Capturing the heat from below

Foundations for a new building at the Green Village contain 40 metre long tubes to exchange heat with the soil. Researchers will monitor the ‘energy piles’ in action.

Phil Vardon and Ivaylo Pantev at an energy pile's terminals. (Photo: Jos Wassink)

Newly built Dutch houses without connections to the gas grid may use stored summer heat to keep warm during the winter. Piles with water-heating/cooling, called energy piles are a promising form of seasonal heat storage. A field test will be conducted on the Green Village by the end of the year.

“Eighty percent of a building‘s energy use goes on heating and cooling,” says Dr Phil Vardon (Faculty of Civil Engineering and Geosciences). “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.”

Together with Dr Rafid Al Khoury, Vardon has been leading the NWO funded project ‘Energy Piles in the Netherlands’ since 2016. Last week, their team reached a milestone when huge cranes and drills entered TU Delft’s Green Village to install nine 18 metre long energy piles. These will be part of the foundation beneath a new glass building called the Co-Creation Centre, or CCC for short. Green Village project manager Tim Jonathan expects the spectacular all-glass building to open early next year.

Caging and 40-metres long tubes. (Photo: Jos Wassink)

PhD candidate Ivaylo Pantev, who will be conducting the field test of the energy piles, explains how the system works. Inside the iron caging for the reinforced concrete, there are two long hoses, or tubes, that go all the way to the bottom and back. Larger diameters may require more hoses.

Lowering reinforcement and tubes into the steel casing. (Photo: Jos Wassink)

The cage with the tubes is first carefully lifted, then tilted till it is vertical and is finally slowly lowered into a steel casing that has been inserted inside a drilled hole in the ground. After filling the hole with concrete, the steel casing will be lifted out and the reinforced concrete energy pile will harden in the ground.

In the summer, heat will be extracted from the CCC building by heat pumps. The warm water will circulate through the energy piles and transfer the heat to the surrounding underground layers of soil. In the winter, this process will be reversed. Circulating cold water will pick up heat from the soil and bring it into the heat pump, which then increases the temperature to an appropriate level for heating.

Roughly, one metre of pile delivers 100 Watts of heat. Double tubes in nine 18 metre long piles would produce 16 kilowatts of heat. “We want to see if we can achieve this,” says Pantev. He explains that four piles have been equipped with strain gauges and temperature sensors to monitor the process.

“It’s not the first time energy piles have been installed in the Netherlands,” says Vardon. “But it is the first time that their thermal and structural performance will be measured.” Similar research has been done in London, Switzerland and in Australia, but this is the first test in soft soil with high groundwater levels.

The research programme will monitor temperatures in the ground, but not the microbial activity. According to Vardon similar studies have been performed earlier for shallow geothermal systems (without piles).

Monitoring of the energy piles will not only assess their performance, but also the thermal expansion. Vardon estimates that temperature changes may cause 10 metres of pile to vary by 1-3 centimetres in length. If that figure proves to be correct, it will be essential to restrict differential settlements in the foundation.

Another uncertainty is the effect of freezing the ground through excessive heat withdrawal followed by subsequent thawing. Can we be sure that an energy pile under load will stay in place in soggy soil? Such extreme regimes will be tested in a stand-alone pile that is not part of the CCC foundation.

Other project members are PhD candidate Ali Golchin who studies the material behaviour of soils, and Dr Mehdi M. Arzanfudi who takes care of the computational part of the project. Willy Spanjer looks after this scientific project for the Green Village. FUNDEX installed the piles as part of their contribution to the project and along with Deltares and Diana FEA are the industrial part of the consortium.

The Co-Creation Centre will have another form of sustainable heating as well, says CCC project manager Hubert Linssen. A wood-clad ‘Climate Tower’ will be built beside the glass box. By making clever use of phase-changing materials (PCM), air entering the building will either be pre-heated or pre-cooled.

In fact, the all-glass building will have two experimental heating systems, both of which should be able to maintain a comfortable indoor climate. Without gas, evidently.

Science editor Jos Wassink

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