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Page Two December 2005 SMU Geological Sciences Chairman’s Report Taking the Earth’s Temperature: Heat Flow a Renewable Source of Energy By Robert T. Gregory This newsletter features some of the activities of our Geothermal Laboratory, operated by Professor David Blackwell since 1968. The Geothermal Map of North America, 2004, edited by Blackwell and Maria Richards of the Geothermal Laboratory is the centerfold feature that summarizes our knowledge of heat flow in North America. Our cover story reports on a novel idea for using waste heat from deep wells, mainly oil and gas wells, to generate electricity that was recently published in the Oil and Gas Journal by SMU’s Research Professor Jason McKenna, Dave Blackwell and Christopher Moyes and R. Dee Patterson of Moyes & Co., Dallas. Heat flow is simply the energy output of the Earth, formerly reported in heat flow units, but now measured in milliwatts per meter squared. Recall that a watt is a joule per second; it takes 4.18 joules to make a calorie and a 1000 calories to make the Calorie that appears on the package of your favorite food. In the old experimental units, the average square centimeter of the Earth’s surface released a bit more than a microcalorie per second, or about 1 heat flow unit. The conversion factor to the standard international units is 41.84 milliwatts per meter2 = 1 heat flow unit. The green areas of the map on pages 5 and 6 represent average values of terrestrial heat flow. The product of the thermal conductivity and the change in temperature with depth is the heat flow (Figure 1).The latter factor is called the geothermal gradient or geotherm. When there are no sources or sinks for heat, the geotherm is a straight line; the gradient is reported in degrees per kilometer (oC/km). A curved geotherm means that there are sources or sinks for heat or that the temperature is changing with time (see the inset, Figure 1). Lord Kelvin made use of the latter phenomena to estimate the age of the Earth from measurements of the geotherm. Because he didn’t know about radioactivity, he misinterpreted the significance of the curvature of the geotherm as only the result of time dependent cooling of the Earth and not in terms of internal heat sources. In the 1960’s, David Blackwell was one of the co- discoverers of the relationship between measured heat production in continental surface rocks due to their internal radioactivity and the surface heat flow. This empirical relationship when analyzed mathematically gives an equation for calculating curved geotherms in the crust. Through the surface of the continental crust, the heat flow is due mainly to the sum of a background mantle heat flux and the heat produced from the in-situ decay of heat producing elements that are concentrated in the continental crust, in particular potassium 40 and uranium. Heat flow varies from maximum values greater than150 mW/m2 in active volcanic terrains, most notably mid-ocean 2 ridges, to values of less than 20 mW/m , either billion year old cratonal crust, forearcs or mature oceanic crust. From the patterns on the map, there is a clear relationship between plate tectonics and the distribution of heat flow on the Earth; this works particularly well for the oceanic crust. The symmetry of the heat flow around the mid-ocean ridges is predicted by mathematical models for the time dependent cooling of the oceanic lithosphere (examine the Mid-Atlantic ridge south of Iceland to observe the effect). Hot new lithosphere cools as it spreads away from the ridge crest. Mantle convection (the solid state flow of mantle rocks) is critical for moving heat from hotter deeper regions towards the surface so that time dependent cooling dominates the oceanic heat flow distribution. The small amount of heat coming out of the Earth locally is not too impressive, about the energy output of a night light shining on a bathroom floor. However, the Earth by comparison is a big place; its surface area is 5.1 X 1014 m2 or 510 million kilometers DEPARTMENT OF GEOLOGICAL SCIENCES Southern Methodist University P.O. Box 750395 Dallas, Texas 75275 214 768-2770; Fax 214 768-2701 E-mail: geol@.smu.edu http://www.smu.edu/geology Figure 1: For a slab with uniform conductivity and no internal heat sources or sinks, the heat flow or the energy flow through the top surface is the conductivity times the temperature differ- ence, here 25 oC, divided by the thickness of the slab, 1 kilometer. With this typical geothermal gradient, and remembering that the kilometer thickness of slab must be converted to meters before doing the multiplication, the heat flow, q, is 62.5 mW/m2. The inset shows curvature of the geotherm from changes in surface T. Photo by Hillsman B. JacksonPDF Image | Geothermal Energy from Deep Wells
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