The Multi Rotary Compressor Pack represents the successful combination of rotary compressors with Green & Cool’s control software to ensure the compressor pack operates reliably and efficiently. The plant unit uses CO2, a natural refrigerant which offers significant environmental benefits.
CO2 as a refrigerant has a global warming potential of 1 compared to a traditional refrigerant such as R404a, which has a GWP of 3922. In practice, this means that for each kilogram of R404a that enters the atmosphere it is the equivalent of 3922kg of CO2. Additionally, the benefits of using CO2 as a refrigerant in the United Kingdom are lower energy consumption against traditional HFC refrigerant plant.
It was at Euroshop 2017 that Tesco approached Green & Cool with the opportunity to trial an early production unit at one of their Express stores. Carter Synergy, as the UK distributor of Green & Cool plant and a Tesco installation and maintenance partner, was chosen for delivering the project.
Brian Frankland, refrigeration and HVAC engineering manager at Tesco, explained: “The installation of the new system at our Express store at Shaw was the culmination of a great deal of planning and effort on the part of all concerned. The system has outperformed all expectations for efficiency and we have been delighted with every aspect of the system to date. This installation simply reinforces the view of Tesco engineering as to the future of natural refrigerant use within the estate both on convenience and main stores.”
As the project was a trial for Tesco, it was benchmarked against a previous traditional HFC based installation at an Express store in Cinderhill. This uses a three scroll compressor HFC pack. Both projects have integrated fan condensers or gas coolers, and both have similar meterage of chilled cases to cool within the stores. The trial of the installation of the Multi Rotary Compressor Pack has been very successful with initial performance and energy consumption figures better than expected. Although the rotary CO2 units currently have a 10 percent higher installation cost, there is a much reduced payback period as energy consumption is lower. On average, it has been found over the trial period to the 01 September 2018 that the Green & Cool Multi Rotary Compressor Pack at Tesco Express in Shaw is consuming significantly less energy than the HFC scroll pack at Tesco Express in Cinderhill.
The installation by Carter Synergy comprised the Green & Cool Multi Rotary Compressor Pack, LT CO2 condensing unit, Carter Retail chilled cabinets with doors, Carter produce cases (open fronted), as well as HT and LT evaporators. Oil system management was considered by Green & Cool to be the biggest challenge in designing the new multi rotary compressor packs. The technical challenges were solved through a combination of technology application and software control. The trial unit is an integrated close coupled pack and gas cooler eliminating any onsite discharge and return pipework simplifying the installation and pressure testing.
The Green & Cool Multi Rotary Compressor Pack has performed extremely well to date operating at temperatures in excess of 36o and temperatures below zero and across all duty requirements with no alarms being generated. Both Carter Synergy and Tesco Maintenance have provided feedback on the unit’s accessibility for maintenance, general layout, quality and finish and this information has been used by Green & Cool to refine the design for full production.
How the new system developed
Rotary compressors have been used in air conditioning and small scale refrigeration for nearly 100 years. CO2 first entered use as a refrigerant more than 100 years ago. Both the refrigerant and technology fell out of favour with the development of chemical refrigerants – CFCs, HCFCs, HFCs, all being engineered to have beneficial characteristics across the required temperature / pressure ranges.
A significant challenge of using CO2 as a refrigerant is the low critical temperature and the related high pressures inherent in a mid-temperature system. The development of new materials and improvement of existing technology has allowed CO2 to be used in large scale low, mid and high temperature applications.
Predominantly used in air conditioning systems the rotary compressor was never fully adopted for refrigeration, with scroll compressors being the standard and more prevalent for smaller HFC systems. A scroll compressor design is very difficult to adapt for use at the higher discharge pressures used in CO2 refrigeration systems with only LT low pressure machines having so far been brought to market. Rotary compressors are relatively low cost when compared to reciprocating compressors due to their simple design and small size. The Green & Cool unit uses multiple rotary compressors, each approximately 10kW coupled with individual inverter drives. The gas cooler fans are EC variable speed as standard, further increasing the energy efficiency and reducing noise levels.
Carter Synergy, together with Tesco, is continuing to monitor the performance and energy consumption of the new system. In the meantime, a larger capacity mid-temperature Green & Cool Multiple Rotary Compressor unit is currently being developed as well as an LT version, and these are likely to be trialled on main store refits later in the year.