Geotimes - July 2002 - Highlights

Geotimes

Highlights

Geothermal Energy
Marnell Dickson

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By the end of 2001, the installed geothermal capacity world-wide had reached 8,183 MWe, showing an increase of about 2.6 percent over the 7,974 MWe installed in the year 2000. This increase is a modest achievement, and can be blamed in part on the fact that some geothermal plants are currently being replaced or upgraded. For example, 90 MWe have been temporarily disbanded in Italy and 60 MWe in Indonesia. Research in the future will tend to focus on the restoration or maintenance of "old" or "exhausted" geothermal fields.

The Energy Centre at Husavik, Iceland. Photo by Hreinn Hjartarson, courtesy of Orkuveita Husavikur

At The Geysers in California, one of the largest geothermal fields in the world, production collapsed dramatically about 10 years ago because of a lack of steam. The Santa Rosa Geysers Recharge Project, which is scheduled to become operative this year, will enhance steam production and bring new life to this field by pumping about 41.5 million liters of tertiary treated wastewater daily into The Geysers geothermal reservoir. The wastewater comes from the Santa Rosa regional sewage treatment plant and other cities through a 66-kilometer pipeline. The project, costing $132 million, will make a valid contribution to solving the energy crisis of the state of California. At the same time, new plants will come online in the near future. A further 100 MWe is currently under construction at the Los Azufres II field in Mexico. At Amatitlan, in Guatemala, a 20-MWe plant will replace the dismantled 5-MWe plant. Fifteen MWe will come online in Guadeloupe, 64 MWe are under construction at Olkaria II in Kenya, 75 MWe in Kamchatka, Russia, and 56 MWe in the Philippines in Northern Negros and at Montelago. Allowing for the inevitable delays, we can still expect a further 450 MWe, bringing the total installed capacity to about 8,633 MWe.

Electricity from geothermal sources represents a small part of the total amount of electricity produced from the world's combined conventional and renewable energy sources. In some countries, however, it plays a significant role in the energy balance, such as on the island of Sao Miguel in the Azores, where it accounted for 40 percent of the total energy produced in 2001, or in Iceland (17.2 percent in 2001), the Philippines (16 percent in 1997), Nicaragua (15.3 percent in 1997) and El Salvador (19.7 percent in 1997).

I. Fridleifsson reports (GRC Bulletin, vol.30, no.4, pp.139-144, 2001) that, at a meeting of the World Energy Council in Houston in 1998, J. Bjornsson and colleagues estimated that geothermal energy could potentially account for as much as 7.9 percent of the world's primary energy production from renewables by 2020. This goal, however, can only be reached by improving exploration methods and reducing exploration costs, by increasing the efficiency of geothermal plants, and by developing new technologies.

There has been considerable progress in most of the disciplines involved in geothermal exploration (geology, geochemistry, etc.), but the most significant has been in the geophysical sector. The CSAMT method (Controlled Source Audiomagnetotellurics), for example, has produced far better results than the traditional geoelectric techniques, and has resulted in a 10-15% decrease in exploration costs (Meidav, GRC Bulletin, vol.27, no.6, p.178-181, 1998).
Progress has also been made in the application of microearthquake seismology to the identification of active fractures. Seismic reflection and refraction techniques are now capable of providing sharper structural resolution, especially in rocks with the chaotic, non-bedded, and poorly bedded characteristics typical of geothermal reservoirs. These improvements have helped to reduce exploration time and costs, and the drilling risk.

The progress made in plant engineering over the years is reflected in gradual improvements in plant efficiency. Recently substantial benefits have also been gained from using the Kalina cycle in the binary-cycle generation of electricity. The binary system utilises a secondary working fluid, typically n-pentane, which has a low boiling point and high vapour pressure at low temperatures when compared to steam. This secondary working fluid is usually operated through a Rankine cycle. Using an appropriate working fluid, binary systems can be designed to operate with inlet temperatures between 85 and 170 degrees Celsius. Heat is transferred from the geothermal fluid to the binary cycle via heat exchangers where the binary fluid (or working fluid) is heated and vaporised before being expanded through a turbine to some lower pressure/temperature. The Kalina cycle, which uses a water-ammonia mixture as working fluid, has a higher efficiency than "conventional" binary plants, and is smaller in size (the technical data are proprietary, so precise details are not available). Kalina plants are already operating at Husavik, Iceland, and in Japan. A pilot plant is at an advanced stage in Canoga Park, California. The project is funded by California Energy Commission.

The cutting edge of geothermal technology is the Hot Dry Rock concept, which aims at creating artificial geothermal systems by injecting water underground at high pressure so as to produce fractures in deep, hot rocks. One borehole is drilled to inject the water, and another to recover the hot water after its passage through the manmade fractures in the deep hot rocks. The first project, launched in 1970 by Los Alamos Scientific Laboratory in New Mexico, was swiftly followed by others in Australia, France, Germany, Japan and Britain. After languishing for a few years, these projects have been given renewed impulse. The greatest progress has been made in Japan and in the European project at Alsace, France. The European project is financed by the European Union and by public and private agencies in a number of European countries. Some 20 teams from the different countries work on the site at various times. A number of HDR projects were launched in Japan in the 1980s, with large-scale funding from both the government and industry, and have produced excellent scientific and industrial results.

The European project has completed the drilling of two wells, the second of which has its bottom-hole at 5060 meters. The results of the geophysical and hydraulic tests are promising. At the moment the European project seems to be the most successful.

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Dickson works at the Istituto di Geoscienze e Georisorse in Pisa, Italy, and is associate editor of the international journal Geothermics. E-mail

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