A test is being carried out by an HS2 contractor that could see heat being extracted out of the ground to heat buildings and railway stations.
Extracting heat from the ground is not a new technology, but it has struggled to be incorporated into large commercial construction sites. Considering that a heat pump needs lots of pipes drilled into the ground to pump cool water down, and warmed water back up, and as most construction sites dig large deep piles into the ground to support the building, it seems that combining the two would be an obvious improvement.
However, embedding ground pump heat exchangers into solid concrete piles rarely seems to work that well, often getting damaged during construction, and when they do work, concrete is a fairly poor conductor of heat from the ground to the pipes inside the pile.
What’s being tested is a hollow pile, called a HIPER Pile, that was developed by the University of London and has been recently licensed to Keltbray.
The hollow pile should get around a lot of the design problems as the heat pump pipes are inserted into the hollow core, not embedded into the concrete, making post-piling installation much easier, and then the idea is to fill the empty core with water, which is a much better thermal conductor than concrete.
According to Keltbray, their tests show the hollow piles outperform traditional solid concrete piles by some 60% in capturing heat from the ground and transferring it to the surface. A recent case study by Keltbray calculated that a conventional heat pump would have extracted around 1,650MWhrs/yr of energy from the ground, but using their hollow piles means that it is actually pulling nearly double that amount of heat energy out of the ground.
That translates into less energy needed to heat the building on cold days, and the system can be reversed on hot days to soak heat out of a building and pump it into the ground instead. Where it can be removed in the winter again.
In addition to being better as heat-pumps, the hollow piles need a lot less concrete in their construction, which is better — or at least, less bad, for the environment. At the moment, piles are often made from pre-assembled steel wire columns delivered to the building site and dropped into the hole, then concrete is delivered to the site to be poured down to create the solid pile.
One of the other advantages of reducing the amount of concrete used in the pile is that it’s physically lighter. Light enough that the entire pile could be built offsite and delivered, so no need to send in two sets of deliveries – one for the steel cage and one for the concrete. The other advantage of remote construction is that it’s easier to use a type of concrete made from steel industry waste products that reduce the carbon-intensive cement content by up to 70%.
The pre-cast pile segments are delivered to the construction site, lowered into a hole dug slightly wider than the piles, and then to form a tight seal, grout is poured into the gap between the ground and the pile.
Once the piles are set, they can then install the heat-pump equipment.
The deployment of these hollow piles is part of HS2’s Innovation programme, and soon to be trialled by contractors Mace Dragados at the project’s Euston station site. Keltbray estimates its technology will harness enough energy to supply 80% of the building’s heating and hot water.
At the moment, this is being tested on just the construction site office, but if it proves its case, then there’s always the possibility that it could be used later in the railway station construction as well.
HS2’s innovation manager, Heather Donald said: “The potential benefits of this innovation are obvious. By harnessing ground heat this technology has the potential to provide both heating, cooling and hot water to HS2 stations – increasing sustainability by reducing their carbon footprint and running costs.”