CO2 Refrigeration Applications

With the proposal and gradual implementation of China's "dual carbon" goals, "carbon dioxide refrigeration" has become one of the focuses of attention in the industry.

This article mainly introduces its application status, advantages and disadvantages.

Application status of CO2:
First, in the field of automotive air conditioning. Due to the large amount of refrigerant emissions and great harm to the environment, environmentally friendly refrigerants must be adopted as soon as possible.
A typical CO2 automotive air conditioning system is the same as the R134A automotive air conditioning system.

The main components include a compressor, heat exchanger, throttling mechanism, and gas-liquid separator. In addition, in order to further cool the high-temperature gas at the outlet of the gas cooler with the low-temperature gas at the outlet of the evaporator, an intermediate heat exchanger (regenerator) is generally provided in the CO2 automotive air conditioning system.

Second, heat pump water heaters. Under supercritical conditions of CO2, there is a considerable temperature slip during heat release, which is conducive to heating hot water to a higher temperature of 65-95°C.
The application of CO2 in heat pump water heaters all uses transcritical cycles. Due to its high pressure difference, the irreversible loss in the expansion process is the main loss of the CO2 heat pump system.

Therefore, reducing irreversible losses is a research hotspot of the CO2 heat pump system. The improvement of cycle performance can be achieved by a combination of one or several of the following methods: multistage compression, adding a regenerator, and recovering expansion work.

Third, commercial refrigeration systems. CO2 has good low-temperature flow and heat transfer characteristics, and is used for fresh storage, refrigeration, and low-temperature freezing of supermarket foods. In addition, CO2 is also used as a coolant or secondary refrigerant in refrigeration systems with strict safety requirements such as food display cabinets. CO2 is also widely used as a low-temperature refrigerant in commercial cascade refrigeration systems. Supermarket refrigeration, especially the refrigeration application of large and medium-sized supermarkets, is the most important application field of CO2 in commercial refrigeration.
There are mainly three basic ways for CO2 to be applied in large and medium-sized supermarket refrigeration systems:
(1) Used as a secondary refrigerant for the secondary circuit of the main refrigeration cycle, or a secondary circuit with a phase change.
(2) Used as the main refrigerant and applied in a subcritical system, adopting a subcritical cycle. Generally, it operates as a cascade system together with other independent medium-temperature circuits.
(3) Used as the main refrigerant and applied in a transcritical system, adopting a transcritical cycle.

CO2 process as a secondary refrigerant:
The liquid CO2 flowing out of the liquid storage tank is pressurized by a liquid pump and then flows through the heat load (display cabinet or cold air coil in the warehouse). After absorbing heat and vaporizing, it forms a gas-liquid two-phase fluid with a certain dryness. It returns to the evaporator of the main refrigeration system and is recondensed into a liquid and then returns to the CO2 liquid storage tank for the next cycle.

Although the superiority and reliability of carbon dioxide refrigeration technology are good, at present, there is still a coexistence of multiple refrigeration methods, and the status quo of traditional refrigeration methods being dominant.

Compared with Freon and ammonia refrigeration, the advantages of carbon dioxide refrigeration:
First, it is green, safe and reliable. The ozone depletion potential (ODP) of carbon dioxide is 0, and the global warming potential (GWP) is 1, which is much lower than that of Freon refrigerants. At the same time, carbon dioxide is non-toxic, non-flammable, and has stable chemical properties. During the operation of equipment, it can ensure the safety of personnel and food. In the future, there will be no problem of refrigerant recovery and treatment.
Second, it has good heat transfer performance, compact structure, and small equipment footprint.
Third, carbon dioxide is a by-product of industrial product production and is easy to obtain. Its price is only one-tenth of that of conventional Freon.

Compared with current alternative refrigerants, its price is very low.

CO2 as a refrigerant has two relatively obvious disadvantages:
The operating pressure of the refrigeration system is high and the refrigeration efficiency is relatively low. The resulting technical difficulties and cost increases have become the main reasons restricting the development of CO2 technology.
The working pressure of the CO2 transcritical refrigeration cycle is about 6 to 8 times that of the traditional refrigerant CFC or HCFC system pressure. Therefore, in the process of designing, manufacturing, and installing compressors, heat exchange equipment, auxiliary equipment, valves, pipelines, and pipe fittings, it is necessary to ensure their pressure resistance. For efficiency, further strengthening is still needed in the design and development of key components such as compressors, gas coolers, and evaporators. At present, the method of using expanders to recover throttling loss energy to improve the refrigeration cycle efficiency is relatively common in transcritical cycles. In the research of other accessories, economizers, ejectors, etc. are introduced to improve system performance.