Many retailers are not aware of the fact that CO2 transcritical refrigeration systems are a viable option in warm ambient climates. While it is true that previously these systems did not perform as efficiently in higher ambient temperatures, the game is changing, and quickly, too. ATMOsphere Europe 2015 offered a unique opportunity for system and component suppliers, policy makers and end users to discuss new developments in the industry, and explore the technology advancements pushing the CO2 equator out of existence.
Parallel compression and ejector technology take centre stage
Presenting in several sessions, Torben Hansen, Commercial Product Director of Advansor, focused on defining the hot climate challenge, mainly, how to remove flash gas at higher temperatures, and presented a number of solutions for improving efficiency of transcritical CO2 commercial refrigeration systems in high ambient temperatures, including high-pressure subcoolers, water spray systems, adiabatic cooling curtains, parallel compression systems and gas ejectors.
According to Hansen, combining parallel compression with ejector technology achieves the highest performance. In fact, he believes it could remove the CO2 equator all together. Combining parallel compression and gas ejector technology is expected to be able to achieve peak savings of 22-27% and annual savings of 12-16%; the data will be validated during 2015.
There is a bright future for enhanced CO2 solutions in hot climates,” concluded Hansen.
Davide Refosco also highlighted parallel compression, citing SCM Frigo’s successes with the technology to improve the performance of CO2 systems. The company has in operation 20 CO2 transcritical units using parallel compression and heat recovery to achieve efficiency increases of 15% and higher throughout Belgium, Spain and France. Of particular focus for the company is continuous product development, a major aspect of which is boosting efficiency of CO2 systems in warmer climates and providing a range of natural refrigerant solutions to meet the varying refrigeration needs of end users across the market.
Torben Funder-Kristensen of Danfoss confirmed that conventional booster systems fall short in terms of performance at around 27°C, compared to systems using R404A, mentioning however that with parallel compression, CO2 systems are able to keep up with R404A systems up to approximately 38°C, which has been confirmed in stores in Spain and Italy. Funder-Kristensen also introduced the application of ejector technology, highlighting that the ability to adapt to a wide range of capacities is the “key to success”.
This technology has been around for more than 100 years; we’re not reinventing the wheel here,” said Funder-Kristensen
According to Funder-Kristensen, operational test sites are showing expected results, noting in particular the positive performance of enEX systems using gas ejectors.
How do ejectors increase efficiency?
Ejectors move gas from the medium temperature suction to the parallel compressor, and in some cases all the gas can be moved, resulting in increased performance in high ambient temperatures or 100% heat recovery. Funder-Kristensen also discussed liquid ejector systems whereby the technology allows the medium temperature evaporator to be flooded; in this case the savings are not so much derived from the parallel compressor or the ejector itself, but from the higher suction pressure. Trials have been running since 2013, also with good results; on average the evaporation temperature is raised by 5-10°C. The savings of the liquid ejector can even be used in parallel with a gas ejector to further improve efficiency. In a comparison of systems using gas and gas and liquid ejectors to systems using R404A, the ejector technologies show superior performance to R404A, even at high ambient temperatures, according to predictions in test trials and laboratory tests.
Efficiency doesn’t depend so much on the ambient temperature anymore,” said Funder-Kristensen, “This is very encouraging because it means the limit for CO2 basically disappears.”
Multijet can achieve 25-30% higher efficiency than standard CO2 system
Armin Hafner of SINTEF Energy Research presented the organisation’s MULTIJET project, a collaboration with enEX and Danfoss, and underway since 2013. It embodies the application of an innovative expansion work recovery system with multiple ejectors to improve energy performance in R744 refrigeration installations for supermarkets. The project includes two supermarket test campaigns, one in Trondheim, Norway, and one in Spiazzo, Italy. The long-term objective is to make ejector-equipped CO2 refrigeration systems thermodynamically, operationally and economically competitive with HFC systems over the entire range of operating conditions, namely, high ambient temperatures.
Commercial refrigeration is in a very innovative period,” said Hafner.
The multi-ejector block is compact and utilises two liquid ejectors and four vapour ejectors, allowing for a wide range of capacity control. The gas cooler is divided into two parts and acts as a heat pump for winter operation. The system maintains flooded evaporators all year round and the evaporating temperature is around 3°C.
Currently, supporting parallel compression with an ejector is state of the art,” stated Hafner, “though adding pressure lift is something new; we need to integrate air conditioning.”
In sum, Hafner commended the high level of flexibility ejector technology can achieve and that the market is quite competitive, allowing end users to make choices that best fit specific situations. In addition, Hafner highlighted that the use of ejector technology is a cost effective and efficient way to integrate AC in CO2 commercial refrigeration applications.
The CO2 equator is moving…actually, it’s gone. We will see this in the US and Asia, I’m sure of it,” said Hafner confidently.
Sergio Girotto of enEX highlighted the importance and interconnectedness of efficiency, cost, reliability and ease of service in designing as well as choosing a refrigeration system. enEX has for years been developing solutions for warmer climates. Using parallel compression and ejectors in unison, the company has realised significant efficiency gains and cost (first cost + energy cost) reductions. While the initial cost of the Multijet ejector solution is higher than an R404A system, the lifetime cost is significantly lower. The company’s “full saving” system consumes 25-30% less energy than a standard CO2 solution. Girotto also highlighted that refrigeration needs of end users vary on a case by case basis, to which enEX is able to cater to maximise return on investment of a system. enEX has installed more than 20 systems with its ejector technology since 2012.
In addition to improving performance of CO2 systems in southern climates, enEX has also realised efficiency improvements in colder climates by implementing heat recovery.
Exclusive video interview with Sergio Girotto at ATMOsphere Europe 2015.
Comparative study suggests 25% less energy consumption possible in Southern Europe
Erik Wiedenmann of Frigo-Consulting presented a comparative study of two CO2 booster refrigeration installations using ejector technology in Migros supermarkets in Switzerland. The installation in Bulle has been in operation since summer 2013, while the one in Ibach was inaugurated in November 2014. In both cases, liquid in the suction allows for the reduction of superheat in the cabinets, which increases heat exchanger efficiency and the evaporation temperature. However, the 2014 installation was enhanced with two more ejectors (for a total of five), allowing for increased evaporation temperature in both the medium and low temperature sides, shifting mass flow to the parallel compressor. Based on the evaluation of these installations, Wiedenmann predicts similar systems in central Europe could achieve 20% less energy consumption and 25% less in southern Europe.
ATMOsphere Europe 2015 event page
Torben Funder-Kristensen: Barriers and opportunities for selection of best low GWP technologies
Armin Hafner: Italian supermarkets: R744 multi-ejector enhanced parallel compression system
Torben Hansen: Case study: Transcritical CO2 retail technologies taking southern tracks
Erik Wiedenmann: CO2 refrigeration system for tropical regions
Davide Refosco: Nature and technology working as one
Sergio Girotto: Reliability, efficiency & cost of CO2 commercial refrigeration