10 Questions on….Geothermal power


With the introduction of various Government incentives to turn your home ‘green’, including Feed-in-Tariffs introduced on 1 April 2010 and Renewable Heat Grants on 1 August this year, we are taking a look at the different options available to homeowners to produce their own energy and reduce their bills.

This month, we look at geothermal power:

1. What exactly is geothermal power?

The ground beneath our feet has a significant amount of energy in the form of heat. This, of course, steadily increases the closer to the Earth’s core you travel. Even just a few metres below the surface the earth maintains a temperature of 11-12°C and this heat can be harnessed and used either to generate electricity or contribute to a heating system. On a domestic scale, this process is usually used for heating and is best achieved via a ground source heat pump.

By warming the water in a heating system, the boiler becomes more energy efficient as it no longer has to boil the water from cold.

2. How does a ground source heat pump work?


A geothermal heat pump works by circulating a carrier fluid (usually a water and antifreeze mix) through pipes buried under the ground around the property. As this fluid circulates it absorbs heat from the warm ground. On its return to the property, the heat pump uses electricity to extract the heat from the fluid. This is added to the heat gained as a by-product from the pump appliance itself and is used cumulatively to help heat the house.

The newly chilled water then continues on its loop. The addition of this loop therefore means that more heat is generated than if electricity alone had been used directly for heating.


3. What are the basic requirements I must fulfill to have a ground source heat pump installed?

Unlike solar power where a particular aspect is necessary, geothermal power will, in theory, work anywhere. However, a household considering installation does need a certain amount of land in order to accommodate the piping needed. The rule of thumb in this case is: heating capacity of the heat pump (kW) x 10m.

In cases where land is more scarce boreholes can be used instead, although the cost for this will be greater. Alternatively, an air source heat pump might be used. With no digging, borehole or slinky requirement, air source heat pumps can provide significant installation cost benefits over ground source systems, although they are generally less efficient.

Generally speaking, the wetter the land the better as water carries heat very well, although the land needs to be firm enough for the trenches and pipes to be laid safely.

There may well be planning restrictions for developments of this type in your area. Particularly if you live in a Conservation Area or your building is listed, it is advised that you consult your local planning authority.


4. How large and noisy would the in-home part of the installation be?

A modern system will only be the size of a fridge, in most cases, and will generate noise at approximately the same level as one. It is generally recommended that they are located in a garage or outbuilding.

5. What would the most compatible type of heating be?

Any type of heating can be connected to a ground source heat pump, but some more effectively than others. Radiators, for example, generally operate at a water temperature of approximately 70°C and are thus less suitable than under floor heating which operates at a comparably low floor temperature of between 25-29°C. The lower the heating temperature (ie. the closer to the earth surface temperature of 11-13°C), the more effective the outcome and the greater the saving.

6. How long have humans been harnessing geothermal energy?


Hot springs have been used for bathing since Paleolithic times (prehistoric period including the development of rudimentary stone tools) at the least. The oldest spa known of is a stone pool on China’s Lisan mountain built in the Qin dynasty (3rd century BC).

In the first century AD, Romans conquered Aquae Sulis and used the hot springs there to feed public baths and under-floor heating. As these baths required a fee of entry, they are likely the first commercial use of geothermal power.

The world’s oldest geothermal district heating system in Chaudes-Aigues, France, has been operating since the 14th century.The earliest industrial exploitation began in 1827 with the use of geyser steam to extract boric acid from volcanic mud in Larderello, Italy.


7. When was the first use of geothermal technology for a residential development in the UK?

Surprisingly, the first use of geothermal power in a UK residential development was as recent as May 2009, when a luxury apartment complex conceived and realised by Oakmayne Properties, Londonewcastle and CZWG architects reached completion in St John’s Wood, London. Planning permission for this development was granted in May of 2006.

8. How much of the world’s power is generated via geothermal means?

As of March 2010, geothermal power made up a total of 10GW installed capacity worldwide. The largest producer of geothermal energy, the United States, creates around 3.15GW of this capacity.

9. What advantages does geothermal power have?

First of all, geothermal energy exploits a naturally occurring phenomenon that does not involve the burning or use of any finite materials. Therefore, it is both an entirely green energy, producing no pollution of greenhouse gases, and an entirely sustainable energy source.

Secondly, geothermal has a significant advantage over its competition in that it does not fluctuate. Solar panels rely on the sun shining and their power diminishes in times of cloud cover (all to frequent in this country). Wind turbines rely on the wind blowing and are frequently seen inoperative if the wind is too low or too high. Unlike these, geothermal power relies on the warmth of the earth itself, a factor which never fluctuates beyond one or two degrees and is always a ready source of energy.

Thirdly, geothermal power is simply better value for money than many of its counterparts. In 2001 the International Geothermal Association estimated that the cost for a geothermal installation was between US $0.02 to $0.10 per kilowatt-hour, with potential future costs of $0.01 to $0.08 per kilowatt-hour.  The association estimated that investment costs would be around $800 per kilowatt of capacity. For comparison, the association estimated wind costs at $0.05 to $0.13 per kilowatt-hour, with potential costs of $0.03 to $0.10 per kilowatt-hour and investment costs of $1,100 to $1,700 per kilowatt of capacity.

10. What are the disadvantages of geothermal power?

Firstly, although domestic installations can be placed anywhere, a sizeable geothermal installation needs a specific site with hot rocks at a depth you can drill to. The type of rock also matters, as a harder rock will be more difficult, and more expensive, to drill through.

Secondly, in some instances a geothermal site might run out of ‘steam’ and become inoperable for perhaps even decades.

Thirdly, pockets of hazardous gases or minerals might come up from underground. These can sometimes be difficult to dispose of.