Abstract
Shallow geothermal energy systems are known to efficiently provide renewable energy for heating and cooling purposes. Energy geo-structures constitute a relatively recent application of these systems where the use of traditional purpose-made boreholes or trenches as ground heat exchangers (GHEs) is minimised or avoided by incorporating geothermal piping in underground structural elements such as piles, retaining walls and tunnels. This study explores the application of energy tunnels in the M4 – M5 Link project in New South Wales, Australia, which includes the construction of twin motorway tunnels of around 7.5 km in length for up to 4 lanes of traffic. The presented work examines this premise in detail by utilising advanced numerical modelling approaches and high-performance computing applications to investigate the long-term applicability of energy tunnels for this case study. The results indicate that thermally activating the entire tunnel could provide up to about 38.6 GWh per year for heating and cooling. A number of pipe configurations are also investigated, suggesting that placing the pipes only on either side of lining of the tunnel (as opposite to top or bottom) can result in a better thermal performance due to thermal interference and in this case provide up to about 17.5 GWh per year.