Evaluation of Efficiency Increase Methods With CO2 as a Refrigerant
Efficiency increase of CO2 refrigeration systems
Carbon dioxide has been used as a refrigerant since circa 1920, when the only refrigerants were some natural fluids.
Since the beginning of the use of CO2, many studies have been done to improve cycle efficiency under conditions of high temperature heat transfer. The low critical temperature penalizes this refrigerant in high temperature ambient conditions. This argument is interesting now due to high summer temperatures in Europe.
The modified cycles analyzed were, briefly, of this type:
a) 2-stage compression, for evaporation temperatures below -20°C
b) Voorhees
c) Plank (post-compression and cooling)
d) Auxiliary compression (flash gas recompression)
e) Energy recovery from expansion with turbine or expander
f) Post-gas cooler subcooling
g) Evaporative cooling of the gas cooler
Recently, ejectors for semi-flooded feeding of evaporators and flash gas pre-compression have been introduced.
I participated directly in the years 2011-2014 in the conception and development of concepts using ejectors, but also, previously, in the development of the 2-stage compressor, a), and in the recompression of the flash, d).
Some solutions were proposed for expander e), but at the date (January 2026) no reliable solutions are available yet. The main problems are the following:
- the number of hours during one year when the energy recovery is significant is limited
- the expander must work with a 2-phase fluid, with obvious lubrication problems
Moreover there is already a simple (mechanically) expander device wthout moving parts - the ejector - with isentropic efficiency in the range of 0,5-0,6 circa in expansion process, so this is the reference to consider.
Separate considerations must be made for the Voorhees method, as I will say later. Apart from a) and d) the other solutions, specifically c), e) have not been proven technically yet.
The subcooling, f), was the first thing that came to mind but the problem is the cost.
In fact, some devices proposed to improve performance have rather high payback times.
What else remains? At present, there is nothing else realistic and immediate.
I will come back with another blog on the Voorhees method.
To increase efficiency in the immediate future, all what remains is to work on apparently secondary aspects but whose benefits are obtained at low cost and, above all, produce effects all year round:
- more precise capacity control
- to avoid or limit losses that are normally neglected, such as leakage and pressure drops
- to limit electrical losses (consumption of electrical panel and control equipment)
- to limit the consumption of “auxiliary” components, such as oil heaters, fans and more