Geothermal energy can supply heating for thousands of years

15 June 2009
By Steen Hartvig Jacobsen, journalist

Geothermal energy is a renewable form of energy utilized to a very limited extent in Denmark to date. However, the subsoil in the metropolitan region actually contains so much hot water that it can cover up to half the region’s district heating requirements for several thousand years. Many areas are likely to have corresponding reserves in the rest of the country, where similar surveys have not yet been carried out.

Since 1984, Denmark has had a geothermal pilot plant at Thisted, which is connected to the town’s waste-based CHP plant and exploits 45˚C hot water from a depth of about 1,250 metres. This plant can produce the equivalent of the annual heat consumption of approximately 2,000 households. In Copenhagen, a geothermal plant on Amager began producing heat in 2005. A third plant is being established in Sønderborg, but more are underway, including a large geothermal plant envisaged by the Copenhagen Climate Plan.

Three exploration licences

Geothermal heat is generated by the hot water in the interior of the Earth. The temperature in Denmark’s subsoil layers typically increases by 25-30 ˚C for every 1,000 metres of depth. The hot water that can be utilized is extracted from highly permeable, porous sandstone layers and pumped to the surface. The sandstone layers become less porous and permeable the deeper they are located, and extracting geothermal heat therefore ceases to be cost-effective at a certain maximum depth. In Denmark, this depth is assessed to be at about 2,500 metres.

At this depth, the water in the subsoil will have a temperature of 70-75 ˚C, as is the case in the demonstration plant built by the metropolitan heat production companies together with DONG VE. For this reason, geothermal energy cannot be used to generate electricity directly, the water temperature not being sufficiently high, but it is highly suited for generating district heating.

Exploring for and extracting geothermal heat require a licence according to the provisions of the Subsoil Act. In 1983, DONG Energy was granted an exclusive licence covering the whole of Denmark until 2013, but the company has since relinquished some areas, thus making room for new players. The DEA has issued three exploration licences to date. Thus, one licence covering most of Northern Zealand was granted in 2001 to the Metropolitan Geothermal Alliance (abbreviated HGS, from its Danish name), which consists of the metropolitan heat production companies and DONG VE.

DONG and Sønderborg Fjernvarme were granted a licence in 2007 that covers the whole of Als and an adjoining area in Southern Jutland. Under this licence, two wells are expected to be drilled in Sønderborg in autumn 2009, to be used in constructing a plant to replace the natural-gas-based district heating system. The plant is expected to go into operation in 2011 and will be connected to the town’s waste-based CHP plant.

The last licence was granted to Dansk Geotermi in 2008, and under this licence drilling expert Sigurd Solem will explore six areas in Jutland jointly with local district heating companies. The DEA is currently considering another application from Dansk Geotermi, covering seven areas in Jutland.

Large potential in the metropolitan region

Since 2005, the HGS companies have operated a demonstration plant at Magretheholm in conjunction with the Amager power station. Hot water at a temperature of about 73 ˚C is extracted from a depth of approximately 2,600 metres. The water is cooled to about 17 ˚C in a heat exchanger before being pumped back into the subsoil. The heat generated by this plant is equivalent to the annual heating requirements of 4,600 households.

In 2008, the HGS companies made an assessment of geothermal reserves in the metropolitan region. The conclusion is that the region has geothermal reserves of about 60,000 PJ, which can cover 30-50 per cent of district heating requirements for several thousand years. Following this assessment, the City of Copenhagen has begun incorporating a geothermal plant into its Climate Plan, with the aim of making the City of Copenhagen C02 neutral in 2025. This plant will have a capacity five to six times higher than that of the Magretheholm plant, and can go into operation in 2015.

Together with the Geological Survey of Denmark and Greenland (GEUS), the DEA is reviewing the potential for new geothermal plants in the rest of the country.

- When GEUS has assessed the geothermal reservoirs, the DEA will review the local conditions of supply and other framework conditions for the optimum exploitation, in environmental and economic terms, of this exciting renewable energy source, says Søren Frederiksen, engineer, who represents the DEA in this investigatory work.

Like other district heating plants, geothermal energy plants require very heavy investments. However, they require no expenses for fuel, but merely operating expenses for pumps, etc. The plant projected in Copenhagen is estimated to cost about DKK 1 billion.