Experimental study of reduced space cooling load and thermal comfort with stratified air distribution

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Experimental study of reduced space cooling load and thermal comfort with stratified air distribution


Author: Shiu, Chui-shan Angel
Title: Experimental study of reduced space cooling load and thermal comfort with stratified air distribution
Degree: M.Eng.
Year: 2011
Subject: Office buildings -- Air conditioning -- China -- Hong Kong.
Displacement ventilation.
Cooling load -- Measurement.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: ix, 163 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2451597
URI: http://theses.lib.polyu.edu.hk/handle/200/6379
Abstract: Air conditioning systems in Hong Kong offices are used to maintain thermally comfortable environment for occupants. Stratified air distribution system, which is not popular locally, has better control of temperature in the occupied zone within acceptable level, while supplying cooled air at or near the floor level and returning at the middle or ceiling levels. Hence, this innovative system can provide high ventilation effectiveness but requires less energy to maintain comfort indoor thermal environment than traditional mixing ventilation system. Although the fact that stratified air distribution system is being increasingly applied in buildings, there is a strong need for a better understanding of several key performance features, e.g. space cooling load reduction and thermal comfort, of this system. This research aims to investigate the ventilation performance of a stratified air distribution system in a typical Hong Kong office with different locations of return air grilles and supply air velocities. In this study, the effects of locations of return air grilles and changes in the supply air velocities on the cooling load reduction and thermal comfort performance were analyzed. A full scale model, stainless-steel thermal chamber with two thermal manikins, has been used to examine the thermal condition in a two-person office with stratified air distribution system under Hong Kong conditions. Comparisons have been made to the cooling load and thermal comfort performance of the stratified air distribution system with high and low supply air velocities in this study. Vertical temperature gradient and difference between temperatures of return and exhaust air have critical effects on the thermal comfort performance and cooling load reduction estimation respectively. Objective measurements such as supply, return and exhaust air temperatures, thermal manikin skin surface temperature and air temperatures at different heights were measured in the chamber. Regarding the two-person office in this study, the temperature gradient between return and exhaust air is significant (1-1.35°C) in both scenarios of high and low supply velocities with locating return air grilles and exhaust air grille at 1.5m and 2.5m respectively. These operation settings also provide 6.65-9.37% reduction in cooling load. The vertical temperature gradient has an insignificant impact on thermal comfort sensation in both cases of high and low supply velocities. Draft perceptions at foot, thigh, back, arm and head are slightly affected by local thermal sensations with both summer clothing. Further study is required to analyze the effect of horizontal position of return air grilles on thermal comfort. Some assumptions were made in the experiments, such as no heat loss through walls of the chamber, effects of distribution ratio of radiant heat and heat sources interaction on cooling load reduction and thermal comfort sensation. More accurate estimation in the future studies is required on the radiant heat distribution ratio, heat sources interaction, the changes in horizontal locations of return air grilles and thermal manikins.

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