The refrigeration efficiency of carbon dioxide air conditioners is generally lower than that of common refrigerant systems under the same working conditions, and it is much lower. Whether heating can really be more efficient is doubtful. I have seen this statement in many places, but I don’t think a consensus has been reached, and I haven’t seen a really convincing comparison. I don’t see anyone using systems and components that are as close as possible to compare CO2 and commonly used For refrigerants, if you simply compare the efficiency results of different research groups without caring whether the specific system and component selection are really comparable, then the comparison results are not very meaningful.
Heating is closer to the efficiency of common refrigerants than cooling, and the low-temperature conditions perform better than common refrigerants, or can provide higher temperatures than common refrigerants. I think these statements are relatively more reliable.
Advantages of carbon dioxide as air conditioner/heat pump working fluid:
1. With high pressure and high density, the carbon dioxide system can be more compact and lighter (suitable for vehicles) with the same cooling and heating capacity requirements.
2. Low viscosity coefficient and small flow loss.
3. Good heat transfer performance.
4. Under the same working conditions, the compression ratio of the compressor is lower, and the efficiency of the compressor is higher; this may show advantages in the low temperature working condition of the heat pump.
5. The high temperature at the outlet of the compressor (can be greater than 100 degrees, which is not a good thing in most cases) can be used to do some things that cannot be done by conventional refrigerant cycles. That might include faster defrosting, whether it’s the car’s windows or the heat exchanger. Advantages can also be found in applications where high temperatures are required (water heaters).
6. Under extremely low temperature conditions, the saturation pressure of the low-pressure side of common refrigerants will be lower than atmospheric pressure, so that air may enter the system, but carbon dioxide will not because of the high pressure; this is also a possible advantage in heat pump applications.
7. Using the internal heat exchanger (IHX) and ejector (ejector) to recover the expansion work, the efficiency can be further improved. IHX may not be expensive, but ejector is expensive.
8. The temperature glide of the high-pressure side of the carbon dioxide transcritical cycle is not a good thing in itself, but for applications that require large temperature changes (such as water heaters), if it happens to match, compared with conventional refrigerants, it is inherently impossible The efficiency loss caused by the avoided temperature glide is relatively small, and this assist in specific applications can also help the efficiency of carbon dioxide heat pumps to approach or even exceed that of conventional refrigerants.
Post time: Mar-03-2023