How Crops Affect Building-Integrated Agriculture Design

 From ASHRAE Journal Newsletter, April 13, 2021

Include crops as an additional internal heat gain/loss when designing an HVAC system for building-integrated agriculture spaces. Otherwise your system may not be sized adequately, advises researcher Marie-Hélène Talbot, ing. This and other results are the subject of a recent Science and Technology for the Built Environment article, which Talbot discussed with ASHRAE Journal staff.

   1. Briefly, what is a building-integrated agriculture space?

A building-integrated agriculture (BIA) space is a controlled-environment agriculture (CEA) building space dedicated to crop production in which optimal indoor conditions such as temperature, humidity, carbon dioxide (CO2) concentration and lighting are maintained using HVAC and other systems. BIA spaces can be in or on top of buildings that have a different main use than agriculture, such as office, school or retail. Vacant spaces in urban areas could be converted to accommodate BIA spaces and create synergies with other spaces of the building.

   2. What is the significance of this research?

The design of HVAC systems for BIA spaces is challenging and requires improved estimation of the peak loads. In this research, a modeling approach to integrate crops as an additional internal heat gain/loss to a building performance simulation (BPS) tool was proposed to estimate the impacts crops have on peak loads using a case study of an interior BIA space. The results demonstrated that not considering the crops could lead to inadequate sizing. Thus, the proposed modeling approach could improve the sizing process of the HVAC system and provide the basis for a valuable dialogue between growers and HVAC designers during the early design stage of BIA spaces.

   3. Why is it important to explore this topic now?

Growing interest exists to make year-round food production possible in cold climates and urban areas, with CEA, such as BIA spaces, being among the proposed solutions. This is because it can lead to yields being a hundred times higher than in open fields. However, these advantages are often offset by high energy use and the associated costs. Improving BIA space modeling is of utmost importance to improve the equipment sizing and energy use of such spaces.

   4. What challenges are particular to calculating peak loads for crops?

Crops are a complex internal heat gain/loss to consider when peak loads of a BIA space are estimated. However, to our knowledge, no guidelines are available on how to consider crops when completing load calculations. In addition, crops’ heat gains/losses are influenced by several parameters that need to be specified such as the indoor air setpoints (temperature and humidity or vapor pressure deficit (VPD)), the crop growth management method, the lighting intensity, the CO2 concentration, irrigation method and the type of crops. Specifying the right crop parameters to estimate peak loads was challenging since they vary according the growers’ requirements.

   5. What lessons, facts and/or guidance can an engineer working in the field   take away from this research?

Crops stand out from existing internal gain categories and should form a new category of internal gains/losses to be considered when designing the HVAC system or completing the energy analysis for any CEA space. Also, the results of the research demonstrated that the loads of those spaces are mainly influenced by internal gains, and the impact of the external gains are minimal. Thus, evaluating internal gains/losses carefully is critical.

   6. How can this research further the industry's knowledge of this topic?

In this research a modeling approach to include crops as an additional internal heat gain/loss for load calculations was proposed, which has not been previously done. The results also highlighted the very distinct energetic behavior of crops and how they stand out from existing internal categories, i.e., the space sensible heat ratio varies significantly depending on the design and operating conditions of the space. The proposed modeling approach can also be used by the industry to compare the performance of different HVAC systems and the energy performance of a BIA space.

   7. Were there any surprises or unforeseen challenges for you when  preparing this research?

While generating the results, I was surprised by the magnitude of the dehumidification loads, which were, even with the more conservative approach, quite high. The dehumidification loads were comparable to the sensible cooling loads caused by the lighting heat gains. This was unexpected, as I initially thought heat gains from the lights would be, by far, driving the loads. However, at a high cultivated density, crops and lights both drive the loads. This finding totally reinforces the need to develop a robust approach to include crops as an additional internal heat gain/loss to load calculations, but also when completing performance analysis over a growth cycle.