How Subcritical CO₂ Heat Pumps Can Transform Cold-Climate Electrification

There’s a well-known expression that says in a car “the windshield is bigger than the rearview mirror, because what’s in front of you is more important than what’s behind you.” However, sometimes looking backwards can help an industry to move forward.

More and more building owners are looking to remove gas-fired equipment and electrify their building’s heating systems. Not only does this transition reduce greenhouse gas (GHG) produced on-site, but when it can be accomplished with heat pumps, it can often lower operating costs. Up until now, there have been limited options for heating with heat pumps in cold-weather climates. Geothermal heat pumps and their well fields are costly and come with operational challenges, while the refrigerants in most air-source heat pumps (ASHPs) restrict their use as a primary heat source to milder climates. That leads to the question: Is there an ASHP approach that could provide year-round heating in cold climates?

Choices, choices…

The first step in finding an ASHP that can provide heat in low ambient temperatures is identifying the best refrigerant, and any analysis of HVAC refrigerants must assess key factors such as  hazards, ozone depletion potential, and  long-term refrigerant availability The ever-expanding list of refrigerants includes many next-generation refrigerants that are becoming more widely used, as well as hundreds of previously used refrigerants going back as far as the 1700s.

When it comes to finding modern refrigerants that are safe, efficient, and environmentally friendly, some innovators are looking into the past and bringing carbon dioxide (CO₂) into the 21^st century, positioning it as a refrigerant for the future. CO₂ is non-hazardous, non-flammable, has zero ozone depletion potential, and is a naturally occurring and relatively inexpensive refrigerant. Previous advancements in refrigeration equipment have made CO₂ one of the leading refrigerants in commercial refrigeration, but there is more to be excited about: Subcritical CO₂ heat pumps can be designed and implemented, creating substantial energy savings and environmental benefits while providing a source of heat, even throughout the cold winter months of the Northern US and Canada. This advancement could have a substantial impact on energy use and GHG production, particularly in climates where typical air-source heat pumps (ASHPs) are unable to provide heat during the coldest months.

Looking Backward – Carbon Dioxide as a Refrigerant

The use of carbon dioxide (CO₂) as a refrigerant dates to the 19th century when it was first introduced as an alternative to toxic and flammable refrigerants such as ammonia and sulfur dioxide.

The thermophysical properties of CO₂ make it well suited for use as a refrigerant as it has a high enthalpy of vaporization and no temperature glide. Among refrigerants with similar characteristics, CO₂ stands out for its low global warming potential (GWP) and an ozone depletion potential (ODP) of zero.

By the early 20th century, CO₂ had become widely used in marine refrigeration systems and industrial applications thanks to its non-toxic and non-flammable properties. However, CO₂ systems typically operate at very high pressures, requiring robust components, which made it less desirable compared to emerging mid-20th century synthetic refrigerants like chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs).

As environmental concerns over ozone depletion and global warming potential (GWP) of synthetic refrigerants grew in the 1990s and 2000s, CO₂ experienced a resurgence as an eco-friendly alternative. In the 1990s, innovations led to the development of transcritical CO₂ refrigeration systems, overcoming previous technical challenges. Today, CO₂ (designated as R-744) is widely used in supermarket refrigeration, large-scale commercial refrigeration, and automotive air conditioning, benefiting from its low GWP and excellent heat transfer properties. With advances in system design and efficiency, CO₂ refrigeration continues to gain traction as a sustainable solution for reducing GHG emissions in cooling applications.

In heating applications, CO₂ is becoming widely available in domestic water heaters. Less common is the use of CO₂ refrigerant in HVAC heating applications. While there are manufacturers that offer these products, they operate at transcritical pressures, limiting the heat recovery potential, making the equipment more expensive, more prone to failure, and more costly to maintain when compared to traditional refrigerants.

Subcritical CO₂ heat pumps operate at lower temperatures and pressures and therefore are not burdened with all of the same costs and complexities as transcritical systems. However, innovative and deliberate system design is needed to overcome technical challenges in order to integrate subcritical CO₂ heat pumps into commercial heating systems effectively. Overcoming these challenges can provide benefits akin to those seen in refrigeration applications: more efficient performance with equipment that has a much lower GWP.

Let’s look at some of the challenges and potential benefits of using subcritical CO₂ as a refrigerant in an ASHP.

Transcritical vs Subcritical

Transcritical CO₂ systems can supply outlet water temperatures above 180 °F, which lends to their application for domestic water heating and industrial process heating. In sections of the transcritical circuit, operating pressures are above the critical point pressure (1069 psia for CO₂). These high operational pressures also contribute to an increased cost of maintenance and higher likelihood of refrigerant leaks. Widespread adoption of smaller-capacity transcritical CO₂ heating equipment, such as domestic water heaters, appears more likely than large high-capacity equipment for which the additional costs of the increased pressures are not outweighed by the benefits of transcritical operation.

Conversely, subcritical CO₂ systems are effective in applications where temperatures are lower, staying below the critical point of CO₂. Because the pressures and temperatures are lower, subcritical systems have lower initial costs (approximately half), consume less energy, and have reduced maintenance costs compared to transcritical systems.

Additionally, because of the thermal properties of CO₂, a subcritical ASHP can effectively extract heat at low temperatures down to -22 °F (-30 °C). This makes it an excellent choice for integration into heating systems across the northern United States and Canada.

Still another benefit of the subcritical refrigerant cycle is that, unlike in a transcritical cycle, the condensing portion of the cycle occurs as a phase change at a constant pressure and temperature. This makes for a more efficient process which translates into energy savings and a higher COP.

For more on the differences between subcritical and transcritical CO₂, see the article “Subcritical and Transcritical Refrigeration Cycles.”

Designing for Low Temperature

Unlike conventional transcritical heat pumps, a subcritical CO₂ heat pump can be designed to operate effectively on heating loops with a low-temperature differential. The limitation of this is the outlet temperature of the heat pump. A subcritical cycle will not exceed the temperature of the critical point. For CO₂, this temperature is 87.8 °F (31.1 °C). In real-world applications, a maximum ASHP outlet temperature of 75 °F (24 °C) is reasonable. While the low-ambient capability broadens the range of potential applications, the low outlet temperature limits the ability of a subcritical CO₂ heat pump to fully replace the primary heating source in most existing systems where design water temperatures may be between 140 and 190 °F. Innovative solutions are required to incorporate the benefits of subcritical CO₂ heat pumps, taking advantage of the equipment’s energy efficiency and low-ambient capabilities to provide heat.

The most likely applications will be in a cascade arrangement with a water-to-water heat pump. Taking that idea to the next step, there are some exciting advantages when we consider the ASHP in place of a geothermal well field. Geothermal wells are employed for heating because of their ability to supply a relatively constant low-grade heat, even in the winter. Traditionally, geothermal works as a heat source in the winter in locations where ASHPs do not. However, a subcritical CO₂ ASHP can provide the same function as geothermal wells: supplying at least 60 °F year-round even in northern climates.

The capex savings between these two scenarios is substantial — likely on the order of millions of dollars, depending on the system capacity. Geothermal well fields also come with a risk associated with the uncertainty of drilling into the earth. Unexpected soil and bedrock conditions can result in underperforming wells. Additionally, geothermal wells typically require thermal recharging during the summer, which can become difficult if the heating and cooling loads are not adequately balanced throughout the year.

In contrast, year-round ambient air temperatures are known, and ASHPs can be reliably selected to provide the required amount of heat with less risk than a geothermal system. In these arrangements, the addition of the subcritical CO₂ heat pump takes advantage of its excellent low-outside air temperature performance, accommodating the limited outlet water temperature of the subcritical CO₂ heat pump while maintaining the overall heating water temperature required. Overall heating system COPs, which will vary with load and outside air temperature, should be in the range of 2.0 to 5.0.

Exciting Possibilities

There is more potential to explore  when considering the applications of subcritical CO₂ heat pumps, especially in colder regions where traditional air-source heat pumps struggle to operate during the coldest conditions. As more buildings look to electrify and reduce greenhouse gas (GHG) emissions, subcritical CO₂ heat pumps can be an important part of the solution.

Ecosystem is always looking for innovative ways to improve efficiency and help meet clients’ goals. To that end, a partnership has been established with a heat pump manufacturer and, with support from the government of Quebec, work has begun to develop a large-capacity subcritical CO₂-based ASHP specifically designed with cold climate operation in mind. (See article Large-capacity Subcritical CO₂-based ASHP Product Development for more details.)

Equipment Challenges

To design and develop a high-capacity air-to-water heat pump using CO₂ in a subcritical cycle, Ecosystem has partnered with RefPlus through their affiliate Ceptek, a Canadian manufacturer of custom refrigeration equipment. Their extensive experience with CO₂ as a refrigerant in many applications as well as their design and manufacturing capabilities makes them a perfect development partner for this technology.

Since this new ASHP is intended to provide heat in subfreezing temperatures, its design and development will have to address the challenges inherent with this novel application:

Split System

Taking advantage of the low-temperature heating ability of subcritical CO₂ ideally means that the equipment will operate in subfreezing temperatures. Any design must consider the freeze potential of the hydronic system. Most ASHPs currently available require an intermediate heat transfer fluid (such as glycol) to circulate between the heat pump and the heating system, if located in northern climates.

Designing the equipment as a split system, where the hydronic piping remains inside and the refrigerant is piped between the indoor and outdoor equipment, eliminates the need for glycol or other freeze protection measures for any outdoor hydronic piping. This simplifies installation, reduces energy losses, and enables indoor placement of the heat pump, protecting it from harsh environmental conditions and improving its durability.

Defrost Cycle and Condensate Management

In an ASHP that is providing heat, the air-side evaporator coil will often be at a temperature below the dew point of the ambient air, leading to condensation. Because this heat pump will provide heat at sub-freezing temperatures, condensation is likely to occur when temperatures are below freezing. Left unaddressed, condensate will turn to ice on the evaporator coil. To defrost the coil, preventing damage and maintaining performance, a defrost cycle must be integrated into the design and operation of the heat pump.

A defrost cycle that utilizes the discharge gases from the compressors can defrost the evaporator without relying on electric heating elements. This approach may also reduce the installed cost of the heat pump, potentially avoiding an increase in the required capacity of a building’s electrical system. For end-users looking to avoid electrical peak charges, it also eliminates the potential for electrical power surges caused by electric heaters.

Once removed from the evaporator coil, condensate must remain liquid until it can be drained. This will likely rely on basin heaters and piping heat trace, either electric or hydronic.

Horizontal Evaporator Coil

The traditional V-shaped coils of ASHPs can become problematic in winter operation where winds may build up snow drifts that reach the coils and overwhelm the normal defrost cycle. With a horizontal evaporator coil, snow is less likely to pose an operational problem. This coil orientation is also less subject to the variability in heat transfer that comes with wind gusts in the winter. Baffled enclosures that are commonly used to address wind and weather conditions are less likely to be needed with a horizontal evaporator coil.

Expanding Future Possibilities

There are exciting opportunities for electrification of heating when we look at novel applications of CO₂. While there are unique challenges in both the development of the equipment and the integration into a building’s heating system, the benefits can be significant, both financially and environmentally. CO₂, a refrigerant with a long history, has a bright new future in building electrification and heating.