Comparison of energy use efficiency of greenhouse and indoor plant factory system

Energy use efficiency in greenhouse and indoor plant factory system is greatly affected by outdoor climates, structure properties, operating conditions and product types. Energy use efficiency for two greenhouse cases and two indoor plant factory cases were compared in simulation with annual lettuce biomass yield and energy consumption including lighting, heating, and cooling for six geographic locations with different climates (Duluth, Minnesota; Seattle, Washington; Phoenix, Arizona; Miami, Florida; Abu Dhabi, UAE; and Riyadh, KSA). The energy balance of a greenhouse was simulated including Greenhouse 1 (GH_1) with a shading curtain (50% shade percentage) deployed during high radiation seasons and Greenhouse 2 (GH_2) with a constant daily light integral level (DLI) of 15 mol m^-2 d^-1 achieved by supplemental lights and variable shading controls. In indoor plant factory model, two cases Indoor Plant Factory 1 (IPF_1) and Indoor Plant Factory 2 (IPF_2) were simulated with EnergyPlus, a building simulation program, with the DLIs of 13 mol m-2 d^-1 and 15 mol m^-2 d^-1 respectively. Using LED lights with high efficacy (2.5 µmol J^-1), indoor plant factories were determined to be superior to greenhouses in cold climates with energy use efficiency as high as 0.13 kg kWh^-1 and 0.14 kg kWh^-1 respectively in Duluth and Seattle compared to greenhouse cases with 0.10 kg kWh^-1 and 0.11 kg kWh^-1 respectively. In hot climates greenhouses are significantly more efficient than indoor plant factories, with the highest energy use efficiency of 0.35 kg kWh^-1 in Miami. Parameters of indoor plant factory systems, including heating, ventilation, and air conditioning (HVAC) economizers, the number of tiers of production shelves, lettuce plant transpiration rate, and light efficacy, were evaluated for their impacts on energy usage and effectiveness for energy savings. Choosing LED lights with high efficacy was recommended to effectively reduce the electricity usage and to enhance energy use efficiency for indoor plant factories.