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Thursday, 16 February 2012

Ground Source Heat Pumps – Tapping the Energy of the Universe

Geothermal energy – the life blood for ground source heat pumps (GSHPs) – is often described as solar power stored in the ground but that is not even half the story. From a depth of about 15 metres (49 feet) into the earth temperatures become quite stable. In other words they don’t fluctuate much throughout the year and are independent of the variations we experience on the surface with the seasons. In Britain the range is between 7oC – 15oC (45oF – 59oF). From hereon down this increases by 2.6oC (4.6oF) every 100m.

While the sun might seem the obvious reason, the main sources lie elsewhere – deep inside as well as far away. When the solar system formed and the Earth emerged from a disc of dust this clumping together produced enormous heat, just like the innumerable impacts by asteroids and comets that followed, not to forget the massive body that is now thought to have created our moon. That same energy is still with us today in a molten core thousands of miles below the planet’s crust, which constantly radiates upwards and is quite visible in form of volcanoes.

It is joined by heat from natural nuclear decay (don’t worry, you’re not going to start glowing in the dark any time soon), that caused by the gravitational pull of the moon, and yes – the sun’s rays.

If we are talking about GSHPs we are linking into a power that originated billions of years ago in outer space. How is that for a thought when you dig your trench or bore a whole into the ground? Which is exactly what is needed, but before we get ahead of ourselves let’s look at the fundamentals.

What is a Ground Source Heat Pump Actually?


It is a system that consists of

  • Pipes (in the ground)
  • Compressor
  • Heat exchanger
  • Pump
  • So-called ‘working liquid’

The pipes are either laid into trenches or inserted into a borehole. The question which one you should choose rests on the space and the funds available. While it is not necessary to have several acres of ground just to install this particular kind of heating arrangement there will need to be at least enough space to accommodate a digger. Boreholes are more expensive but obviously need less room horizontally and access more constant temperatures.

As a guide, 50m – 80m (164ft – 262ft) of pipe are needed per kW and typical installations provide 6kW-12kW. The resulting 300m – 960m (984ft. – 3150ft.) of plastic tubing might sound frightening but laid out flat that can be done in coils. This reduces the length to 10m (32.8 ft.) per kW. The trenches are 300mm wide (roughly 1 ft.) and 2m (6.6 ft.) deep. If more than one is needed they should be 5 meters (16.4 ft.) apart. Boreholes on the other hand are only 100 – 150mm (~ ½ ft.) in diameter but can be over 100m deep. Several lengths of pipe can be sunk into the cavity if required. Therefore, the space requirements really do stay within manageable limits, although you will have to have a place for pump and control equipment. Outhouses, garages and cellars are suitable locations.

The Way it Works

The basic idea is to pick up heat somewhere and then release it at another point. The central role falls onto the ‘working liquid’ that is most often a mixture of water and antifreeze.

When this substance is pumped into the pipes it is colder than the nearby soil or rock. Since nature seeks to even out differences, warmth naturally wanders to the cooler, less agitated state. It follows that the fluid absorbs the heat from the surroundings and evaporates. Just put the kettle on to see what I mean.

As a gas, it now reaches a compressor. Here it is squeezed together, which increases the heat further. The next stage is a heat exchanger where the gas can release the energy it has stored to the central heating or hot water systems. In cooling down again the gas condenses and returns to its liquid state, ready to go back into the cycle.


Is it Right for Me?

Unfortunately, this is a question that prompts the answer: ‘It depends’. GSHPs do not fit every situation equally well.

Generally, buildings need to be highly insulated and draught proof. Of course, that is always a good idea for saving energy, but in these cases it is particularly important because they run at lower temperatures than conventional heating installations, 45oC – 55oC (113oF – 131oF) compared to 65oC (149oF) in a boiler. This is also the reason why it is possible to combine a GSHP with radiators but these need to be of higher capacity. It does not mean automatically huge, unwieldy monsters. Innovations have led to much more effective and efficient radiators with very little, if any, increase in size. You just need to be aware of it.

The common recommendation is to run it with underfloor heating, though. This technology requires lower temperatures anyway. More importantly, because warmth coming up from the floor keeps feet nice and toasty we feel comfortable despite a cooler room. This linked to proper insulation keeps a GSHP at normal working levels, where it does not become overstretched by having to run above its optimum output.

The Energy Saving Trust in Britain provides an excellent buyer's guide and has investigated 83 systems in a field trial for efficiency, customer satisfaction, ease of use, costs and installation practices. The findings were quite revealing. The measured coefficient of performance was easily 3.0. That meant that a heat pump could produce 3 times as much heat than it used in electricity. However, efficiency and output were often compromised by less than optimal installation and the difficulties owners had in using the GSHP to the best effect. Obviously, standardisation was needed, which the Microgeneration Certification Scheme has largely addressed. But operation needed to be simplified as well to enable users to adjust their habits better and understand how to handle the system in its day-to-day operation.

How much does it cost?

GSHPs are not exactly cheap with price ranges of £7,000 - £17,000 ($11,000 – $27,000). In Britain the Renewable Heat Incentive, which I will look at more closely in an upcoming article, provides some financial support provided by the government. Nevertheless, it is a major expense. Pay-back is not achieved quickly. At the current levels of energy prices especially modern gas boilers compare quite favourably. A much more significant difference can be seen if it is electric, LPG or coal heating that is being replaced.

Not only that, it is recommended to have a complimentary installation, such as solar panels or a wood stove to make sure the user has all the hot water he needs. Typical GSHPs provide between 6kW – 12 kW. For a three bed-room house a standard gas central heating is rated at 28kW. Admittedly, a well-insulated house and underfloor heating would make that gap less important but there is an additional cost for the back-up. On the positive side, depending on your country’s incentive schemes, if the reserve is also renewable such an expense could be minimal or zero even.

What often cannot be avoided is the regulatory side. In many countries it will be required to get planning permission. This can go way beyond ordinary building laws and require consent from water authorities or even holder of mining rights.

Does It Make Sense?

Yes, it does. You will hear claims that gas boilers are greener than GSHPs. That is then related to the fact the latter needs electricity for its pump and that power still is generated by fossil means in most places. It is an argument that is as short-sighted, as it is illogical. Electricity increasingly comes from renewable sources and studies have shown that GSHPs do reduce carbon emissions. Natural gas heating burns fossil fuel – today, tomorrow and in ten years’ time. It will never be a renewable energy system, whereas geothermal sources are. Fair enough, the pumps need to be fitted and operated properly but how is that different to any other system, conventional or not?

All in all it is easiest to plan for a GSHP in a new property where it can be integrated right from the start into an energy efficient building as part of a complete low-carbon concept. Retrofitting is not impossible but requires commitment and planning.

If you have the right conditions and regard it as an investment for the long term, it is a proven technology that can take you towards your very own zero carbon future. Most importantly, do not cut corners in the preparation. Again, it will depend on where you live. However, independent organisations that work in the interest of the consumer are establishing themselves everywhere. They are a mine of information with the sole purpose to help you step around the traps. Not to forget, installers are far more experienced and monitored now. They should be integrated into the process as early as possible rather than met with suspicion.

Further information

Knowledge is power and there is a lot to consider when looking at such a project. If you have a question either ask me or try the sources below.


ð  Britain

ð  America

ð  America and Canada

ð  Germany

ð  Austria

Info Sources

ð  Britain

ð  America

ð  Germany

ð  Austria

ð  Switzerland


 See you next week


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Geothermal heat pumps;
Air-water heat pumps;
Exhaust air heat pumps.

They are energy efficient complete systems for heating and cooling which is both economic and environmentally sustainable .
This technology is available both for residential single / multi-family and industrial and commercial buildings.

The geothermal heat pump uses as a heat source the subsoil or ground water and allows a cost savings for the heating up to 80%.
You do not need to reside in a thermal area to enjoy the benefits of underground heat: from 10 m depth the soil temperature remains almost constant throughout the year, and around 100 meters the temperatures are around the 10-12 °C constituting a heat source for the optimum operation of the heat pump.

The air-water heat pump uses as a source of heat the outside air and allows a cost savings for the heating up to 50-60%.
It can produce hot water for heating or sanitary uses up to temperatures of 65 °C with only the compressor and work up to temperatures of -25 °C!

The exhaust air heat pump uses as a heat source stale air present in the house, allows a cost savings for the heating up to 50%.

Heat pumps are often installed in homes to serve both heating and cooling need. Heat pumps are safer than systems relying on combustion. They are clean, quiet and odorless.

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