Advanced manufacture methods for form stable composite phase change materials (FSCPCMs) for thermal energy storage applications

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Jiang, Zhu (2021). Advanced manufacture methods for form stable composite phase change materials (FSCPCMs) for thermal energy storage applications. University of Birmingham. Ph.D.

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Abstract

With the annually increasing worldwide energy consumption and carbon emissions, there is an urgent need to enhance overall energy utilisation efficiency and improve renewable energy penetration. Thermal energy storage (TES) is considered as an effective method to effectively recover industrial waste heat and to address the intermittency of renewable energy. Large-scale deployment of TES technology, however, requires the use of a massive amount of high-performance and cost-effective TES materials. This PhD study concerns the form stable composite phase change material (FSCPCM), which is a composite phase change material that can maintain structure stable during phase change. It is one of the most promising types of TES materials for their favourable thermal properties. However, only a few FSCPCMs are currently at an early stage of commercialisation, owing to low production yield, high costs, and lack of large-scale manufacturing methods and associated expertise. This forms one of the reasons for a slow rate of FSCPCM deployment and the motivation of this research.
This PhD study aims to address the gaps in FSCPCM manufacturing. The focus is on the advanced manufacturing technologies for the FSCPCMs. The investigations include a mix-sintering method and a continuous hot-melt extrusion method. The two methods are scalable, flexible, easy-controllable and potentially very cost-effective. For the mix-sintering method, a modified mix-sintering method with an additional preheating process is proposed and compared with the traditional fabrication route. The microstructure development of the FSCPCM is studied and the criteria for using the modified mix-sintering method to produce high-quality products are investigated and verified. With the hot-extrusion method, a novel HDPE-pentaerythritol (PE)-graphite FSCPCM was firstly extruded by a single screw extruder. Effects of different processing parameters on the extrusion operation stability and rheological behaviour were studied. These include optimal particle loading, particle size and processing temperature. The particle dispersion of the extruded FSCPCM was correlated with different processing parameters. Finally, thermal properties and thermal cycle stability of the extruded FSCPCMs were investigated to assess their suitability for TES applications.
The work on the mix-sintering method shows that both the physical and thermal properties of the diatomite-KNO3 FSCPCM have been improved significantly with the use of the preheating process. The modified mix-sintering method contributes to the formation of a denser structure of the FSCPCM with an increment of ~20% in density and a reduction of up to ~14.67% in the porosity. The thermal conductivity has also been enhanced up to 54.91%. It is found that the melted salt KNO3 is absorbed into the porous structure of diatomite as well as the interparticle voids during the preheating process, leading to a less porous structure of the FSCPCM after compression and sintering. Moreover, a better wettability between the liquid phase and solid surface is found to be an important criterion to capture the benefits of the use of preheating process, while the porous structure is not an essential condition. This finding is verified by both a graphite-KNO3 composite and a SiO2-KNO3 composite.
A novel HDPE-pentaerythritol (PE)-graphite FSCPCM was successfully extruded. The work on the continuous hot-melt extrusion of the HDPE-pentaerythritol (PE)-graphite FSCPCM shows that the particle loading, particle size, processing temperature strongly affect the extrusion stability and rheological behaviour of mixtures for making the FSCPCM. Two extrusion instabilities were observed during the process: sharkskin defect and oscillating defect. A periodic pressure oscillation phenomenon along with surface distortion evolution is found in the unstable extrusion process. The stable and continuous extrusion can be achieved by reducing the mass ratio of particle loading, decreasing the particle size and increasing the processing temperature. Besides, the particle dispersion of the FSCPCM is found to be related to the extrusion conditions. The particle dispersion increases with the extrusion speed until a certain value before it starts to decrease. This is because the particle dispersion is affected by the shear stress and the mixing time, with the former increasing with increasing extrusion speed and the latter showing the opposite trend. The porosity of the FSCPCM is found to increase when the processing temperature increases due to PE vaporisation. The extruded FSCPCM is found to have an energy density as high as 426.17 kJ/kg from 100 oC to 200 oC. Thermal cycling of the CPCM shows a slight decrease of latent heat of the CPCM by 5.72% after 100 cycles.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):