Using Heat from the Earth
By “Awake!” correspondent in El Salvador
August 7, 1975, was a memorable day for a small Central American country with a population of a little more than four million and an area of just 21,000 square kilometers (8,100 square miles). A thermoelectric plant, using neither coal nor oil, started operating that day, freeing the country from the need to import fuel for generating electric power. What, then, operates the electric generators? Steam lying very deep under the surface of the ground.
EL Salvador is a country of many volcanoes. Along nearly a longitudinal axis of the land, which also follows a line of geological faults in the earth’s crust, there are 18 volcanoes. At least four of them have been active in the more recent past. Volcanic action also has manifested itself in the form of visible steam leaks, geysers or cavities in the earth (called ausoles in the native Indian language).
How is the steam produced? Scientists tell us that rainwater filters into the ground through craters and other highly permeable areas and finally reaches rock. Sometimes, especially in volcanic regions, rock heated by lava lies near enough to the surface of the earth so that the water is heated to temperatures high enough to convert it into steam. In Ahuachapán, in the western section of El Salvador, earth cavities in the form of mud ponds eight to 10 meters (26 to 33 feet) in diameter have existed for some time. These are full of boiling mud varying in color from reddish brown to yellow, and they give off vapors having a strong sulfurous odor. For many years these cavities were merely a tourist attraction, no thought being given to the use of them for any practical purpose.
The Start of Geothermal Studies
In the 1950’s, however, the CEL (Comisión Ejecutiva Hidroeléctrica del Río Lempa), an autonomous governmental body in charge of the development of hydroelectric power, heard about the first developments of endogenous (internally developed) power from the earth. This was in the Larderello region of Italy. Also, New Zealand was trying to generate electricity by using such resources at Wairakei.
This news created much interest. El Salvador was just starting the development of its hydroelectric energy from rivers. But to make the best use of such energy, it would eventually be necessary to generate thermal power. To do so with fossil fuel would require importing oil or coal. On the other hand, natural steam could be harnessed to accomplish the same thing.
The year 1953 saw the start of the first geothermal investigations in the Ahuachapán area. In 1958, 11 shallow exploratory wells were dug in the fields of Playón de Ahuachapán and El Salitre. More serious geological, geophysical and geochemical research began in 1966, covering an area of 200 square kilometers (77 square miles).
The investigations gave promise of excellent potential for energy sources. Therefore, in 1968, experimental wells were dug to depths of 865, 981 and 1,192 meters (2,838, 3,218 and 3,911 feet) respectively. One of these wells was dry, but the other two began producing steam at temperatures of 231 degrees and 208 degrees Celsius (448 degrees and 406 degrees Fahrenheit) respectively, and at pressures of 10 kilograms per square centimeter (142 pounds per square inch). These wells were kept at full steam production for more than one year to test their capacity for maintaining stable pressures and temperatures. In January 1970, six more wells, varying between 700 and 1,400 meters (2,300 and 4,600 feet), were dug in the fields of Playón in Ahuachapán to obtain data for technical and economic feasibility studies, with a view to creating the first 30-megawatt geothermal plant. This plant was intended as the first-stage development for utilizing to the full the estimated capacity (100 megawatts) of the geothermal Ahuachapán field.
A Program for Nonfuel Power Plants
Construction of the first power-generating plant, with a capacity of 30 megawatts, started in 1974. This plant was inaugurated and put into service in August of 1975. A second 30-megawatt generating unit began to be built in 1975. This required boring five additional wells to depths between 600 and 850 meters (1,970 and 2,790 feet). The second unit was put into service in 1976. A third generating unit, with 35-megawatt capacity, is now under construction and will utilize the steam from the first two units. Thus, the geothermal field in Ahuachapán will be generating 95 megawatts of continuous power starting this year. Because of not having to use fossil fuels, this represents savings of 28.5 million colones ($11.4 million) annually for the country.
The good results obtained until now have prompted a vigorous program of exploration and research in the eastern part of the country. New wells are being opened in that area to implement the expansion program for geothermal power.
Electric-power generating plants using fossil fuels, like coal or oil, or employing atomic fuels, give rise to pollution problems. Ashes, smoke and gases may contaminate the atmosphere. Lakes and streams may be polluted from the discharge of water used for cooling. Also, the final disposition of residual products from these plants presents a serious problem to the community.
On the other hand, geothermal plants, burning no fuel at all, could be expected to offer fewer environmental pollution problems. Nevertheless, the steam, gases and water produced by geothermal wells can bring about ecological difficulties.
In the case of geothermal fields producing dry or superheated steam, there are natural discharges containing high concentrations of sulfates, some acidity and traces of chloride. Some of these waters may be slightly alkaline, with a predominance of sulfates and bicarbonates. There may also be a high concentration of carbon dioxide, boron and ammonia. Another element, hydrogen sulfide, is highly toxic and could cause ecological problems.
Fields producing water steam yield large volumes of residual water. This water usually has a high salt content that is harmful to plant and creature life. The boron content always exceeds what has been established as tolerable by resistant crops. Arsenic is generally associated with these waters, too, making them unfit for human consumption.
So, the final disposal of these residual waters presents serious problems. Principally, the following disposal methods have been employed: (1) Dilution into the sea, (2) dilution into rivers, (3) reinjection into the subsoil and (4) evaporation in ponds.
Dilution into the sea could be expensive and difficult if the geothermal field is far away from the seashore. Dilution into rivers depends on the amount of water flowing, so that tolerable concentrations of toxic elements are not exceeded. During the dry season, rivers often have so little water that this would be impossible. Reinjection into the subsoil strata could be obstructed by the salts carried in the residual water, as these salts form deposits on the walls of wells. Evaporation in ponds is possible only if large areas of flat land are available to construct the ponds and if the rainfall is minimal in the region.
At the Ahuachapán geothermal plant, residual water passes through an open canal, to be diluted in the sea. Also, there have been successful experiments with reinjection into the soil.
It is interesting to compare the costs of a geothermal power plant with those of conventional plants, either hydroelectric or thermal plants powered by fossil fuel. Investment costs for the Ahuachapán 95-megawatt geothermal power plant have been estimated at $700 for installed kilowatt. The generating cost for this plant is $0.005 for each kilowatt hour generated, whereas the cost for the biggest hydroelectric plant in El Salvador, Cerron Grande, is $0.004 per kilowatt hour. However, the cost of generating power in oil-fired plants presently runs $0.024 per kilowatt hour. This is almost five times as much as the cost for geothermal plants. No wonder El Salvador is eager to develop its endogenous power!
Now that the energy crisis is increasing in the world, many countries are looking for new sources of power to substitute for the scarcer and more expensive imported oil. Endogenous power—heat from the deep layers of earth’s crust—certainly is a useful source of energy. It may, therefore, be expected that other countries having cavities in the earth that emit smoke, or where there are some other signs of volcanic activity, will start to tap these hidden treasures under their feet.