Recycling mines into heat pumps could hold key to geothermal energy future

Recycling mines may help heal the scars of the past and heal our future. Coal mining has played an enormous role in contributing to the challenging climate in which the world now finds itself. But reusing mines may be part of the solution – if they can be turned into giant geothermal heat pumps.

The concept isn’t new. Heat will always want to jump to a cooler destination. Touch a cold glass, and your fingers will be left chilled.

Likewise, geo-exchange exploits the natural imbalances within the Earth. Deep soils and waters retain stable temperatures year-round. They are, in essence, thermal batteries.

Pumping water through these depths “transfers” that temperature back to the surface. Here, a heat pump compresses that temperature difference before redirecting it to a heat exchanger – such as an air-conditioning system.

Heat can be drawn out of a building and pumped into the ground. Heat can be drawn out of the ground and pumped into a building.

The challenge is digging deep enough – and extensive enough – holes for a large-scale effect. And then transferring it to where it’s needed. But what if that hole has already been dug?

Reusing mines

University of Melbourne engineering PhD student Mauricio Carcamo is exploring the energy potential of decommissioned open-cut mines.

The hole has been dug. Remediation efforts are going to take place anyway. But is there economically viable potential to extract an energy advantage out of this process?

“Geothermal reuse of decommissioned mines appears an attractive option,” Carcamo says.

“The idea is to minimise the risk for the nearby population and wildlife while providing an advantageous end-use for the local community.”

Disused open pit mines are graded to reduce the risk of collapse. Backfill layers are added if there are any acidic rocks or toxic materials. And the depression is gradually filled with water to create a lake for recreational and irrigation use.

It’s a long-term process that creates enormous layers of temperature differential – a scenario ideal for shallow geothermal heat exchange.

“It depends heavily on the local conditions,” Carcamo says. “And while the solutions might share some core ideas, every mine will require a specific tailored decommission plan.”

His feasibility study focuses on a group of mines in North Rhine, Westphalia, Germany, and a similar set of mines in the Latrobe Valley, Victoria.

Existing shallow geothermal technology involves burying heat exchanges or setting them in deep water. But the scale is generally limited to that of a single house or commercial building.

“In general, shallow geothermal systems benefit from high efficiency and low operational cost, but have a high up-front capital cost,” Carcamo says. “If we want to upscale the use of shallow geothermal energy to an entire district with a large source such as the decommissioning of an open pit mine, a District Heating-Cooling System is required.”

Carcamo is coming from Germany to Australia to establish the viability of piping large-scale geothermal heating and cooling from decommissioning coal mines in the Latrobe Valley to the towns of Moe, Morwell and Traralgon.

Ultimately, it comes down to how many customers it takes within a given distance for the cost of installing such a system to become viable – and what engineering challenges must be confronted to improve that equation.

Carcamo says he’s convinced the potential is there. “The post-mining landscape can help us transition to clean energies while giving an economic boost to the nearby communities that had been dependent on the operating coal mine,” he says.