The towering “Cheese Grater” office development in London will include a geothermal component in its energy regime, reports Adrian Greeman
Small additions to the foundation works for the new Leadenhall office tower building in London could become commonplace in future City structures, and perhaps elsewhere in the country too.
They are heat pump loops in the massive recently completed piles.
Geothermal energy pumped from the ground is not new - it was first used in India nearly a century ago - but the use of foundation structures to make heat transfers to and from the ground is still relatively unusual, at least in the UK, although Europe and the US are increasingly using the technique.
Subsidies and tax breaks in these countries mean energy coils and conduits are almost routinely included in piles and diaphragm walls.
But some of the major developments, which are restarting in the City as finance and banking recovers somewhat, are including at least an element of this form of sustainable energy in their designs.
One such project, destined to become a highly visible part of the City is 122 Leadenhall, already christened the “Cheese Grater” due to its tapered shape.
“Each of those piles contains around 200m of HDPE tubing,”
Designed by architect Rogers Stirk Harbour the office and retail building for British Land and its joint venture partner Oxford Properties will be one of London’s largest, standing 225m - 48 stories - high when complete in 2014.
As might be expected it has fairly hefty foundations, the main component being 30 bored piles, 30m deep and with diameters of 2.1m and 2.4m.
These have been installed over the last 10 weeks by subcontractor Skanska Cementation at the site opposite the Richard Rogers-Renzo Piano Lloyd’s Building.
Working alongside Skanska as the big pile rigs have bored into the London Clay, is the relatively new and still small firm Gecco2, which is specialising in geothermal and heat pump work.
It is installing a ground energy system that makes use of three of the piles.
The firm does both the detail energy design and fitting work.
“We do various projects in the microgeneration and heat pump sector,” explains managing director Matt Love.
The company designs and installs complete heat transfer systems from the basic pipework in the ground to the heat exchangers and plant that connect with a building’s main heating and air conditioning systems.
“That means we have to come on to a project at several stages,” says Love. “When the piles go in, when the ground is excavated and then later to fit and connect the pipework and plant.”
For a scheme like Leadenhall, which is using a small number of its huge piles for the system, that also means co-ordinating carefully with other contractors.
Piles are very suitable for ground heat transfer says Love, particularly in London where the firm clay ground means that piles are usually quite deep.
Leadenhall’s are initially 30.5m deep, though part of that will be excavated later.
The project began more than two years ago, but was held up by the effects of the credit crunch.
A temporary slab was installed in a two-storey deep excavation at the site during the hiatus, but this will be excavated another 7m down before a huge 3m raft is constructed.
Skanska Cementation has installed the now completed piles through the thin temporary concrete.
Three of the piles will be used for the geothermal system with one more also fitted with heat transfer tubing as a reserve in case of any problems.
“Each of those piles contains around 200m of HDPE tubing,” explains Love.
This is looped, running up and down eight times.
Inside the tubes there will be a mix of water and 25% non-toxic propylene glycol - antifreeze effectively - which will either absorb or discharge ground heat depending on the season.
For installation the loops are fixed to the inside of the pile reinforcement cage before it is lowered into the excavated bore.
“We have to co-ordinate very carefully with the pile contractor for this because we don’t want to cause any hold-ups,” says Love. “I am a former piling man myself and I know how expensive it can be to keep large plant waiting.”
Gecco2 did some of its work in advance at Cementation’s steel fixing yard where part of the cages were made up.
Cages are completed on site just before they are lifted in.
It is critical to ensure that the tubes are not damaged in any way during installation, which is one reason to fix the tubes to the inside of each cage.
This is especially important at the top, the tubes are also protected with Armourflex “and over that we slide bright yellow plastic gas piping”, says Love.
This latter precaution is for the next phase when surrounding ground will be excavated and the tops of the piles, which are only concreted to about 7m below the slab level, are broken back.
During pile concreting and later excavation the tubes are kept at about 5 bar with pressurised water.
This is to test for any leakage and to keep the tubes from pinching or deflating.
“We test with pressurising at several stages in fact,” says Love.
The HDPE pipe has a capacity pressure capacity of nearly 16 bar.
Severing the pipes is a risk, he says, “though we can cut them back and weld on new sections”.
He is most worried about small stones or aggregate falling into a cut on the pipes and then jamming lower down, which would be “nearly impossible to get out”.
That is one reason for a spare fourth pile.
An additional problem to watch for, he says, is holding potentially stagnant water in the pipe loops. Bacteria can build up, creating a small health risk.
More important, the build up can cause pipes to clog, so a bactericide is added.
“We test with pressurising at several stages in fact,”
When the Gecco2 team comes back to site for the excavation work, it will monitor the work to keep the pipes safe.
Assuming all goes well the tops of the piles will be connected to a manifold on one side of the site before the slab is concreted.
“We come back again later on once the plant room is being fitted out, to install the heat exchange and ground monitoring equipment,” says Love.
The heat flows in and out of the ground have to be controlled quite carefully; there is only so much heat in the ground and it can be transferred only at a certain speed.
Take out too much and the ground would freeze.
Once operational, the contribution the system will make to the overall energy requirements of the building is likely to be fairly small.
But the system gives the building additional “green” credentials.