Heat pipe thermally enhanced phase change material

Khalifa, A 2015, Heat pipe thermally enhanced phase change material, Doctor of Philosophy (PhD), Aerospace, Mechanical and Manufacturing Engineering, RMIT University.


Document type: Thesis
Collection: Theses

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Title Heat pipe thermally enhanced phase change material
Author(s) Khalifa, A
Year 2015
Abstract Concentrating solar power (CSP) is increasingly considered because it facilitates harvesting a large amount of solar thermal energy. In order for CSP plants to be operationally cost competitive and efficient, they must be able to harvest and store the maximum solar thermal energy during the period of solar availability and to utilize the stored heat for continuous electrical power generation during times when the sun is not available. Hence, efficient and high-density thermal energy storage is an essential aspect of CSP system development. Latent heat thermal energy storage (LHTES) systems can directly reduce the development costs of CSP plants, in terms of smaller installation space and reduced heat storage material requirements. However, the low thermal conductivities of phase change materials (PCMs) have limited their thermal potentials.

This research focuses on increasing the overall thermal conductance of PCMs used in LHTES units through using heat pipes (HPs). Specifically, three new designs of LHTES units, which utilize HPs to enhance the energy transfer, were numerically and experimentally investigated. Three different numerical models along with three different experimental configurations were developed to assess the thermal performance of the proposed units. Also, a preliminary system sizing was conducted to estimate the size of each unit required for 50MW electrical power output. After calculating the size of each unit, the units were economically assessed.

Firstly, the advantages of utilising axially finned HPs in LHTES units were numerically and experimentally quantified. The experimental measurements were conducted on a bare heat pipe and on an identical heat pipe with four axial fins. The results have shown that the energy extracted from the PCM increased by 86% and the heat pipes effectiveness increased by 24 %. Secondly, finned heat pipes were held, in suspension, adjacent to the heat transfer channel in order to increase the overall heat conductance of the PCM and act as effective heat spreaders. The results have shown that the effectiveness of the twelve-heat pipe configuration reached 2.4 after 5h of simulated operation. Thirdly, the thermal performance of a micro heat pipe-phase change material (MHP-PCM) composite was studied. A 3D numerical model was introduced to firstly predict the effective thermal conductivity of MHPs-PCM composites, and secondly to simulate heat transfer and phase change processes in a high-temperature LHTES unit. The model takes into consideration the effects of the MHP orientation as well as the MHP volume fraction. The results have shown that the thermal conductivity increased by approximately 35 times at a MHP volume fraction of 10%. Finally, the last portion of this research focused on size estimation of each of the designs proposed in Chapters 4 and 5. Economic assessments of each design are then presented. Compared with the existing two-tank system, the capital cost of the unit presented in Chapter 4 was found to be 8% lesser. In contrast, the unit presented in Chapter 5 is found to be significantly more expensive than the two-tank system. That is, its total cost was found to be 5.6 times higher than the two-tank system.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Aerospace, Mechanical and Manufacturing Engineering
Subjects Computational Heat Transfer
Energy Generation, Conversion and Storage Engineering
Keyword(s) Heat pipe
Solidification
Phase change materials
Latent heat energy storage systems
Concentrating solar power plants
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