Low Temperature Inorganic Phase Change Materials and the Applications in Buildings

低溫無機相變材料及其在建築中的應用

Student thesis: Doctoral Thesis

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Award date18 Jun 2021

Abstract

The building sector is responsible for over 40% of the global energy produced worldwide, causing an irreversible environmental problem. Precisely, the energy use in space heating and cooling and domestic hot water, estimated to account for roughly half of building energy-consumption, puts high pressure on the power grid. Currently, phase-change materials (PCMs)-based energy storage technologies and systems provide a significant opportunity for better economic benefits and enhanced energy efficiency in buildings. There are four systems, including PCMs-based thermal/cold energy storage systems, PCMs-based solar heat collection system, and PCMs-based building envelope system, that are being widely concerned.

The literature has demonstrated that organic PCMs applied in the above systems are not economical enough. Inorganic PCMs are very low in price and have high latent heat, which is considered more suitable. However, they suffer high supercooling and phase separation, which limit their applications. Therefore, this thesis aims to design and develop various low-temperature inorganic PCMs for applying different energy-saving systems in buildings.

In Chapter 3, concerning domestic hot water and space heating, sodium acetate trihydrate (SAT) was developed as energy storage media into the PCMs-based solar heat collection system (~58.0 ºC) and PCMs-based air conditioning thermal energy storage system (~45.0 ºC) for solving the mismatch between heat supply and demand. Because multi-wall carbon nanotubes (MWCNTs) have a high specific surface area, high thermal conductivity, and solar absorption capacity, it was selected as nucleating agents, thermal conductivity enhancers, and photothermal materials for SAT. The experimental results showed that the supercooling degree of SAT decreased by 0.9 ºC with the addition of 2.0 wt.% of MWCNTs. Meanwhile, the thermal conductivity improved by 36.9%, and the photothermal charging efficiency increased by 3.0 times. Furthermore, to satisfy the application requirements of air conditioning thermal storage systems, the phase change temperature of the SAT/MWCNT composite was reduced from 57.5 ºC to 45.1 ºC by adding 10 wt.% temperature regulator of NH4Cl, and its influencing mechanism was clarified.

In Chapter 4, sodium sulfate decahydrate (SSD) and NH4Cl composite with a crystallization temperature of 7.9 ºC was developed for cold thermal energy storage (~8.0 ºC). Nanoclay (NC) was used as a nucleating agent and thickener to mitigate the supercooling degree and phase separation of the SSD–NH4Cl composite. The results indicated that with the addition of 18.0 wt.% NC, the supercooling degree of the SSD–NH4Cl composite reduced by 0.3 ºC from 18.5 ºC, and its nucleation mechanism was also clarified. In addition, the SSD–NH4Cl composite achieved a shear-thinning characteristic with the addition of 18 wt.% NC; thus, pumping technology is suitable for filling the SSD–NH4Cl–18NC composite into a container. Moreover, this composite did not exhibit any phase separation after 1000 thermal cycles, and the latent heat remained up to 95.2 J/g. These results indicated that the prepared SSD–NH4Cl–18NC is very suitable to apply in air-conditioning cold storage systems.

In Chapter 5, a stabilized binary salt hydrate of a calcium chloride hexahydrate (CCH)–potassium chloride (KCl) composite (~26 ºC) was also prepared for application to building envelopes. Strontium chloride hexahydrate (SCH) was used as the nucleating agent, and a melamine sponge (MS) was used as a carrier to load the CCH–KCl composite to suppress phase separation. The results indicated that the supercooling degree of the CCH–KCl composite was reduced to 1.7 ºC from 20 ºC with the addition of 3 wt.% SCH. Meanwhile, the CCH–MS–3SCH–2KCl composite had a supercooling degree of only 2.7 ºC, and its latent heat retained 143.6 J/g after 600 thermal cycles. In addition, the thermo-regulated performance and electricity consumption of industrial paraffin and the CCH–MS–3SCH–2KCl composite in buildings were evaluated using a numerical method. The results indicated that the static payback periods for a building to which the CCH–MS–3SCH–2KCl composite was applied were only 11.6 and 16.5 years in Changsha and Hong Kong and 30.2 and 57.7 years for industrial paraffin, respectively. Meanwhile, the carbon dioxide emissions could be reduced 16.0 and 25.3 kg/year/m2 in Changsha as the addition of industrial paraffin and the CCH–MS–3SCH–2KCl composite. While for Hong Kong, the carbon dioxide emissions only reduced 6.5 and 10.4 kg/year/m2. Overall, it can conclude that applying the CCH–MS–3SCH–2KCl composite to buildings is feasible.