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HS2: High speed cooling A common consideration for all new or extended railway projects is balancing the benefits that the railway brings with the impacts it will have on the surrounding environment. In rural locations the impacts may be dominated by visual or noise considerations. In city locations these considerations also exist, but perhaps more challenging constraints arise in the availability and cost of land. Collectively, this tends to result in railways being placed in tunnels below ground. HS2 is no exception to this, with the proposed Phase 1 alignment resulting in 12 tunnels ranging from 325m long to 13,400m between Euston and Birmingham. Placing a railway underground introduces challenges in providing an acceptable tunnel environment for passengers and staff. This environment must address the situation when the trains are running through the tunnels normally and when the trains stop for operational reasons. A safe environment must also be provided in the very unlikely, but very serious, event of a fire within the tunnel. Some of these challenges are common to all railways and are well understood by specialists in tunnel ventilation and cooling. The proposed high speeds, and services frequency, of HS2 generate a unique set of challenges that require consideration. In addition to these ‘internal’ challenges, ‘external’ challenges arise due to the impacts of reasonably foreseeable climate change. This is a real consideration for major infrastructure projects such as HS2 where the structures are planned to last for over 120 years. Parsons Brinckerhoff is delivering the preliminary railway systems design for HS2, including the tunnel fire and ventilation systems which manage the tunnel environment during normal and emergency operations. During normal operations control of tunnel temperature, and in particular maximum temperature, is a key challenge and is discussed further in this narrative. High speed cooling challenge requires complex considerations for HS2. The temperature of a tunnel is a balance between the heat sources and absorbers of the heat (termed ‘heat sinks’). The trains and train operations are the most significant sources of heat within a tunnel, and by some considerable margin. For an accelerating train, these heat sources are dominated by the losses and inefficiencies of the traction package while it controls the amount of power fed to the train motors to within acceptable limits. When the train reaches its speed limit (termed ‘line speed’) the heat sources are a mixture of the electrical losses in the traction package and motors, the train’s aerodynamic drag, the frictional losses in the gearboxes and at the wheel rail interface. But they also include heat emission from the air conditioning systems and other auxiliaries such as power filters, power conversion equipment, communication systems, lighting, small power and braking systems. It is, however, during braking that the heat emissions can become high. Braking Energy Regenerated Power Friction Braking Vehicle Drag Electrical Losses Heat Electrical Equipment Essential Engineering Intelligence for Transport IET Sectors Case Study A moving object contains kinetic energy which is equal to its mass multiplied by its speed/velocity squared. During braking this kinetic energy must be absorbed. Most modern trains use their motors in a reverse cycle to act as generators to slow the train down. In doing so electricity is generated that can be used by other trains. HS2 is no exception to this and is planning to use this sustainable technology to the maximum extent possible. What makes HS2 different to many other railways are the two prime factors in the kinetic energy equation: mass and speed. The HS2 trains are proposed to be up to 400m long which represents a very large train mass. The trains are also being planned to operate at a speed of 360 kilometres per hour; higher than existing high speed rail services. Since the kinetic energy is a function of speed squared, a train travelling at half of this speed (for example) would have only a quarter of the kinetic energy. When the trains are running according to timetable it is envisaged that much of this kinetic energy can be regenerated with minimal braking heat rejection caused by the naturally occurring inefficiencies in converting mechanical work to electrical energy. Because of the high speeds and mass, even these inefficiencies can amount to an appreciable amount of heat. When recovering from potential disruptions the braking rates may need to be faster, meaning that some of the braking may need to be achieved by friction, which ultimately ends up as heat. The heat rejection needs to be managed to provide acceptable tunnel conditions. Parsons Brinckerhoff has brought to HS2 its understanding of how railways operate as complex inter-related sub systems along with its experience in methods of managing tunnel temperatures. These methods have been applied in numerous railways around the world and combine a mix of practical experience and detailed numerical modelling using such software as the Subway Environment Simulation software (co-developed by PB). During normal train operations the intent is to manage the tunnel temperatures to below 35°C which would allow the train air conditioning systems to provide a comfortable experience for the passengers without excessive amounts of cooling being installed on the trains. This is achieved by a mixture of natural ventilation using draught relief shafts, mechanical ventilation using fan shafts, and optimisation of the train operating profiles to result in train speeds and braking locations that balance the heat rejection with the timetable needs. Climate change provides an external threat to the railway operations. A range of climate change scenarios are being considered by the team to balance the potential risks associated with warmer tunnels with the cost and viability of mitigations. A tunnel warming in excess of 5°C is predicted for some of the climate change scenarios being considered. Whilst more cooling might be fitted on the future trains, the tunnels need to be cool enough to allow maintenance and passenger evacuation during an emergency. This raises the potential need for future cooling of the tunnels. A logical and important question arises as to whether the waste heat from the tunnels could be re-used. Such waste heat is at comparatively low grade for domestic uses such as heating, but with the use of heat pumps it could achieve a useful temperature for injection into district heating networks in nearby communities or businesses. In this way there is an opportunity to use similar technology that would be required to cool the future tunnels in summer, to assist in recovering and utilising the waste heat during winter. In the coming design stages it is proposed to investigate methods to recover the heat from the tunnels which could include cooling pipes or embedded pipes within the tunnel liner. These would be mapped to locations that have the potential to re-use the waste heat for district heating networks. Where cooling might be required, methods to gradually increase the cooling provision of the life of the railway would be investigated. Sustainable cooling using borehole water, such as that adopted by London Underground at Green Park station, would also be reviewed. This IET Transport Sector Case Study was written by Mark Gilbey, principal engineer and tunnel ventilation specialist at Parsons Brinckerhoff. Parsons Brinckerhoff is a Corporate Partner of the IET. www.theiet.org/transport The IET is a world leading professional organisation sharing and advancing knowledge to promote science, engineering and technology across the world. The professional home for life for engineers and technicians, and a trusted source of essential engineering intelligence. The Institution of Engineering and Technology is registered as a Charity in England and Wales (No. 211014) and Scotland (No. SC038698). Michael Faraday House, Six Hills Way, Stevenage, Herts, SG1 2AY.