Using the buildings piles as an energy source can be an attractive option in certain cases. The ground collector pipes are incorporated within the piles during their construction meaning this can be a cost effective option compared to drilling a separate borehole array. It may also be interesting to consider incorporating the collector within the building footprint especially where site space is very limited. The Econic team are experienced in the design and installation of these so called thermal piles.

Ground collector piping can be incorporated in both CFA and Rotary piles. In the case of rotary bored piles, the collector pipes are installed directly into the bore before cage installation and the concrete pour. With CFA piles, the collector pipes are attached to the steel reinforcement cage and ‘plunged’ directly into a freshly poured concrete pile. Where cage lengths do not extend to the full depth of the pile, single rebar extensions are used to support the collector pipe down to the full pile depth. The installation depths depend how much heat energy is to be collected or ‘dumped’ for cooling purposes.

Two types of collector pipe are used in thermal piles: ‘concentric’ and ‘U-pipes’. The concentric collector consist of one pipe installed within another to provide a ‘flow’ and ‘return’ The U-pipe can consist of single pipes (1 flow, 1 return) or double U-pipes (2 flows and 2 returns). Once the pipes are incorporated in the thermal piles they are connected together and led back to the heat pump in the plant room. A heat transfer liquid (monopropylene glycol and water mix) is then circulated through these pipes to form the collector array.

Design and Correct Sizing of Thermal Piles

It is critical to ensure that the sizing and installation of a thermal pile based collector is done properly. There are a number of things that need to be considered:

1) Vertical ground collectors should ideally be deeper than 15m and preferably +20m. The average UK pile depth is approximately 15m, so not all installations are suitable.

2) Conventional thermal piles use a continuous pipe wrapped around the pile cage. However, as the pile cage normally only extends to 2/3 the pile length, we use a coaxial (pipe within pipe) collector. This is connected to re-bar which extends to the base of the pile. Pile diameter is also an important factor. The volume of the collector pipe should clearly not have a structural impact on the pile.

3) Care must be taken when pumping in the concrete when the cage and collector pipe have been installed first or when installing the cage and collector into a filled pile. The collectors must be pressurized to 6 BAR and capped with a valve prior to installation in order to ensure that there are no leaks during the construction of the pile.

4) It’s important to be realistic about the heat yields that can be achieved from thermal piles. In mainland Europe, they tend to use an average of 35W/m of pile installed. In the UK this can be reduced to as little as 15W/m. The principal factors controlling yield are: soil type, groundwater level, pile diameter, depth and number of vertical collectors within each pile. To ensure an accurate heat yield figure Econic would always recommend that a trial thermal pile is installed and a thermal conductivity test performed. The trial pile isn’t wasted money as it can be used at a later date; either as a structural 'working' pile, or as a structural 'expendable' pile used in a pile load test.

5) People often worry about whether the ground collector piping will affect the structural performance of the pile. Studies have shown that provided the thermal pile is correctly designed both structurally and thermally, no undue settlement or loss of load carrying capability has been experienced.

Contact Econic if you are interested in this innovative area.