|Experimental and numerical investigation of integration of cooled-ceiling, nocturnal sky radiative cooling and MPCM slurry thermal storage
|FCE Awards for Outstanding PhD Theses
|Buildings -- Cooling.
Buildings -- Energy consumption.
Heat -- Transmission.
Phase rule and equilibrium.
Hong Kong Polytechnic University -- Dissertations
|Department of Building Services Engineering
|xviii, 190 leaves : ill. ; 30 cm.
|The energy consumption of buildings for cooling purposes has increased considerably during the last decades. Especially in hot climate countries, the penetration of conventional air conditioning units is extremely important, having a serious impact on the peak electricity load. In recent years, new type phase change materials (PCM), microencapsulated phase change materials (MPCM) slurry, are investigated to be used as the thermal storage medium for building cooling applications to achieve the purpose of peak load shifting and energy saving. In this study, paraffin was chosen as the core material of MCPM slurry. The effect of supercooling, one of the major problem for PCM application, on the thermal properties of MPCM slurry was experimental investigated. In addition, the study investigated the proper method to prevent supercooling of PCM by using carbon nanotube (CNT) particles as nucleating agent. Finally, the utilization potential of nocturnal sky radiator combined with MPCM slurry as thermal storage medium is simulated and discussed. At the beginning of the thesis, latent heat of fusion, melting and solidification temperatures and supercooling degree of the material of PCM and MPCM slurry were investigated by using differential scanning calorimeter (DSC). An experimental test device with whole hybrid air conditioning system was also set up to measure the dynamic heat transfer performance of hybrid system combined with cooled ceiling system and MPCM slurry TES under the practical operation scheme. The water slurry of microencapsulated n-hexadecane with a melting temperature of 15.7 ℃ was cooled to 5 ℃ and heated to 25 ℃ cyclically in a storage tank of 230 litre. Melting and crystallization behaviours of MPCM slurry running in a thermal storage test system were investigated experimentally. Furthermore, supercooling and its effect on effective latent heat of MPCM slurry with different experimental conditions were also calculated and evaluated. Empirical equations were obtained to describe the relationship between the effective latent heat and the charging temperature, which can be used for determining the medium mass needed for a specified working temperature range.
Afterwards, CNT was used as the nucleating agent for prevention of supercooling. Various surfactants are firstly tested as additives to overcome the rapid aggregation and sedimentation of the nanoparticles in the organic liquid. Stable and homogenous dispersion was finally attained through surface modification of the MWCNT particles with strong acids H₂SO₄ and HNO₃, plus the addition of 1-decanol as a surfactant to hexadecane organic liquid. The average hydrodynamic diameter of MWCNT-1-decanol in hexadecane was measured by dynamic light scattering (DLS) analysis and the morphology of nano-additives was observed by transmission electron microscopy (TEM). The visible aggregation was negligible even after seven days. Thermal analysis of the n-hexadecane with well dispersed MWCNT particles by DSC indicated that the supercooling of n-hexadecane was significantly decreased with the concentration of 0.1% and 0.5% but only slightly with the concentrations over 1.0%. It appears that well dispersed nanoparticles provided stable foreign nuclei of proper size to promote the heterogeneous nucleation process and accelerate crystallization process, thus the supercooling was significantly reduced. The obvious effects of MWCNT particles on the decrease of supercooling of n-hexadecane provide promising way of improving the performance of system energy efficiency in building cooling and heating applications. At the end of the thesis, a mathematical model of the combined system of MPCM slurry, nocturnal sky radiator and cooled ceiling was built. The cooling energy consumption and the effect of energy-free nocturnal radiation application were simulated using the energy simulation code ACCURACY and MATLAB model based on hour-by-hour calculations in five typical cities across China. It can be drawn that MPCM slurry appears to be a good medium for the combined application of passive cooling technology and the nocturnal radiation in air conditioning system due to its relatively high working temperature. The results showed the energy saving potential in Lanzhou and Urumqi can reach 77% and 62% for low-rise buildings, which exhibits strong attractions for building energy conservation and emission reduction. Hong Kong has the weakest effect in the five typical cities under the same operating condition due to the hot and humid climate condition. The present hybrid system is recommended to be used in northern west and central China cities where the weather is dry and the ambient temperature is low at night.
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