Innovative heat transfer enhancements for thermal energy storage systems based on phase change materials

Rapid economic growth has led to increased use of fossil fuels as an energy source. This has a major impact on greenhouse gas emissions. Scientists around the world are investigating new, more economic and renewable sources of energy. A complementary approach is to improve existing energy storage systems, which are as important as the development of new energy sources. Thermal energy is widely available in nature as a by-product of numerous energy conversion plants. Thermal energy could be accumulated using latent, sensible and thermo-chemical storage methods. The latent heat thermal energy storage method that utilizes phase change materials (PCMs) is the most promising energy storage technique. The method has a high energy storage density and is characterized by a slight variation in temperature during the process of energy storage. The low thermal conductivity is a major drawback of the PCMs and limits their application as latent heat thermal energy storage materials. That low thermal conductivity of PCMs increases their thermal energy storage and release times. The present study investigates various PCMs-based solutions to enhance the overall storage system thermal performance. The solutions proposed in this thesis comprise an innovative design of metallic fins and the introduction of innovative heat exchanger geometries. Numerical simulations that monitor melting (charging) and solidification (discharging) processes, are conducted to evaluate the proposed solutions. This manuscript is divided into four main parts that report the results of the investigations seeking to improve the latent heat energy storage systems utilising PCMs. In the first part of this study, innovative fin shapes were introduced to enhance the thermal performance of PCM in a triple tube heat exchanger. These fins included: tee fins and plus fins. To evaluate the thermal performance of these fins, a comparison was made with the traditional longitudinal fins. The results showed significant improvement in the PCM thermal performance when using the plus fins shape as compared to the longitudinal fins and tee fins configurations. When utilizing the plus fins, the PCM melting time reduced by 22% and the solidification time decreased by 25% compared with using the longitudinal fins. In the second part of this study, novel fin shapes were introduced to improve the thermal performance of PCM in a shell and tube heat exchanger. These fins included: tee fins, and tree fins. To evaluate the thermal performance of these fins a comparison was made with the traditional longitudinal fins. Similar to the triple tube heat exchanger, the results showed significant enhancement in the PCM thermal performance when utilizing the tee fins as compared to the longitudinal fins. For the tee fins, the PCM melting time reduced by 33% and the solidification time decreased by 33% when compared with using the longitudinal fins. In the third part of this study, an innovative heat exchanger geometry (webbed tube heat exchanger) was introduced to improve the thermal performance of PCM. To demonstrate the value of the webbed tube heat exchanger design, its thermal performance was compared against conventional heat exchangers, which included: triple tube heat exchanger, shell and tube heat exchanger, and multitube heat exchanger. When utilizing the webbed tube heat exchanger, the PCM melting time reduced by 50% and the solidification time decreased by 45% as compared to using the triple tube heat exchanger. Moreover, by utilizing the webbed tube heat exchanger, the PCM melting time reduced by 72% and the solidification time decreased by 66% when compared with using the multitube heat exchanger. Finally, the webbed tube heat exchanger, introduced in the third part of this thesis, was employed instead of the inner and middle tubes of the triple tube heat exchanger to produce the modified webbed tube heat exchanger. The modified webbed tube heat exchanger was proposed as a novel heat exchanger design to improve the thermal performance of the latent heat thermal energy storage systems that are based on PCMs. The study proved that by utilizing the modified webbed tube heat exchanger, the PCM melting time was reduced by 38% and the solidification time was decreased by 41% when compared to using the webbed tube heat exchanger. Comparisons were made with previous related experimental studies. The numerical results showed good agreement with the available experimental results and justify the investigations of cases that have not previously been studied. This systematic approach could constitute guidance for any practitioner looking to improve PCMs-based solutions