The United States Environmental Protection Agency (EPA) estimates that commercial supermarket refrigeration systems using synthetic refrigerants lose an average of 20% of their charge per year. The effects of synthetic refrigerants on global warming are 3,000 to 4,000 times higher compared to carbon dioxide (CO2). Therefore, using CO2 as a refrigerant will have a significant impact on greenhouse gas (GHG) emission reduction.
For a CO2 cooling system operating under transcritical conditions, a pressure-relief valve is required at the gas cooler outlet to lower the CO2 pressure below the breakeven point prior to entering the tank, thereby enabling CO2 condensation. In the past, there was no way of recovering the energy lost during this pressure relief. The insightful addition of an ejector to replace the pressure-relief valve makes it possible to achieve the required depressurization, while creating a Venturi effect that will drive part of the gas normally aspirated by the main compressors. This shifting from part of the system’s compressors makes it possible to efficiently recover a large part of energy that is typically lost. The technology had not yet been demonstrated in the supermarket sector, which offers significant potential savings in electrical energy. Recognizing the opportunity , Carnot Refrigeration proposed the “Refrigeration System Using an Ejector” project for ecoENERGY Innovation initiative funding. The project received $850 k to integrate an ejector into a CO2 refrigeration system operating under transcritical conditions and to measure the energy gain obtained.
The project’s very first achievement was the development of a calculation tool to size the ejector based on the estimated mass flow, with flow varying between refrigeration systems based on total cooling capacity. The calculator was developed by a project partner - the CanmetENERGY team at Natural Resources Canada.
The second achievement was the design of an ejector manufacturing method that was both accurate and cost-effective. The following requirements had to be met: design that would resist CO2 working pressure with a safety factor of at least 5; accuracy to ten thousandths of an inch needed for certain ejector cavities; possibility of adjusting the spout position to optimize the ejector operation; and commercially viable cost to manufacture. In order to meet these requirements, the final design uses a combination of common parts (clamps and high-pressure lines) and parts designed by Carnot Refrigeration. These parts are made by sub-contractors, while the final welding and assembly is done directly at the Carnot Refrigeration plant.
The third main achievement was measuring, on an operational system, the actual energy gains produced by the ejector. The initial goal was to conduct tests and measure performance directly at the supermarket for which the ejector was designed. This approach would have placed all of the supermarket’s fresh produce at risk. The decision was therefore made to conduct the tests on an industrial site where products were insensitive to occasional fluctuations in cooling capacity. The tests conducted on this site made it possible to reproduce the design conditions and measure the ejector’s performance. The results obtained were in line with the results initially calculated by CanmetENERGY engineers.
The first tests were conducted in 2014 on a refrigeration system installed in a Sobey’s Quebec supermarket. The final tests were conducted in January 2016 on a refrigeration system in a warehouse used for freezing cranberries.
Benefits to Canada
This project is a first in the world for commercial refrigeration systems. The increased efficiency of refrigeration systems in supermarkets, through the proper integration of ejectors, will generate energy savings and reduce GHG emissions and benefit both the industry and Canada as a whole.
The principle of defrosting using an ejector should be implemented in a commercial system. The demonstration systems will help to improve the control sequences, measure performance over a long period of time, and confirm the energy gains for the specific application.