Humidity Control for Indoor Farms

Indoor farms have many advantages over traditional outdoor farming and are becoming more widespread. However, they are energy-intensive operations, and heating, ventilation, and air conditioning (HVAC) often accounts for 45% of total energy use due to the high dehumidification requirements. Improving HVAC energy efficiency for indoor farming is therefore an important energy-saving opportunity. Dehumidification is typically accomplished by cooling air to its dewpoint at a refrigerant-based evaporator coil, which condenses moisture. This process overcools the air, and it usually needs to be re-heated after moisture removal. This study evaluates a novel approach to dehumidification that consists of a conventional refrigerant-based air conditioning system coupled with an air-to-air heat exchanger. The heat exchanger transfers sensible heat from the return air to the supply air, reducing the need for re-heat energy. To evaluate this approach’s real-world performance, we identified two indoor container farms growing lettuce located in California Climate Zone 4 and retrofitted one farm with the novel approach. The other farm remained equipped with a conventional air conditioning system where re-heat was provided with an electric resistance heater; this farm served as the reference for determining energy savings and cooling performance. HVAC power draw and energy use, as well as indoor temperature and humidity, were monitored over the course of the study. The retrofit provided 37-41% energy savings and 25-30% peak demand reductions relative to the baseline. However, the retrofit equipment occasionally over-dehumidified the farm and required modifications to better meet the cooling loads in the farm. These findings show that the retrofit can provide substantial energy savings for the indoor farming industry, but it may not be a one-to-one swap for existing HVAC equipment.