Many contractors now offer geothermal piles as part of their foundations solutions, but Balfour Beatty Ground Engineering must be one of the few to have designed, constructed and operated them.
Ground source heat pumps appear to offer the holy grail of delivering something for nothing – well almost – when it comes to combining such systems with the construction of new piles. However, the experience of owning such a system does not always match up to the expectations but one piling contractor decided to put the concept through its paces to prove the technology.
For the last three years Balfour Beatty Ground Engineering’s (BBGE) precast concrete pile factory at Balmore near Glasgow has been benefiting from heating and hot water generated from the ground below the factory. As well as cutting energy costs by 75% for operating the kitchens, showers and drying rooms, the company has also gained a greater understanding of the technology.
Discussions on harnessing the geothermal potential at Balmore began in 2006 when BBGE started to look at redevelopment of the factory to increase efficiency and productivity.
“Extensive testing regarding the suitability of the site was undertaken in October 2007 and included Thermal Response Testing of installed trial piles plus testing within a borehole with a coaxial pipe,” says BBGE design manager Derek Lennon. “Two types of piles were tested – a 270mm section precast concrete driven pile and a 244mm diameter steel driven tubular pile.”
The test results showed that precast concrete piles and steel piles of the same length would provide very similar energy output.
“Balmore lies in the valley of the River Kelvin to the north of Glasgow and the area is underlain by drift deposits comprising glacial till and fluvioglacial sands and gravels,” says Lennon. “At the factory sands and gravels are predominant with ground water only 0.75m below ground level.
“There are published tables showing suggested thermal outputs for certain soil conditions, which were referenced as part of the trial. Our tests demonstrated that the thermal conductivity of the underlying ground at Balmore was as expected in these ground conditions, with an average conductivity of 2.1W/m/K.”
Information from these tests, along with the energy requirements of the proposed building and details of the insulation and glazing, were then used to design the length of heat recovery pipe in the actual geothermal pile installation, which was carried out in June 2009 using the precast concrete solution.
The final solution used 22 275mm square section precast piles, which were driven from 8.5 to 11.5m depth on a 1.5m grid. The piles were driven using conventional piling equipment but are not designed to be load bearing so were driven to the depth required by the geothermal design.
Pipes from each pile are connected in a series of trenches that connect into a manifold at the side of the building. In the tests BBGE used 32mm diameter HDPE (High Density Polyethylene) pipes in the piles, which were incorporated into the reinforcing cages on the precast test piles, however in the actual pile installation a single loop of pipe was attached externally over the full length of the pile as this was shown to offer greater efficiency.
Three years on and the system is delivering savings for BBGE. “The heat pump costs approximately £1050 per year in electricity for heat and hot water to kitchens and showers and heating to drying rooms,” says Lennon. “Previously we used a diesel generator for heating and hot water for the kitchens that cost up to £2800 a year and the drying room fans cost around £1000 a year in electricity.”
According to Lennon, putting the geothermal concept into practice at Balmore has helped BBGE gain further understanding about energy pile output and the benefits this can bring to precast piling projects such as housing and commercial schemes.
“When closed loop heat pipes are installed with the piling operation the only cost is the pipe itself and a slight lengthening of the piling programme,” he says. “For a typical commercial office block, we may be installing in the region of 200 piles to 20m depth, so there is potential to provide 120kW of heating energy for this building. A typical two storey office block with 24 rooms requires around 50kW of energy, assuming insulation and glazing are of a good standard, so if piled foundations are needed it is very likely that the energy gained through the geothermal piling system will be sufficient to meet its heating needs.”