The State of Technology: Refrigeration

Chris Zimbelman, Max Ma, and Bryan Kilgore, ERS, for Zondits

March 22, 2016. Image credit: Marco d’Itri

As part of the state of technology series, the refrigeration team pulled together a few different updates on some new technologies in the refrigeration world. Here are a few happenings that we thought were interesting.

Magnetic Refrigeration

Keeping things cold is not a new concept. From keeping food fresh to cooling industrial processes, refrigeration is required in various forms all around us every day. To date, almost all of this refrigeration that we so commonly use has worked off the vapor compression cycle, which uses a refrigerant that undergoes a series of phase changes to create the cooling effect. This technology has proven to be effective, although it carries with it the use of energy-intensive compressors as well as refrigerants that can pose risks to the environment or directly to people.

An alternative to the vapor compression cycle is magnetic refrigeration. This process takes advantage of something called the magnetocaloric effect, which is the heating and cooling of certain materials by exposure to a magnetic field. This concept has been around for more than a hundred years, but it was only recently adapted to common refrigeration applications. Several companies are working to change that and have made headway in developing commercially viable products. Efficiency impacts will certainly need to be evaluated, but initial vendor estimates are that the technology could be 20%–30% more  efficient than the more traditional vapor-compression refrigeration and without the use of potentially dangerous refrigerants.

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Secondary Refrigeration Systems
Secondary refrigeration systems (SRSs) work by using a fluid other than the primary refrigerant to transfer heat between the space and the compressors. Chilled water systems serve as typical examples, where the chilled water serves as the secondary fluid. Ideal secondary fluids are economical, non-corrosive, and non-toxic, and they have a low viscosity, high heat capacity, and low freezing point. Water, the common choice for HVAC applications, would freeze in most refrigeration applications; thus, secondary refrigeration systems are not as common in low temperatures. With the development of new secondary fluids, typically referred to as brine, secondary refrigeration systems now offer the following advantages in new construction projects:Reduced compliance requirements on the primary refrigerant (especially for ammonia systems)

• Reduced compliance requirements on the primary refrigerant (especially for ammonia systems)
• Reduced cost to recharge the refrigerant
• Allowance for lower-cost and lower-leakage piping materials (since there is no phase change and little pressurization)
• Reduced maintenance due to oil return and refrigerant return issues, especially during winter or windy conditions
• Reduced drying of produce stored in warehouses, according to anecdotal evidence

Now the fun part: how does the secondary refrigeration system fare on energy efficiency? You guessed it – it depends on design conditions and requirements. The general pros and cons are listed in Figure 1 below.

I hope by now you are all excited for your next encounter with secondary refrigeration systems!

CO2 Refrigeration Systems
In an effort to reduce the global warming impact and the ozone depletion potential of refrigerants, alternative refrigerants have been developed and implemented. An emerging refrigerant that is starting to establish itself in the commercial market is carbon dioxide (CO₂) used directly as a refrigerant.  CO₂, or its refrigerant designation R-744, has minimal environmental impact compared to typical commercial refrigerants and the possibility of efficiency improvements depending on the region and application. Table 1 shows the global warming potential (GWP) for various refrigerants.

R-744 can be used in a secondary system as the secondary fluid, but two types of systems use R-744 directly, transcritical systems and cascade systems.

Transcritical systems use only R-744 and generally are configured as a two-stage system. At high ambient temperature conditions the system operates in transcritical mode, where R-744 exits the high-stage compressor as a supercritical gas and does not condense during heat rejection. Only after the pressure is decreased does R-744 condense into a liquid. At lower ambient temperatures the system operates as in subcritical mode, and operates similarly to traditional systems with a condensing heat rejection step.

Cascade systems use R-744 in a low-stage refrigeration system. However, the R-744 condenser rejects heat to the high-stage refrigerant instead of to the outdoors. The high-stage system typically uses a traditional hydrofluorocarbon (HFC) refrigerant. 744’s condensing  temperature is maintained below the critical point, allowing subcritical operation regardless of ambient temperature.

The general advantages and disadvantages of R-744 are shown in Figure 2.