Iceland is unique among industrial nations in that more than 50% of its energy comes from renewable geothermal energy. The word geothermal comes from the Greek words geo (earth) and therme (heat).

Iceland uses geothermal energy for a diverse area of services, including electricity production, spas, home heating, fish farming, and industrial process heat. More than 95% of Iceland’s urban population heat their homes and buildings with geothermal energy. By a great margin this is the largest contribution geothermal energy makes to a national energy budget.

Geologic setting

Iceland lies over a plume of hot material upwelling from great depths and the Mid-Atlantic Ridge, the boundary between the North American and Eurasian tectonic plates where new crust is being created on the order of 45 cubic km per 1000 years. Iceland formed by the coincidence of the spreading boundary of the North American and European plates and a hotspot or mantle plume. As the plates moved apart, excessive eruptions of lava constructed volcanoes and filled rift valleys. Subsequent movement rifted these later lava fields, causing long, linear valleys bounded by parallel faults. These movements continue today, accompanied by earthquakes, reactivation of old volcanoes, and creation of new ones. On top of hot spots is a 20-100% molten layer at a depth of 5 to 20 km.

These conditions produce an unusually large geothermal gradient beneath Iceland (the rate of increase in temperature per unit depth in the Earth).  The average geothermal gradient is about 35°C/km, but Iceland is characterized by much more rapid temperature increase with depth. For instance, the poorly permeable basaltic bedrock in Arskogsstrond (North Iceland) exhibits a temperature gradient over 200°C/km.  This geothermal energy produces about 30 volcano systems and 600 hot springs and geysers.  The island has about 130 volcanic mountains, of which 18 have erupted since the settlement of Iceland. Over the past 500 years, Iceland's volcanoes have erupted a third of the total global lava output.

Figure 1. Heat flow in Iceland crust, measured in terrawatt-hours per year(TWh/a).

The same conditions provide Iceland with some of the world's richest geothermal resources. When magma comes close to the surface it heats ground water found trapped in porous rock or water running along fractured rock surfaces and faults.   Such hydrothermal resources have two common ingredients: water (hydro) and heat (thermal).  Naturally occurring large areas of hydrothermal resources are called geothermal reservoirs. The heat in these reservoirs can be tapped to generate electricity or to provide heat for a wide range of industrial, commercial, municipal and residential purposes.

Figure 1 illustrates the magnitude of heat flow through the crust beneath Iceland. Scientists estimate that about 59 terrawatt-hours (TWh) per year of heat can potentially be captured for human use. By way of comparison, Iceland currently generates about 1.7 TWh of electricity from geothermal resources.

Figure 2. Location of Iceland geothermal fields. Source: Icelandic Energy Authority.

Geothermal areas in Iceland are divided into high temperature fields and low temperature fields (Figure 2). High temperature fields have temperatures of at least 150 degrees Celsius at a depth of one kilometer and are only found in the active volcanic zone along the tectonic plate boundary. Most of the high temperature fields are rich in gases and minerals and therefore cannot be used directly for space heating and bathing. How ever the fields'  high pressure and high thermal energy make them well suited to heating fresh cold water which then can be used for space heating and the generation of electricity.

The low-temperature areas are distributed widely across Iceland because they are located in areas flanking the active volcanic zone. The water temperature decreases with distance from the volcanic belt and is below 150 °C to 100°C   at a depth of about 1 km. The largest resources are located in southwest Iceland, especially in Reykir and Reykholt. The surface activity is usually restricted to hot springs; some systems have no surface manifestations at all. 

Primary energy use

Geothermal energy has captured an increasing share of total primary energy use in Iceland (Figure 3). The development of geothermal resources  was enhanced by the oil price shocks of the 1970s and 1980s. Until that time, Iceland relied heavily on imported oil that suddenly became very expensive, prompting the government to adopt policies and programs to encourage the development of domestic renewable sources such as geothermal an hydropower.  geothermal energy now accounts for about 55% of total primary energy use.

Figure 3. Primary energy use in Iceland. Source: Icelandic Energy Authority.

Note that primary energy refers to the energy from the Earth prior to any conversion or transformation process.  Accordingly, the primary energy consumed in producing electricity from geothermal energy is about ten times the electricity produced, which means that the efficiency of the generation process is  about 10%.  When geothermal energy is utilized for heating, as for instance in district heating systems, the primary energy is calculated as the energy extracted by cooling the water to 15°C.

End use of geothermal energy

Electricity generation

In 2006, about 27% of electricity generation in Iceland came from geothermal sources.  The major plants are described below. The first two power plants produce both electricity and hot-water for heating purposes, whereas the other three produce only electricity.

Figure 4. Nesjavellir Geothermal Power Plant in Iceland. Credit: Gretar Ívarsson.

1. The Svartsengi Power-Plant (Figure 4), situated in the south-west of the country, near the International Airport at Keflavík on the Reykjanes peninsula. It  produces about 76 MW of electricity, and about 475 liters/second of 90 °C hot water (ca. 80 MW). Surplus mineral rich water from the plant fills up a nearby lake and popular tourist bathing resort Bláa Lónið (Blue Lagoon).
2. The Nesjavellir Power-Plant, situated in the south of the country, near the lake Þingvallavatn and Hengill volcano. It currently produces 120 MW of electricity, and about 1800 liters/second of heating water (ca. 300 MW).
3. The Krafla Power-Plant, situated in the north-east of Iceland near lake Mývatn and the volcano Krafla - hence the name. It produces 60 MW of electricity, with an expansion to 210 MW on the drawing boards.
4. The Reykjanes Power-Plant, situated in the south-western tip of the country (to the west of Svartsengi), went on line end of 2006, two turbines are producing 100 MW.
5. The Hellisheiði Power-Plant, to the south of the Hengill volcano is being built, two turbines with together 90 MW went on line end of 2006 and one 34 MW low pressure unit end of 2007.

Space heating

Utilization of geothermal energy for heating is accomplished largely through district heating, a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating and water heating. District  heating using geothermal energy began in 1930 when the first house, a schoolhouse, in Reykjavik, received hot water from wells close to the old thermal springs in Reykjavik. Soon after the main hospital, a swimming pool and about 70 private houses were connected. In 1943 geothermal water from a very large thermal field located about 17 km from the city was piped to Reykjavik. By the end of 1944 a total of 2850 houses were connected to the district heating utility in Reykjavik. Following the oil price increases of the 1970s, the government took the initiative in expanding district heating utilities.  Now there are 26 municipally owned district heating system in Iceland, and pace heating now accounts for about 57% of geothermal energy end use.

Swimming pools

Figure 5. A pool in Iceland heated with geothermal energy.

The heating of swimming pools is the third most important direct use of geothermal water in Iceland (Figure 5). There are at least 160 swimming  pools in operation;  based on their  surface area about 90% are heated with geothermal energy. Most of the public swimming pools are open-air pools in constant use through out the year.  Swimming is very popular Iceland--the average person makes about 15 visits per year to a public pool. A new, middle-sized swimming pool uses as much hot water as is needed to heat 80- 100 single-family dwellings.

Snow melting

The use of geothermal energy for snow melting began to spread in the 1980s.  Spent water from heating of houses, about 35 °C, is commonly used for deicing of sidewalks and parking spaces. Most systems have the possibility to mix the spent water with hot water (80°C) in periods when the load is high. At the time of  an extensive rehabilitation of streets in downtown Reykjavik few years ago, a snow melting system was installed under pavements and streets covering about 40,000 m². Many streets in a new construction area in the eastern part of Reykjavik have snow-melting systems installed. The total area now covered by snow melting systems in Iceland is at least 350,000 m² , of which about 250,000 m² are in Reykjavik. The annual energy consumption is dependent on the weather conditions, but on average equals about 325 kWh/m². Of that about two third come from spent water from the houses and one third from 80 °C hot water.

Industrial Uses

The diatomite plant at Lake Myvatn uses more direct geothermal energy than any other industrial enterprise in Iceland. The plant, which has been operational since 1967, produces some 27,000 tonnes of diatomite annually. in recent years year the plant has used about 220,000 tonnes of geothermal steam  for drying applications. The seaweed product manufacturer Thorverk, at Reykhólar in West Iceland, also uses geothermal heat directly in its production. The plant produces 2,000-4,000 tonnes of rockweed and kelp meal annually, using 28 l/sec of 107°C hot water in its production. A salt production plant was operated on the Reykjanes peninsula for a number of years but its operation has been intermittent. Since 1986, a facility in Grímsnes, South Iceland, has produced carbon dioxide (CO₂) from geothermal fluid. The plant uses approximately 6 l/sec of fluid and produces some 2,000 tonnes annually. The production is used in greenhouses, for manufacturing carbonated beverages and in other food industries. Among the other uses of geothermal energy is drying of fish, which is carried out in many areas of Iceland. 

Greenhouses

One of the oldest and most important usage of geothermal energy in Iceland is for heating greenhouses. Before it was used first in 1924, naturally warm soil has been used for growing potatoes and other vegetables. Most greenhouses are located in the south, and the majority is enclosed in glass. The growing season and improved greenhouse utilization has been increased by artificial lightening and the use of additional CO2-enrichment in recent years. The total area under glass was about 195,000 m2 in 2002. Of this area, 55% is used for growing vegetables and 45% for flowers. Outdoor growing at several locations is enhanced by soil heating though geothermal water, especially during early spring. Soil heating enables growers to thaw the soil so vegetables can be brought to market sooner. It is estimated that about 105,000 m² of fields are heated this way.

Fish farming

Geothermal energy is also extensively used in the fish farming industry. Geothermally heated water is used mainly in the drying and hatchery stages.

Sources

• Gawell, Karl and Griffin Greenberg, 2007 Interim Report Update on World Geothermal Development, May 1, 2007, Accessed 12 September 2008.
• International Energy Agency, 2005 Energy Balances for Iceland, Accessed 12 September 2008.
• Kranz, Kathrin, Geothermal Energy in Iceland, Institute of Hydrogeology, TU Bergakademie Freiberg, Germany, Accessed 12 September 2008.
• Ragnarsson, Árni, Geothermal energy in Iceland, Geo-Heat Center Quarterly Bulletin, Vol 17, No. 4 (1997)
• Stesky, Robert M.  A Tour of Some Iceland Geology, Pangea Science, Accessed 12 September 2008.