In response to the Kyoto Protocol and the rising concern about greenhouse gas emissions, there is a general consensus that the use of hydrofluorocarbons in refrigeration must be reduced, which can be achieved by reducing leakage and introducing more environmentally friendly fluids. To reduce refrigerant leakage, the use of secondary refrigeration is being promoted. Secondary loop systems thus employ two separate heat transfer loops: one for the primary refrigerant and one for the secondary refrigerant. The principle of secondary refrigeration consists of reducing the volume of the primary cooling unit (and therefore its leaks) as much as possible while using a secondary loop containing a safer fluid to distribute cooling power to users. It has several advantages such as, factory-built units can be used, local construction of primary refrigerant piping can be avoided and installation work can be made in a simpler way. It is also possible to design the refrigeration unit in a compact way and with an extremely small refrigerant charge.
Followings are some desirable properties of secondary refrigerants:
- Enough freezing security
- Higher density (lower charge volume)
- Higher specific heat capacity
- Lower viscosity (low pressure drop and pump size)
- Good thermal conductivity (high heat transfer)
- Good chemical corrosion inhibiting
- Chemically stable, no separation or degrading
- non-toxic, non-flammable
- Food grade for food refrigeration.
To take care of growing demand of energy density, the miniaturised devices as well as large devices require more efficient cooling systems with greater cooling capacities and decreased sizes. Thus, heat transfer capacity of secondary refrigeration loop needs to be increase and this need must be met by enhancing the heat transfer capability of secondary refrigerant. Hence, many alternative and advanced secondary refrigerants are being promoted.
There are two kinds of secondary refrigerants, namely, single-phase fluids and two-phase fluids. Single phase fluids generally consist of some kind of antifreeze solution, corrosion inhibitor, and biocides. The single-phase secondary refrigerant can be further divided into two categories, aqueous and non-aqueous fluids. Two-phase secondary refrigerants (phase change materials in liquid carrier) take advantage of the high latent heat during the phase change process from the liquid to the solid or from the liquid to the gaseous state.
Aqueous fluids as secondary refrigerant
Aqueous liquids are basically water-based solutions containing glycols or salts, which are used to decrease the freezing point of water. Aqueous solutions of ethylene and propylene glycol, ethyl alcohol and chloride salts have long been used as secondary refrigerants. For example, magnesium and calcium chloride water mixtures have been widely used in refrigeration applications since a long time ago. Ethylene glycol is also widely used as an automotive anti-freezing coolant. Lately, potassium acetate and potassium formate have been employed in low-temperature applications because they show advantages in terms of corrosion and desirable physical properties as compared to other mixtures.
Non-aqueous fluids as secondary refrigerant
A number of non-aqueous heat transfer liquids are also used as secondary refrigerant, which may be natural inorganic, natural organic or synthetic organic fluids. HFE-7100, referred to as a hydrofluoroether (HFE), has recently been introduced. The freezing point of this fluid is listed to be below (–) 100-degree C, making it suited for both low and medium temperature applications. It is orally non-toxic, non-flammable, and it has been found to be compatible with most common materials. Several synthetic organic heat transfer fluids (Dowtherm J, Syltherm XLT, Baysilone KT 3, Gilotherm D12 and Tyfoxit) are also available. One of these fluids was found to offer freeze protection to (–) 73.3-degree C, thereby providing another option for low temperature systems. This synthetic fluid provides superior low temperature viscosity when compared to ethylene glycol. Like ethylene glycol, it is orally toxic and would therefore require special precautions. CO2 is attracting attention as the ideal secondary refrigerant due to minimum viscosity change even at low temperatures [below (−) 30-degree C]. The latent heat of vaporisation is large and heat transfer efficiency is excellent when CO2 is used as secondary refrigerant. Therefore, the bore of the pipe can be designed to be 5 times smaller because the CO2 mass flow rate is smaller than that of the conventional secondary loop cooling system using conventional brines. This also reduces the pressure drop triggered by friction inside pipes. The energy consumption can be reduced by about 40 per cent when CO2 is used as the secondary refrigerant. It is possible to reduce the pump’s power consumption by 90 per cent when using CO2 as secondary refrigerant compared to other systems those adopted other brines as secondary refrigerant.
Nanofluids as secondary refrigerant
Nanofluids are dilute liquid suspensions of nanoparticles with at least one of their principal dimensions smaller than 100 nm. Nanofluids, consisting of ceramic particles, pure metallic particles and carbon nanotubes suspended in typically conventional heat transfer liquids (water, ethanol, brine solution, refrigerant, oil, lubricant, etc), have been shown to enhance the convective heat transfer performance of the base liquids. An increase in thermal conductivity, turbulence due to nanoparticle embedded rough surface (nanoporous and nanofin), various slip mechanisms are the reasons behind the increase in performance till optimum concentration. Nanofluids have the potential to reduce thermal resistances and it is expected that these fluids will play an important role in developing the next generation of cooling technology. Hence, conventional aqueous solution (brine) based nanofluid or hybrid nanofluids have been promoted as secondary refrigerant. Nanofluids allow the refrigerator to operate with lower condensing and higher evaporating temperatures, thus increasing the system COP. As the addition of nanoparticles to the secondary fluid does promote the enhancement of the refrigeration cycle performance as it flows through the evaporator. No doubt, the use of nanofluids as secondary refrigerant will improve the performance and reduce the system size and space needed. However, most of the nanofluids yield the payback period higher than the component life and hence not beneficial for present scenario. In future, the payback period can be reduced by reducing in the cost of nanoparticles and increasing suspension stability, hence may be a alternative.
Phase change material slurries (PCS)
PCS is two-phase secondary refrigerants, mixture of carrier fluid (basically aqueous solution used as a continuous phase) and solid PCM particles. Contrary to current secondary refrigerant fluids such as brine solutions or ethylene glycol which have a low energy density, these slurries have a higher energy density than single-phase secondary refrigerants, due to both sensible and latent heat capacities of the materials. PCS may be ice slurry, microencapsulated phase change material slurry (MPCS) as well as clathrate hydrate slurry (CHS).
Ice slurry (suspension of ice crystals in a carrying liquid phase) is considered as a very promising secondary refrigerant. Ice slurries consist of a number of ice particles in an aqueous solution and the diameter of ice particles is equal or smaller than 1 mm. The smaller the ice particles in the ice slurry are, the better the slurries can be transported. Besides the reduction on the charge of primary refrigerant associated to any secondary refrigerant, ice slurry allows a reduction in energy consumption compared to single phase secondary refrigeration systems as well as the possibility of thermal storage. It is obtained in two different ways: firstly, the energy efficiency of an ice slurry plant is greater than that of a plant using a single-phase secondary refrigerant; secondly, the energy consumption on the pumps used in the secondary refrigerant distribution system can be reduced compared to the energy consumption necessary to pump the traditional single-phase secondary refrigerant. The optimal ice concentration depends on specific operation conditions and the heat exchanger type (smooth or corrugated tube). As a general rule, the optimal ice concentration increases as the heat exchanger length increases. Although in most cases the direct use of ice slurry improves the heat exchanger’s performance, there are some cases, especially for low heat exchanger length, where the direct use of ice slurry is inadvisable. The refrigeration system pipeline diameter can be reduced by using ice slurry as the secondary refrigerant. The cooling capacity of an ice slurry is four to six times higher than that of conventional chiller water, depending on the ice fraction.
MPCS is a kind of solid–liquid suspension consisting of the carrier liquid and small particles of phase change material (PCM) with a thin shell. Paraffin waxes are often used as the core materials due to its extended melting temperature range. The shell material is usually natural or synthesised polymer, such as polyester and polyethylene, which has high strength and flexibility. The carrier liquid is usually water. Compared to non-encapsulated PCM slurry, MPCS owns several advantages. A Phase Change Material Emulsion (PCME) is a multifunctional fluid consisting in a Phase Change Material (PCM) as paraffin, dispersed in an aqueous surfactant solution, usually water. PCMEs became popular because of their interest for comfort cooling applications as they perfectly fit with the required range of temperatures (0–20-degree C) owing to the use of paraffin. In addition, this type of fluid is specifically studied, because paraffin is chemically inert and stable and considered safe and reliable. Furthermore, PCMEs compared with other fluids show significant advantages such as a high phase change enthalpy and no capsule for the paraffin particles, which makes them cheaper and easy to produce.
Clathrate hydrate can be formed in some aqueous solution of tetra-alkylammonium salts with simple an ions (such as halides, sulphate, formate, etc.). Clathrates hydrates are solid structures similar to ice but able to trap gas molecules such as CO2 or CH4. They also have the advantage of being able to form by gas injection, thus without mechanical processes contrary to ice slurries. CO2 hydrates have a high dissociation enthalpy of 374kJ/kg, which is higher than that of ice (333kJ/kg). Their formation temperature mostly lies above 0 °C and, when present, the gas pressure may be above atmospheric. Their latent heat of fusion is often lower than that of ice, except for the hydrate of CO2. Mixed hydrates may also form, associating a gas and a salt. Because of the wide variety of hydrates, plus the effect of gas pressure when present, the melting temperature can be tailored to the application for greater overall energy efficiency. Gas hydrates are crystalline solids resulting from the arrangement of water molecules linked by hydrogen bonds constituting cages around stabilising gas molecules. Gas hydrates are solid structures able to trap gas molecules and have a high dissociation enthalpy so that they can store and transport huge quantities of cold energy. When used as secondary refrigerants for cold storage and refrigeration applications, hydrate slurries offer high-energy densities due to their significant latent heat of fusion. The potential presence of gas in secondary refrigeration processes with cold storage is thus a new feature. Heat transfer coefficient of CO2 hydrate slurry is nearly 2.5 times higher than liquid water.
Applications and Challenges
There are several application areas of secondary loop refrigeration system including ice plant, cold storage, refrigeration warehouses, milk chilling, fish freezing, supermarket display cases (the primary refrigerant is contained within the primary loop in the machine room and does not enter the retail sales floor), etc. Single-phase brines such as ethylene glycol, propylene glycol, ethyl alcohol, methyl alcohol, glycerol, potassium carbonate, calcium chloride, magnesium chloride, sodium chloride, and potassium acetate are widely used as secondary refrigerants for these applications. Many synthetic fluids are also being used as secondary refrigerants. Nanofluids are not being used as secondary refrigerants may be due to its cost and operational issues. Ice slurry has been used for some applications. Supermarket systems using ice slurry (a mixture of water, ice and ethanol) have also been evaluated in some European countries. CO2 has also great potential as secondary refrigerant and supermarket installations with CO2 as the secondary refrigerant have been installed with good results. It can be concluded that phase change slurries (ice slurry, microencapsulated phase change material slurry and clathrate hydrate slurry) and CO2 have great potential as secondary refrigerants.