Indoor farming (including vertical and horizontal methods) involves growing food crops under sole-source electric lighting inside buildings, including warehouses. This system is gaining importance globally, especially near urban areas (Rabara et al., 2017). Leafy greens, such as lettuce, are among the group of popular crops for indoor farming (Kozai, 2013). Crops are produced using electric lighting for an average of 16 h·d−1 (Agrilyst, 2017) in indoor farms. Thus, electrical energy-use costs can be quite high in indoor farming (Barbosa et al., 2015; Shimizu et al., 2011). Due to their relatively low thermal emissions (Nelson and Bugbee, 2014; Poulet et al., 2014), LEDs can be placed close to a canopy to reduce photon losses (Bourget, 2008; Poulet et al., 2014), and subsequently are desirable for indoor farming. Despite using LEDs, estimates indicate that energy costs for electric lighting can be as high as 25% to 30% of operational costs in indoor farming (Agrilyst, 2017; Voss, 2011), which significantly reduces profits. At present, this is one of the major issues challenging profitability and sustainability of the indoor farming industry.
Broad-spectrum LED lights generally are preferred in horticulture production as they enable better visual assessments of plants, and crop growth under these lights is comparable or better than that under monochromatic LED lights (Mickens et al., 2018; Park and Runkle, 2018). Broad-spectrum light usually is produced by a process called “phosphor conversion” (pc) in newer commercial LED fixtures. This involves coating blue LEDs with “phosphor” (a synthetic material with luminescence property), which absorbs blue light and re-emits light of lower energy (phenomenon known as “down conversion”) wavebands (Huang and Yang, 2013; Pattison et al., 2018; Ryu and Ryu, 2015; Yam and Hassan, 2005). This method is more economical as opposed to combining red, green, and blue monochromatic LEDs to produce broad-spectrum light, mainly due to the low quantum efficiency of monochromatic green LEDs (Pust et al., 2015). By fine-tuning the phosphor chemical composition and concentration, different percentages of green and red light can be produced from blue light.
When generating lower energy light from greater energy blue photons in pc-LEDs, some electrical energy is lost as thermal dissipation inside the phosphor (Huang and Yang, 2013), which increases phosphor temperature. Greater phosphor temperature lowers phosphor efficiency (Ryu and Ryu, 2015). This suggests that EUE can be lower when a greater proportion of red light is emitted by phosphor due to a larger “down conversion” involved (blue vs. red) and increased wastage of electrical energy in the form of heat as a result. In contrast, a greater proportion of red light in the incident light may be preferred for lettuce production as it increases leaf expansion and shoot growth (Kang et al., 2016; Lee and Kim, 2013; Wang et al., 2016; Zhang X. et al., 2018). Thus, the spectral quality of LEDs can potentially influence EUE by affecting both SDW and EEC in opposite ways. However, studies simultaneously examining the effects of the spectral quality of LEDs on lettuce SDW, EEC, and EUE are limited in terms of the net effects on productivity.
Manufacturers of LED fixtures commonly use PE to characterize light fixtures (Both et al., 2017; Nelson and Bugbee, 2014; Park and Runkle, 2018). Greater PE values are associated with lower energy costs of producing artificial light (Kubota et al., 2016) and can thereby increase EUE (Park and Runkle, 2018). This measurement is specific to a given fixture, and as such it is broadly useful for light-fixture selection for different species and purposes. An issue with using PE for maximizing EUE is that it is solely based on light intensity, or the number of photons generated by the fixture for a given input of electrical energy, but does not indicate spectral composition of light. Thus, PE does not account for plant response to differences in spectral quality of light emitted by a fixture. Several reports indicated a reduction in lettuce growth when spectral quality of supplied light is not optimized for a given light intensity (Johkan et al., 2010; Tosti et al., 2018; Zhang T. et al., 2018; Zhen and van Iersel, 2016). Moreover, effects of light spectrum on plant dry-matter production can be pronounced under low light intensities (Nelson and Bugbee, 2014), suggesting that spectrum of electric lighting can be an important determinant of EUE for indoor production. The disconnect between PE and SDW can be seen in recent reports on cucurbits (Hernandez and Kubota, 2014) and bedding plants (Park and Runkle, 2018). Thus, greater PE may lower EEC, but it is unclear whether it translates to greater EUE in indoor lettuce production when spectral quality of light is not optimized.
To better understand the effect of spectral quality of light on EUE and bridge the disconnect between PE and EUE in indoor lettuce farming, we conducted two experiments in which our objectives were to 1) study the effects of spectral quality of LEDs on lettuce SDW, EEC, and EUE; 2) verify whether greater PE can increase EUE for indoor lettuce production; and 3) assess other efficiency metrics associated with EUE for indoor lettuce production. The purpose of Expt. 1 was to study the aforementioned objectives using select lettuce varieties. The findings from Expt. 1 were then validated in Expt. 2 using a vertical hydroponic farm and three different lettuce varieties.
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