The greenhouse industry, including floriculture and greenhouse vegetables, is an important part of U.S. agriculture, with a 2012 farm gate value of ≈$6.5 billion (U.S. Department of Agriculture National Agricultural Statistics Service, 2014). Crop production in greenhouses has long been used to overcome natural barriers to crop growth and allow crops to be grown year round and in areas where they otherwise could not (Jensen and Malter, 1995; Woods and Warren, 1988). Providing crops with an optimal growing environment increases yields and allows inputs such as water, fertilizer, and energy to be used more efficiently.
In greenhouses, most environmental variables can be monitored and controlled to provide optimal conditions for plant growth (Bot, 2001; van Iersel et al., 2013; Vox et al., 2010). Light typically is less controlled than other environmental conditions, which can result in great spatial and temporal variability in light levels. The temporal variability occurs on short (within a day), intermediate (day to day, based on weather conditions), and long (seasonal) time scales. For example, the northern parts of the United States typically receive ≈3–5 times more sunlight in June than in January (Korczynski et al., 2002). Thus, for efficient year-round production in greenhouses, supplemental light is often needed from late fall through early spring (Clausen et al., 2015; Gómez et al., 2013). High-pressure sodium (HPS) lamps are still the most commonly used lamp for providing supplemental lighting. These lights are expensive to use. Combining the ballast and bulb, a single 400-W HPS light consumes ≈465 W of electrical energy (Nelson, 2003). To provide supplemental light at a PPF of ≈85 μmol·m−2·s−1 requires ≈1500 400-W lamps/ha, assuming the lamp provides 1.4 μmol·J−1. If these lamps are used for 16 h·d−1 and 180 d·yr−1, at an electrical cost of $0.12/kWh, the annual electricity cost is ≈$230,000/ha. That cost accounts for ≈30% of the average annual farm gate value of $700,000/ha for vegetable greenhouses (U.S. Department of Agriculture National Agricultural Statistics Service, 2014). Clearly, more efficient supplemental lighting will have a major impact on the sustainability and profitability of greenhouses.
There appears to be little past research on optimizing the economic return of supplemental lighting methods for greenhouse production. Heuvelink and Challa (1989) used a crop photosynthesis model to predict assimilate production, assimilate allocation toward salable products, the value of those products, and electricity prices to calculate when supplemental lighting was cost effective. Albright et al. (2000) showed that maintaining a consistent DLI can result in predictable, year-round lettuce production. They developed a set of rules that can be used to control shading and supplemental lighting, based on real-time electricity prices, weather conditions, and location, with the goal of achieving consistent DLIs throughout the year. More recently, Clausen et al. (2015) developed a dynamic control system for supplemental lighting that takes into account real-time electricity pricing, the weather forecast, and photosynthetic responses to light. Part of the rationale behind this dynamic control system is that, because of the nonlinear relationship between PPF and photosynthesis, supplemental light increases photosynthesis more when it is provided when ambient PPF levels are low. Using such a dynamic system to control HPS lights resulted in electricity savings of ≈25% with little effect on production (Clausen et al., 2015; Kjaer et al., 2011).
LEDs provide important advantages over HPS lights. For example, they can provide light with a spectral distribution that can be used efficiently by plants (Bourget, 2008; Goto, 2012; Liu, 2012; Morrow, 2008). It is also possible to manipulate plant morphology or secondary metabolites in crops by using specific spectra (Ouzounis et al., 2015; Stutte, 2015). Although the efficiency of LEDs, expressed in micromoles of PPF produced per Joule of electricity, has long been touted, both Nelson and Bugbee (2014) and Wallace and Both (2016) found that the most efficient HPS and LED lights had similar efficiency (≈1.6–1.7 μmol·J−1). However, recent improvements in LED technology have resulted in major increases in efficiency with the most efficient LED grow light now at ≈2.4 μmol·J−1 (A.J. Both, personal communication; PLOS ONE, 2016).
Although LEDs are becoming more efficient and less expensive, LED lighting for large-scale horticultural production is expensive (Bourget, 2008; Pimputkar et al., 2009). The high price of LED lights has slowed adoption of this technology and the cost effectiveness of LEDs for crop production is debated. Nelson and Bugbee (2014) reported that, because the capital costs for LEDs are 5 to 10× higher than for HPS, LED lights are more expensive per mole of PPF provided. In contrast, Ouzounis et al. (2015) argued that the payback time for LEDs is now realistic, especially if growers take advantage of the ability to control intensity and spectra.
Current LED grow lights do not take full advantage of the capabilities of LEDs. Specifically, the dimmability of LED grow lights has received little attention. The light output from LED lights can be controlled in two different ways. Limiting the current powering the LEDs essentially dims the lights and provides a steady light level, below the maximum light output. Pulse width modulation (PWM) turns LED lights on and off at high frequency (10,000 s of times per second). Within each on/off cycle, the fraction of time that the LEDs are on (duty cycle) can be precisely controlled. Reducing the duty cycle creates the perception of dimming, even though in reality, the light is on at full power for a shorter period, followed by a longer off period. Implementation of PWM control is cheap and easy and can be used to adjust the light output from LEDs based on real-time ambient light conditions. Tennessen et al. (1995) showed that leaf photosynthesis of tomato (Solanum lycopersicum) responds to the PPF averaged over the on/off cycle, unless the on/off cycle is excessively long (>20 ms). Weaver and van Iersel (unpublished data) confirmed that PWM control at high frequency does not affect the light use efficiency or leaf photosynthesis and that leaves respond to the average PPF. Tennessen et al. (1995) concluded that photosystems I and II can store a certain amount of excitation energy during the on cycle, which can be used during the subsequent off cycle of the LED light.
Our goal was to develop an automated, adaptive control system for LED grow lights that can prevent the PPF at the canopy level from dropping below a user-defined threshold. We used two different approaches to do so: 1) turn on the LED grow light at full power when the PPF from sunlight drops below a certain threshold PPF and off again when sunlight PPF exceeds the threshold or 2) use a PWM controller to only provide enough supplemental light from LEDs to reach the threshold PPF. This adaptive control approach to lighting control should reduce energy use, by providing supplemental light only when needed and, in the case of PWM control, in the amount needed. To quantify reductions in energy use, the two threshold-based lighting control methods were compared with 14 h·d−1 of continuous, full power supplemental light. Although only one type of LED grow light was tested in this study, the principles can be applied to any LED grow light and control can regardless of whether current or PWM control is used to adjust the light output from the LED grow light.
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