Integration of a three-stage expander into a CO2 refrigeration systemIntégration d'une turbine à détente triétagée dans un système frigorifique au CO2
Introduction
Environmentally friendly refrigerants like CO2 are only of value, if the associated refrigerator has at least the same COP as systems with other refrigerants. Research and development at many places during the last 15 years have identified applications, where CO2 is as good or even superior as other refrigerants, e.g. as secondary refrigerant or as refrigerant in the lower stage of a cascade. But in higher temperature applications, where with CO2 the critical pressure is exceeded, CO2 is normally not competitive concerning COP. The reasons for this are the rather high end temperature of the compression and the large thermodynamic losses associated with the throttling.
So far there are two exceptions, where the transcritical CO2 cycle is competitive concerning COP: the warm-water heat pump and the car air conditioning system. In the warm water heat pump one takes advantage of the high end temperature of compression and the gliding ‘condensing’ temperature. The COP of passenger car air conditioning systems is strongly influenced by high heat exchanger losses due to the limited space available for condenser and evaporator, and by the pressure drop in the interconnecting pipe work. Here one can take advantage of the good heat transfer properties of CO2 and the compact design of the CO2 channels, which leave more space for the air side heat transfer surface.
But in the majority of applications like commercial and industrial refrigeration as well as in air conditioning systems, CO2 is not a good refrigerant, if one restricts oneself to the standard vapour compression cycle. The reasons for the low COP, as mentioned above, are the high end temperature of compression, which can only be ‘cured’ by two-stage compression with intercooling, and the large losses associated with the throttling. The latter can be ‘cured’ by a two-stage throttle expansion—which has to fit to the two-stage compression—or—and this is the topic of this paper—by a work extracting expander, which replaces the throttle valve.
Section snippets
Work extracting expander
Fig. 1 shows a flow sheet, where an expander drives directly the second stage of compression, thus ‘curing’ both disadvantages of the CO2 supercritical cycle: due to the enthalpy reduction in the expander, the specific rate of refrigeration is increased and the work of compression is reduced, because the power for the second stage is ‘free of charge’. This arrangement has another important advantage: it reduces the exhaust pressure of the main compressor for example, from 100 bar to less than 80
Testing of the expander–compressor
The expander–compressor was installed in a refrigeration plant and after steady state was reached, all pressures were recorded in function of the piston rod stroke (Fig. 3). In the two figures on the left and the right of Fig. 3 those cylinders are combined, which are passed by the same 50% of the total flow.
In the diagrams the curves start with the movement of the piston rod (Fig. 2) to the right. Besides the pressures inside the cylinders also the external pressures are recorded.
The meaning
Integration of the expander into a refrigeration system with several evaporators
How should such an expander be integrated into a refrigeration system with several evaporators? The expander replaces the throttle valve. Throttle valves are normally located decentralized close to the evaporators. On the other hand, an expander, which drives the second stage of compression, has to be placed close to the main compressor or close to the condenser.
Initially we saw the two options shown in Fig. 5. Either one could expand only to a pressure, which is quite higher than the
Conclusions
For a three-stage expander, which drives an auxiliary compressor in a transcritical refrigeration cycle, a method has been identified, how it can be integrated into a system with several evaporators. The new feature is a liquid–vapour separator vessel between the second and third stage of expansion.
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2022, Applied Thermal EngineeringCitation Excerpt :Galoppi et al. [18] tested the radial piston expander for a heat pump with the R134a as the working fluid. Nickl el al. [19] studied a three-stage expander for replacing the throttle valve in a CO2 refrigeration system. Liquid turbines can be applied in various industrial systems and classified into several types [20].
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Member of IIR Comm. B1.