Carbon dioxide R744 (CO2)

CO2 has a long history since the middle of XIX century, but it was completely removed from appliance after 1950 leaving his place to refrigerants which are safe, nontoxic and more efficient. Today, CO2 has successful comeback in refrigeration in different applications. Improperness by this refrigerant is its low critical point (31°C) which means there is not a process of condensation above that temperature. The refrigeration cycle which is taking place above the critical temperature is called a transcritical cycle, and compared with the conventional one it is energetically less efficient because of the relatively less latent heat. Unfavorable side at transcritical cycle with CO2 is also very high pressures (up to 100 bars).

However there are few ways how this unfavorable to be surpassed, and in few cases to take advantage of it. Energy efficiency of transcritical cycle can be increased with installing regenerator, i.e. heat exchanger before the expansion in which subcooling will take place using the cold vapor from the evaporator outlet. Taking in account increased cooling effect from one side and increased power consumption from other side, the result is still positive. Other way for improvement of the cycle is subcooling with previously throttled refrigerant made for that cause, i.e. usage of economizer which can be done at mid pressure with screw, scroll or two stage reciprocating compressor.

Temperature of the vapor at the compressor discharge is relatively high (around 90°C), and heat rejecting to the environment in transcritical area is taking part with big temperature changes. This could be very useful for making (sanitary) hot water, which is well confirmed in practice in Japan where a mass usage of heat pumps with CO2 for hot water started. Here new possibility opens for appliance of CO2 for air conditioning in buildings because together with producing sanitary hot water, the energy efficiency increases.

A good perspective for CO2 application is present in vehicle air conditioning. Lot of prototypes were made, but because of the low energy efficiency they are subject to further improving. The advantage is that the equipment (compressor and heat exchangers) and tubes are pretty smaller. High pressures require special equipment construction which causes increasing of the price. But when everything is taking in account, probably the ecological side will have a deciding role. CO2 made a great breakthrough in usage regarding systems for low temperatures (-30 do -50°C) as a lower part of the cascade system. At the freezing tunnels and industrial systems in the high cascade ammonia is used. At supermarkets and other commercial systems in the high cascade is propane (or some of the HFC refrigerants).

Pretty attractive results are achieved by appliance of CO2 as a secondary coolant where the dimensions of the piping and pumps are several times smaller, and because of that the energy for transport is also lower. However, the piping network works at much higher pressure, which today is not a problem.