Current market data shows a strong increase in the number of CO2 booster systems especially in Northern Europe, where the cost of this technology has declined rapidly. “In Norway for example, since last year an HFC system is more expensive than a CO2 one. Now there is no question any more regarding what to apply,” Armin Hafner of SINTEF pointed out.
While CO2 booster systems have become increasingly popular in the mild climates of Northern and Central Europe, there is still a need to identify CO2 system configurations suitable for hot climates in Europe and beyond. One of SINTEF’s GL 2014 papers investigates the energy efficiency of different types of CO2 systems for supermarkets in higher ambient temperature climatic locations in China in comparison with a standard R404A system based on measurement results presented by Carrier at the ICR2011.
Comparison of three CO2-based system configurations with R404A in warm climate
The three CO2 system configurations evaluated by SINTEF include:
- A standard CO2 booster system
- A CO2 booster system with a mechanical subcooling unit using hydrocarbons
- A CO2 ejector supported parallel compression system
Taking into consideration the recorded temperatures of four Chinese cities (Beijing, Shanghai, Guangzhou and Hangzhou), the energy efficiency of the medium temperature part of different CO2 system configurations was assessed.
R744 ejector supported units with parallel compression achieve up to 25% energy savings
According to Armin Hafner of SINTEF, “innovation related to CO2 units will enable the technology to outperform commercial refrigeration units using HFCs even in hot climates. You can do the same with air conditioning and other applications if you focus and invest in the right technology.” He emphasised this latter point saying, “We are able to improve the COP at high heat rejection temperature significantly, if we do it the right way.”
On the basis of an annual energy consumption, the results indicate that compared to a standard R404A system, a CO2 ejector supported system with parallel compression can achieve higher energy savings in a warm climate, requiring around 25% less energy for its operation. The CO2 booster system with a mechanical subcooling unit utilises around 14-16% less energy than a R404A system, while the standard CO2 booster system can achieve around 5-10% energy savings.
When comparing relative differences in energy consumption between the different regions in China, the analysis indicates that the refrigeration systems in the southern region require more energy compared to those installed in the North, due to the larger number of operating hours at higher ambient temperatures in the South. However, while the energy consumption of a CO2 ejector supported commercial refrigeration unit in the warm climate of Guangzhou consumes a similar amount of energy as an R404A system in Beijing, the R404A unit in Guangzhou consumes 34% more energy than in Beijing.
Integrated CO2 systems could reduce energy consumption by 30%
Another paper presented by SINTEF described an integrated energy system built in a supermarket in Trondheim, Norway, and presented measurement results from the first year of operation. The key parts of the energy system consist of:
- An innovative HVAC system with airside bypasses for the rotating heat recovery device as well as the heat exchangers
- Three floor heating loops
- Four energy wells (heat source for the heat pump, heat sink for air cooling)
- R744 booster system with three gas coolers and heat pump option
- High temperature energy storage units
- Snow melting exterior heat exchangers
In operation since autumn 2013, measurement data indicate that during three seasons (autumn, winter, and spring) the energy consumption of the store was reduced by 30%, compared to similar supermarkets of the same chain in Trondheim. The data recorded for the summer indicates a large saving potential which can be solved by active shading, which will be addressed and solved before the summer of 2015.