Beneath our feet there are rich reserves of heat and energy stored in rocks and groundwater. On average, with increasing depth, the temperature increases by around 3°C per 100m. In the upper surface layers the heat in the ground comes from the sun. A number of quantitative models from geothermal low activity areas (i.e. on stable platforms outside tectonic and volcanically active areas) show that at shallow depths down to a few hundred meters, mean annual surface temperature is the main factor controlling subsurface temperature. Geological variation in the underground, such as heat flow, heat production and thermal conductivity, first become significant around 1,000m and deeper below the surface.
At depth, some of the heat comes from the cooling of the Earth’s core, but most is from decay of radioactive elements, mainly uranium, thorium and potassium, in rocks of the crust. This means that the geothermal gradient (the increase in temperature with depth) varies with the chemical composition and age of the rocks. As one example we could mention the radioactive Løvstakken Granite, near the city of Bergen in western Norway, which shows an average heat generation value of 8.03?W/m³, which is more than twice as much as normal levels for Norwegian granitic basement rocks. On a global scale the Earth’s surface heat flow averages 82 mW/m², and the estimated total thermal energy above mean surface temperature to a depth of 10 km is 1.3x1027J, equivalent to burning 3.0x1017 barrels of oil.
The potential for utilizing geothermal energy from deep underground is generating increasing interest, as previously discussed in GEO ExPro Vol. 4, No. 5. Exploration drilling and geothermal heating plants already on steam in Europe have documented that many areas have good potential for using geothermal for heating, but also that some areas outside the well documented `high-temperature fields` may have temperatures at depths sufficient for generating electricity.
Heating from Shallow Sources
Geothermal energy is extracted from within the earth via water, occurring either in liquid or steam phase. By using geothermal heat-pumps, geothermal energy can be obtained from low temperature sources, and used to heat workplaces, hospitals, schools and our homes. More than 80 countries around the world today use geothermal energy for heating. Since ground source heat for house hold warm ing is commonly extracted from shallow boreholes between 100 and 200m deep, the key factors controlling the effect and economy of installations for extracting geothermal energy at shallow depths are mainly linked to the overburden, hydro-geological activity underground, and the capability of the rocks to act as reservoirs and water carriers. We therefore need information on the spatial distribution, the porosity and the permeability of the geothermal reservoirs to evaluate the geothermal potential of a certain area.
With a few exceptions, geothermal energy for heating has been used only rarely in Europe. In the Paris region, a limestone reservoir with an area of 15,000 km² provides temperatures ranging between 56 and 85°C and has been exploited to heat the equivalent of 150,000 homes for the past 20 years. For a geothermal heating plant, water at less than 100°C would be sufficient, and 60-70% of the energy used in Europe is for low temperature applications. Recent studies carried out by GeoForschungsZentrum Potsdam (GFZ) have shown that large areas of the North German basins, the foothills of the Alps and the Rhine Graben are suitable for extracting heat from the ground. A geothermal heating plant established in Neustadt Glewe in Mecklenburg has been using water at a temperature of 98°C from a depth of 2,300m since 1995. GFZ has estimated that, looking at the geological and the geotechnical requirements, it would be possible for 17,000 plants to generate heat from the North German basin alone. However, one limitation is that geothermal energy must be used close to where it is generated, as it is not economically viable to transport such energy over long distances.
In areas with high thermal gradients, like the volcanic active zone in Iceland, geothermal steam and hot water can be used to generate electric power. But even in low-temperature areas water with high temperatures can be found at depth. Water pumped from the depths at temperatures of 100-150°C can transfer heat via a heat exchanger to a heating circuit containing a liquid with a low boiling point. The gas pressure generated in this way drives a turbine to produce electricity. Going for the deep heat seems to become more and more technological feasible and economic attractive.
In Kirchweidach, in southern Germany, a 3,900m deep borehole was completed in summer 2011. The target was a karstic carbonate Upper Jurassic reservoir with good permeability and with water temperature reaching 130°C. This is used to produce electricity, in addition to the heating power plant capacity. The plan is to produce up to 13,000 MWh per year. The drilling was carried out with GFZ’s InnovaRig, which proved very efficient. The drilling started in November 2010, and after one month the drill-bit had reached 2.5 km. The well is deviated, so that by 3,800m the horizontal deviation is almost 600m. The total concession area for the geothermal drilling is 76 km², with two deep boreholes planned. After completion of the two Kirchweidach boreholes, new drilling for geothermal energy will be started in the nearby district.
The use of geothermal heating and energy produces no nitrogen oxides, sulphur dioxide or carbon dioxide. One megawatt of power will provide enough electricity for about 1,000 households. This would prevent about 3,000 ton of carbon dioxide from entering the atmosphere every year. There is a green light for further developments of the use of the heat beneath our feet.