Geothermal Energy in Finland

The use of geothermal energy in Finland is restricted to the utilization of ground heat with heat pumps. This is due to the geological conditions as Finland is a part of the Fennoscandian (or Baltic) Shield. The bedrock is Precambrian covered with a thin (<5 m) cover of Quaternary sediments. Topography is subdued and does not easily produce advective re-distribution of geothermal heat by groundwater circulation systems. Due to crystalline character of the bedrock, rock porosity and its water content are low. This practically excludes geothermal systems utilizing hot wet rock. The lithosphere is very thick in Finland (150-200 km), and heat flow is mostly below continental average (< 65 mW m-2). Measured heat flow density values in the uppermost 1 km of bedrock range from very low (<15 mW m-2) values to 69 mW m-2, whereas an average value of 46 sites (53 boreholes) is 37 mW m-2. Geothermal gradient is typically 8-15 K km-1, and the annual average ground temperature at the surface ranges from about +5oC in the southern part to about +2oC in the northern parts of the country. Temperatures at 500 m below surface are usually between 8 and 14oC. At 1000 m the temperature ranges from 14 to 22oC. Values either extrapolated from geotherms or calculated with thermal models suggest that temperatures exceeding 40oC should be encountered at 1-1.5 km depth. However, in order to reach 100oC, depths from 6 to 8 km are required. These numbers suggest that Finland is not a good candidate for either wet or dry hot rock systems, although some formations with either anomalously high heat production rate or thermal blanketing effects due to low thermal conductivity should be investigated in more detail. Nevertheless, promising applications can be found for small-scale use of ground-stored heat in all parts of the country. About 10,000 heat pumps have been installed in boreholes, lakes or Quaternary deposits since the early 1980's. About 70 % of them are horizontal ground coupled systems, 20 % are using lake water and 10 % are vertical ground coupled systems. Typical vertical installations are in small family houses using a shallow 100-200 m deep borehole. The order of magnitude of energy extraction from such holes is 50 W/m. The use of ground-heat with heat pumps is currently increasing in Finland.