How Geothermal turns up the heat -- cleanly
By Carey Callaghan
Of all of the renewable energy sources, geothermal is unique for not being an indirect form of solar power. Wind, wave, tidal, and even hydro are indirectly powered by the sun. Geothermal power, by contrast, comes from the latent thermal energy from the earth’s formation, combined with heat released from the decay of radioactive isotopes in the earth’s crust.
Ever want to journey to the center of the earth? Maybe not – the temperature at the earth’s core is estimated to be at 7,000 degrees Celsius! Harnessing geothermal energy is all about getting a fraction of that energy to the earth’s surface, where it can be tapped as steam or hot water. Not surprisingly, the areas with the best high energy geothermal resources are those along the boundary lines between the lithospheric plates of the earth’s crust. Along these boundaries, volcanic activity is more prevalent, and so is the geothermal resource. As a result, Pacific nations or regions such as Japan, the Philippines, Indonesia, New Zealand, Chile, Central America, Mexico, and the western U.S. are good areas for geothermal exploitation. Europe, Italy, and Iceland also have good resources.
It’s no coincidence that the largest and easiest geothermal resources to develop were harnessed decades ago. Locations like the Geysers in California or Lardarello in Northern Italy had substantial surface manifestations of geothermal activity. Tapping these early fields, especially initially before steam pressures subsided, was often as easy as piping the steam to a steam turbine, which spun a rotor and thereby produced electricity. Today these fields are managed to maximize their long-term viability, with waste steam or condensate being re-injected and production metered to allow the geothermal resource to recharge.
A more complex geothermal plant, which envisions using a caustic heat resource such as brine, is shown in the schematic below. Here, the brine is used to heat either water or another fluid (butane in this case), which is kept in a separate closed loop. Fluids such as butane, which have much lower boiling points than water, can be used to take advantage of cooler resources where water would not be viable. After spinning the turbine to generate electricity, the working fluid is condensed and then re-circulated. These units are often used as pressurized systems, in which the liquids are superheated (prevented from boiling by the high pressure). For lower temperature resources, pressurization can boost overall efficiency despite the power demand of producing the pressure.
Not withstanding the title of this Technology Spotlight item, geothermal is not as hot in terms of economic activity as one might imagine as compared to solar and wind power. Geothermal is thought of as a “mature” industry, since the technology is well-established and its application dates back many decades. This, however, is starting to change.
The development of new geothermal power-plant systems, such as United Technologies’ Power PureCycle system (not a Fund holding), capable of using low-grade geothermal resources (as low as 165 degrees Fahrenheit), are vastly expanding the addressable market. For instance, at those temperatures, the waste water associated with oil and gas production, which amounts to over 10 billion barrels per year in the U.S. alone, could become a viable resource for electric generation. Thousands of megawatts of potential lie untapped in oilfields in the U.S., and much more abroad. Additionally, much acreage in the western U.S. has low-grade thermal resources that could support power production at these temperatures. Some recent efforts have simply focused on exploiting differentials between surface and subsurface temperature, in areas removed from geothermal resources. The simple use of vertical wells and heat exchangers for residential heating and cooling is still in its infancy, but potentially promises to be a strong area of growth given the rising cost of utility bills for consumers.