Geothermal Energy

Krafla Geothermal Power Station, Iceland, picture courtesy of Landsvirkjun

Renewable energy souces have one thing in common, they all existed before man appeared on this planet. Wind, Wave, Hydro, Solar, Geothermal and Tidal power are all forces of nature and are mostly intermittant energy sources, Geothermal is the only consistant phenomonon. Geothermal renewable energy sources where probably the first to be fully utilised by man. Early civilisations tapped this heat to cook, fire clay pottery, create baths and spas and even heat their homes. Roman villas had under floor heating from natural hot springs over 2000 years ago.

Where does the heat come from?
The main source of this heat is from the molten centre of our planet which remains hot due to the vast pressure exerted by the Earth's crust. The Sun also contributes to this heat gained by the Earth during daylight hours which then radiates away during the night. Only a few metres down the temperature tends to remain constant at about 10-16°C and usually varies by a few degrees (more in the winter) from the surface and air temperature. This is the principal on which a ground source heat pump (GSHP) works. The amount of temperature variance depends on the geology of the soil, climate and seasonality.

How is it used?
In Iceland whole townships are heated using the geothermal heat from subterranean volcanic systems. 85% of all homes in Iceland are heated by hot spring water and Reykjavik Energy runs the largest district heating system in the world. Iceland is the only Western country that has a full renewable energy electricity generation system, 83% produced by hydro power stations and the other 17% by geothermal power stations. In geothermal power stations such as the one in Landrello, Italy (world’s first) or Wairekei in New Zealand, a pipe drilled down to the heat source delivers water and another nearby drilled pipe collects the resulting hot water under pressure. When it reaches the surface this water turns to steam and is used to turn turbines in a generator to produce electricity.

What if you have no volcano to tap?
As mentioned earlier there is always a temperature difference between the surface and a few metres underground and this can be utilised by ground source heat pumps. These normally consist of three different elements. A loop buried in the ground (approx six feet) in a horizontal configuration or down to 150 feet in a vertical configuration. The length of the loop depends on the heat requirement. The ground loop carries a water or water/anti-freeze mixture which is pumped around the system by the second system element which is a heat pump. This in turn passes the heat onto a system which transfers the heat throughout the house. (This is the reverse of how a refrigerator works)
In Argyll this has taken the form of under floor heating (ground floor) and radiators in upper floors. In systems where space heating is used the system can also work in reverse, acting as a heat sink, in the summer and cool the house. Due to the lower ambient temperature of ground source heat pump systems under floor heating is the best complementary distribution option for heatpump/exchanger combinations which in turn is best installed while constructing a building. Retrofit is possible and a large social housing project which did just that was recently finished in Cornwall. 
Only the pump requires electricity to drive the system therefore this form of heating is very cost effective over its life cycle as it produces more energy than it consumes (3 to 1 ratio). If a solar photovoltaic panel or a roof mounted wind turbine is also include in the project then it becomes a completely renewable energy powered system.
Ground source heat pump systems are used extensively in Sweden, Germany and North America and have been used to heat domestic homes in these countries for nearly 30 years.

Ground Source Heat Pumps in Argyll and the Islands area
West Highland Housing Association have built twelve houses in Bowmore, Isle of Islay which will be heated by GSHP systems with under floor heating and radiators on the 1st floor. As it is more cost effective to install this type of system as new build, this tends to be the method adopted by the housing associations and community groups as they consider new developments across Argyll and the Islands.
The new Antlers community development on Jura and the playing fields at Port Charlotte (Iomairt Chille Chomain) on Islay are both considering this type of heating system for their new buildings with support from AIE and ALIenergy.
Community groups and domestic homes (domestic installations now benefit from a reduction in VAT duty to 5% announced in the March 2004 Budget) can apply for assistance for this type of installation from our Scottish Community and Household Renewable Initiative Scheme (SCHRI) by contacting ALIenergy in Oban.
Companies in the Agricultural field, such as growers and nurseries who use greenhouses or poly tunnels can make considerable savings on their energy bills by switching to GSHPs, in some cases up to 80%.  
Businesses should review the assistance available to companies listed on the Renewable Energy Initiatives  page of our web site (see RH side panel) for information on Enhanced Capital Allowances and 0% interest loans.