Modeling resource consumption and carbon emissions associated with lettuce production in plant factories

Abstract

Plant factories (PFs), multi-layer production in opaque warehouses, are novel production systems with many potential benefits and some key impediments, notably, energy consumption. The economic viability and environmental footprint of PF facilities depend on energy consumption associated with lighting, heating, ventilation, and cooling to the production space. Light and energy-system modeling can inform designers and investors about the energy requirements and economics related to a proposed or existing facility. Previous models have focused on comparing energy consumption between PFs and greenhouses (GH) in a limited number of production scenarios. There is little information regarding water consumption and carbon emissions at PF facilities and how energy conservation measures (ECMs) may impact water use efficiency, upstream emissions and purchased carbon-dioxide. The objective of this study is to model energy and water use in PFs growing lettuce in five U.S. locations (New York, NY, Los Angeles, CA, Seattle, WA, Chicago, IL, and Atlanta, GA) with three types of ECMs: installing lighting fixtures with high photon efficacy, introducing outdoor air for cooling through use of an air-side economizer, and maintaining elevated levels of carbon dioxide in the crop production space to enhance photosynthesis. Building energy modeling (BEM) software EnergyPlus was employed to simulate environmental conditions and associated energy consumption. Energy consumption ranges from 6.2 to 12.0 kWh kg−1 fresh weight of lettuce produced depending on design and operational choices. Water consumption for irrigation and production operations ranges between 2.0- and 9.8-L of water per kg (FW) lettuce produced. Beyond operating parameters, the primary emissions associated with energy consumption at the facility varies widely due to the energy mix of each location's grid, ultimately ranging between 0.6 and 3.9 kg carbon dioxide equivalent emissions per kg (FW) lettuce produced. We conclude that locating PFs in areas with a cleaner grid and employing these ECMs can reduce the energy consumption associated with growing lettuce indoors, though there are important tradeoffs to consider between water, energy, and supplemental carbon-dioxide in design.