PPFD on the plant canopy. The LRC found that, on average, approximately three times as manyLED horticultural luminaires would be needed to provide the same PPFD as a typical 1000-watt HPS horticultural luminaire layout.The results show that intensity distribution plays an important role, illustrated by the fact that two of the tested LED luminaires had higher luminaire efficacy than the HPS luminaires but still had a higher total power demand in the greenhouse application. Among the LED luminaires that could provide the target PPFD, the median LPD was the same as that of the 1000 W HPS base case
When choosing a lighting system for a greenhouse, growers should consider the size and number of luminaires needed, because luminaires block daylight from reaching the plants. The LRC shading analysis found an increase in shading from LED luminaires compared with HPS luminaires due to the size of the luminaires and the fact that more are needed to provide the same PPFD. The shading from LED luminaires reduces daylight by 13â55% compared with a 5% reduction in daylight from HPS luminaires, thus more electric energy could be needed for lighting with the LED systems, depending upon the available daylight.The greater number of LED luminaires and their equivalency, on average, in application power demand impacted their life-cycle costs. The LRC found that three of the tested LED horticultural luminaire lighting systems had lower life-cycle costs and the remaining seven had higher life-cycle costs than either of the two 1000-watt HPS lighting systems that were tested. The results show that specifiers should not make overly simplistic generalizations about the energy usages and life-cycle costs of LED and HPS lighting systems used in controlled-environment horticulture, but they also show that energy use and life-cycle costs can be lower for some LED lighting systems relative to some HPS lighting systems.
Importantly, as has been known for many years in architectural lighting, growers must have accurate and complete system energy and cost analyses to make meaningful comparisons of lighting systems that might be used in controlled environment horticulture.In fact the results suggest that with some design modifications, LED horticultural luminaires would have an energy and economic advantage over their HPS counterparts. The results suggest that LED horticultural luminaires should have increased PPF (around 1500 Îźmol s-1) and have a luminaire efficacy of at least 2 Îźmol J-1 to compete on a one-for-one basis with 1000 W HPS luminaires. These luminaires could even cost more than they do presently, if fewer are needed, and still achieve lower 20-year life-cycle costs than the tested 1000 W HPS luminaires. A hypothetical LED luminaire with a luminaire efficacy of 2.5 Îźmol J-1, a PPF of 1500 Îźmol s-1, and requiring 24 luminaires to meet a target PPFD of 300 Îźmol m-2 s-1, could cost as much as $3300, and still have a lower 20-year life-cycle cost than the tested 1000 W HPS luminaire with the lowest 20-year life-cycle cost.