History of CO2 as a refrigerant
As a refrigerant, CO2 (R744), ammonia (R717), SO2 (R764), and methyl chloride (R40) were widely used before the end of the 19th century to the 1930s. In addition to CO2, the above refrigerants are all toxic or flammable. CO2 is non-toxic and non-combustible. It was originally proposed by Alexander Twining of the United States in 1850 and was granted a British patent.
The use of CO2 as a refrigerant has considerable advantages, which are summarized as follows:
a) Excellent environmental performance. Its ozone depletion potential (ODP) is zero and its greenhouse effect potential is minimal (GWP=1)The HGP and its mixture as recommended alternative working media have zero ODP, but GWP is 1000 to 2000 times higher than CO2.
b) Low self-cost. No need to recycle or regenerate, operating and running costs are lower.
c) Good chemical stability. CO2 is weakly acidic when mixed with water and can corrode common metals such as carbon steel but does not corrode stainless steel and copper metals.
d) It helps to reduce the device volume. The high working pressure makes the suction specific volume of the compressor smaller, so that the volumetric cooling capacity is larger and the compression size is reduced.
e) safe and non-toxic, non-flammable. Even at high temperatures it does not decompose to produce harmful gases. Because CO2 is the highest oxidation state of carbon, it has very stable chemical properties.
Of course, using CO2 as a refrigerant also has its disadvantages. For example, it cannot sustain life. When its concentration in the air is less than 2%, there is no obvious harm to humans. However, if it exceeds this concentration, it can cause respiratory damage and even suffocation. In addition, the high critical pressure of CO2 and the low critical temperature also bring many problems. Whether the subcritical cycle or the transcritical cycle, the operating pressure of CO2 refrigeration system will be higher than the traditional refrigeration and air conditioning system, which will inevitably bring many new requirements to the design of the system and components. At the same time, there is still a relatively low efficiency of the CO2 refrigeration system. In the CO2 automotive air conditioners currently under study, transcritical refrigeration cycles are basically used. This avoids system performance degradation due to excessive heat source temperature under subcritical cycle conditions. Moreover, due to the special thermophysical properties of the fluid under supercritical conditions, it has unparalleled advantages in terms of reflow and heat transfer. The complete supercritical cycle is only used when atomic power is generated, and this cycle method is not used in refrigeration and air conditioning applications.