systems, are the controls,” Ferlin said, adding that this difference offers another learning opportunity. The team decided to use Emerson programmable controllers for the front of the racks rather than conventional parametric controls. “Programmable means everyone needs to have a fundamental understanding of what the transcritical cycle is and how it’s supposed to operate,” said Ferlin. “You are obliged to do that when you’re forced to program the controller to manage the cycle.” Though choosing a parametric control system might have been the simpler choice, “that would have come at the cost of not learning, understanding and developing that local transcritical CO 2 knowledge base.” Woolworths’ pilot transcritical CO 2 system is designed to operate efficiently in ambient temperatures of up to 113°F (45°C), with a view to replicating it almost anywhere in Australia across various temperature conditions. “This is a template, one that is robust, energy-efficient, and simple to service in the years to come,” Ferlin said. The key to maintaining the system’s efficiency and reliability is managing the flash gas – a side-effect resulting from the operation of the system in transcritical mode in Australia’s blistering summer temperatures. The solution chosen was parallel compression – a well-known flash-gas management method tested in numerous transcritical CO 2 systems around the world.
FLESHING OUT THE DESIGN The transcritical CO 2 rack supplier for this project, Bitzer Australia, played a key role in educating stakeholders early in the design process. The system consists of two independent racks, each one serving 50% of the freezer load and 50% of the medium temperature load. “Throughout the design process,” said Ferlin, “we had forums where the Retail FM design and installation team would come in, Bitzer Australia would come in, and the engineering department of Woolworths would come in, and we would actually flesh out the design that Bitzer was proposing, and it offered everyone an opportunity to challenge each other.” Among the questions asked, said Ferlin: Why are you doing this? Isn’t there a better way of doing it? “So, having that local content being delivered by a local technology partner is part of that training – part of that skill set investment that we are trying to achieve.”
However, that method comes with its own challenges, Ferlin noted. “You are balancing the use of parallel compressors with the flash-gas bypass valve, adding a bit more commissioning complexity.” But the decision was made with a long-term view, looking ahead to the transcritical CO 2 installations to follow around Australia. Once the design process was complete, Bitzer Australia facilitated hands-on transcritical CO 2 training for technicians. “The design and install contractor, Retail FM, actually had an opportunity to charge up a test rig at Bitzer’s facility and assist in actually starting up the test rig,” said Ferlin. “And this is following a theoretical seminar, leading up to that hands-on experience.” “The training that the rack manufacturer has done has been really good,” noted Merry. “It helped make sure that the technicians were comfortable [working with transcritical CO 2].” Both Ferlin and Merry highlighted the importance of training contractors to help sustain this transcritical CO 2 system, and the others to follow.
Another key choice in the design of the transcritical CO 2 system was the controls. “What is particularly different about transcritical CO 2 systems, as opposed to conventional Accelerate America