Controlled Environment Horticulture: Space Conditioning Systems for Indoor CEH

Measure Overview

This proposed measure would establish mandatory sizing requirements and controls requirements for space conditioning systems providing heating, cooling, and dehumidification to support plant growth in CEH indoor growing spaces and would prescriptively require these systems meet key functional and performance metrics.  

These requirements would only apply to space conditioning systems serving CEH spaces with a lighting power density greater than 30 watts (W) per plant canopy square foot in indoor CEH facilities with at least of 5000 ft2 of plant canopy area. (Plant Canopy Area is the aggregate horizontal surface area occupied by actively growing vegetation within a cultivation facility. It is measured by the boundaries of the specific grow trays/benches utilized, excluding non-productive spaces such as aisles, walkways, ancillary equipment zones, and structural columns).  

The Statewide CASE Team has developed a new indoor CEH prototype and building energy model for facilities meeting these conditions. The Statewide CASE Team is recommending that these capabilities be introduced to the California Building Energy Code Compliance Software (CBECC) software. Should this space type be available in the CBECC software, facilities subject to these new prescriptive requirements would also have the option to comply via the performance pathway.   

These proposed requirements recognize that functions of cooling and dehumidification equipment are highly interactive and must be considered together, as a part of a space conditioning system that manages both temperature and humidity in CEH spaces.  The prescriptive requirements would require the applicant to identify a primary space conditioning system on the design documents. This primary system would be required to have a variable sensible heat ratio, to be capable of modulating the amount of dehumidification process heat recovered to reheat dehumidified air between 10 percent and 90 percent, and to be sized to meet at least 80 percent of the peak combined sensible and latent load.  

These prescriptive requirements of variable SHR (sensible heat ratio), modulating heat recovery, and minimum size of a combined cooling and dehumidifying system would be used to set an energy budget for compliance via the performance approach where any system that can be modelled could comply if it requires less long-term energy cost than the base system. 

Because the prescriptive requirements are based on a functional description of range of required sensible heat ratios, range of heat recovery modulation and minimum combined cooling and dehumidification size to design capacity, a variety of primary systems could comply with the requirements such as: an integrated direct expansion (DX) system, a desiccant-based system, or a heat recovery chiller system.  

An example compliant integrated DX system is repurposed DX Dedicated Outdoor Air System (DOAS) or pool dehumidification equipment that has configured, sized, and programmed for 100% recirculated air. Integrated DX systems use variable speed or staged compressor systems in combination with variable speed fans to modulate the ratio of sensible cooling to dehumidification that occurs at the evaporator coil, depending on supply air conditions and humidity and temperature setpoints. The dehumidified air then passes over an indoor reheat coil to reheat the air to desired supply air temperature using waste heat from the dehumidification process. These systems modulate the amount of waste heat that goes to the indoor reheat coil versus rejecting the heat to an outdoor condenser coil, in a process known as modulating hot gas reheat. 

An example of a compliant desiccant system is Mojave HVAC’s liquid desiccant cooling and dehumidification system that cools incoming air and dehumidifies with a liquid desiccant. This system uses condenser heat for the regeneration (desiccant drying) process.  

A four-pipe heat recovery chiller system uses a chilled water loop and fan coils to dehumidify and cool return air, with a separate hot water loop that reheats the dehumidified air to the desired temperature. Water in the hot water reheat loop is heated using waste heat generated through the process of cooling the water supplied to the chilled water loop.      

The mandatory sizing requirement would require the submission of calculated sensible and latent loads and sizing calculations for space conditioning systems serving indoor CEH spaces with a lighting power density greater than 30 W per canopy square foot. The method for calculating the load and the sizing requirements would be defined in a new Nonresidential Appendix.  

In addition to the primary space conditioning system, the proposed requirements would allow the use of supplemental equipment to meet sensible or latent loads exceeding the primary system’s capacity.  

Mandatory requirements would require the installation of a central control system that fully utilizes the capacity of the primary space conditioning system, modulates heat recovery to the provide the minimum heat needed, and when additional capacity is needed, stages the minimum amount of supplemental capacity.  

Mandatory requirements for the primary system would require that it be controlled to do the following: 

• Modulate sensible heat ratio (SHR) in response to room conditions and temperature and humidity setpoints; 
• Modulate reheat to reject or recover dehumidification process heat, as needed to meet supply air setpoints; and 
• Limit the use of primary heating (i.e. resistance, furnace, or boiler) heat to periods when the use of recovered process heat cannot meet supply air setpoints. 

Controls for supplemental unitary dehumidification equipment without variable SHR or heat rejection capability would be required to do the following:  

• Integrate and stage supplemental equipment with primary system to meet temperature and humidity setpoints; 
• Stage unitary dehumidifiers in response to humidity sensors (including those that measure relative humidity, dewpoint, or wet bulb);  
• Only activate dehumidification equipment when either: 1) all waste heat can be used in the space without creating additional cooling load, or 2) the primary system cannot satisfy 100 percent of cooling, heating, and dehumidification loads.  

The requirements would be the same for all 16 California Climate Zones. The proposed requirements would apply to new construction, additions, and major alterations. Replacing a single piece of equipment in a larger system would not trigger the requirements. The code would be triggered by additions or alterations that increase plant canopy area by at least 5000 square feet. The code would also be triggered by alterations that replace greater than 50 percent of the total capacity of heating, ventilation, air conditioning, cooling, and dehumidification systems serving at least 5000 square feet of plant canopy area in indoor growing. 

This code change proposal would also revise requirements for outdoor air ventilation requirements, that may negatively impact plant growth or efficiency in CO2 enriched spaces.    

Title 24, Part 6 does not currently include prescriptive measures for CEH spaces, and CBECC does not currently include an indoor CEH prototype. To provide facility designers with flexibility in equipment selection, the Statewide CASE Team recommends that CBECC is updated so it can be used to demonstrate compliance using the performance approach.   

Table 1 summarizes the scope of the proposed code change. Note that this proposal includes acceptance testing performed by a field technician. Because the field technician is typically a member if the installation team, this acceptance testing is not considered “third party verification.”