Optimizing Carbon Dioxide Concentration and Daily Light Integral Combination in a Multi-Level Electrically Lighted Lettuce Production System

Abstract

There are many issues that will make producing sufficient food for the growing global population a difficult task. Controlled Environment Agriculture (CEA), integrating environmental control and hydroponic technology, can efficiently produce more food with less inputs. The new production practices of vertical farms have precision environmental control and subsequently more consistent and higher productivity with the advantage of being located almost anywhere, especially closer to population centers. A vertical Farm can be described as a fully indoor production system that uses electrical lamps for photosynthetic lighting and high density crops grown in multiple layers. CEA technology of supplementing atmospheric carbon dioxide (CO2) in greenhouse applications to compensate for low light levels, maintain plant photosynthesis, and enhance profits is practiced. However, due to the amount of ventilation generally required in greenhouse environments, maintaining CO2 concentrations can be expensive and impractical. The closed configuration of vertical farms can enhance CO2 use efficiency, however, the use of electrical lighting results in a large electrical power requirement. The goal of this study was to evaluate the level of daily light integrals (DLI) and atmospheric CO2 concentrations that would provide savings of electrical power usage and CO2 supplementation while producing a marketable head lettuce (Butterhead, cv. Fairly) product. Experiments were conducted in a 45 m2 environmentally controlled (air temperature, PPF, DLI, CO2, DO, EC and pH) vertical farm research facility with six values of DLI (9, 11, 13, 15, 17, 19 mol m-2 d-1) and six CO2 concentrations (400, 550, 700, 850, 1000, 1300 ppm), which were maintained constant from transplant through harvest. Plant shoot fresh and dry weights were measured at harvest and compared with resource use accounting of electrical energy for LED lighting, heat pumps for air conditioning, water pumps for nutrient solution circulation, and air pump to maintain dissolved oxygen in the nutrient solution for the roots. It was demonstrated that 1) a linear relationship of increase biomass to increase of DLI existed for all treatments; 2) plants within the 850 ppm CO2 concentration yielded the largest average fresh and dry shoot weights and yields decreased as CO2 was further elevated; 3) the physiological disorder tip burn was more pervasive and appeared sooner for either larger CO2 concentrations and larger DLIs. No tip burn was observed at 400 and 550 ppm CO2 concentrations within any DLI; 4) lettuce grown in lower light intensities had larger physical size dimensions, but were less dense and had less biomass, compared to lettuce grown in higher light intensities which had a smaller physical dimension, but were more dense and thus greater biomass; 5) the metrics for the average overall resource use efficiencies of plant production for fresh weight edible biomass were 69 gfresh kWh-1, 147 gfresh LCO2-1, 20.7 LH2O kgfresh-1 y-1 , and 86.0 kgfresh m-2 y-1; 6) the potential electrical savings from changing the DLI (mol m-2 d-1)/CO2 (ppm) combination from 17/400 to 13/850 in the small scale research facility, to which this study was conducted, is $59 per harvest and $762 for the year (14.4% savings). Larger commercial vertical farm operations lowering the DLI and increasing CO2 concentrations could have a much greater electrical savings potential.