Ventilation strategies for bus passenger waiting areas and boarding passages : final report

Abstract

With the proposed green deck design, the tunnel exit will be extended further north, and so will the discharge of vehicle pollutants. The air pollutants levels on the deck will be much reduced and pedestrians will be free from direct exposure to the emission from tunnel vehicles. Also, a large open area of the green deck will induce high wind, in comparison with the existing street canyon situation on the road level. With relatively warm weather conditions in Hong Kong, the higher wind is desirable, leading to better thermal comfort. The current environment is a street canyon situation, with unfavorable wind and pollutant dispersions, and both the bus passengers and drivers are exposed to relatively high road-side pollutant concentrations. However, once covered by the green deck, it is expected that the semi-tunnel situation will be resulted in, and higher exposure level will be encountered if on special ventilations are provided. On the other hand, because of the favorable wind and air quality above the green deck region, new opportunities arise. Bus waiting areas will be allocated to the mid-level, with semi-enclosed staircases leading to the open doors of the stopped buses downstairs on the ground level. The bus waiting areas will be provided with natural ventilation using wind catchers, which will pressurize the staircases to minimize pollutants migrated from the covered bus lanes. To assure the air quality and thermal comfort at low-wind conditions, mechanical ventilation will be provided in parallel. In addition, enclosed passageways are proposed to avoid high pollutant exposure of passengers and drivers during the boarding period. The overall objective of the ventilation strategy is to prevent the vehicle emissions from escaping into the adjacent areas, and this will be realized via a holistic engineering approach by: a) provision a central mechanical exhaust with filtration; b) use of wind catcher to utilize the favorable natural wind to pressurize the mid-level and the enclosed staircase and passage way. In such a way, it is expected that the central mechanical extraction air volume will be designed to be the minimum required to maintain tunnel air pollutant level, yet better air quality in the passenger zone and the buses will be maintained by the clean air introduced from the wind catcher. The outcome is the reduced air pollutant exposure of bus passengers and drivers during the waiting and boarding, and reduced air-conditioning and ventilation fan energy use in the mid-level.