Background to the Calpine Geysers Geothermal Field
The Calpine Geysers Geothermal Field is located in Northern California, covering over 29,000 acres. In total the complex houses 15 geothermal power plants, 10 located within Sonoma County and five in Lake County.
These 15 Calpine plants have a 725MW capacity, supplying approximately 6000 GWh of electricity to the grid each year, representing 21% of California’s renewable electricity generation capacity; enough to power over 750,000 homes in the USA.
At the geysers, water deep below the Earth’s surface is exposed to hot porous and permeable rock resulting in superheated steam. Steam production wells are then drilled down (some in excess of 2 miles) to access this superheated steam, which then comes to the surface where it is used to drive conventional steam turbines.
The first commercial steam well was dug within the geothermal field in 1954, and since then a further 584 steam wells have been drilled – although now only 333 are currently used.
How Do They Keep Geyser Producing Steam?
Recognising the importance of the geyser reserve, Calpine have since 2003 been pumping reclaimed waste water from local sources down into the ground via 75 injection wells, replacing the water (steam) that is lost. Steam is lost as it has to travel to the surface to produce the electricity. If this weren’t to happen, the superheated steam reservoirs would eventually dry up, leaving the steam production wells unusable.
In total 17.8 million gallons of water are pumped down into the ground per day to keep a steady supply of superheated steam to drive the turbines. This operation has actually has had a positive impact on the local environment as prior to 2003, most of this waste water would have been pumped into local rivers
Other Environmental Impacts of the Geyser Geothermal Field
In terms of other environmental impacts of the geothermal field, evidence points to increased seismic activity as a result of the deep-well injection in the area, which is a concern for the potential to initiate earthquakes. The seismic activity resulting from operations in the region are described as micro-seismic events though, as the resulting events tend to only be between 0.5 – 3 on the Richter scale, with the largest recorded event in 1973 (where it was between 3 and 4.6).
Since this date the number of seismic events has stayed relatively constant despite an increased rate of injection. The chance of a large earthquake resulting from the drilling activities is extremely unlikely, as none of the Geothermal fields lie over a major fault line.
Geothermal Power – Iceland
You don’t have to back far in time, in fact only back to spring 2010, to remember how Icelandic geothermal activity impacted the UK and north-western Europe. Volcanic activity on the island caused ash clouds to build over the UK and grounded the majority of flights coming in and out of the continent. Although this wasn’t a comfortable experience, in the UK we can learn much about Iceland harnessing this natural energy source into geothermal power to create both electricity and district heating solutions.
The History of Iceland Harnessing Geothermal Power
For centuries, the people of Iceland have been using hot springs for washing and cleaning, however it was not until 1907, when a farmer from the west of Iceland used a concrete pipe to direct steam from a hot spring into his house, taking advantage of this naturally occurring product for heating. Since then, Iceland has been taking ever increasing advantage of it’s unique geology. It has even got to the point where in the capital Reykjavik, the pavements are heated.
What Makes Iceland So Unique to Exploit Geothermal Power?
Well it lies directly over the mid-Atlantic ridge, which is essentially the crack in the Earth’s crust where the Eurasian Plate and the North American Plate diverge from one another. As such Iceland is an area of extreme tectonic activity, with regular occurrences of both volcanic and seismic activity.
In the volcanic region of the island, there are 200 active volcanoes, so essentially Iceland can be considered as one super volcano, formed over millions of years from molten rock bubbling up from the seabed; as a result, the earth is particularly porous.
Iceland also has abundant precipitation, which as it falls gets absorbed by the sponge like earth. Since the Earth’s crust is relatively thin, magma chambers are positioned relatively close to the surface, which heats this water. There are approximately 20 high-temperature areas within the volcanic area of Iceland, where underground temperatures reach 250°C at a depth of only 1,000m.
This combination of factors makes Iceland the ideal place for geothermal power, with approximately 66% of primary energy coming from this source (both heating and electricity). In 2008, close to 90% of the country’s heating came from hydrothermal sources.
Geothermal Power Station Examples in Iceland
There are currently 9 geothermal power plants in Iceland, providing in excess of 5000 GWh of electricity per year, the largest being the 303MW Hellisheidi power station, operated by a utility company owned by the capital city of Reykjavik.
This geothermal power plant is a flash steam, combined heat and power (CHP cogeneration) plant power station, which besides producing electricity, provides hot spring water to the city, via a 20km pipe, which is used for heating. This plant consists of 50 boreholes varying in depth between 1000m and 2200m to get access to the hot steam fields. The two other large CHP plants are the Reykjanes plant (100MW) and Nesjavellir plant (120MW).
It is important to note that while only 30% of total electricity demand is met via geothermal power, the country provides the rest of its power using hydroelectric so is operating on a 100% renewable basis. In addition only a very small proportion of both its hydroelectric and geothermal power potential is currently been tapped into, so as the country continues to grow it will easily be able to maintain this 100% renewable position.
Applying Icelandic Geothermal Expertise to the UK
The UK has by comparison a very low percentage of clean, renewable energy being satisfied by geothermal power, with majority coming from wind farms or hydroelectric. To do more to exploit this geothermal energy source, on 30th May 2012, a Memorandum of Understanding was signed between the UK and Iceland that will see the two countries look at developing the deep geothermal sector in the UK. This initiative is set to explore whether an electricity cable can be laid between the two countries allowing Iceland to export it’s electricity, helping the UK hit its green energy targets.
