CO2 (R744)

GWP	1
ODP	0
Flammability	A1 (non-flammable)
Max outlet temperature	~90 °C
Cycle type	Transcritical
Operating pressure	Very high (100–130 bar)

Carbon dioxide (CO2 or R744) is considered a good refrigerant for heat pump water heaters for several reasons:

1. Environmental Friendliness: CO2 has a low Global Warming Potential (GWP) of 1, making it an environmentally friendly option. It does not contribute significantly to climate change compared to some traditional refrigerants with higher GWPs.
2. Natural and Abundant: CO2 is a natural substance that is abundant in the atmosphere. It is non-toxic and non-flammable, contributing to its safety and ease of handling.
3. Stable and Inexpensive: CO2 is stable and does not decompose easily, contributing to system reliability.
4. Energy Efficiency: CO2-based heat pump water heaters can achieve high energy efficiency, helping to reduce energy consumption and associated costs.
5. Growing Industry Adoption: The use of CO2 in heat pump water heaters is gaining popularity globally, with increasing research and development efforts to optimize its performance and expand its application in the heating and cooling sector.

Under what conditions are CO2 HPWHs most efficient?

Carbon dioxide (CO2) is most efficient when the following ng operating conditions align with its thermodynamic properties:

1. Transcritical Operation: CO2 operates efficiently in transcritical cycles (see below), where the refrigerant undergoes both a liquid-to-gas phase change and a gas-to-gas phase change. However, high pressures are required.
2. Low inlet water temperature: CO2 HPWH can output the desired outlet temperature in a single pass even with low inlet water temperatures, by using a variable speed pump to have a variable flowrate.
3. High Temperature Output: CO2 heat pumps are particularly efficient when used for medium to high-temperature applications (60-90degC), such as domestic hot water heating.
4. Cold climate: CO2 can perform very well in cold climates with low ambient temperatures.

Downsides to CO2

CO2 is not perfect. With its low critical temperature (31.1 ℃) and high critical pressure (7.37 MPa), the requirements for compressors and pipelines are particularly high, which increases the costs of product manufacturing. Nevertheless, the world’s top compressor manufacturers are studying improvement methods, such as the use of trans-critical circulation systems.

1. High Ambient Temperature: In regions with consistently high ambient temperatures, CO2 HPWHs operating in transcritical cycles may experience reduced efficiency. The system’s performance is impacted as it must reject heat to the environment at elevated temperatures.
2. Complex System Design: CO2 HPWHs often require a more complex system design compared to traditional refrigerants. Components must withstand much higher pressure (100-130 bar).
3. Higher Initial Costs: CO2 HPWHs may have higher upfront costs compared to conventional water heaters. The specialized components and technology required for CO2 systems can contribute to the initial investment.
4. Limited Industry Experience: limited manufacturing capability in China. Most manufacturers are in Japan (Chofu, Rinnai).
5. Performance in Low Ambient Temperatures: CO2 HPWHs may face challenges in extremely cold climates where low ambient temperatures can impact their efficiency.

Understanding CO2 Transcritical Nature

The terms subcritical, transcritical, and supercritical refer to different states of a substance, such as a refrigerant, in a thermodynamic cycle. These states are commonly used to describe the behavior of substances, including gases like carbon dioxide (CO2) used in refrigeration systems.

1. Subcritical State:
   • Description: In the subcritical state, a substance exists entirely as a liquid or a vapor. It doesn’t undergo a phase transition (from liquid to vapor or vice versa) under the given temperature and pressure conditions.
   • Example: When water is below its critical temperature (100 degrees Celsius at atmospheric pressure), it is in a subcritical state. The water remains in either liquid or vapor form.
2. Transcritical State:
   • Description: In the transcritical state, a substance operates at pressures and temperatures above its critical point, where it experiences both liquid-to-vapor and vapor-to-vapor phase changes. This occurs without a distinct separation between the liquid and vapor phases.
   • Example: CO2 is often used in transcritical cycles. In a transcritical CO2 refrigeration cycle, the refrigerant goes through a phase change without clear distinctions between liquid and vapor, especially above its critical point (31.1 degrees Celsius and 73.8 bar).
3. Supercritical State:
   • Description: In the supercritical state, a substance is above its critical temperature but still below its critical pressure. It exhibits properties of both a gas and a liquid, with density and compressibility characteristics that fall between those of a gas and a liquid.
   • Example: CO2 can exist in a supercritical state when the temperature and pressure exceed its critical point. In this state, it possesses gas-like and liquid-like properties simultaneously.

Summary

Overall, CO2 stands out as a high-performing refrigerant for heat pump water heaters.