Secondary refrigerants allow the amounts of environmentally harmful primary refrigerants to be minimized and contained in a restricted area. Examples of secondary refrigerants include water, air, hydrocarbons, ammonia and carbon dioxide, which are more environmentally benign than traditional refrigerants such as HCFCs. They are safer (some are even incombustible and non-toxic) and generally suitable for refrigeration systems. Brines are often chosen as secondary refrigerants for large refrigeration systems, such as those supplying supermarkets, the most common brines being water-glycol solutions, water-ethanol solutions and acetate solutions.
The traditional method of refrigeration for medium- and low-temperature display cases in supermarkets is to circulate primary refrigerants (CFCs, HCFCs and HFCs) from a centrally located plant room. The charges of primary refrigerants for such systems are large, and they are contained in extensive ducts with many potential sources of leakage. The refrigerants are expensive and have negative environmental impacts when released to the atmosphere. One way of reducing the amount of these refrigerants and the risk of leakage is to chill a secondary refrigerant, such as glycol or brine, in a centralized plant room and circulate this rather than the primary refrigerant.
Secondary loop systems thus employ two separate heat transfer loops: one for the primary refrigerant and one for the secondary refrigerant (see Figure 5.11). The secondary refrigerant is circulated through the display cases in the supermarket using a pump. The primary refrigerant is contained within the primary loop in the machine room and does not enter the retail sales floor. A heat exchanger is required to cool the secondary refrigerant with the primary refrigerant. Because the primary refrigerant is no longer in close proximity to the customers, it is possible to use alternative fluids, such as ammonia. A number of supermarket installations with CO2 as the secondary refrigerant have been installed with good results. Supermarket systems using ice slurry (a mixture of water, ice and ethanol) have also been evaluated in some European countries.
The disadvantage of this kind of system is the thermodynamic penalty from using an extra heat exchanger and a pump for the secondary loop, both of which add to the system's power consumption. This penalty may be reduced by new developments in refrigeration technology.