Gustav Lorentzen 2010: CO₂ supermarkets - Part 1

With CO₂ moving to areas of higher ambient temperatures and pressures (transcritical), 31 CO₂ systems have been employed in Australia in Woolworths alone in the last 5 years - since 2005, when the first CO₂ supermarket was built in the country. On the other hand, all new systems today in Denmark are CO₂.

EVOLUTION OF CO₂ SYSTEMS DESIGN BASED ON PRACTICAL EXPERIENCES FROM SUPERMARKET INSTALLATIONS IN NORTHERN EUROPE, H. O. MATTHIESEN, Danfoss

Though the primary focus of the presentation by Matthiesen was on the Nordic experience with CO₂, where a total of 500 transcritical CO₂ systems are installed, in the opening of his presentation he provided figures showing the global potential distribution of CO₂ supermarket refrigeration based on energy savings realised by transcritical CO₂ systems compared to a single stage R404A system.

He then moved on to discuss winning configuration including:

• Transcritical Booster: this is currently the winning configuration in cooler climates, Matthiesen maintained, although there are some discount markets that choose to run only the high temperature cases on a single stage transcritical cycle, while the low temperature cases are stand alone hydrocarbon integrals. The costs are coming down as the number of systems built is increasing.

• Cascade with Volatile Brine and DX Low Temp: This is an efficient solution for all climates and geographic locations, and it is expected to gain ground in temperate regions where frequent transcritical operation will render transcritical CO₂ systems inefficient or where secondary coolants are already being used.

Before concluding, the presenter referred to how economies of scales are beginning to affect component prices as well as to future trends, including: cascade systems with ammonia or hydrocarbon chillers and smaller systems to cover gas stations and the mini market segment. Transcritical CO₂ may be expected to become relevant in applications such as larger systems where ammonia may have previously been chosen, as well as large AC systems.

CO₂ SUPERMARKET REFRIGERATION SYSTEM FOR HOT CLIMATES, S. GIROTTO, Enex S.r.l.

Girotto of enEX presented a six month data set regarding the operation of a CO₂ supermarket in central Europe for which a parallel compression design with independent compressors was chosen in order to avoid the drop in capacity and efficiency during summertime. The system employs semi-hermetic compressors, a heat recovery heat exchanger before the gas cooler, while a water spray system is activated above 30°C. The data presented demonstrated high energy efficiency for the system, however, the presenter noted that this has been running mainly in subcritical mode also during the summertime.

Girotto, hence then moved on to discuss identified improvements that can enable the effective transfer of such technology to hot climates. He proposes optimising the intermediate pressure by acting on the parallel compressor capacity. Before concluding he stressed that design temperature needs to be carefully considered, since it influences the compressor size and hence the capacity range where the volume ratio can be optimized at each ambient temperature.

CO₂-ONLY REFRIGERATION SYSTEMS WITH TWO-STAGE GAS COOLERS FOR THE AUSTRALIAN CLIMATE, M.BELLSTEDT, eCO2 Technologies Pty Ltd

Bellstedt presented results from 2 modelling exercises regarding the efficiency of CO₂ systems for Australian climate conditions. The first exercise showed that CO₂ systems outperform conventional R404A plant at all locations during cool weather (subcritical operation), and in combination with two-stage gas coolers also during warm weather. R404A systems with large pre-cooled condensers can provide lower power consumption under typical summer conditions in dry regions, but at a significant water consumption penalty.

A second modeling exercise showed that all CO₂ systems modeled are more efficient than well-designed air-cooled R404A systems in cool (<+15°C) and hot (>+25°C) ambient conditions, with similar efficiencies achieved at moderate (+15°C to +25°C) conditions. The resultant total annual power consumption profile for various locations gives mixed results, but generally indicates that the dry-boosted CO₂ only system is a good choice for all Australian climatic conditions, Bellstedt concluded.