Fluctuating environmental policies in the U.S. tend to obscure the fact that the transition toward refrigerants with a low GWP continues. HVACR manufacturers, researchers, and facility owners worldwide are making significant progress, thanks to consistent public policies but also from recent development of proven technologies.
Technologically, it’s worth looking at recent moves away from refrigerants with high GWPs—like HFC-134a and HFC-404A—and toward low-GWP formulations, like natural hydrocarbons and HFOs with GWPs below 5. The HFOs have a low GWP because they break down in the atmosphere over a few days. Furthermore, HFOs together with HFCs form low-GWP hybrids that have GWP ratings below 150.
Innovation is the key that is unlocking the potential of the new refrigerants to create a low-GWP future. Important developments include: Utilizing the thermodynamics of CO2 refrigerant—specifically, its expansion energy and heating properties—to make CO2 even more feasible for commercial refrigeration; the ongoing decarbonizing of the electric power supply; and boosting flexibility in electric consumption with demand response and thermal storage technologies. These developments are driving forward the adoption of low-GWP refrigerants in food and commercial refrigeration today.
Innovation Accelerating Transition
In the earlier transition from high to low ODP refrigerants, manufacturers could generally use a “drop-in” approach—that is, simply replacing one refrigerant for another after modifying seals and other minor components. That is not always possible with the switch to low-GWP refrigerants. Natural refrigerants, such as ammonia and CO2, operate with different parameters compared to HFCs and require specially designed system components. Other natural refrigerants, such as propane, use components similar to those employed with HFCs, but special safety precautions need to be taken.
Depending on corporate goals and local regulations, manufacturers and end users worldwide are adopting low-GWP-refrigerant systems employing various technological innovations.
As a practical example of how innovation accelerates the transition, it’s worth examining the case of a military supermarket that successfully implemented a CO2 refrigeration system in a hot-climate region. The region experiences 90°F and higher ambient temperatures in the summer. Compared to using HFCs at those temperatures, employing CO2 refrigerant within its normally high operating pressures was not energy efficient. Nevertheless, several advanced technologies were employed to go beyond CO2’s traditional ambient temperature limitations. The system used the Danfoss Multi Ejector Solution™, consisting of a high-pressure valve, integrated pressure transmitter, and an intelligent system controller in conjunction with the compressor rack.
The system operates reliably in the transcritical zone of the compression cycle, where pressures can exceed 1,067 psia. In the transcritical zone, heat rejection can efficiently occur even at high ambient temperatures—even at 36°C (97°F). The bottom line: The CO2 system supplying display cabinets has an extremely low GWP of 1.0 and provides up to 30 percent energy savings compared to the incumbent solutions based on R-22 and R-404A refrigerants.
Several HFO refrigerant formulations have been readily adopted for a number of applications. R-1234yf is used by automotive manufacturers for car air conditioners, and R-1234ze(E) is used in innovative centrifugal compressors employing magnetic bearings, such as Danfoss Turbocor® compressors.
Some low-GWP refrigerants are flammable, some are toxic, and some are both. ASHRAE 34 and International Standards Organization (ISO) 817 standards have been developed to classify flammability and toxicity. Lower GWP properties tend to correlate with higher flammability/toxicity characteristics. For example, HFC refrigerant R-410A (with a 2,088 GWP) is non-toxic, non-flammable, and classified A1. The HFO refrigerant R-1234yf (with a 4 GWP) is non-toxic, mildly flammable, and classified A2L. Hydrocarbon refrigerants (R-600a isobutane and R-290 propane) are highly flammable (A3). Ammonia (R-717) has higher toxicity and flammability (B2), while CO2 (R-744) has no flammability or toxicity (A1).
The correlation between GWPs, refrigerant density (weight per volume), and safety classifications of various refrigerants is of great concern to HVACR system manufacturers and specifiers. Blends of HFOs and HFCs are being developed to optimize performance and safety. Whether using HFOs, natural refrigerants, or blends, HVACR manufacturers must innovate technologies that maximize efficiency and minimize risk for the selected refrigerant.
Development of new technologies makes it easier for manufacturers to balance safety and environmental responsibility. Some leading retailers have been using R-290 (propane) in equipment for over a decade. Taking advantage of R-290’s excellent thermodynamic properties, systems are operating successfully with a charge limited to 150 grams (5 ounces) for safety. Further research and product development led the International Electrotechnical Commission (IEC) to propose a 500-gram (1.1-pound) limit for R-290 in single commercial refrigeration appliances. The new IEC 60335-2-89 standard was passed in May 2019.
As time passes, it’s logical to think that Europe and the U.S. will continue to explore the balance between safety, efficiency and the environment—but not at the expense of endangering users and technicians. The service industry, in particular, will need to become more familiar with flammable refrigerants.
“Installation and service are the areas where proper guidelines, training, certification, and standards are the most important,” explains Bill Goetzler of Navigant Consulting. “The lines carrying flammable refrigerants may be opened during various servicing operations, and service personnel also may be working with a high-temperature ignition source. Those are really places to be very careful and to be sure we've got standards and guidelines in place to minimize the risk.”
Safety is a vital factor relative in a refrigerant sustainability triangle that includes environmental and economic factors. Other factors come into play depending on the properties of the refrigerant. With some HFO refrigerants, for example, flammability can present a safety issue, as previously discussed. Cost can also be a factor, as HFO formulations are more expensive to produce, and supplies are constrained. Finally, environmental concerns can still be an issue in several countries, because some HFOs break down in the lower atmosphere, forming fluorinated products. Trifluoroacetic acid (TFA) is an HFO breakdown substance but also occurs naturally in seawater. Hydrogen fluoride (HF) is another HFO breakdown substance that is very toxic. The small amounts of TFA and HF are not expected to pose global or regional problems.
The search for refrigerants that perfectly balance environmental, economic, and safety concerns is continuing. Unfortunately, this quest throws customers and end-users off balance -- planning is stressful when the future is subject to change. Fortunately, in recent years, technology has developed to the point that it is possible to build a long-lasting platform for low-GWP commercial refrigeration. Proven technologies are now available in four areas:
1. Developments in CO2 refrigeration
Until recently, it’s been impossible to apply the energy efficiency, safety, and environmental benefits of transcritical CO₂ systems in all climates. However, as previously explained, Danfoss Multi Ejector technology enables a CO2 refrigeration system to operate in a transcritical zone, even in warm climates. The result is up to 10 percent annualized savings compared to prior transcritical parallel compression systems and up to 30 percent energy savings during the hottest hours of the year compared to booster systems. Moreover, compressor discharge temperatures in transcritical operation can exceed 250 °F. This heat, which is usually rejected by a gas cooler, can be reclaimed by heat exchangers mounted before the gas cooler. This combination of technologies brings all the safety, economic, and environmental benefits of CO2 to many large and mid-size U.S. commercial refrigeration applications, regardless of location.
2. Developments in demand response
The high usage of electricity by supermarkets can make it very advantageous to participate in demand response programs. In this type of program, the utility sends a signal to a central control unit such as the Danfoss ADAP-KOOL® System Manager AK-SM 800 Series, which can then reduce power to motors, compressors, and other electrical equipment. The signal triggers a reduction in electric consumption for brief periods ranging from less than one minute to long periods within a 24-hour interval, depending on the application.
Not every utility offers demand response programs, but those that do pass on the prospective savings to participants. Economic benefits are based on the customer’s commitment to respond. A carefully designed demand response program using appropriate refrigeration control and management systems can achieve substantial energy savings while protecting food integrity.
3. Developments in thermal storage and related thermal-shifting technologies
Thermal storage makes it possible to shift electricity consumption from expensive peak rates during the day to lower rate periods during the night. Thermal storage tanks hold a liquid medium chilled at night for cooling display cases, reducing equipment run time.
In a supermarket application, the display cases can themselves function as a form of thermal storage. Lowering display-case temperatures during off-peak hours enables compressors to run less during peak hours. Thermal storage capacity can be used to produce a revenue stream when it's connected to other stores through a district heating and cooling (DHC) system. Heat recovery technology can also be used to capture heat otherwise rejected by the refrigeration system’s condensers. For stores using CO2 with a heat-reclaim system, the size of a separate heat source—such as a boiler—can be greatly reduced or even eliminated.
4. Developments in the decarbonized grid
Called “the largest machine ever built,” the more than century-old electric grid is the largest producer of greenhouse gasses. Fortunately, technology exists today to re-engineer that machine to cut CO2 emissions dramatically. Increased use of renewable power (wind and solar), continued use of stable nuclear power, and use of responsive hydropower are low-carbon resources that are gradually creating a decarbonized electricity supply. Models of a moderately decarbonized electricity system for the year 2050 estimate CO2 emissions intensity could be 60 to 80 percent lower than today’s U.S. grid.
Recent technological developments are supporting the transition to low-GWP refrigerants. While policies may fluctuate, research and innovation are bringing proven products and platforms to market that can be implemented today in more food and commercial refrigeration applications than ever before.
HFOs in conjunction with oil-free system developments are very viable options in air conditioning and, eventually, heat pump applications. Advances in transcritical CO2 systems are also providing an attractive value proposition for many supermarkets. Other developments—demand response, thermal storage, and heat reclaim—complement the trend toward a decarbonized electric grid and significantly reduce commercial refrigeration’s carbon footprint.
Utilities and regulators play a vital role in creating an HFC-free future. Incentives and policies can reassure supermarket owners operating on tight margins that their investment will pay off by boosting profits in the short term, as well as the environment in the long run.