Taking heat from below the streets: unlocking the power of the London Aquifer

Deep beneath the vibrant streets of London, where history and modernity meet, lies an untapped resource—the London Aquifer. For centuries, this underground reservoir has quietly served as a vital water source for the city. But at Citigen, our innovative yet hidden power station in the heart of The City, we've discovered a new way to harness this ancient asset—by transforming it into a sustainable heat source for homes and businesses across the capital.

Our vision was simple yet ambitious: to draw on the natural heat stored in the earth beneath London’s busiest streets.

Tapping into the depths of London

Imagine extracting heat from below the very streets we walk on every day. To bring this vision to life, we drilled 200 meters beneath the capital using three industrial-sized heat pumps to reach deep into the aquifer. The challenges were immense—drilling through layers of history in the square mile, navigating the logistical nightmare of London’s busy streets, and even dealing with the eerie possibility of disturbing remnants of the plague.

The first borehole took three months to complete, the second - three weeks - but by the time we drilled the third, we’d got to grips with what we were doing; the team were able to carry out more accurate flushing on the drill head, we corrected the right speed variations, and knew the exact depth we could drill to – which meant we were able to complete the final borehole in just three days. Practice certainly makes perfect.

A modern twist on historic grounds

This site where our Citigen plant sits has a rich history, beginning in the 1890s as a fossil fuel-powered electric generating station. By 1914, some of the site had become the ice store for Smithfield Market, and, in a fascinating historical twist, that very same room now houses Citigen's plant room – a place reborn as a hub for modern, low-carbon heating and cooling technology.

Three heat pumps now operate at the heart of our system, extracting thermal energy from the water drawn from deep within the aquifer. These heat pumps are specifically designed to generate hot water (85 °C) from relatively low-grade energy sources, all while maintaining outstanding energy efficiency. 

Each of our borehole pumps extracts 33 litres per second. Which means that with all three of our borehole pumps running (supplying the heat pumps), you could fill your bath at home in just one second.

The ancient foundations beneath our feet

North of the River Thames, London is predominantly built on London Clay, a sediment laid down 56 to 34 million years ago during the Eocene epoch. This clay can be up to 150 meters thick, providing a stable foundation for the city’s buildings and infrastructure.

But beneath this clay layer lies something even older—a chalk bed dating back to the Late Cretaceous period. This chalk acts as a major aquifer, providing essential groundwater resources for the city.

Now, we’re tapping into this ancient foundation, not just for water, but for heat—a revolutionary step towards a more sustainable future. And Citigen isn’t the only borehole project we’re undertaking here in the UK. As part of our 15-year energy efficiency partnership with Nottingham University Hospitals NHS Trust, we’re drilling 64 boreholes beneath the Queens Medical Centre (QMC), helping the hospital deliver its ambitious environmental targets of achieving net zero carbon operations for a heating and cooling system emissions by 2040.