Hillphoenix's ChargeSecure system, which helps preserve CO2 charge during a power outage

What’s the Best Way to Prevent Release of CO2 Refrigerant in a Power Outage?

Hillphoenix is launching a new product and partnering with food retailers and others to address charge preservation in CO2 systems.

CO2 (R744) is used in commercial refrigeration because of its excellent thermophysical properties (including high energy content, excellent heat transfer coefficient and low viscosity of the liquid phase).

One aspect of these properties is that when the system warms during an extended power loss, the pressure increases.  To protect the piping system, a charge release may occur through a pressure relief valve. The amount of charge released depends on the length of the power outage, but if enough charge is lost it may disrupt the restart and operation of the system.

As transcritical CO2 systems gain more adoption in North America – following their dynamic growth in Europe – OEMs, contractors and end users are increasingly focusing on the need to limit or prevent releases of CO2. The North American Sustainable Refrigeration Council (NASRC) has recently convened a working group of stakeholders to tackle this important issue. There are different strategies to help manage charge preservation and different stakeholders have different views on how to address it.

However, U.S. OEM Hillphoenix is releasing a product designed to help with charge preservation for CO2 systems in a cost-effective manner.

ChargeSecure coming this summer

Hillphoenix is a leading provider of transcritical CO2 systems, with over 1,500 installed in North America (18,000+ units globally). It has already provided some solutions to the charge-release challenge and this summer will release another system to help address charge preservation called ChargeSecure. Powered by an uninterruptible power supply (UPS) battery backup or emergency power, this device will prevent pressure buildup by maintaining a CO2 circuit from the flash gas receiver to a compressor and the gas cooler.

The benefit for end users is their CO2 system can maintain a its charge during a power outage or service event, including the normal restart of the system and its operating capabilities. The new ChargeSecure is a 100% natural solution because it uses CO2 from the rack, noted Scott Martin, Senior Director Industry Relations and Compliance for the Georgia-based Hillphoenix.

Hillphoenix and its European affiliate Advansor have also pioneered the use of a UPS on the rack to close all valves in the system – including the expansion, high-pressure and flash-gas valves – during a power failure. This will “isolate the refrigerant and not let it migrate and cause relief valves to open prematurely,” said Martin. “It slows the pressure increase.”

Higher pressure ratings debated

Concerns about CO2 charge preservation have led some U.S. foods retailers to ask if the pressure rating of piping in transcritical systems should be raised significantly – up to 90bar (1,305psi) – throughout the system, including in suction piping.

Through science-based and practical approaches, the industry is discussing where in the system pressure rating should be raised. However, more work is believed to be required before a conclusive decision can be made.

There are many factors that could impact the decision to go with high pressure systems. As an example, high-side rack pressure ratings are already at a higher pressure – 120 or 130bar (1,740 or 1,885psi) – but the industry standard for in-store piping such as liquid supply lines is currently 45bar (653psi), with medium-temperature suction piping also at 45bar and low-temperature suction piping at 30bar (435psi).

Martin agrees that liquid-line piping could benefit from a higher-pressure rating – at least 60bar (870psi) and up to 80bar (1,160psi). The liquid line is the most susceptible part of the system to pressure buildup during an outage because the liquid expands to a gas, he noted. By contrast, suction lines already contain mostly gas, which doesn’t increase in pressure nearly as fast.

As a comparison, the majority of European OEMs use 60bar in liquid lines, while 20% employ 80bar. For suction lines, the majority have 60bar medium temperature and 30bar low temperature.

An important factor is the cost-benefit of going to 90bar. Martin pointed out that raising the pressure rating for all in-store piping to 90bar would require the use of copper-alloy piping, upgraded evaporator coils and medium-temperature compressors for low-temperature applications. All of those would add to costs. Other strategies for minimizing charge loss, such as placing the flash-gas receiver in a walk-in cooler, would also increase the price of a transcritical CO2 installation.

“There are many design options that have various application costs; therefore, we should balance effort versus reward,” Martin said.

To further address this issue and provide more information on ChargeSecure, Martin will be participating in a case study presentation on approaches to mitigating CO2 charge loss during power or service events at the ATMOsphere America conference, to be held June 10–11 in Washington D.C. Martin’s presentation will begin at 3:15 p.m. on June 10.

“There are many design options that have various application costs; therefore, we should balance effort versus reward.”

– Scott Martin, Senior Director Industry Relations and Compliance for Hillphoenix.

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