2019 ASHS Annual Conference

The bulk of energy costs during winter hydroponic production are associated with supplemental lighting and heating. Our previous research indicated that hydroponic lettuce grown using heated nutrient solution (21 °C) and cooler greenhouse air temperature (16 °C) resulted in good growth and reduced energy costs of heating. Light emitting diodes (LEDs) are increasingly becoming popular in greenhouses due to their increased energy efficiency. Thus, a combination of energy efficient LED lighting and heated nutrient solution can further maximizing electrical energy use efficiency (EUE), defined as the ratio of biomass produced to energy consumed (g·KWh^-1), in lettuce production. However, differences in spectral composition of LEDs can potentially affect crop growth and energy use, thereby affecting EUE. In addition, electrical energy costs of heating may vary between nutrient film technique (NFT) and deep flow technique (DFT) of growing lettuce due to differences in volume of nutrient solution used between two production systems. Our objective was to compare EUE differences between NFT and DFT systems under LED lighting treatments with different spectral composition and using heated nutrient solution. An experiment was conducted for four weeks during winter using eight different varieties of lettuce belonging to four major groups. Plants were grown in two production systems (NFT and DFT; split levels) and under two supplemental lighting treatments (broadband or white and narrow band or red/blue; main levels) provided at nighttime. Heated nutrient solution (21 °C) was used to grow plants while the greenhouse air was maintained at 16/10 °C (day/night). Both LED fixtures consumed similar energy (1.15 KWh·d^-1) and produced similar light intensity (12.5 mol·m^-2·d^-1). Results indicated that the interaction between light and production system was significant for EUE. Regardless of variety, EUE was higher in DFT compared to NFT and difference between DWT and NFT was higher under red/blue than white LED fixture. In spite of higher electrical energy consumption for heating under DFT than NFT (approx. 15% higher), EUE was higher under DFT. This may be likely due to better nutrient uptake in DFT than NFT due to increased root exposure to nutrient solution and increased crop growth. Red/blue LED fixture likely optimized light for photosynthesis, which may have further increased crop growth, thereby increasing EUE. In conclusion, our research indicates that maximum EUE in lettuce production could be achieved in a DFT system using heated nutrient solution and narrow band (red/blue) nighttime supplemental lighting.