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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.
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.
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.