Improved transcritical CO2 refrigeration systems for warm climates.

Starting in the first 2000’s CO2 refrigeration systems were initially designed for the North Europe climate, where ambient temperature is rarely above 25°C and a CO2 refrigeration system works in practice for most of the time in subcritical conditions. Low ambient temperature allowed the use of simple circuit layout, and such refrigeration systems were already enough efficient if compared to state of the art solutions with HFCs. When the CO2 “transcritical” concept was moved to southern European countries it proved to be unsatisfactory for several reasons. First of all the efficiency was lower than conventional systems with HFC (and of course HCFC) for a significant part of the year. Secondly, some issues emerged, like high discharge temperature and some concerns regarding reliability. Due to the above it became common to speak of a “CO2 equator”, an imaginary line representing the latitude below which the CO2 systems were inefficient and not convenient. Indicatively the line showed the latitude of Paris: the zone in which such systems are efficient, in practice Northern Europe and North America, is too limited for a general use of CO2 - only refrigeration systems. Since 2008 a lot of work has been done and new solutions were introduced: ejectors for energy recovery and flash vapour recompression have demonstrated to be a viable option to overcome the main issue associated with the use of CO2 in warm ambient conditions, thus allowing the diffusion of CO2 also in Southern Europe.
The paper summarises the proposed and available options for increasing efficiency of direct expansion CO2 systems, focusing not only on efficiency but also on some technological aspects. The paper presents as well some results from the field and faces the issue of moving towards a simplified design. Simplification and standardisation of the high-efficiency CO2 solution is considered essential to successfully spread the proposal to countries where synthetic refrigerants are nowadays the first choice, also for economic reasons.