Sustainable Refrigeration and Heat Pump Technology Conference - part 3

By team, Jun 21, 2010, 11:36 4 minute reading

This final article reporting on the ‘Sustainable Refrigeration and Heat Pump Technology’ Conference in Stockholm, provides an overview of second-day presentations that focused on commercial refrigeration, including on an ongoing project measuring the performance of existing CO2 supermarket installations and trends in commercial refrigeration.

Current Trends in Commercial Refrigeration, Prof. Dr.-Ing. habil. Michael Kauffeld, Karlsruhe University of Applied Sciences

Global warming is driving development of commercial refrigeration technologies. Indeed, 1/3 of HFC emissions are coming from the commercial refrigeration sector. At the same time, 50-80% of emissions associated with centralised systems are related to the energy used to power them (indirect emissions). Kauffeld hence discussed different ways of implementing measures for the reduction of direct and indirect emissions.
  • Tight refrigeration systems: Kauffeld referred to the approach used in Denmark and Norway, where a high greenhouse gas tax is imposed on all refrigerants, encouraging users to keep refrigerant leaks to a minimum.
  • Reduced refrigerant charge: The refrigerant charge inside the heat exchangers can be reduced by up to 80 % using minichannel heat exchangers well known within the automotive air conditioning industry. Another possibility to reduce refrigerant charge is the application of indirect refrigeration systems, very common in Sweden where refrigerant charge per system has been limited to some 30 or 40 kg for many years now. The use of distributed systems is gaining considerable market share in the USA (15% of all new supermarket refrigeration in 2006). They allow for 5 to 8 % lower energy consumption and about 30 to 50 % lower refrigerant charge than comparable R404A direct expansion (DX) systems.
  • Refrigerants with low GWP: Looking at the Global Warming Potential (GWP) of refrigerants, Kauffeld referred to the current trend of “going back to the future”, meaning going back to the use of natural refrigerants, including R744. Regarding newly developed low-GWP HFCs he pointed out their expected high prices as a main disdvantage. He then moved on to discuss different system configurations, including: high energy efficient R744 cascade systems; or systems with DX HFC for MT plus R744 cascade for LT, a common configuration in Europe (several hundred such systems) that is cost competitive for larger supermarkets (over 2000m2), as it entails reduced refrigerant charge and energy consumption. Central multiplex systems with transcritical CO2 are also gaining popularity in Europe and slowly in the rest of the world. The typical direct expansion system for MT and LT applications uses a two stage compression for the LT side. Pressure inside the store is usually limited to 40 bar but there are now developments underway of components that can sustain 75 bar. Over 300 such stores have been built to date in Europe. Energy efficiency is usually better than for a comparable R404A system during outdoor temperatures below approximately 12 °C, equal to R404A between 12 and 26 °C and slightly lower at higher ambient temperatures, which makes such systems a good choice for many locations north of Lyon.
Kauffeld then listed measures that could be employed during the design and construction phase of a supermarket refrigeration system in order to reduce energy consumption, such as the use of glass doors or lids instead of open cabinets, hot gas defrost instead of electric defrost and flooded evaporators. The selected measures have the potential of saving a total of 70-80% of energy. When asked later by the audience whether flooded evaporators have the potential to lead to significant savings, the speaker confirmed that there indeed gains as there is no superheat, while the heat transfer coefficient is better.

Finally, he pointed out the potential to use renewable energy in supermarkets as these have rather large roof areas. Indeed several supermarkets already use renewables in Germany and the UK. He then finished off his presentation stating that in the future customers will set the trend and producers will have to listen more to them while he expressed his vision of supermarkets becoming an energy source rather than a sink.

Field Measurements and Comparison of Supermarket Refrigeration Systems, J. Rogstam, Sveriges Energi och Kylcentrum (Swedish Energy and Refrigeration Centre)

Based on field measurements and computer simulation models, the performance of different supermarket refrigeration systems were compared under a project financed by the Swedish Energy Agency. Two supermarkets were CO2 transcritical, one was a CO2 cascade system with R404A in the high stage. They were compared to reference conventional systems that are considered best practice systems with advanced control and robust design: these were indirect for the medium temperature working with R404A and partly R407C as refrigerant, while the secondary refrigerant was propylene glycol. For the low temperature solely R404A was used in direct expansion. All motors serving compressors, pumps and fans were frequency controlled, while all systems had floating condensing and evaporating temperatures.

The field measurements concluded that transcritical CO2 systems require improvement to reach the most advanced “conventional” systems on the market, but that the required improvement for the COsystems is within close reach. The researchers assess that CO2 systems should be able to reach or exceed the efficiency of the conventional systems through applying internal subcooling, improved heat exchangers and improved controllability. Furthermore, the inherent advantages of CO2 systems should be utilised such as direct use on cold and warm side; thus avoiding extra temperature differences. A next phase of the project will include more and newer generation CO2 systems and more manufacturers. 


By team (@r744)

Jun 21, 2010, 11:36

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