Innovative method of engineering phase change within a bulk fluid (Energy efficient way of manufacturing ice)

Lead Research Organisation: University of Bristol
Department Name: Mechanical Engineering

Abstract

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.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509619/1 01/10/2016 30/09/2021
1815230 Studentship EP/N509619/1 01/10/2015 30/06/2019 Samuel Brooks
EP/R51245X/1 01/10/2017 30/09/2021
1815230 Studentship EP/R51245X/1 01/10/2015 30/06/2019 Samuel Brooks
 
Description Ice slurries are formed from a combination of ice and water containing freezing point depressant. Despite the proven potential uses and energy saving capacity, they remain underutilised. Improving the efficiency of slurry production could increase adoption in many industries. Applications requiring greater hygiene have also developed in food production and medicine. Currently scraped surface generators are the favoured generation method; a large body of work has focused on finding an effective replacement. The work undertaken investigated a new method, utilising plastic pipes to slow ice growth and reduce adhesion.

A coiled nylon pipe was cooled to cool an internal brine (saltwater) flow until ice nucleated in the flow. Initial results showed that ice slurry above 15% could be produced, at refrigeration temperatures from -8 to -13C. Refrigeration temperatures that are warmer than scraped surface generators, indicating potential improved efficiency. Changing the flow direction, coil orientation, and fluctuating the flow rate in the pipe, failed to increase ice production. A control system, designed to set flow rate of the fluid, maintained ice fraction above 10%, despite rising refrigeration temperatures. A comparison between aluminium, silicone rubber, PTFE and nylon HCHXs was conducted. PTFE produced the highest ice fractions and production rates, possibly due to its high contact angle (hydrophobic) surface. Further experiments with silicone rubber pipe cooled in a tube-in-tube heat exchanger were conducted. The pipe was flattened in order to help remove ice formed in the tube. Small improvements in ice production were observed.

Successful improvements noted in previous chapters were combined with a smaller design. Fluid phase was recirculated in through the pipes to generate ice fractions up to 23.5% after 6000 seconds of operation. Possible designs for implementing this method in a working generator with a refrigeration loop were proposed. A working prototype of one of these methods would be required to confirm the improved efficiency over scraped surface generators. Nevertheless, this method does offer more hygienic ice slurry generation, which could be exploited in the food and drinks industry or medical cooling. Many remaining avenues of research still exist which could optimise ice production in plastic heat exchangers further.
Exploitation Route The method can produce ice slurry more hygienically than many other previous methods and has the potential to be used in the medical or food production/dispensing industries.
For example, it could be used to make ice slurry from beer to create self-cleaning beer lines. Or used for hygienic fast cooling or organs and tissues in medicine.
Sectors Agriculture, Food and Drink,Healthcare,Pharmaceuticals and Medical Biotechnology

 
Description Phase change conference and ice slurries 2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Attended Phase change material conference in Canada.
Year(s) Of Engagement Activity 2018