The system, estimated to save 30% in energy, experienced one leak, but the contractor believes it’s safe.
The PnP Strand CO2 refrigeration rack arriving on-site.
Over 10 years ago, Pick ‘n Pay (PnP), a South African food retailer, installed ammonia/CO2 (NH3/CO2) cascade systems in three stores, discovering various benefits and challenges to this refrigerant combination, which has never taken off globally. Only a handful of supermarkets are known to use NH3/CO2 cascade refrigeration. The PnP installations are among the first.
“With all the buzz around transcritical CO2 systems, industry newcomers often overlook other efficient natural refrigeration options for retail applications,” said Marius La Grange, Manufacturing Manager at Energy Partners Refrigeration Solutions, the contracting company that manufactured and installed the refrigeration system at one of these pilot projects.
“Like many good things, cascade systems need to be applied correctly and in suitable applications to have the best potential results,” he said. “But a potential saving of 30% or more really justifies this option and makes it worth considering.”
La Grange considers an ammonia system safe to use, with only one leak experienced in a decade.
In 2009, German development agency, GTZ (now GIZ), funded the three ammonia/CO2 cascade installations in South Africa for PnP at stores based in Strand, Rand Park Ridge and Hurlingham, which came online in that order.
The PnP Strand, located in the Western Cape province, near Cape Town, installed a NH3/CO2 cascade system with a secondary circuit (water/ glycol) for medium-temperature loads. The ammonia part of the system uses open-drive reciprocating compressors and an evaporative condenser.
A similar system is used at PnP Rand Park Ridge in Johannesburg. The only difference was that the medium-temperature and low-temperature loads varied slightly between the two stores. Both were retrofits of F-gas systems.
At the Strand location, one of the biggest challenges was that the store could not be closed down for the revamp, explained La Grange. The new equipment, with all its reticulation, had to be installed and connected to the new cabinets and evaporators in under a few days, he said.
In addition, the store had to identify a new machine room area, one that would comply with all health and safety regulations. An open area on the first floor was selected. “The area was rather limiting, so ample planning had to go into what specific [equipment] options would satisfy the requirements and also fit into the space,” said La Grange. The available space had to be narrow and long, instead of square, to accommodate the evaporative condenser, for example.
“Applying a TEWI [total equivalent warming impact] view and looking at the refrigerant’s direct GWP figures, this type of installation is cleaner than a similarly sized transcritical installation,” explained La Grange. “When considering indirect emissions from the energy consumed to operate the plant, this type of installation is also highly likely to come out on top.”
Ammonia is the most energy-efficient solution for medium-temperature applications "by some margin,” he said. The CO2 low-temperature system is also operating within a set of operating conditions where it is very energy efficient, he added. Then, of course, there is the added bonus of zero ODP and a GWP of only 1. In both cases, the charge of the refrigerant was very limited.
With no recorded energy consumption from the F-gas system before the revamp, the reduced energy consumption could not be verified. “But we believe that the new installation saved significantly on the electricity bill the store had,” said La Grange.
He estimated that the installation would offer a 30% or higher rate of saving in energy consumption over an HFC-charged DX installation. “Maintaining an ammonia system is more costly than an HFC system, but the saving in operational costs offset that additional cost by some margin.”
The medium-temperature air coolers (heat exchangers) that were made to fit into the cabinets and rooms have a very low delta T (temperature difference between the coolant and the air tempera- ture). This offers a few advantages. “One of the biggest is the fact that very little ice buildup on the outside of the air coolers takes place,” explained La Grange.
Ammonia’s pungent odor and potential toxicity are what keep most supermarkets from using it. Needless to say, a suitable sensor is required in any NH3/CO2 system.
In the 10 years since the installation of the PnP systems, only once, when a heat exchanger failed internally, did ammonia leak into the secondary cooling circuit. Since ammonia has a corrosive effect on any copper components, it is potentially very damaging unless it is detected and rectified immediately.
The store did experience an ammonia smell with some of the secondary coolants leaking at a connection, so it was evacuated for a few hours until it was well vented. Fortunately, there were no injuries.
But fixing the issue was easier said than done. “We had a very challenging task to replace the heat exchanger,” La Grange recalled. “In the end, we replaced it with a unit that Vatherus made in a few days, and we air-freighted it to get the store in operation again.”
Despite this incident, La Grange is not worried about the perceived safety risks of working with ammonia in a retail installation. “I don't believe it is a risk, provided that all the required precautions are taken during the design, construction and, maintenance of such a plant,” he said. “The ammonia has a limited charge and is confined to the machine room only, so there is no ammonia where the customers and staff are.”
Another challenge with a secondary cooling system is removing air from the system, said La Grange. A centrifugal pump is constantly circulating the secondary coolant around the pipe network. “The pipe design and installation should therefore be done in a way to remove air from any part of the pipe networks as easily as possible.” A small make-up tank at the highest point of the pipe network, used to return liquid into, and draw liquid from, the pump station, solves many of these problems, he noted.
The pump station is also something that needs careful consideration, according to La Grange. What should the supply pressure from the pump station be? As low as possible. “We simply need to circulate constantly at below 3.3ft/s (1m/s) at full load where possible.” The lower the energy consumption to circulate, the better the overall plant COP (coefficient of performance). A variable-speed drive could be made to fit the pump to alter speed.
This article was originally published in the January 2020 issue of Accelerate Magazine.