Development of novel form-stable composite phase change materials and thermal energy storage concrete

新型定形複合相變材料及儲熱混凝土的研發

Student thesis: Doctoral Thesis

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Author(s)

  • Shazim Ali MEMON

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date15 Jul 2014

Abstract

The building sector is the dominant energy consumer with a total 30% share of the overall energy consumption and is also responsible for one-third of the green house gas emissions around the world. Moreover, in recent years the energy demand for buildings have increased very rapidly due to population growth, enhancement of building services and thermal comfort levels, and the rise in time for people spending inside buildings. Furthermore, it is predicted that fossil fuels will continue to produce 75-80% of the world’s primary energy by 2030. Thus, the increase in energy demand, shortage of fossil fuels and environmental concerns has provided impetus to the development of sustainable building and renewable energy resources. Thermal energy storage is a simple and effective technique for application to building envelops to enhance the energy efficiency of buildings. This, in turn, reduces the environmental impact related to energy use. Thermal energy storage can be accomplished either by using sensible heat storage or latent heat storage or reversible chemical reaction heat storage. Among these methods, latent heat storage utilizing Phase Change Material is the most promising technique because of its advantages of high energy storage density and small temperature change from storage to retrieval. Moreover, organic PCMs are the preferred choice because they are generally chemically stable, do not suffer from super cooling, non-corrosive, non-toxic and have high latent heat of fusion. However, for successful utilization of PCMs in building envelop, the phase change temperature should be in the human comfort zone. Therefore, Dodecyl alcohol and paraffin, which have melting temperature in the human comfort zone and have high latent storage capacity, were selected as PCM for this research. Combination of building materials and PCM is an efficient way to increase the thermal energy storage capacity of building components for the purpose of direct thermal energy storage in buildings. Therefore, in the first part of this research, the focus was on the development of novel-form stable composite PCMs by incorporation of PCM through vacuum impregnation in widely used construction materials such as Kaolin, granulated blast furnace slag and cement. The composite PCMs were characterized by using SEM and FT-IR. Thermal properties, thermal stability and reliability of the composite PCM were determined by using DSC, TGA and thermal cycling test. Moreover, thermal performance of cement paste composite PCM panel was evaluated by self-designed heating system. Test results showed that maximum percentage of PCM retained by construction materials without leakage was found to be 24% and this composition was named as form-stable composite PCM. From ESEM, it was found that PCM was well confined into the pores of construction materials through capillary and surface tension forces which, in turn, prevented the seepage of melted PCM. FT-IR results showed that the form-stable composite PCMs are chemically compatible and the interactions between the components of form-stable composite PCMs are physical in nature. The phase change temperatures of the form-stable composite PCM determined by DSC were in the human comfort zone and they possessed considerable latent heat storage capacity. TGA results showed that the form-stable composite PCM are thermally stable and they did not show any sign of degradation below 100ºC. From thermal cycling test it was revealed that the form-stable composite PCMs are thermally reliable. Thermal performance test showed that in comparison to the control room model, the room models prepared with form-stable composite PCM reduced the temperature fluctuations and maximum indoor center temperature by up to 4.9ºC. Moreover, the temperature curves of the room models prepared with form-stable composite PCM were right-shifted. This aspect could be beneficial in reducing the peak demands on gas and power utilities. It can therefore be concluded that the prepared form-stable composite PCMs are potential candidates for thermal energy storage applications in buildings. Normal weight and Lightweight aggregate concrete are an important and versatile building material in all areas of construction worldwide. The large thermal mass of NWAC and LWAC buildings can be advantageous especially in moderate climates where it can be used to store energy during the day and release it during night time therefore reducing the need for auxiliary cooling and heating. In addition, the energy storage capacity of NWAC and LWAC can further be enhanced by incorporation of macro encapsulated PCM-LWA into it. Therefore, in the second part of thesis, the focus was also on the development of thermal energy storage concrete by incorporation of prepared macro encapsulated Paraffin-LWA into NWAC and LWAC. The macro encapsulated Paraffin-LWAs were prepared by incorporation of Paraffin into porous LWAs through vacuum impregnation and they were characterized by using MIP, SEM and FT-IR. DSC and TGA were used to determine the thermal properties and thermal stability of the macro encapsulated Paraffin-LWAs. The sealing performance of the epoxy was evaluated by thermal cycling test while the thermal conductivity of macro encapsulated Paraffin-LWAs was improved by incorporating different percentages of graphite powder into epoxy. The compressive strength and shrinkage strain of thermal energy storage concrete was also evaluated. Moreover, indoor and outdoor tests were carried out to evaluate the thermal performance of concrete containing macro encapsulated Paraffin-LWA. Finally, the economic and environmental aspects of application of macro encapsulated Paraffin-LWA in a typical floor area of public housing rental flat in Hong Kong were evaluated. According to MIP and vacuum impregnation results, it was estimated that Paraffin can penetrate into the pores space with diameter of 0.9μm and the maximum percentage of paraffin absorbed by LWA was found to be 70%. The thermal conductivity of the macro encapsulated Paraffin-LWA with 15% graphite powder was 162% higher than that of epoxy. FT-IR results showed that the interaction between the components of macro encapsulated Paraffin-LWA is physical in nature. DSC results showed that the phase change temperature of the macro encapsulated Paraffin-LWA is in the human comfort zone and it has substantial latent heat storage capacity (102.5J/g). TGA results showed that macro encapsulated Paraffin-LWA did not show any sign of degradation below 150ºC while thermal cycling test revealed that the macro encapsulated Paraffin-LWA sustained 1000 cycles of melting and freezing and did not show any sign of leakage. The compressive strength of NWAC with macro encapsulated PCM-LWA at 28 days varied from 33.29 to 53.11MPa therefore it opens an opportunity for structural purposes. Moreover, the compressive strengths of LWAC incorporating macro encapsulated Paraffin-LWA was also higher than mixes without macro encapsulated Paraffin-LWA at all ages. From shrinkage results, it was found that with the increase in the percentage of macro encapsulated Paraffin-LWA in LWAC the shrinkage strain reduced. In comparison to LWAC without macro encapsulated Paraffin-LWA and at the age of 57 days, LWAC with 100% macro encapsulated Paraffin-LWA showed 41.8% reduction in shrinkage and therefore has a beneficial effect on the volume stability of LWAC. From indoor thermal performance test, it was found that in comparison to control room models, the room models with macro encapsulated Paraffin-LWA in NWAC and LWAC reduced the maximum temperature measured at the center of room by up to 3ºC and 4.7ºC respectively. These macro encapsulated Paraffin-LWA incorporated rooms were also found to be effective in reducing the temperature fluctuations within the room. Therefore, NWAC and LWAC incorporated with macro encapsulated Paraffin-LWA have a function of reducing the energy consumption by decreasing the indoor temperature; flatten the fluctuation of indoor temperature and shifting the loads away from the peak periods. From outdoor thermal performance test on thermal energy storage LWAC, it was found that in comparison to control room model, the room model prepared with macro encapsulated Paraffin-LWA showed gentle fluctuations and reduced the maximum temperature measured at the center of room by up to 2.9ºC. The surface temperatures measured on the wall panels were reduced by up to 4.7ºC. Therefore, macro encapsulated Paraffin-LWA can improve the indoor thermal environment. From economic evaluation of application of macro encapsulated Paraffin-LWA in a typical floor area of public housing rental flat in Hong Kong, the recovery or payback period was found to be 29 years for macro encapsulated Paraffin-LWA incorporated in LWAC and 28 years for macro encapsulated Paraffin-LWA incorporated in NWAC. Therefore, the incorporation of macro encapsulated Paraffin-LWA in NWAC and LWAC building walls is economically feasible. Finally from environmental prospect, a saving of 465 kg CO2-eq/year or 12.91 kg CO2-eq/year/m2 was achieved. This reduction would contribute to mitigate Greenhouse Gases emissions over the life span of building. It can therefore be concluded that the prepared macro encapsulated Paraffin-LWA is a potential candidate for thermal energy storage applications in buildings.

    Research areas

  • Concrete, Change of state (Physics), Building materials, Thermal properties, Heat storage