Impact of Crops on the Thermal Loads of a Controlled Environment

Crop-Environment Interactions: an Overlooked Impact

 “Heating, ventilation, and air conditioning (HVAC) systems are at the heart of the energy consumption of CEA spaces.” However, there are no guidelines to properly size such systems for this type of application. To size HVAC systems, heating and cooling load calculation must be completed. This calculation involves determining the maximum demand for sensible heating, sensible cooling and latent cooling, also called space dehumidification (Spitler, 2014). Modelling tools can be used to perform this step, however, when modelling controlled environment agriculture (CEA) spaces, crop-environment interactions are often neglected (Sethi et al., 2013). In the case of a Building-Integrated Agriculture (BIA) space, a type of CEA space, crop-environment interactions have a significant impact (Talbot & Monfet, 2018) and should therefore be included in load calculations.

Assessing Crop-Environment Interactions

Energy interactions between crops and their environment (shown in Figure 1) take place during the photoperiod—when plants photosynthesis occurs while the lighting is on—and the dark period—when plant respiration occurs while the lighting is off. During the photoperiod, interactions are based on the particular ability of crops to photosynthesize the energy flux emitted by lighting (radiative energy) by converting it into a latent heat flux toward the environment, i.e. transpiration. In addition, depending on indoor conditions, the temperatures of the crops can be up to 10 °C higher or lower than the ambient temperature (Downs, 2012). In fact, when the temperature of crops leaves is below the ambient temperature, the crops cool their environment and, conversely, they warm it when the temperature of crops leaves is higher than their environment. During the dark period, crops transpire and cool their environment.

Figure 1. Energy interactions between crops and their environment.

The heat flux induced by crops can be determined by solving the energy balance between crops and their environment, as presented in Figure 1. Several factors impact these heat flux: crop type, leaf size, indoor conditions—temperature, humidity, CO2 concentration, lighting—and irrigation.

Underestimation of the Heating and Dehumidification loads if crop-environment interactions are neglected

A case study using a BIA space model generated with a building performance simulation tool was conducted to estimate heating and dehumidification loads. The space is located inside a building and may contain one or more tiers of hydroponic lettuce beds stacked vertically (Figure 2). The number of lettuce bed tiers defines cultivated density: one tier corresponds to a cultivated density of 60%—since it covers 60% of the floor—and 10 tiers correspond to a maximum cultivated density of 600%.

Figure 2. Interior configuration of a BIA space.

For load calculations, the critical heat flux induced by crops were considered according to the Leaf Area Index (LAI). The maximum value of this index depends on the crop growth management method; it is set at 2.1 for lettuce growing under a diversified growth management method and at 10 for lettuce growing under a single growth stage management method.

Figure 3 details heating and dehumidification loads obtained from different cultivated density for the baseline case, which does not include crop-environment interactions in the load calculations, and two other cases (LAI of 2.1 and 10) according to the crop management method. The loads were determined for various indoor conditions: temperature (in °C), relative humidity (in %) influencing the vapour pressure deficit (in kPa), a variable that acts on plant transpiration rates.

Figure 3 Left: BIA space heating loads for the baseline case (blue), leaf area index (LAI) of 2.1 (red) and 10 (orange). Right: BIA space dehumidification (1) loads for the baseline case (blue), leaf area index (LAI) of 2.1 (purple) and 10 (green).
(1) A negative value means that the space must be humidified to control indoor conditions.

Results show that the baseline case underestimates the heating load caused by the cooling effect of the crops. They also show that a dehumidification system is required when considering crops transpiration. Crops transpiration increases with cultivated density and is one of the most important loads in a BIA space. If loads are incorrectly estimated, the HVAC equipment may not be able to maintain indoor conditions at all times. Since this is a type of space that is quite humid, insufficient heating or dehumidification capacity of the HVAC systems could lead to condensation problems on colder surfaces in the BIA space. The approach developed in this study, which includes the heat flux induced by crops to load calculations, is a basic tool for a valuable dialogue between growers and HVAC system designers—right from the preliminary design of a CEA space.

The study highlights other important aspects in the development of this emerging field of research: 

• Lighting and crops have the highest impact on loads in this type of space, demonstrating the importance of including heat flux induced by crops when modelling a CEA space.
• Some decisions made by growers regarding operations—cultivated density, crop growth management method, indoor conditions—have significant impacts on loads.
• This study contributed to the development of guidelines for HVAC systems sizing of CEA spaces, by taking into account the heat flux induced by crops in load calculations.

Additional Information

For more information, please refer to the following paper:

Marie-Hélène Talbot & Danielle Monfet (2020) Estimating the impact of crops on peak loads of a Building-Integrated Agriculture space, Science and Technology for the Built Environment, 26:10, 1448-1460, DOI: 10.1080/23744731.2020.1806594