Heat extraction in underground railway tunnels

The Electrical Installations and Safety Systems Subcommittee of the UITP Metropolitan Railways Division has launched a study concerning ventilation and smoke extraction based on observations made regarding the production of heat in metro tunnels.

It soon became apparent that it was possible to recover some of the heat dissipated in metro tunnels. Several networks have undertaken trials geared to energy savings and the urban ecology policy advocated by the UITP.

General network context

Inside a metro tunnel, heat is emitted by:

• Trains: in particular starting and braking, which account for 80% to 90%
• Lighting: which releases roughly 20W/m2
• Electromechanical equipment
• Passengers (emissions of 140W per stationary standing passenger in an ambient temperature of 26°C)

The biggest proportion of heat energy is generated near and in stations.

Three types of equipment are necessary in order to allow:

• Thermal comfort
• Ventilation
• Smoke extraction

Ventilation

Role of ventilation

The role of ventilation is to establish and maintain an atmosphere that is satisfactory in terms of health and thermal comfort in metro tunnels and stations. Natural ventilation is provided by air displacement produced by moving trains and air circulation emanating from the access and structural openings linked to the outside. If natural ventilation is recognised as insufficient in some metro networks, forced ventilation becomes necessary and may also be used for the purpose of smoke extraction in the event of fire. This ventilation must satisfy comfort objectives in the field of temperature, humidity, air speed and air replenishment. The main influencing factors are the external climatic conditions, train movements, passenger numbers, and heat loss from electrical and electromechanical equipment.

Heat uptake essentially stems from the heat energy dissipated by trains (the biggest proportion being generated during the traction and braking phases), lighting, electromechanical equipment and passengers.

Ventilation principle

Longitudinal ventilation is the type of ventilation that is most often found in the railway field. This type of ventilation artificially accelerates the natural air displacement that passes through the tunnel.

Since the biggest proportion of thermal energy is produced near and in stations, it may prove necessary to superimpose longitudinal ventilation in tunnels with transversal ventilation in stations. This solution involves having a shaft beneath the platform that allows air to be aspirated at train level, i.e. as close as possible to where the thermal energy is produced, while a ceiling duct allows fresh air to be blown into the station (cooled, if necessary).

Smoke extraction

In the case of smoke extraction, the principle used involves introducing:

• Drop in pressure within the affected zone in order to extract the smoke
• Boosting the pressure in the protected zone to stop the smoke from spreading to it

This principle, when combined with longitudinal ventilation, is quite efficient in terms of smoke extraction. By increasing station size (metro or rail), ventilation needs in terms of smoke extraction become greater than those for comfort. This has led to the reinforcement of the capacity of smoke extraction plants. Ventilation includes one to two fans.

Influence of platform edge doors

There is a strong link between ventilation/smoke extraction systems and platform edge doors:

• In Hong Kong, these doors provide complete insulation between the station and the tunnel, allowing for savings on the replenishment of station air-conditioning systems and their operating costs.
• In Tokyo, local regulations do not allow the doors to rise up to the ceiling.

Survey of potential uses for dissipated heat

Recovery of heat dissipated in metro tunnels

The benefits from recovering the heat dissipated in tunnels depends on:

• The traffic and the size of the underground network, which can be considered as a ‘low temperature heat source’.
• The value of the tunnel temperature cannot be too low, and the temperature difference with the outside must be significant. For the networks that have heat recovery equipment, the tunnel temperature in winter time is at least 18°C and the difference with the outside at least 11°C.

In terms of heat recovery, the ventilation plants are of great importance in view of the rates of airflow passing through them: 22 to 120m3/s.

The user must be the shortest possible distance from the extraction point.

Heat pump

The heat pump provides one of the technical possibilities for heat recovery. The extraction source, known as the cold source, is the air extracted from the metro, while the restitution point is known as the hot source and uses water (or air) to feed the user’s heat circuits.

A few examples of existing equipment

Munich metro

• Heating of service premises using a 55kW heat pump
• Heating of service premises on line U3 using three identical 40kW units
• Heating of service premises on line U6 using a 30kW heat pump

Vienna metro

• Heating of service premises on line U1 using a 60kW heat pump
• Heating of service premises on line U3 using three identical 40kW units
• Heating of service premises on line U6 using a 30kW heat pump

Paris metro

• Heating of a gymnasium and medical buildings using a 90kW heat pump
• Heating provided for a residential building using a 22kW heat pump

Conclusions

Increasingly complex ventilation equipment is being used to provide passenger comfort. This equipment is designed in a very similar way to the smoke extraction units used as part of fire protection.

For the time being, the integration of additional applications that might allow heat recovery is still highly unusual. However, the potential is there, provided that certain technical conditions are met. This is an opportunity that should be seized within the energy-saving framework.

With regard to the economic balance, the following steps are worthwhile:

• Contemplating the additional cost of recovery compared to the cost of ventilation and smoke extraction equipment
• Looking at matters within the context of an energy-saving policy that might possibly be subsidised

Examples show that the return on investment may be spread over four to 10 years depending on the application.