Von Ness, Ryan Michael (2018-09). Experimental Investigation of a Compact Heat Exchanger for Thermal Energy Storage in Sustainability Applications (for Mitigating Energy-Water Nexus Issues). Master's Thesis.
Thesis
The application of a compact heat exchanger (CHX) as a latent heat thermal storage system (LHTESS) was explored in this study by employing phase change materials (PCM), for supplementing cooling loads of a dry-cooling platform. The melting and solidification times for a fixed mass of a chosen PCM was estimated from temperature measurements (recorded experimentally using a digital data acquisition system) performed in this study. A salt hydrate (lithium nitrate trihydrate) was chosen as the PCM for this study. The experiments were performed by varying the inlet temperature and flow rate, as well as the flow direction of the heat transfer fluid (HTF). Furthermore, the chosen CHX was fortified by filling it with aluminum porous foam that is impregnated with the chosen PCM. The "Cold-finger" technique was implemented during the experiments to enhance the reliability of the TES system. This unique technique is implemented by leaving a fraction of the PCM mass solidified (which ensures partial melting of the PCM instead of complete melting) while thermal cycling experiments were performed involving repeated partial melting and complete solidification of the fixed mass of PCM in the CHX. These thermal cycling experiments were performed to monitor the range of subcooling required to initiate solidification. The "Cold Finger" technique enhances the reliability of these systems by enabling low values of subcooling. Complete melting can often cause the degree of subcooling required to initiate solidification to increase with each thermal cycle - thus compromising the reliability of the LHTESS. Hence, the Cold Finger technique is an effective strategy for mitigating the reliability issues endemic with various PCMs, particularly salt-hydrates. The results obtained in this study demonstrate that subcooling of less than 1 ?C was achieved by combining the Cold Finger strategy with counter-flow configuration of the CHX, thus realizing a LHTESS with enhanced reliability. The experimental results also show that flow rate and inlet temperature of the HTF do not significantly affect the level of subcooling. The repeated thermal cycling experiments performed in this study show that the melting and freezing time are sensitive to both flow rate and inlet temperature of the HTF. The values for melting time and freezing time are more sensitive to the variations in the inlet temperature (than that of the flow rate of the HTF). Increasing the flow rate to achieve the same levels of instantaneous power ratings are associated with higher values of pump penalty to achieve the same goals. Hence, varying the inlet temperature of the HTF is a more effective strategy for enhancing the power rating of the LHTESS explored in this study. The experimental results also show that the uncertainty in estimating the energy storage capacity rating of the LHTESS in this study increases due to parasitic heat transfer (heat loss or gain from the surroundings). For the design conditions explored in this study, the achieved level of cooling exceeds 7 ?C (which is more than the specified value of 5 ?C targeted in this study). The effectiveness during melting was ~0.8 and during solidification was ~1 for the design conditions explored in this study (which is deemed to be adequate for the program requirements of this sponsored research project).
