VC-21A-C024 - Estimated Energy Demand and Sensible Heat Ratio of a Controlled-Environment Agriculture Space for a Growth Cycle

Growing food in controlled-environment agriculture (CEA) spaces have made year-round food production possible in cold climate and urban areas. However, designing and operating heating, ventilation and air conditioning (HVAC) systems required to maintain the indoor growth conditions (temperature, humidity, carbon dioxide (CO2) concentration and light intensity) is still challenging since the sensible heat ratio (SHR) can significantly vary between the photoperiod and the dark period as well as throughout the growth cycle. In this study, the energy demand and the SHR of a small high-density CEA (CEA-HD) space is estimated for a growth cycle using a building performance simulation (BPS) tool. The results are presented for two types of light-emitting diode (LED) lamps as well as for two different crops (lettuces) models, an existing steady-state lettuce model and a modified version of that existing model that includes a crop growth model (dynamic model). For the LED lamps, the impact on the energy demand is assessed for different heat fractions (short-wave radiative / long wave radiative /convective) for high electrical efficiency (HEE) and low electrical efficiency (LEE) lamps. The results showed that the energy demand is influenced significantly by the heat fractions of the LED lamps and the stage of growth of the crops. The sensible cooling demand is more important in the second part of the growth cycle for the LEE LED lamps, which have a higher convective heat fraction. This is explained by the fact that as the crops grow, the leaves absorb (and convert) more short-wave radiation thus, lowering the lamps heat gains in the space. Also, the SHR of the space during photoperiod in the second part of the growth cycle is 70% (HEE lamps) and 47% (LEE lamps) lower than the SHR in the first part of the cycle. The use of a crop growth model is more suitable to predict the sensible and latent energy demand of a CEA space in operation, while a steady-state model could be appropriate to complete load calculation.