Tunnels being built in the UK will deliver transport and utility improvements but they could also help meet renewable energy targets. Claire Symes reports
Using the ground to provide energy in the form of heat is gaining popularity in the UK with commercial developers thanks to government incentives. Until now most systems have used purpose built boreholes or piled solutions to extract the geothermal energy but Crossrail could be on the verge of taking the concept to a new level.
Crossrail is currently in discussions with concept developer Rehau to incorporate geothermal tunnel linings into four 250m sections of the eastern running tunnels and a decision is expected this coming spring. “The idea started in Germany, where the geothermal industry is more established than in the UK, but has undergone trials in other European countries and has real potential for projects like Crossrail,” says Rehau commercial manager Mike Moseley.
There are two types of system that have been trialled - open loop systems and closed loop systems, which is the type currently being considered for Crossrail.
In an open loop system, the latent heat of naturally draining groundwater is used as a heat source. “Water temperatures can be as high as 16 to 19ºC,” says Moseley. “In Switzerland, water from one tunnel has been used as a heat source for a town, while water from another, which was more remote, led to the development of a fish farm in order to make use of the heat source.
“This type of system is limited by the geology and does need long tunnels in rock to be successful, so these do not offer much potential for the UK market.”
Moseley believes that the opportunities in the UK lie with use of closed loop systems. Rehau developed the concept with German contractor Zublin and looked at the potential to incorporate pipes into the tunnel lining itself, either during refurbishment or during the construction phase.
The system was put through a short term trial in Copenhagen in 2008 but the first long-term use of the concept is now entering its first winter in full use on a scheme in Austria.
“The opportunity in Jenbach was driven by the local mayor who wanted to find a heat source for the town’s community centre,” explains Moseley. “The scheme is using a closed loop system installed into the tunnel linings on one of the Brenner Base Tunnels.”
According to Moseley, the discussions at Jenbach happened at a late stage in the construction programme so the system only uses a 50m length of the tunnel. Pipes fitted into 10 sections of tunnel lining with pipes carrying the water out of the tunnel buried below the cast insitu concrete railway track bed.
“Installation of the scheme started in 2010 and will be fully operational for its first full winter this season but so far the system has worked well.”
Moseley explains that there are two types of tunnels: “In a cold tunnel the temperature of the tunnel itself isn’t going to rise but you can use the tunnel structure to draw the heat from the surrounding ground. A hot tunnel is where the movement of the trains and utilities result in a higher ambient air temperature so a ground source system can extract this heat, as well as that of the surrounding ground.”
The main benefit for the rail operators in the UK comes from the cooling effects, rather than the ability to sell heat as the district heating market is not as developed here as in other parts of Europe. Moseley adds that a heat pump is also needed in order to maximise the benefits from the heat extracted. “There is lots of energy potential in UK tunnels but it is a low grade energy,” he says.
The other drawback for hot tunnels is that most operators would not want to use their network as heat dumps as the buildings that benefit from winter heat look for cooling solutions during the summer months.”
Nonetheless, Moseley believes that the heat could be used for station buildings or dumped into the atmosphere. “Use of geothermal systems to draw the heat out of hot tunnels would help make air conditioning systems more effective,” he says.
Moseley hopes that Crossrail will take the opportunity to become a pioneer for geothermal tunnel linings but says that discussions are also underway with two metro developments in France and some projects in the Middle East are looking at the potential of the concept. Moseley believes that HS2 and the Thames Tideway tunnels could also benefit.
“The cost of installing the system can be around 1% of the construction costs but the operational cost benefits over the lifecycle of the tunnel could be huge,” says Moseley. “The real driver of the geothermal linings in UK tunnels will be a development of a market for heat.”