How does geothermal heating work?

With global warming rearing its ugly head and geopolitical tensions contributing to substantial fluctuations in the costs of hydrocarbon energy sources, a lot of people are looking for more affordable, sustainable heating alternatives to conventional gas boilers. 

One of the most popular solutions that has emerged is geothermal heating systems, but how exactly do they work? Let’s explore the main parts involved, from ground temperature to refrigerant cycles.

The basics

Geothermal heating systems work by transferring natural heat stored in the ground beneath us, through a closed loop full of fluid, into a heat pump. 

That pump then amplifies that heat, and can be used to heat water or heating systems. Of course, there’s a bit more to it than that, so let’s take a closer look. 

How the system works

There are a few critical parts of the system that facilitate the transfer of heat from the ground into a property.

The earth’s heat

What makes geothermal heating systems possible is the fact that if you dig down just a few metres, the temperature of the earth remains pretty much the same year-round (about 10 to 15°C), no matter what happens to the air temperature. 

This might not seem very hot, but it’s the stability of this temperature that matters, not how hot it is. You can transfer a tiny bit of that heat into a fluid in a pipe (more on that later), pump that fluid up to the surface, and the mass of the Earth around it means that there’s still plenty of heat energy left over.

Geothermal boreholes

To tap into that heat, geothermal heating systems use a system of boreholes. These holes are dug to a depth of between 50 – 200 m, and then a U-shaped pipe is installed. 

Grout is poured in to seal the gap between the ground and the pipe, and facilitate an efficient transfer of heat. That pipe can then be filled with a water-antifreeze mixture, which can be pumped around the system. As it passes through the loop, it absorbs some of the surrounding heat, bringing it back up to the surface.

The refrigerant cycle

The warm fluid that comes up through the loop runs through a heat exchanger, warming a refrigerant that turns into a gas. A compressor pressurises the gas, which causes it to heat up substantially. 

The now very hot refrigerant goes through a second heat exchange, which can be used to either directly heat a building’s air or to heat water. Once this is done, the refrigerant cools, turns back into a liquid, and goes back to the beginning of the cycle. 

In summer, this system can be reversed, taking heat out of the building and transferring it down into the loop, while transferring some of the cold from down in the earth back into the building.

Geothermal heating may have some complicated aspects, but overall, it’s relatively simple. As a result, it is also incredibly reliable. So long as the system is maintained, you should be able to benefit from a productive geothermal borehole for at least 50 to 100 years after the system is installed.