*2.4. Power Consumption and Biomass Efficiacy*

The LEDs consumed the least electricity with 257.7 W m−<sup>2</sup> , followed by the FLs with the use of 299.4 W m−<sup>2</sup> , whereas the HPS lamp consumed the highest amount of electricity with 374.9 W m−<sup>2</sup> . At the end of the cultivation period, the power consumptions per m<sup>2</sup> of LED and FL lighting system resulted in high energy savings of 31.3% and 20.1%, respectively, when compared to the consumption of the HPS system. While each LED system enabled ± 1.92 g of fresh thyme per kWh and square meter, the HPS and FL enabled only ± 40% and ± 16% of these yields per kWh and square meter, respectively. Accordingly, the dry thyme production per kWh and square meter under LED (±396.3 mg) was significantly higher than the dry thyme production under HPS (±155.2 mg) and FL (±39.1 mg). Further, the production of VOCs per kWh and square meter was significantly elevated underneath the LEDs (±5.4 mg) as compared to HPS (±1.9 mg) and FL (±0.4 mg). Results and calculations are combined in Table 4.



\* LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light, *na* = not applicable. \*\* Presented are calculated average fresh and dry thyme productions per power consumption of each light fixture type within a square meter during the cultivation period (g or mg per kWh and m<sup>2</sup> ) of four independent spatial replications per light treatment (*n* = 4) ± *SD* of 32 harvested plants per spatial replication and light treatment (*N* = 384, *n* = 128 plants per supplemental light treatment, *n* = 32 plants per spatial replication). Significant differences (*p* ≤ 0.01) were determined according to Dunnett's T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (*p* ≤ 0.001). Different letters indicate significant differences. \*\*\* Presented are calculated average productions of volatile organic compounds per power consumption of each light fixture type within a square meter during the cultivation period (mg per kWh and m<sup>2</sup> ) of four independent spatial replications per light treatment (*n* = 4) ± *SD* of 16 harvested plants per spatial replication and light treatment (*N* = 192, *n* = 64 plants per supplemental light treatment, *n* = 16 plants per spatial replication). Significant differences (*p* ≤ 0.05) were determined according to Dunnett's T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (*p* ≤ 0.004). Different letters indicate significant differences.

Our power consumption results and thus the potential of LEDs for reducing energy costs coincide with numerous studies and reviews [6,9,59], stating energy reductions up to 70% compared to traditional light sources while producing similar crop yields at equal light intensities, and confirm the current trend of LEDs' increasing photon efficiencies: While HPS and LED fixtures had nearly identical photon efficiencies until ~2015 [6,35], the best evaluated LED fixture was 40% more photon-efficient than HPS due to technological improvements of LEDs within the PAR region soon after [35,59]. A current study by Hernandez et al. (2020) confirms the corresponding increase in biomass efficacy of LEDs, as their LED treatment led to a 2.4 to 3.1 times greater biomass efficacy than HPS, which matches our findings [60]. Another study in which LED and FL treatments were compared, reported a biomass efficacy three to five times higher under LED than under FL lighting [61]. In contrast, the LED system used in this current study greatly exceeds their findings, as the LED enabled a biomass efficacy 6 to 10 times higher than the FL system (Table 4) under our experimental conditions.

Further, in our study, plant growth may have been limited by nutrient availability, and an adjustment of fertilization based on the differing thyme growth rates may further increase biomass efficacies under HPS and especially under the broad-spectrum LED system. Nevertheless, when using the broad-spectrum LED lighting system, the significantly more inhomogeneous light intensity distribution compared to HPS and FL lamps (Figure 3) must be taken into account when light uniformity is necessary for the greenhouse application as it demands more LED light fixtures per area. *Plants* **2021**, *10*, x FOR PEER REVIEW 10 of 17


**Figure 3.** Irradiance profiles (W m−2 nm−1) of the experimental plots (1 m<sup>2</sup> ) underneath each supplemental lighting system. **Figure 3.** Irradiance profiles (W m−<sup>2</sup> nm−<sup>1</sup> ) of the experimental plots (1 m<sup>2</sup> ) underneath each supplemental lighting system. (LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light).

Nevertheless, at their edges, where the lowest light intensities occur, the LEDs achieve values of 16 W m−2 nm−1, which are sufficient for high-quality thyme production. Therefore, if homogeneous plant development is not necessarily required, plants of mar-Nevertheless, at their edges, where the lowest light intensities occur, the LEDs achieve values of 16 W m−<sup>2</sup> nm−<sup>1</sup> , which are sufficient for high-quality thyme production. Therefore, if homogeneous plant development is not necessarily required, plants of marketable quality are also available with the LED setup used here without additional lamps.

ketable quality are also available with the LED setup used here without additional lamps.
