Phil Vardon is Professor of Energy Geo-Mechanics in the Department of Geoscience and Engineering at Delft University of Technology. His research is aimed at complex coupled processes in geomaterials. He focuses on how soils and geomaterials are impacted by and control various processes including temperature and fluid movement and the stresses and strains, and applies these to practical applications such as the production of geothermal energy.
The Geothermie Delft (GTD) project is centred around the dual objectives for the university it is really important to have a dual aim, not first energy and then research of providing clean sustainable and affordable energy for heating the TU Delft campus and the city of Delft and providing needed knowledge to the geothermal industry. The system is made of two wells – a doublet – where one well is used to extract hot water where the heat is used for heating and the cooled own water reinjected into the subsurface via the second well. The fact that this geothermal heating system is located on the TU Delft campus is crucial for this dual aim, because the campus and surrounding areas have a high energy need for heating and this geothermal plant also facilitates a large scale research programme. Having commenced operations in February 2026, the well is systematically monitored by an interdisciplinary team from TU Delft.
We sat down with Phil Vardon, Professor of Energy Geo-Mechanics at TU Delft, to explore what his team is "seeing" and monitoring more than 2.3 kilometres beneath our feet and hoe TU-onderzoekers en technisch vernuft de capaciteit van het project gaan uitbreiden in de volgende fase.
The well started operating in February 2026, but the research programme started some time ago. What have you discovered so far?
P.V.: "The early revelation has been the sheer variability of the rock samples. Millions of years ago, this area around Delft shifted between being a river delta and a shallow sea. This created a highly complex, layered reservoir that is beautifully challenging to predict."
What do you do in the summer when the city of Delft doesn't need that heat?
P.V.: "That is one of the exciting parts of our system we are developing. It you have too much energy available, you want to store it – like in a battery. From our geothermal system this would mean a lot of hot water. But you can't easily store hundreds of thousands of cubic metres of water on the surface. If you built an insulation tank that large, it would be astronomically expensive—picture three or four of the tallest faculty buildings like the EWI-building at TU Delft campus!
Instead of a tank, we use the subsurface itself for high-temperature heat storage (HTO). Later this year (2026), we are installing a shallower system than the geothermal doublet—about 180 metres deep. In the summer, when energy demand is low but our geothermal doublet can still produce heat, we will inject hot water into this shallower aquifer. In the winter, we can pump it back up for extra sustainable heating."
Has this been successfully done elsewhere?
P.V.: "Historically, globally, most high-temperature underground storage projects have failed. They often suffer from chemical clogging when water temperatures change abruptly, or the heat simply dissipates into the surrounding ground if the project is too small.
Ours is uniquely positioned to succeed because we have mapped the subsurface layers, the chemistry, we understand the groundwater flow, and crucially, we have an unprecedented amount of monitoring equipment installed at those shallower depths. We can watch the heat move in real time and adapt."
It sounds like a massive undertaking. Is there anything else that makes this project stand out to you?
P.V.: "In the academic world, researchers often work in isolation—one professor with a couple of students in a silo. What we have created here at TU Delft is a massive, interdisciplinary team of about 20 people. We have geologists, geophysicists, geochemists, and computational modellers all working in lockstep.
It is an incredibly powerful way to work. It pulls people from completely different backgrounds together with a single, urgent shared aim: making clean, renewable energy a reality for society."
