Refrigeration tends to be overlooked in energy consumption studies, however, air conditioning, cooling and refrigeration are major energy hungry activities. It is inefficient to generate cooling on demand. The idea of generating cold during 'off-peak' periods and using it whenever there is a demand is very attractive. The easiest way of generating a constant temperature cold sink is to use phase change materials which are able to absorb large quantities of heat whilst undergoing phase change. One of the most attractive phase change systems is pumpable slurry ice. Slurry ice is an excellent thermal storage medium which is inherently environmentally friendly. All commercial ice generators have a cold surface on which ice is allowed to form, the ice is then either mechanically or thermally removed. To increase the ice generation rate, the area of the cold surface has to increase or its temperature has to decrease. Both of these have economic penalties; large surfaces mean large expensive units. Lower temperatures imply lower energy efficiencies. This project is about the generation of ice in a continuous reliable manner in the bulk fluid (away from solid surfaces). This requires study in the following:
1. Phase change in super cooled environments
2. Nucleation sites and their effect on phase change
3. Mixing solvent streams of different temperature containing different amount of solutes
4. Heat and mass transfer in miscible fluids
5. Optimisation of heat transfer with small driving temperature differences
The proposed new ice making technique will have a brine cold sink. Heat will continually be removed from the cold brine by a conventional refrigeration unit, this will chill the brine to say -15 oC, but not freeze it. Chilled water (at 0 oC) is then introduced into the cold brine, this results in freezing of some of the water. The amount of water which freezes depend on the degree of mixing of the pure water with the brine before it freezes. This is a complex phenomenon; we require high heat transfer between the water and the brine, whilst needing low mass transfer and very low mixing.
This is a demanding field of work as the area between the water and the brine is ill defined and strongly affect by mixing and diffusion. Nonetheless the ice maker's performance will be dependent on the ratio of heat transfer to mass transfer between the two fluids (this is ratio is generally referred to as the Lewis number). The work will be underpinned by experimental work which will provide data to assist in understanding and in developing scaling rules. The initial experimental work will initially be undertaken in a simple chilled container to demonstrate the ability to generate ice within a miscible fluid and not at solid interfaces. The next experimental work requires more sophisticated equipment, with pumped chilled brine streams and the ability to harvest the ice generated within the liquor.