Over the last couple of years I have been fortunate to collaborate with Maria Alberdi-Pagola from Aalborg University in Denmark on the conversion of driven piles to ground heat exchangers.

Maria is working with the piling company centrum to characterise their square concrete driven piles. You can see a summary of the product here.

Different types of ground heat exchanger: a) horizontal; b) boreholes; c) square pre-cast energy piles

Maria has conducted a range of thermal response tests on these piles which are now written up (see the White Rose repository here). While (the open access version of) this work is under embargo until the end of the year, I will give a summary below.

Five thermal response tests were carried out at two different sites. The piles are 300mm x 300mm square pile, with thermally active lengths between 10m and 17m. They contain either one or two U-loops of heat transfer pipes installed in a factory environment. The test duration were between 50hrs and 150hrs.

The tests involved injecting heat into the piles at a constant rate and measuring the resulting temperature change. The idea is that you can use the rate of change of temperature to back calculate the thermal properties of the ground around the pile and the pile itself. While this method is well developed for traditional borehole heat exchangers, it is still a matter of uncertain application for piles. This is because the piles are large in diameter (and therefore have a significant internal heat capacity) and shorter in length (which means that end effects become more important more quickly). These points are of relevance since traditional thermal response test interpretation methods assume (1) no thermal capacity in the heat exchanger and (2) and infinite source of heat.

Simulation of the square pre-cast energy piles

Maria used 3D numerical simulation to back calculate the ground and concrete thermal properties. The results compared favourably to earlier laboratory tests.  Then she tested a number of well known and more recent analytical techniques to determine the ground and pile properties, along with a simpler 2D numerical simulation. The results showed that:

– Methods that did not consider the pile thermal capacity always over estimated the ground thermal conductivity by between 10% and 40%.

– 2D methods that ignored the end effects of the piles also overestimated the ground thermal conductivity, by between 5% and 30%

– Only methods that include both the pile thermal capacity and the actual pile length gave results close to the 3D numerical simulation. This includes the semi-empirical G-function approach which I developed with Professor William Powrie.

– Pile thermal resistance (a lumped parameter including concrete thermal conductivity and pile/pipe geometry) was much harder to predict. 2D numerical simulation was the only approach to come within 10%. This reflects the fact that all of the other models are based on cylindrical boreholes or piles, and only the 2D numerical models capture the correct geometry.

Consequently, it is possible to use thermal response tests on these small diameter square driven piles to determine the ground thermal conductivity to within 10%. However, for the pile thermal properties, it is better to calculate based on the known factory construction conditions of these particular piles.