Supermarket Savings

Supermarkets are notorious energy guzzlers. They use huge amounts of fresh air for ventilation to counteract the smells that percolate from the produce on the shelves and from backroom service areas such as the butcher’s shop and kitchen. Then there are the coolers and freezer cases. These refrigeration units account for up to 40% of a store’s total energy consumption.

But the refrigeration units also have a disproportionate impact on the amount of greenhouse gases stores emit into the atmosphere. Thanks to the Montreal protocol, most supermarkets are now using non-chlorine-based refrigerants rather than CFCs. So the problem isn’t related to ozone depletion. The problem is that the replacement refrigerants now used, typically R507A and R404A, emit massive amounts of greenhouse gases. These synthetic substances emit up to 4,000 times more than the equivalent mass of carbon dioxide, according to Daniel Gigure of Natural Resources Canada.

Gigure is a technological expert for a federal government program dedicated to reducing the energy use and environmental impact of refrigeration units. The Refrigeration Action Program is run by Natural Resources’ CANMET Energy Technology Centre in Varennes, Quebec.

725 vs. 3,000 kilograms of synthetic refrigerant

The CETC-Varennes program has been instrumental in helping a Loblaws supermarket install an unconventional refrigeration system in its 10,000-sq.m store in Repentigny, a suburb east of Montreal. In this pilot project — a first in Canada — the store’s freezers and refrigerators are cooled by two independent secondary fluid loops that contain environmentally benign coolants. As a result, the synthetic refrigerant is confined to equipment in the mechanical room.

The secondary loop approach means that only 725 kilograms of synthetic refrigerant is used, compared to 3,000 kilograms in a conventional system where the synthetic refrigerant is circulated throughout the store area to the display cases. And because the synthetic refrigerant is confined to equipment in the mechanical room, there is much less opportunity for it to leak into the atmosphere. Synthetic refrigerants are very volatile, explains Gigure, and their pressure is much higher than atmospheric pressure so they leak from the many brazed joints in a conventional piping network.

Another important feature at Repentigny is the integration of the building’s HVAC and refrigeration systems. Heat recovered from the refrigeration system is used to warm the store’s ventilation air and heat the domestic water supply.

As a result of these innovations, the store has reduced its overall energy consumption by up to 37% compared to a conventional system in a comparable store. And when the reduction in synthetic refrigerant leakage is taken into account, the greenhouse gas emissions from the HVAC and refrigeration systems together are reduced 55%, or equivalent to about 2,200 tonnes of carbon dioxide (CO2) per year.

Same old, same old

For decades supermarket refrigeration in North American has relied on the technology known as the multiplex system, explains Gigure. The manufacturers offer differences in price and small differences in equipment he says, but basically their systems are built on the same principles.

But while the refrigeration industry has been slow to adopt new systems and approaches on this continent, society’s growing environmental awareness is driving the demand for change.

Repentigny’s secondary loop system

There are two secondary loop refrigeration systems in the Loblaws Repentigny store. The low temperature system serves the freezer cases and it has a nominal refrigeration capacity of 55 tons (or 200 kW). The medium temperature system serves the refrigerated cases and it has a nominal refrigeration capacity of 110 tons (or 400 kW).

The low-temperature loop serving the freezers contains a brine solution of potassium formate at about -29 C. The medium temperature loop circulates propylene glycol at about -6.5 C. These environmentally friendly secondary fluids are cooled by refrigerant through a heat exchanger and then circulated through pipes to the display cases.

Heat recovery

The heat rejected by the medium temperature system serving the refrigerators, amounting to about 250 kW, is recovered by an air coil located in a rooftop unit and used to heat the cold fresh air supply and return air in the store’s HVAC system. The temperature of the ethylene glycol secondary fluid is adjusted to match the space heating demands, making it work like a heat pump. Most supermarkets are kept at around 20 C, Giguere explains.

Heat rejected by the low temperature system (220 kW) is recovered and upgraded by seven water loop heat pumps. As well, a small heat exchanger, or “desuperheater” on the rooftop discharge compressor produces temperatures of 90 C and is used to heat directly the building’s hot water supply. Excess heat from both loops is ejected outside by fluid coolers.

Other features include mechanical subcooling by 30 C for the low-temperature liquid refrigerant line (from 35 C to 5 C), which is performed by the medium temperature system through a plate-type evaporator.

Variable speed pumps are installed on the cold secondary loops, and there is a control system to optimize the overall performance. A variable condensation pressure (floating head pressure) allows the system to adjust automatically the condensing pressure according to the outdoor conditions and the space heating demand.

Carbon dioxide as an alternative coolant

Engineers might be reluctant to use the above type of secondary loop refrigeration system, suggests Gigure, because it involves more equipment and components. But the systems are reliable, and simple to start and run, he says. They can be controlled and optimized using a simple or a sophisticated automation system. With standard refrigeration systems, by contrast, there are often temperature fluctuations, and they are difficult to balance.

One downside in the secondary loop approach is the use of brine for the low-temperature system. “Brine is very corrosive,” says Gigure, which creates maintenance problems. One solution they are considering is to use carbon dioxide as the coolant. The CETC-Varennes program hopes to have a CO2 demonstration system running sometime next year. Gigure says there are already 200 or so secondary loop refrigeration systems using CO2 in Europe. Carbon dioxide is a high-pressure gas, and so needs small diameter copper tubing with relatively thicker walls.

(According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), carbon dioxide is gaining interest as an alternate, environmentally benign refrigerant, with no ozone depleting potential and low greenhouse gas emissions. Ashrae has added guidance on using CO2 in its 2006 Handbook on Refrigeration.)

It is difficult to retrofit an existing store with a secondary loop system, since it involves extensive changes to the store’s piping systems. The secondary loop technology is therefore better suited to new construction or major retrofits. However, adding heat recovery to an existing refrigeration system is relatively easy, and since the Repentigny project, CETC-Varennes has helped add heat recovery systems to two existing supermarkets in the Ottawa area.

Program gives technical and financial help

The Refrigeration Action Program for Buildings has a dual focus: besides the supermarket stream, it also investigates innovative refrigeration practices for ice rinks and curling rinks. It offers technical and financial support to projects that can significantly reduce greenhouse gas emissions. Because both these sectors are such heavy consumers of energy, the program can help to achieve great energy savings. Combining the reductions in energy use with the elimination of 75% of synthetic refrigerant leaks, the program estimates it can save th
e equivalent of up to 4 megatons of greenhouse gas emissions a year.

CIMA+ of Quebec was the consulting engineer on the Repentigny project (Ren Gingras, project manager). The CETC-Varennes website is http://cetc-varennes.nrcan.gc.ca