The Design of Thermal Tunnel Energy Segments for Crossrail

Summary

In recent years the drive to reduce carbon emissions and reliance on oil and gas has led to the increase in the use of renewable energy heating systems. The ground source heat pump systems are an example of these renewable systems. Recently tunnels have been considered as a way of connecting building heat pump systems to the energy stored in the soil. On Crossrail it is also recognised that heat is also given off by the trains in particular during braking, stopping at platforms and acceleration away from stations. A complex ventilation system including shaft, fans and Under Platform Extraction (UPE), has been designed to manage the rising temperature in tunnels and stations. This conventional approach results in high energy consumption for running the fans and neglects the possibility to use the extracted heat above ground in building, whilst reducing the energy consumption as well as CO2 emissions.
The Crossrail Tunnel Energy Segment (TES) design seeks to provide an alternative solution to cool the tunnel and the surrounding ground, and transfer this heat to adjacent buildings. Heat pumps are used in the buildings to step up the temperature for hot water supply throughout the year and space heating during winter.
This paper describes the technical challenges associated with:

Assessing the available tunnel heat and linking this heat to adjacent buildings.
Modeling the heat transfer from the tunnel heat to the pipe network in the segments.
Design and construction of the segment pipework and box-outs
Header pipe hydraulic design
Access to the tunnels from shafts and boreholes.
Integration of TES heating with adjacent buildings.
Durability and operational considerations of TES including thermal effects and fire
Operational and commercial benefits to cool the tunnel via TES

The paper also outlines the design for an 860m long demonstration tunnel on Crossrail between Tottenham Court Road and Fisher Street shaft.
The TES modeling of the Crossrail system, which is not dissimilar to other metro-style/frequency urban underground services, indicates that this is a viable technology for the future smart cooling of tunnels. It is smart because it extracts heat in a form that can be utilised in buildings, with a potential revenue stream and because it can be used to reduce the reliance on UPE and tunnel ventilation systems, thus saving on electrical energy. These two benefits provide significant carbon and financial savings at a time where there is a focus on carbon reduction and when fuel costs are anticipated to increase between 50-100% decade on decade in real terms.
Installation of a TES system should be considered in any future metro system, and the development of this concept on Crossrail provides a vital building block in understanding the benefits of the system so that it can be fully implemented on future projects.