Groundwater Cooling Pipes in Underground Railway Tunnels

Cooling pipes within underground railway tunnels were identified as a possible cooling technology. Groundwater would be circulated through the pipe to allow heat exchange between the cool pipe and warm air within the tunnel.
Underground railway system usage is growing throughout the developing world and in many cities the underground railway is the most commonly used form of public transport. The heavy use of electrically propelled train carriages within these systems can generate substantial quantities of rejected heat energy. This energy can significantly increase air temperatures within the trains and tunnels. When coupled to high ambient temperatures this can lead to passenger discomfort and health issues. Conventional air conditioning systems have been used in some modern underground railway installations but their operation has had limitations and the units are highly energy intensive solutions. Conventional air conditioning often cannot be included in older systems through heat rejection and unit size problems. Sustainable cooling systems could easily reduce the overall system energy usage and provide an acceptable environment for passengers. Two novel proposals for cooling underground railways are presented here. The first uses cold groundwater to cool the air in the tunnels. This cooler air is then circulated through the tunnel network using the existing ventilation in combination with the piston effect of the trains. The second novel proposal uses the surrounding soil to store thermal energy during operational hours and then to reject this stored heat during nighttime. This geothermal solution would off set the cooling load to the point that the passengers’ thermal comfort would not be negatively affected. This paper introduces a novel mathematical model that has been developed specifically to evaluate the effectiveness and the implementation of sustainable and low energy cooling strategies when applied to underground railway systems. The expected temperatures are calculated for both the train and the tunnel air, together with a profile of the temperature distribution in the surrounding soil. Introduction Underground railway systems can generate enough heat from their operations to raise tunnel and station temperatures as much as 8 to 12K above outdoor temperatures, the greatest differences being at night and in the early morning hours. Passenger discomfort will result in warm weather conditions if the underground railway environment is not cooled.