On January 1st of 2020, new changes go into effect for Title-24, California’s energy efficiency standard for commercial buildings. Neil Bulger has a nice summary of changes to HVAC, and a bit on lighting. But what’s happening with refrigeration in supermarkets?
There are two important changes that are reflecting market shifts and setting the stage for the coming phase out of HFCs.
We’ve seen the influx of adiabatic condensers in the market which are an interesting mix of some of the benefits of air-cooled, and evaporatively-cooled condensers. These typically are set up to run “dry” as air-cooled condensers when it’s cool out, then use evaporative pads to precool incoming air when it is warmer and the extra capacity is needed.
One of the things I like about adiabatic condensers is that they provide much of the benefits of evaporative condensers, while avoiding some of the hassles. Notably, they only wet the pads with enough water to cool the air, using humidity sensors to determine how much water is needed to just wet the pad, and not more. Then you don’t have to recirculate that water – avoiding the need for water treatment, and minimizing water use. You do have to periodically replace those pads, but that’s not a big ticket item.
T-24 2019 sets specific efficiency targets for adiabatic condensers (i.e. 45 Btu/W minimum) as well as capacity requirements for design day operation.
Exceptions for CO2 Transcritical Systems
The other substantive change to T-24 for commercial refrigeration is that the code makes an exception to condenser requirements for transcritical (TC) CO2 refrigeration systems. Essentially this means that if you’re doing CO2 TC, your design is, for the time being, up to you. Presumably that gap will be filled as we gain more experience with those systems. But for the time being, if you’re using this low-GWP type system, you get a pass.
Unfortunately, the code missed any exceptions for other low-GWP options like NH3/CO2 cascade systems, or NH3 + secondary coolants. If you’re using propane as a refrigerant, I think you also fall between the cracks. With the coming HFC ban, I expect that T-24 will need to cover these additional system types, because much of our new construction will be driven that direction.
Still crazy after all these years…
The next two items have been in the code since the 2013 edition, but I bring them up because I believe there is a lot of missed code compliance on these two issues:
All condenser fans must (still) be variable speed
Whether you’re using air-cooled, evaporatively-cooled, or adiabatic condensers, all your condenser fans must be continuously variable speed, with the control of those fans in unison (down to a reasonable minimum cut out of 10 to 20%). There are plenty of less sophisticated contractors out there that have yet to get up to speed on this count.
All condensers must (still) have floating head pressure control
Floating head pressure, or as it’s called in the code, variable-setpoint control logic, is also required for all condenser types. This refers to a continuous reset of the condenser pressure in response to outside air conditions. In practice this means that your condenser fans are going to ramp up or down to provide a leaving temperature that is a fixed difference (aka fixed delta T) compared to the outside air temperature. For air-cooled and adiabatic condensers, this delta T will be with respect to outside air dry-bulb conditions and for evaporative condensers, the delta T is compared to outside air wet bulb. Your variable speed condenser fans should actually make this control more stable than prior, stepped or two-speed, fan control strategies did.
To get the idea of how this works, it’s easiest to see this figure from the CEC’s Title 24 Compliance Manual. In this example, we have an evaporative condenser that responds to outside air wet bulb temperature:
Figure credit: CEC’s 2019 Nonresidential Compliance Manual
As you can see in the diagram, the setpoint is raised in response to the outside air wet-bulb temperature – when it goes up, the setpoint goes up, when it goes down, the setpoint goes down. This means that when it’s cool outside (and drier), the discharge pressure is lower at the compressors, they do less work, and therefore cool more efficiently. As is commonly adopted with this strategy, the code allows for a minimum setpoint of 70 degrees F, to ensure enough pressure for other system components (like TXVs) to work correctly.
The elephant(s) in the room
Overall these requirements should result in more efficient systems, although one wonders how many new stores will actually be affected. New construction rates for supermarkets have been pretty slow, and I’ve heard anecdotally, that the industry may be “overbuilt”. Also, progressive companies are also experimenting with natural refrigerants in anticipation of the HFC phase outs, further decreasing the number of new stores that will be built with halocarbon refrigerants.
If we want to get the most energy savings, perhaps we should be turning to areas of true weakness in the existing building stock. For instance, we know that we have a code compliance issue and it’s rare for city building departments to have the expertise to ensure these advanced control strategies work effectively. Also, we really need to begin concerted efforts towards workforce education so that we can: 1) make sure the stores run like they were design to and; 2) have a trained workforce to install and operate new store designs using natural refrigerants.
Let me know if you’re experience is different – we’d love to hear from you.
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