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Transfer structure

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 too. They are heat pump loops in the massive recently completed piles.

Geothermal energy pumped from the ground is not new but the use of foundation structures to make heat transfers to and from the ground is still relatively unusual.

But some of the major developments that are now restarting in the City include at least an element of this form of sustainable energy. One such project is 122 Leadenhall, already christened the Cheese Grater due to its tapered shape.

Tall building

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. 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 by subcontractor Skanska Cementation.

“We have to coordinate very carefully with the pile contractor”

Matt Love, Gecco2

 

Working alongside Skanska is the relatively new firm Gecco2, which specialises in geothermal and heat pump work. It is installing a ground energy system that makes use of three piles. The firm does the detailed energy design and fitting work.

It 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.

Piles are very suitable for ground heat transfer says Gecco2 managing director Matt Love, particularly in London where they usually go quite deep.

Three piles

Three piles will be used for the geothermal system with one more also fitted with heat transfer tubing as a reserve.

Each of those piles contains around 200m of HDPE tubing. 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 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 know how expensive it can be to keep large plant waiting.”

It is critical to ensure that the tubes are not damaged during installation, which is one reason to fix them 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.

Precaution

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 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 leaks and to keep the tubes from pinching or deflating. The HDPE pipe has a pressure capacity of nearly 16 bar.

The Gecco2 team will monitor the excavation to keep the pipes safe. Assuming all goes well the tops of the piles will be connected to a manifold before the slab is concreted. The team returns again later to the heat exchange and ground monitoring equipment.

Heat flows have to be controlled quite carefully; there is only so much heat in the ground and it can only be transferred at a certain speed. If too much is extracted, the ground could freeze.

The system is only expected to make a small contribution to the building’s energy requirements, but it gives the building additional “green” credentials.

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