| Abstract | As countries transition to a low carbon economy, there are sizable environmental and economic benefits from developing and using efficient, innovative, low carbon heating and cooling technologies that have the potential to reduce energy use and carbon emissions. This thesis focuses on elastocaloric refrigeration technology and ways of enhancing the thermal outputs of Shape Memory Alloys (SMA), which are the core material of the technology.
The thesis includes an up-to-date and comprehensive critical review and evaluation of recent advances in emerging alternative heating and cooling technologies that have the potential to reduce the environmental impacts of the refrigeration, air-conditioning and heat-pumps (RACHP) sector.
The literature review on elastocaloric refrigeration showed that all the designed and manufactured prototypes to date either work under tension or under compression for pipes.
The experiments also showed that using tension loading for a heat pump device is not practical despite its excellent heat transfer potential, as the material under tension tends to deform permanently much quicker, since the cracks on the surface of the material tend to grow and propagate and thus leading to failure; moreover, tension loading is limited to stress between 100 MPa to 200 MPa. On the other hand, although compressive loading requires specific geometric configurations to avoid failuressuch as buckling, it has a longer fatigue life, because the impurities and cracks do not grow and propagate; moreover, compression loading can exceed stresses of 1000 MPa allowing for better material performance.
To overcome the challenges as identified and stated in the literature, it was necessary to establish a thorough understanding of the designated material properties with the aid of COMSOL MULTIPHYSICS modelling. This included looking at methods of altering the material's characteristics by means of heat-treatment, as well as using material characterization equipment to achieve improved thermal outputs. Moreover, the research focused on proposing and studying a range of novel geometric designs and configurations for the material. The best performing configuration was established, and this led to designing the SMA-heat-pump stack and the fluid flow paths. Also, the research focused on modelling the working fluids in COMSOL MULTIPHYSICS to establish the most appropriate means of
enhancing their thermal properties. The first base-fluid to be tested was water, of which was followed by adding 1%, 2% and then 3% concentrations of Graphene Oxide nanoparticles to
compose new nanofluids that had improved thermal properties.
The research included a study of the relationship between the stress and strain, the temperature lift, and the available latent heat. Since the potential design was to stack the plates and compress them, the results showed that applying a compressive loading of 500 MPa on an SMA specimen resulted in a 1.63% of material’s deformation, a 10K temperature
lift, and 1.46 𝐽. 𝑔−1 of latent heat. When the applied compressive loading was increased to 900 MPa, the material deformed by 5.4% and in so doing achieved a 19K temperature lift and 19 𝐽. 𝑔−1 of latent heat. On the plate design front, the results showed that the rectangular shape channels/fluid-path provided the highest Reynolds number which led to higher heat transfer coefficient; and as a result, it was possible to extract 98% of the available heat within the plate.
On the fluids front, the results showed that the channel/ flow-path has different temperatures at different heights, and it was found that there is a lag between the increase
of the material’s temperature and that of the fluid. It was also found that water achieved a temperature span of 2.8K; however, when 1%, 2% and 3% concentrations of the nanoparticles were added to water, the newly formed nanofluids had better thermal properties, as their thermal conductivity increased by 52%, 59% and 65% respectively, and because of that the temperature lift increased by 25.9% and the loading cycle was shortened by 24% with the third nanofluid (water plus 3% of Graphene Oxide), which will have a positive impact on the compactness and the cost of the SMA core.
This research has contributed to knowledge through the following:
✓ Providing a roadmap for SMA modelling in CFD (COMSOL MULTIPHYSICS) and how SMA is
susceptible to different applied stresses and cycle times.
✓ Providing a roadmap of how to design different SMA geometries that can withstand high stresses and thus could potentially be used as the core material for an SMA-based heat pump device without encountering material failure due to high stresses.
✓ Developing an innovative approach to enhance the heat transfer from SMA through using enhanced nanofluids. |
|---|