G : What is Geothermal?

Ground Source Heat Pumps as installed in the UK generally do not utilise the heat radiated by the earths core but rather that which is absorbed and stored by the outermost layer of the earths surface from the radiated heat of our sun.

In general ground source heat installations in the UK are serviced by boreholes of less than 200m depth. The vast majority being between 80m and 150m depth.

Borehole depths of significantly less than 80m are not usually recommended by professionals unless there is an overriding geological reason to do so. These shallower borehole depths are used by those operators with makeshift drilling apparatus.

How do ground source heat pumps work?

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Using a hose filled with liquid, known as a collector, you can bring up solar energy stored deep down (60-150m) in the earth. The liquid in the collector is circulated by a pump and is heated up (1) by the stored solar heat down in the ground. When the liquid passes up into the heat pump, it meets another closed system. This contains a refrigerant that can turn into gas at a very low temperature. Under high pressure, a compressor (2) considerably increases the temperature of the refrigerant. Then, using a condenser, (3) the heat is transferred to the water-based pump driven heating system in the house. Meanwhile, the refrigerant reverts to liquid form (4), ready to turn into gas once more and to collect new heat energy.

If this seems complicated, think of your fridge, if you put your hand round the back and touch the coils (your underfloor heating/radiators), these are warm. The inside of your fridge (the borehole) is cold. Your fridge is a heat pump. That's all there is to it.

The earth absorbs and stores heat from the sun year after year, providing us with a constant source of naturally renewed energy. Just a few feet under the ground, there is a fairly constant average temperature of 4°C to 12°C. This trapped energy represents a vast reserve of low grade heat waiting to be tapped.

Ground Source Heat Pumps need power (electricity) to operate. They are therefore not truly 100% green unless this electricity is solar or wind generated. However for every unit of power they use, they generate 3 - 5 units of heat.

The energy generated by a ground source heat pump is only 100% renewable if the power to operate the pump comes from solar electric panels or a wind turbine.

What will it look like in my house?

You most likely won't be able to see it.

The boreholes, (or slinkies), will be below ground and hidden.

The heat pump (like a fridge) will be in a utilities room, outbuilding, or garage.

Will a GSHP give me hot water?

Yes, the latest GSHP's are fitted with hot water tanks.

Can I also use it for cooling my house?

Yes, in summer the process can be reversed.

Where does it come from?

The word geothermal comes from the Greek words geo (earth) and therme (heat). So, geothermal energy is heat from within the earth. We can use the steam and hot water produced inside the earth to heat buildings or generate electricity. Geothermal energy is a renewable energy source because the water is replenished by rainfall and the heat is continuously produced inside the earth.


Geothermal energy is generated in the earth's core, about 4,000 miles below the surface. Temperatures hotter than the sun's surface are continuously produced inside the earth by the slow decay of radioactive particles, a process that happens in all rocks. The earth has a number of different layers: The core itself has two layers: a solid iron core and an outer core made of very hot melted rock, called magma. The mantle which surrounds the core and is about 1,800 miles thick. It is made up of magma and rock. The crust is the outermost layer of the earth, the land that forms the continents and ocean floors. It can be three to five miles thick under the oceans and 15 to 35 miles thick on the continents. The earth's crust is broken into pieces called plates. Magma comes close to the earth's surface near the edges of these plates. This is where volcanoes occur. The lava that erupts from volcanoes is partly magma. Deep underground, the rocks and water absorb the heat from this magma. The temperature of the rocks and water get hotter and hotter as you go deeper underground. People around the world use geothermal energy to heat their homes and to produce electricity by digging deep wells and pumping the heated underground water or steam to the surface. Or, we can make use of the stable temperatures near the surface of the earth to heat and cool buildings as is the case in the UK.

Geothermal energy as utilised in the UK is mainly the heat absorbed from the sun into the earth, as described above in the last line of the explanation of Geothermal Energy, not the heat radiating from the earth's core. To extract this heat would require deep boreholes in locations where the earth's crust is thinner, e.g. as in some scandinavian countries. Having said that, there is an interesting venture in Cornwall...

The UK government is increasingly eager to promote Carbon Footprint Reduction for both private and public buildings. Conversion of Geothermal Heat as utilised by Ground Source Heat Pumps (GSHP) is a widely accepted, efficient way of achieving this goal.

In the UK, geothermal heat is under-utilised. We lag far behind other northern european countries. Thus, geothermal heating is a real option in the development of future environmentally-friendly energy supplies.

The schematic diagram below shows the construction of a typical borehole for a Ground Source Heat Pump





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