Research Headlines – Profound changes for shallow geothermal systems

Heat from the Earth is in plentiful supply – and innovation could help to reduce the cost and further improve the efficiency of technology for its extraction. An EU-funded project has set out to upgrade shallow geothermal systems and underground thermal storage. Part of the research is conducted in Turkey, where activities focus on the grouting.

©GEOCOND project

Domestic heating and cooling is an area that offers ample scope for energy efficiency gains and decarbonisation. Shallow geothermal systems have particular potential in this respect, notably because they tap into a reliable source of energy that is available everywhere, say the researchers involved in GEOCOND.

Led by the Polytechnic University of Valencia, Spain, this EU-funded project set out in May 2017 to advance the technology and help to reduce its life-cycle cost by a quarter. Work in GEOCOND focuses more specifically on closed-loop systems with vertical borehole heat exchangers, with further attention dedicated to their combination with enhanced underground thermal energy storage technologies to underpin district heating and cooling systems.

Partners from seven countries are cooperating in this endeavour, which involves activities as varied as developing thermally enhanced and longer-lasting pipes with customised thermal properties and analysing the proposed innovations’ environmental and social impact.

Concrete solutions

One component where gains could be made is the grouting: the characteristics of the materials that hold the pipes in place in the borehole greatly influence the performance of the system.

“The grout has to be conductive as this helps to cut the cost of the system, which is currently expensive,” says Ayten Çaputçu, who leads the research contributed to GEOCOND by Turkish cement manufacturer Çimsa. The more conductive the grout, the more efficiently it will transmit the Earth’s warmth to the water running in the closed loop, and the shorter the length of pipe (and the depth of the borehole) required to achieve the expected temperatures, she explains.

The porosity of the grout, the proportion of silica sand and the relative proportions of water and cement involved are key factors in this respect, Çaputçu adds. Geocond is testing different ‘recipes’ in a bid to surpass the thermal conductivity and rheological – flow- and deformation-related – properties of the grouts that are currently on the market, she notes.

Further improvements are to be achieved through the inclusion of innovative additives. “We are developing carbon-based materials for use in cement formulations,” says Burcu Saner Okan, who coordinates the GEOCOND work conducted at Sabanci University in Istanbul. These materials have a hybrid structure combining carbon and silica, she explains.

Two additives are under development, Saner Okan notes. One of these involves expanded graphite, while the other consists of graphene nanoplatelets. The graphene used for the latter is near-prime material derived from waste tyres – or rather from the carbon reclaimed from these via a thermal treatment – by her spin-off company Nanografen, she explains.

As of March 2019, these additives are being generated at lab scale, in quantities of several hundred grams, Saner Okan notes. Production is due to be scaled up in view of the field tests that the project intends to carry out from July, she adds. Trials are planned in Germany and Spain before the project is due to end in October 2020.

Cementing alliances

In GEOCOND, the entities involved in Turkey are interacting with project participants in Germany, Israel, Italy, Spain, Sweden and the United Kingdom, some of whom are directly involved in the enhancement of the grouts. The consortium has set itself ambitious goals that it is tackling as a team, says Çaputçu, emphasising the added value and impetus deriving from this close collaboration.

“We are a good network,” Saner Okan observes, adding that effective cooperation also makes it easier to seize new opportunities to arise. “Every project potentially initiates other projects,” she concludes. “It is important to be part of this type of interdisciplinary environment.”

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