BESTEC

Origin and Potential

by Dr. Jörg Baumgärtner, former Socomine S. A., Soultz-sous-Forêts, Alsace, France (1994)

Natives at a hot spring in today’s Calistoga, California, USA. Some tribes viewed hot springs as neutral ground where battles were forbidden. (Source: Geothermal Education Office)Compared to other, known ‘alternative’ energy resources, such as wind and sun, geothermal energy is not immediately felt. This is why geothermal energy is not as evident to us. Only miners learned over centuries that underground temperatures rise with increasing depth, and that the degree of these temperature increases can vary regionally. Volcanic activity along the continental rupture zones indicates that underground temperatures must be substantially higher in deeper basements. Nordic mythology and its deeply ingrained veneration for nature’s forces can only be fully understood if one has actually been exposed to an active volcano, for instance in Iceland, and if one has experienced the elemental forces that lie dormant in the bowels of the earth.

The question that poses itself at this point is that of the origin of the heat in the interior of our earth. The answer to this question is not simple. Generally speaking, geothermal energy is a form of renewable energy which originates from two main sources:

According to modern science, the earth’s original heat core has been burning for 4.5 billion years and dates back to the earliest formation phase. It is assumed that the enormous gravitational energy produced by the contraction of gas, dust, rock and ice was transformed into heat in a simultaneous collision of masses of matter, a large part of which escaped in the process. Today, scientists estimate the duration of said formation phase to have taken about 200 million years. However, only a small amount of the original gravitational energy was stored in these first rock formations. Geological research indicates that rather early in this process an equilibrium was reached between solar irradiation and heat emission from the earth. Fossilized organisms showed that the earth’s mean surface temperature has not substantially changed in the past 600 million years. This means that the majority of original gravitational energy is no longer contained in the earth’s interior. Nevertheless, much more significant than original gravitational heat is the energy that resulted from the fractionation of natural and long-lasting radioactive isotopes. The most important radioactive isotopes in this process are uranium 238, uranium 235, thorium 232 and potassium 40. Compared to one ton of granite, which contains about 3.3 grams of uranium and 12.5 grams of thorium, only a small fraction of these isotopes actually remained stored in sedimentary rock. Natural radioactivity is therefore a phenomenon that humanity has lived with for thousands of years. Not only does granite run in many regions directly underneath the turf, but due to its resistance to atmospheric conditions, it has always been a favored building material. Like in Germany (for instance in the Upper Palatinate), in many parts of the world there are houses entirely made of granite. Overall, it is assumed that only thirty percent of the heat flow from the hot interior of the earth to its cold surface can be traced back to the earth’s original heat core, while the remaining seventy percent are the result of isotope fractionation.Geyser in Iceland

The attempt to determine the earth’s thermal capacity by adding its original energy supply and radiogenic energy potential, results in a total of twelve to 24 x 1030 joule, depending on the respective temperature estimates for the earth’s core heat. Naturally, those numbers must always remain in the realm of the abstract. The earth’s outermost shell, its crust, on the other hand, conveys a much more concrete notion of its theoretical thermal capacity. Geoscientists define the earth’s crust as the thin, brittle skin of our planet, whose depth varies between 5 kilometers (underneath the oceans) and 40 kilometers (under the continents). Given those numbers, the earth’s crust can of course only be defined as thin when it is compared to our planet’s radius, which adds up to about 6,400 kilometers. Alone the heat stored in the crust underneath the continents amounts to 6 x 1026 Joule.This energy is enough to operate one million power plants with a capacity of 200 MW (electrical) for a period of 10,000 years.

Nevertheless, even though the earth’s crust appears considerably thin compared to its radius and in spite of the possibilities of modern technology, the drilling of depths of more than seven or eight kilometers can only be achieved at very high costs. (The deepest borehole so far is at 12 kilometers.) Considering the technological and geological difficulties posed by such an undertaking, economically, a large-scale recovery of this energy does not seem sensible. Nonetheless, there still remain a number of regions and depth ranges where the conditions for industrial exploitation are favorable. This following equation should render an idea of the potential economic value of these energy resources: When one cubic kilometer of earth crust is cooled down to 100 °C, it releases up to two x 1017 joule of thermal energy.

In formations with adequate temperatures (> 200°C), this energy will suffice to operate a power plant with a capacity of 30 MW (electrical) for a period of 30 years.

The importance of the rock temperature is essential in this equation, since the conversion of heat stored in a formation fluid (such as water) into electrical energy is, in its efficiency, largely temperature dependent. In this field of energy production, formation fluid must have a minimal temperature of about 175 to 180°C to be efficient. Naturally, the first question that arises in this context is that of the gradient of temperature increases in the upper layers of the earth’s crust. In the earth’s crust, temperature increases depend on its depth and vary between 20 and 40°C per kilometer. In Germany, this value lies at 30°C per kilometer, which means that the required 175°C to 180°C could be accessed at a depth of about six kilometer. Nevertheless, regional differences from the mean temperature can be considerable. In volcanic regions, surface temperatures can reach up to 1000°C. Temperatures of more than 200°C can be measured in depths of no more than one kilometer in geothermal fields in Japan and New Zealand. In our native Rhine Rift, volcanic activity is, of course, less extreme, but temperatures are still above ‘normal’. In Soultz-sous-Forêts, for example, a temperature of 160°C can be measured at a depth of only 3.5 kilometers, which equates to a temperature increase of 45°C per kilometer. In this case, temperature anomalies are due to the existence of a large-scale groundwater circulation system in the area, which allows heat to flow from large depths up to the surface.

Literature

Rummel, R. & Kappelmeyer, O. (1993): „Erdwärme, Energieträger der Zukunft?, Fakten, Forschung Zukunft“, Verlag C. F. Müller, Karlsruhe