Energy optimisation of plant factories and greenhouses for different climatic conditions

The trend to localise food production promises reduced reliance on increasingly uncertain global supply chains. Controlled-environment agriculture, in particular indoor vertical farming, is developing as part of this trend, to ensure a year-round supply of healthy food and protection from extreme weather events. However, high energy consumption is a major concern that could greatly impact the environmental sustainability of high-tech farms. Addressing the lack of comprehensive comparisons between different controlled-environment agriculture systems on a consistent basis, this work investigates the favourability of indoor vertical farms (i.e. plant factories) over modern ventilated open and closed greenhouses from an energy intensity perspective. This was based on a flexible yield-energy model incorporating detailed air conditioning system dynamics, which was developed to evaluate the influence of outside climate conditions on energy consumption and vegetable yield. The model was used to optimise the climate control strategy and to minimise hourly specific energy consumption for multiple systems, parameter settings, and locations. The hourly model performance is demonstrated for Stockholm, which indicates that advanced climate control allows for very low-energy operations in summer compared to winter. The results show a strong parametric sensitivity for the thermal transmittance of the cover, the target light intensity and the crop climate preference in all three systems, as well as the efficiency of lighting and water cooling for plant factories. Considering the yearly average for multiple locations, open greenhouses were substantially more energy-efficient than plant factories in all ten locations (from −45% in Reykjavík to −94% in Gauteng). The option to ventilate a greenhouse (open vs closed) had the greatest positive effect on specific energy consumption in less extreme climates (from −36% in Massachusetts to −83% in Gauteng) but increased water consumption considerably (from an average of ~2 l/kg to 26 l/kg). Although local availability of land and water plays a significant role in the choice between growing systems, the results imply that high-tech ventilated greenhouses perform significantly better than vertical farms from an energy perspective in most inhabited regions of the planet.