Case study

Using abandoned mines to tap into geothermal energy

Our researchers are investigating how to best harness geothermal energy found in naturally heated water in the UK's abandoned coal mines.

With the climate change emergency becoming ever more critical by the day, all eyes are turning towards solutions that can help slow that change down. Renewable energy is one of the key solutions to reduce the speed of climate change, and geothermal energy is just one option.

Geothermal energy is beneath the earth’s surface and can be used as a way of either heating or cooling systems, or to generate electricity. Naturally heated water in disused mines is an important source of renewable energy in the UK. There is a large interest in using that renewable energy for homes and workplaces in order to reduce carbon emissions and saving costs.

Looking into a solution

With one quarter of the UK’s homes and businesses above abandoned coal mines, according to the Coal Authority, finding a renewable energy solution that can make the most of mine water is an important step.

Creswell Mine, in North Derbyshire, is 450 metres deep and 6 metres in diameter. The mine shaft therefore offered the University of Derby’s Zero Carbon researchers an opportunity to assess the feasibility of a heating system taking heat from the water within the shaft.

A temperature log carried out in 2021 at Creswell Mine found the mine water temperature was 17 to 22°C. This meant the mine could proved to be a very useful heat source if that heat could be harnessed.

Researchers Dr Hirbod Varasteh, Dr Tugba Gurler, Dr Zaharaddeen Hussaini and Professor Chris Sansom recommended two types of systems to try to capture the heat from the mine water. The team tested both an open-loop and closed-loop system in order to identify the best method for heat transfer.

An open-loop geothermal system extracts water from the ground directly into a heat pump and then into a discharge well.

A closed-loop geothermal system continuously circulates a mixture of antifreeze solution in underground pipes that pass through the warm mine water. The antifreeze solution is what passes through the heat pump.

The benefit of the closed loop system is that the mine water contamination is not mixed with the pumped water and has a low cost. However, the open loop was found to have better heat transfer and can recover heat more efficiently due to the direct contact of warm mine water with the heat exchanger.

The researchers have found that the overall efficiency of the open-loop system is very high. And, by integrating the system with a photovoltaic (PV) system or wind turbine, any electricity required for the heat pump and shaft water pump can be produced in a renewable way.

Next steps

Having established that the proposed system has a very high coefficient of performance score (a ratio that describes the efficiency of a system), meaning it is very efficient, the team propose to design and build a small feasibility demonstrator so the theory can be put into practice. The team are also considering the ability to add storage options to ensure that, if there is no immediate need to heat something, the renewable energy can be stored and used at a later date.

A novel solution for generating electrical power?

There is an additional opportunity for generating electricity when the water needs to be returned to the bottom of the abandoned mine shaft. While these ideas require rigorous testing, they present another effective solution for generating power: