Document Type

Theses, Ph.D

Rights

Available under a Creative Commons Attribution Non-Commercial Share Alike 4.0 International Licence

Disciplines

2.2 ELECTRICAL, ELECTRONIC, INFORMATION ENGINEERING

Publication Details

A thesis submitted for the degree of Doctor of Philosophy, Technological University Dublin, 2021.

Abstract

In the past few decades solid-liquid phase change materials (PCMs) have gained an increasingly important role in thermal energy storage applications due to their ability to absorb or release large amounts of energy during melting or solidification. The precise phase change temperature varies with different conditions, such as external pressure, small variations in the PCM composition in the case of multi-component mixtures and/or material purity. In order to achieve better energy efficiency for the energy storage process, it is necessary to be able to accurately detect the solid-liquid phase changes in the bulk of a PCM.

Optical fiber sensors allow for direct detection of the phase changes in PCMs while also offering the advantages of a passive nature and small size.

The focus of the research presented in this thesis is on the development of a novel approach to detecting the solid-liquid phase changes in selected PCMs using optical fiber sensors. To achieve this goal, initially the correlation between the temperature, changes in the refractive index (RI) and internal pressure acting upon the optical fiber during the phase transitions was studied for the selected PCM, n-octadecane. Based on the results of these studies, several optical fiber sensing structures have been proposed and demonstrated for the detection of phase changes as follows:

An optical fiber Fresnel reflection sensor for detection of phase changes.

An fiber Fresnel reflection sensor for detection of solid-liquid phase change in n-octadecane is proposed and experimentally demonstrated. The sensor probe consists of a single-mode fiber with a cleaved end immersed in the n-octadecane sample under test. The detection relies on measuring the slope of the output power ratio change which is caused by the RI change during the phase transition. The results of this work suggest that such a simple optical fiber sensor can be used for detection of liquid-solid phase changes in other materials with similar thermo-optic properties to n-octadecane. This sensor realized in-situ detection for a solid liquid phase change, which is a significant advantage compared to the traditional phase change detection methods.

A fiber heterostructure based optical fiber sensor for detection of phase changes.

A single-mode-no-core-single-mode fiber optical sensor for the detection of solid-liquid and liquid-solid phase changes in n-octadecane is proposed and demonstrated. The transmission-type sensor probe consists of a short section of no-core fiber sandwiched between two sections of a single-mode fiber. The detection relies on measuring the level of the output power ratio which is caused by the large step-like variations in the RI of n-octadecane’s. Importantly, compared to the Fresnel reflection sensor, the proposed fiber heterostructure is resistant to bending and strain disturbances during the measurements. The results of this work suggest that the proposed sensor is potentially capable of detecting liquid-solid phase changes in other materials whose thermo-optic properties are similar to those of n-octadecane. Moreover, this sensor not only has the advantage of achieving in-situ phase change detection, but also has the ability of working in an environment subjected to mechanical disturbance, which makes it has great potential of industry applications.

Optical fiber Fabry-Perot sensor based on a singlemode-hollow core-singlemode fiber.

An optical fiber Fabry-Perot sensor to monitor the solid-liquid and liquid-solid phase changes in n-octadecane is also proposed and investigated. The sensor probe is fabricated by splicing a short section of a hollow core fiber between two single-mode fibers. By analyzing the changes in the output spectrum of the probe, such as spectral shift of a selected interference dip, the phase change within a material sample in the vicinity of the fiber probe can be accurately detected. The proposed sensor can deal with PCM types whose RI values make it difficult for the other two sensor types to work, and also can be used for detection of the material’s phase state at a particular point of its volume. This work has the potential to better understanding phase change mechanism and its application in energy engineering. Compared to the other sensors developed in the research presented in this thesis, this sensor has the advantage that the application is not limited by the RI of the PCMs.

DOI

https://doi.org/10.21427/wqq6-1705

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