Transcritical CO₂ applications

In transcritical processes, the heat rejection on the high pressure side proceeds isobar but not isotherm. Contrary to the condensation process during subcritical operation, gas cooling (desuperheating) occurs, with corresponding temperature glide. Therefore, the heat exchanger is described as gas cooler. As long as operation remains above the critical pressure (74 bar), only high-density vapour will be transported. Condensation only takes place after expansion to a lower pressure level – e.g. by interstage expansion in an intermediate pressure receiver. Depending on the temperature curve of the heat sink, a system designed for transcritical operation can also be operated subcritically ‒ with higher efficiency. In this case, the gas cooler becomes the condenser.

Another feature of transcritical operation is the necessary control of the high pressure to a defined level. This "optimum pressure" is determined as a function of gas cooler outlet temperature by means of balancing between the highest possible enthalpy difference and minimum compression work. It must be adapted to the relevant operating conditions using an intelligent modulating controller (Example of a transcritical CO2 booster system).

As described before, under purely thermodynamic aspects, the transcritical operating mode appears to be unfavourable in terms of energy efficiency. In fact, this is true for systems with a fairly high temperature level of the heat sink on the high pressure side. However, additional measures can improve efficiency, such as the use of parallel compression (economiser system) and/or ejectors or expanders for recovering the throttling losses during expansion of the refrigerant.

Apart from that, there are application areas in which a transcritical process is advantageous in energy demand. These include heat pumps for heating of sanitary water or drying processes. With the usually very high temperature gradients between the discharge temperature at the gas cooler intake and the heat sink intake temperature, a very low gas temperature outlet is achievable. This is facilitated by the temperature glide curve and the relatively high mean temperature difference between CO₂ vapour and secondary fluid. The low gas outlet temperature leads to a particularly high enthalpy difference, and therefore to a high system COP.

Low-capacity sanitary water heat pumps are already manufactured and used in large quantities. Plants for medium to higher capacities (e.g. hotels, swimming pools, drying systems) must be planned and realised individually. Their number is therefore still limited, but with a good upward trend. Apart from these specific applications, there is also a range of developments for the classical areas of refrigeration and air-conditioning, e.g. supermarket refrigeration. Installations with parallel compounded compressors are in operation to a larger scale. They are predominantly booster systems where medium and low temperature circuits are connected (without heat exchanger). The operating experience and the calculated energy costs show promising results. However, the investment costs are still higher than for conventional plants with HFCs and direct expansion.

On the one hand, the favourable energy costs are due to the high degree of optimized components and the system control, as well as the previously described advantages regarding heat transfer and pressure drop. On the other hand, these installations are preferably used in climate zones permitting very high running times in subcritical operation due to the annual ambient temperature profile.

For increasing the efficiency of CO₂ supermarket systems and for using them in warmer climate zones, the technologies described above using parallel compression and/or ejectors are increasingly used.

Therefore, but also because of very demanding technology and requirements for qualification of planners and service personnel, CO₂ technology cannot be regarded as a general replacement for plants using HFC refrigerants.