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PCM energy storage modeling case study for a solar-ejector cooling cycle.

Author(s) : ALLOUCHE Y.

Type of monograph: Doctoral thesis

Summary

The subject of this thesis is the evaluation of a phase change material (PCM) slurry as a low temperature thermal energy storage medium and as being part of a solar driven air conditioning system using an ejector cooling cycle. The PCM slurry studied is a microencapsulated phase change material (RT15) suspended in water with a concentration of 45% w/w and with a phase change temperature around 15ºC. The ultimate goal of the thesis is to provide scientific background and adequate methodology for assessing and designing latent heat storage systems. The work focuses on two main areas of interest, thermo-physical property characterisation of the PCM and heat transfer study. Both areas are explored using experimental and numerical approaches. A numerical model is also developed that can be used for the dynamic simulation of the performance of an integrated solar driven air-conditioning system depending on design parameters and climatic conditions.
In order to create a fundamental background for the research work carried out within this the-sis, an exhaustive review of the available literature is presented. The existing solar cooling technologies including solar thermal, thermo-electrical and thermo-mechanical systems are summarised with their most relevant performance characteristics. The typical thermal energy storage systems are discussed with a special attention to latent heat storage. Phase change materials for the storage of thermal energy are classified, their advantages and challenges are discussed in details. The most commonly used experimental techniques and existing mathemat-ical models to describe non-linear physical properties of phase change materials are presented.
Although the work is focusing on PCM thermal energy storage, a numerical model of a 5 kW rated capacity steam ejector is developed using computational fluid dynamics (CFD). The objective was to better understand the flow structure inside the ejector and thus the performance of the ejector cooling cycle. The effect of the operating conditions on the entrainment ratio and critical back pressure are analysed. A performance map of the ejector is constructed to be used in a dynamic system simulation tool also developed in this thesis.
The thermo-physical characterisation of phase change materials is crucial for successful design and integration of a latent heat storage unit into a thermal energy system. In the present study, specific heat, specific enthalpy variation, thermal conductivity as well as the density of the PCM slurry are experimentally determined. A rheological study is also carried out using a rotational viscometer with temperature control. A non-Newtonian shear-thickening behaviour of slurry was identified. In order to directly compare the thermal performance of the PCM to a storage system using water (sensible heat storage), an experimental analysis of the heat transfer into a 100 l capacity cylindrical storage tank is carried out during the charging process under identical operating conditions. The results showed that the amount of energy stored using the PCM slur-ry is 53% higher than for water after 10 hours of charging, for the same storage tank volume. The surface heat transfer coefficient between the PCM slurry and the tube wall, used for the energy charging process, increases during phase change, however it remains smaller than the values obtained for water.
A three dimensional CFD model of the PCM storage unit using ANSYS/FLUENT commercial soft-ware package is also developed to evaluate its capability of accurately simulating the energy storage during the charging process. The thermo-physical properties implemented in the model are based on the results of the experimental work carried out within this thesis work. The im-portance and characteristics of the natural convection flow structure inside the storage tank containing the PCM suspension is analysed. The results obtained from the numerical simulation are validated with experimental data. It was found that PCM bulk temperature, rate of thermal energy storage and accumulated energy were generally well within 10% for different mass flow rate of the charging heat transfer fluid.
A dynamic simulation model for the integrated solar-driven ejector cooling system (SECS) in-cluding the PCM storage unit is developed using TRNSYS software package. A case study is pre-sented to satisfy the summer cooling load of a 140 m3 office space located in Tunis, Tunisia. The analysis indicates that the installation of only a small hot storage tank is advantageous, while it is more beneficial in terms of system efficiency to integrate a PCM cold storage tank. Based on the parametric analysis using the dynamic simulation tool developed, design recommendations are given to assure high performance system operation.

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Pages: 248 p.

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Details

  • Original title: PCM energy storage modeling case study for a solar-ejector cooling cycle.
  • Record ID : 30030792
  • Languages: French, English, Arabic
  • Subject: Technology
  • Publication: National Engineering School of Tunis (ENIT)/Faculty of Engineering of the university of Porto (FEUP) - Tunis - Portugal/Tunisia
  • Publication date: 2016/03

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