Application of MPCM slurry with cooled ceiling to realize a low energy building design

Abstract

The objective of this study is to develop a building energy cooling system using microencapsulated phase change material (MPCM) slurry as both heat transfer and heat storage medium, which is integrated into the cooled ceiling (CC) technology for removing room sensible heat. With the increased specific heat capacity and enhanced heat transfer of the slurry working fluid, pump energy consumption can be reduced and more significantly nighttime cooling using off-peak electricity or evaporative cooling can be more effectively stored to reduce the daytime peak electricity demand. First a series of experimental investigations were carried out to investigate the MPCM physical properties, including latent heat of fusion, melting and freezing points, viscosity data and particle diameter distribution. State-of-the-art equipment was used to characterize the MPCM slurry, including a differential scanning calorimeter (DSC), a rheometer, and a mastersizer. The results indicate that the slurry developed by us is homogenous and can be considered as a Newtonian fluid even when particle mass faction is up to 27.6 %. An experimental rig was set up to understand the heat transfer behaviors of MPCM slurry in a horizontal circular tube under a uniform heat flux boundary condition. The slurry consisted of microencapsulated 1-bromohexadecane (C16H33Br) and water, and the mass fraction of MPCM particles varying from 5 % to 27.6 %. The pressure drop and local heat transfer coefficients were measured, and the influence of particle fractions, heating rates and flow structures on heat transfer performance has been studied. Heat transfer coefficients measured for MPCM slurry are significantly higher than those for single-phase fluid flow in laminar flow conditions, and they exhibit more complicated phenomena at low turbulent flow conditions. Up to 1.67 times higher average Nusselt number relative to pure water was found in laminar slurry flow when particle mass fraction was 0.276, and about 1-2.5 time higher average Nusselt number was found in turbulent slurry flow. Two new heat transfer correlations, which predict the average heat transfer data within an error of +-10 %, were proposed for predicting the heat transfer behaviors of MPCM slurry in the phase change process along the flow direction. Air-conditioning electricity use does not only constitute a major portion of global energy consumption but also stresses electricity generation installation capacities. Opportunities exist to alleviate the problems via improving air-conditioning system efficiency and leveling the electricity demand profiles. The application of MPCM slurry for cooling storage in combination with CC was therefore proposed to investigate these aims. With a separate chilled water system for air-handling units to cool and dehumidify the ventilation air, the remaining space cooling load is taken by the radiant CC panels, which use MPCM slurry of a temperature above 18oC obtained from thermal storage using nighttime electricity chilling, which in turn operates at a relatively high evaporator temperature of 13oC. Building energy simulation technique was used to investigate the thermal performance and energy efficiency of the combined system of CC and MPCM storage. The simulation procedure was fulfilled by enhancing the exiting building simulation code ACCURCY. A feasibility analysis for the combined system was made based on the computer calculations. It is found that a small MPCM slurry tank with volume of 0.0283 m3/(m2 floor area) is able to shift the part of cooling load of a typical office room with the height of 2.6 m from the daytime to nighttime. It is also revealed that the combined system of CC and MPCM slurry storage is an energy saving and economy favorable air-conditioning system. Compared with the combined system of CC and ice storage, the combined system of the CC and MPCM slurry consumes 72 % less electricity power and spends less energy cost at a low ratio (<2.4) of day and nighttime tariff. Evaporative cooling technology is able to generate the cooling medium at a temperature approaching the ambient wet bulb temperature. The assessment of evaporative cooling availability was done for five representative climate cities in China, including Hong Kong, Shanghai, Beijing, Lanzhou and Urumqi. Energy saving potential of the hybrid system of CC, MPCM slurry storage and evaporative cooling technologies was examined by using previous validated building simulation software. The results indicate that the hybrid system offers extensive energy saving potential up to 80 % under northwestern Chinese climate and less extensive energy saving up to 10 % under southeastern Chinese climate. The optimal design of the MPCM slurry storage system has also been made based on the monthly/weekly cooling storage behaviors and cooling demand of the ceiling panels. The optimal size of the slurry tank with two-day cooling storage capacity for a typical office room is found to be 0.034 m3/m2 floor area when it is located in Hong Kong and 0.046 m3/m2 floor area when it is located in Urumqi. The results from this study are useful for the development of the new compact heat exchangers for the utilization of the MPCM slurry and the design of the new thermal energy storage system using MPCM slurry as storage medium.