Application of heat recovery to long tunnels

J Thompson, M Gilbey, M LeggWSP | Parsons Brinckerhoff in the UK

Agenda• Case study• Heat recovery methods

– Heat recovery pipes– Geothermal tunnel liners

• Stakeholder analysis• Heat storage• Conclusions

Case study• 13 km uni-directional rail tunnel• Air temperature ranges 25C (winter) to 35C (summer)• Potential for recovery of waste heat

Tunnel cooling pipes• Bare pipes in tunnel bore• Possible to retrofit• Well understood, proven in

service• Potentially limited service life• Precedent includes;

o Channel tunnel• Fins may be used to increase

effective area

Tunnel Airflow Hairpin cooling pipe circuit

Tunnel Airflow

Ventilation shaft

Geothermal tunnel liners• Polyethylene pipes cast into

segmental tunnel liner

• Not normally practicable to retrofit

• Longer service life

• Larger active area for heat exchange

• Limited analytical data available

Geothermal tunnel liners• 2D Finite Element model built

• Parametric study data fitted to curve to calculate surface heat transfer

• Corrections made for inactive perimeter and longitudinal spacing

h= convective HTC (w/m2K)ΔT = Temperature difference between air and pipe (°C)

Stakeholder analysis• Heat maps supplied by local

authority• 500m radius from shaft location

• Heat demand• Peak during winter• Potentially large daily variations

• Heat supply• Peak during summer• Lesser daily variation

• Low grade heat – additional “top up” heating likely required

• Assumed gas fired condensing boiler

Heat storage• Heat storage may be beneficial to

“smear” daily supply profile• Heat wasted on high grade side

therefore poor financial case• Slight improvements for short pipe

lengths

• Low grade storage considered impracticable

• Lag in heat supply• Requires large storage space• Capital cost of equipment• Environmental risk of water storage

Comparison with geothermal liner• Similar heat recovery can be achieved in

shorter tunnel section• Tunnel pipes – 1,525m• Embedded liner – 915m

• Higher return temperature of water• Reduced water flow rate• Improved efficacy of heat pump• Recued need for top up heat

• Order of magnitude capital costs• Tunnel pipes - £450,000• Embedded liner - £1,100,000

• Does not account for replacement of tunnel pipes in service

Conclusions• Potential for significant level of heat recovery from tunnels• Large mismatch between heat supply and heat demand• Financial case contingent on identifying nearby stakeholders

– Users of low grade heat preferential– Poor financial case if displacing gas– Users may value renewable energy as a heat source

• Heat recovery pipes– Attractive based on initial capital cost– In-service replacement of pipes erodes financial case

• Embedded liner– Considerably more expensive– Potentially longer service life (similar whole life cost)– Higher system efficacy, reduced requirement for top up heat

Questions?