Indoor horticulture represents only a tiny fraction of U.S. farmland but accounts for a disproportionately significant portion of farmland revenues. In 2017, more than 40,000 growers operated in more than 1 billion square feet of growing area.
Green plants need light for photosynthesis. Supplemental lighting is often used to grow plants all year because light exposure in a greenhouse can be lower than outdoors. As with other types of lighting, the LED source has become a viable competitor in this market. LEDs promise the usual energy savings and longevity benefits as well as the ability to produce spectral light recipes ideal for healthy plant production.
In December 2017, the U.S. Department of Energy (DOE) issued a report evaluating the energy-savings potential of LED lighting in horticultural applications. These include supplemented greenhouses, which use electric lighting to extend daylight hours, supplement sunlight on cloudy days, or interrupt dark periods to influence plant growth; nonstacked indoor farms, where plants are grown on a horizontal plane under ceiling lighting; and vertical farms, where plants are stacked in shelving. Lighting typically accounts for the highest portion of the electrical load.
Of these applications, LED lighting currently has low penetration in supplemented greenhouses (2 percent of installed base) and nonstacked indoor farms (4 percent). In vertical farms, LEDs now constitute an estimated 66 percent of all horticultural lighting, which is not surprising because the light sources are installed in shelving units close to the plants.
The DOE concluded that LED lighting offers 24–30 percent energy savings per square foot of grow area. If all horticultural lighting switched to LEDs, total market energy savings would reach 40 percent. To download the DOE report, click here.
This energy-savings potential has caught the attention of utilities, for whom horticultural facilities have become a fast-growing application. The DesignLights Consortium, which produces the Qualified Products List used by many utilities to qualify products for rebate programs, has released a new specification for LED horticultural lighting (SSL V4.4). This will qualify it for many rebate programs. Check it out here.
Another research report published by the Lighting Research Center (LRC) in May 2018, however, went further than the DOE report in its analysis and, as a result, produced different conclusions. The LRC based their methodology on application rather than luminaire efficiency. Luminaire efficiency compares wattage required to produce a given output. Application efficiency compares wattage to produce a lighting effect required by the application.
The main criterion used was photosynthetic photon flux density (PPFD), the greenhouse equivalent of foot-candles on a workplane in a commercial building. The LRC claimed it’s only valid to compare power densities of lighting systems when they provide the same PPFD on the plant canopy, which involves looking at luminaire size, light distribution (where the luminaire places the light and at what intensity), and layout (number of luminaires required). Stakeholders such as contractors, specifiers and growers themselves, the LRC argued, can be misled if they only look at luminaire efficacy.
Overall, the LRC studied 14 horticultural luminaires, including 10 LED products, using 11 luminaire-specific metrics and five application-specific metrics to conduct their analysis. The evaluation included photometric testing, application simulations and life-cycle cost analysis.
The LRC found that for a given growing area, energy savings could be achieved with some of the tested LED horticultural luminaires compared to the tested high-pressure sodium (HPS) and metal halide luminaires while satisfying a common PPFD. However, there was a significantly wide variation among products.
What’s more, the LRC found about three times more LED luminaires, on average, were needed to produce the same PPFD as a typical 1,000-watt HPS luminaire layout. This contradicted the DOE report’s assumption that LED luminaires can replace traditional luminaires on a 1:1 basis. The size and number of luminaires block sunlight from reaching the plants. The LRC determined the tested LED luminaires reduced daylight by 13–55 percent compared with a 5 percent reduction from HPS, which in turn could require more electric lighting to make up the difference.
Because of these factors, the LRC determined three of the tested LED luminaires had lower life-cycle costs while the remaining had higher life-cycle costs than either of the two 1,000-watt HPS systems being tested.
The lesson here is that, when looking at the energy-savings benefit of LED horticultural lighting alone, LEDs can save energy. However, one should look at more than just luminaire efficacy to confirm that and determine how much energy is actually being saved. It can be beneficial to consider the luminaires in the intended application, taking into account factors such as light distribution and intensity as well as number of required luminaires needed to produce the desired light exposure for the plants. This may provide a more accurate picture of energy performance. Download the LRC report here.