This paper reports the findings of an investigation into the effectiveness of a low-cost measure for mitigating the problem of deteriorated energy performance of air conditioners in high-rise residential buildings that reject condenser heat simultaneously into recessed spaces outside the buildings. The measure involves installing a horizontal plate at mid-height in the recessed space to direct the hot air from below to flow outside the recessed space, allowing relatively cool outdoor air to flow into the recessed space above the plate, leading to an improvement of the average performance of all air conditioners. The study was performed using a commercial computationalfluiddynamics(CFD)program, with reference to a 30-story residential building in Hong Kong and to the worst scenario in summer with all air conditioners rejecting heat into the recessed space simultaneously under the full-load condition. It was found that the plate would lower the temperature rise of the air, from the fee outdoor condition to the inlet condition at air-conditioner condensers, by around 30% on average. Furthermore, when the plate is moved closer to the next higher (the 16th story) condenser, a further reduction in the air temperature rise by 6% could be achieved.

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