Piling up the heat

The idea is simple enough. Heat or cold can be stored in the ground for heating or cooling purposes later. Most buildings use piles for the foundation, and these piles can easily contain heat exchangers in contact with the soil. The so-called ‘energy pile’ can thus fulfil a double function: foundation plus heat exchange. Heat pumps can then transfer heat very efficiently from the ground to be building interior and vice-versa.

Geotechnical engineer Dr. Philip Vardon at the Faculty of Civil Engineering and Geosciences sees great potential in this technology. “Eighty percent of a building’s energy use is for heating and cooling. Using geothermal heat through heat pumps can reduce the energy consumption considerably. What’s more: buildings can store heat in the summer when renewable power is abundant and almost free for use in the winter.”

So why are ‘energy piles’ hardly used at all? Vardon explains that builders are reluctant to use the technology because of the unknowns. What, for instance, is the effect of repeated thermal cycles on the soil and the piles? What is the optimal size and what the best temperature range? No one knows.

Vardon, who works at Prof. Michael Hicks’ group of geoengineering, received funding from STW for a four-year project (850,000 euros). The most solid part is the pile prototype that will be installed at the TU Delft Green Village next spring. That installation allows for practical experiments and measurements.

Lab experiments will study the influence of temperature cycles on soil properties, and a coupled computer model will be set up to describe the thermo-hydro-mechanical properties of the ground. Dr. Rafid Al Khoury and Professor Bert Sluys from the department of structural mechanics at CiTG are responsible for this part of the project.

At the end of the project, in four years, measurements should provide guidelines for the technical construction while a computer model should adequately describe the impact on the soil. Together, Vardon hopes, these results will stimulate the implementation of this promising geothermal technology.