Improving Climate Uniformity and Energy Use Efficiency in Controlled Environment Agriculture

Abstract

Over the past few decades, a new form of controlled environment agriculture known as indoor plant factory system has been developed. In an indoor plant factory (IPF), a fully controlled environment with a sole source lighting system provides the possibility to grow and harvest crops at multi-tier vertical layers inside buildings. It has become attractive for providing locally grown fresh vegetables in urban areas. There are many benefits to indoor farming including accelerating growth rate of the crop, high space, water and nutrient use efficiency, and providing fresh locally grown produce. However, there are still many challenges, such as high start-up capital costs, high operating costs, limited types of crops grown, and difficulties in microclimate management. Microclimate uniformity and energy consumption in indoor plant factories were studied for enhancing energy use efficiency in indoor plant factories. Energy use efficiency can be defined as fresh weight grams of crop produced per kilowatt-hour of energy. In additional to the energy consumption of indoor plant factories and greenhouse crop production for different outdoor climates were compared to provide insights for effects of climatic locations, IPF operating conditions, and IPF control strategies determine energy consumption and energy use efficiencies. Computational fluid dynamics (CFD) simulations were used to simulate and study transport phenomena of flow fields within plant growth rooms inside a commercial, large-scale indoor plant factory, having the goal to improve climate uniformity and subsequently crop uniformity. Air ventilation systems of various designs were compared for different combinations of air distribution supply vents openings or air distribution perforated air tubes and return air outlets within the plant growth room with steady-state simulation. The climate uniformity was compared for five cases for air current speed (m s-1), air temperature (°C), and vapor pressure deficit (Pa) by evaluating relative standard deviation (RSD) just above the top surfaces of crop canopies. These included Case 1 — with supply air inlets and return air outlets installed on opposite side walls; Case 2 — with inlets and outlets placed in alternating rows on the ceiling; Case 3 — with the same size and location of inlets as in Case 2 but with outlets placed on two side walls at the floor level; Case 4 — with perforated air tubes installed above aisles and outlets placed on the ceiling; and Case 5 — with a perforated air tube installed at each level of shelves. The airflow pattern within indoor plant factories was greatly affected by the locations of airflow inlets and exhausted outlets. With the localized air ventilation (Case 5), the climate uniformity at the crop canopy was improved compared to the control case (Case 1). The RSD for the averages of air current speed, air temperature, and vapor pressure deficit (VPD) were improved by 14.3%, 0.1%, and 1.0% respectively. The average of air current speed just above crop canopy surfaces was enhanced from 0.3 m s-1 in Case 1 to 0.4 m s-1 in Case 5. The averages of air temperature and VPD in Case 5 as a result of air mixing within the system were also increased, with 297.3 K and 1205.98 Pa respectively compared to Case 1 with 295.8 K and 924.0Pa respectively. Energy use efficiency for two greenhouse cases and two indoor plant factory cases were compared for lettuce production yields in kilogram fresh weight including energy consumption for lighting, heating, and cooling as inputs at six geographic locations with different climates (Duluth, Minnesota, Seattle, Washington, Phoenix, Arizona, Miami, Florida, Abu Dhabi, UAE and Riyadh, KSA). The cases for greenhouses included Greenhouse 1 (GH_1) — with 50% of shading deployed during high radiation seasons of the year and Greenhouse 2 (GH_2) — with automatic shading curtains and supplemental electrical lighting to maintain a constant daily light integral level of 15 mol m-2 d-1, while the cases evaluated for indoor plant factories were operated with constant DLI of 13 mol m-2 d-1 and 15 mol m-2 d-1 for Indoor Plant Factory 1 (IPF_1) and Indoor Plant Factory 2 (IPF_2) 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 (in the current study evaluated with Duluth and Seattle) 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. However, in hot climates (in the current study evaluated with Phoenix, Miami, Abu Dhabi, and Riyadh) greenhouses are significantly more efficient than indoor plant factories, with the highest energy use efficiency of 0.35 kg kWh-1 in Miami. The energy use efficiencies for all indoor plant factories are close to each other under different climates. The reason for this might be the fact that the indoor plant factories are tightly sealed and well insulated and the internal growing conditions are maintained same for the crop growth leading to same production yield outputs. Indoor plant factory systems, including using Heating, Ventilation, and Air Conditioning (HVAC) economizers, increasing the number of tiers of production shelves, lettuce plant transpiration rate, and light efficacy, were evaluated for energy loads and energy saving potential. The electricity usage increased linearly with the increased number of tiers in the production system. The energy use efficiency remained the same for multi-tier indoor plant factories. Productivity of lettuce per unit area of footprint of an indoor plant factory increased in proportion to the number of tiers. Potential strategies for energy savings for indoor plant factories were integrating economizers to HVAC system and installing LED lights with high light efficacy. An HVAC Economizer can save up to 87% of electricity usage for cooing in cold climates (Duluth and Seattle) while the energy saving in hot climate is less significant (around 30%) but is still considerable. The cold and dry outdoor climate is ideal for an economizer with differential enthalpy control. Lighting is the major consumer of energy consumption in indoor plant factories, therefore choosing LED lights with the highest efficacy is the best method to reduce the electricity usage and to enhance energy use efficiency for indoor plant factories.