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Heat Transfer Enhancement of Latent Thermal Energy Storage in Rectangular Components

Auteurs : ABDI A.

Type de monographie : Thèse de doctorat

Résumé

Latent Thermal Energy Storage (LTES) is an interesting choice to store thermal energy in a sustainable energy system. The primary benefit of LTES is the relatively high latent heat of fusion of the materials, known as PhaseChange Materials (PCM), used in such a system as the storage medium. However, as the thermal conductivity of PCMs is often very low, there is a need to enhance the rate of heat transfer within the charging/discharging process and to improve the thermal performance of the LTES systems.

This thesis addresses the enhancing effect of extending heat transfer area in rectangular LTES enclosures. A key contribution of this thesis is a comprehensive visualization of the phase change processes for an organic PCM, including solidification and melting, constrained as well as unconstrained, known as Close-Contact Melting (CCM), in a cavity with and without fins. Observations have been carried out for fins of different lengths and numbers with a varying angle of inclination, and a comprehensive analysis in terms of phase change time and thermal power is conducted.

The observations show fins are more influential in solidification than in melting, reducing the solidification time by 80% and increasing the mean power by 395%, at a cost of 10% loss in the extracted energy. In contrast, in melting, fins have a modest effect in enhancing the process. The relative enhancement effect of fin is higher in constrained melting than in unconstrained melting. In a case with maximum enhancement, a reduction by 52% in the constrained melting time and a relative enhancement in the meanpower by 90% is achieved at a cost of 9% loss in the stored energy. As the volume fraction of fin increases, the discrepancies in melting time between the constrained and unconstrained melting diminishes.

A numerical model for solidification and constrained melting is validated based on the experiments, and a more inclusive sensitivity analysis of fin parameters is performed. The enhancing effect of different parameters on the phase change time and the thermal power is analyzed and the relatively more effective measures are identified. Analyzing the simulation data with dimensionless parameters for a cavity oriented horizontally and enhancedwith vertical fins, overall dimensionless groups for solidification and constrained melting have been obtained. The dimensionless groups contribute in general to achieving a better understanding of fins parametersand to facilitating the LTES designs.

In addition, this thesis investigates a novel idea of extending the surface area via incorporating mini-channels into LTES enclosures, used as passages for air as a low thermal conductive Heat Transfer Fluid (HTF). The mini-scale internal hydraulic diameter of the mini-channels and their high external area-to-volume ratios make a potential for dual enhancement on both the PCM side and the HTF side. An existing design and a conceptual one with the possibility of adding fins on the PCM side, capable of being manufactured via production methods of extrusion and Additive Manufacturing (AM), respectively, have been simulated and studied.

The two mini-channel types provide considerable enhancements in the rate of heat transfer for a PCM heat exchanger working with air. The degree of enhancement increases as the air flow rate increases, at the cost of an increasingly higher pressure drop. Regarding this, increasing the number of channels is identified as a more effective enhancing measure than adding fins to the PCM side. In addition, the conceptual design with a higher internal hydraulic diameter and considerably a higher aspect ratio has a lower pressure drop than the existing design, charging/discharging the thermal energy at a similar rate but with a lower fan power. More optimized designs with minimization of pressure drop, contribute to paving the way in facilitation of the utilization of the enhanced air-PCM heat exchanger in various applications.

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Pages : 181 p.

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Détails

  • Titre original : Heat Transfer Enhancement of Latent Thermal Energy Storage in Rectangular Components
  • Identifiant de la fiche : 30033988
  • Langues : Anglais
  • Sujet : Technologie
  • Édition : KTH (Royal Institute of Technology) - Suède
  • Date d'édition : 2022
  • ISBN : 978-91-8040-173-9

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