There is another form of geothermal energy at work in the world today, one that holds great promise for mankind and for a cleaner environment. It’s geothermal electric power (GEP), and its energy source, instead of arriving on earth from the sun that we then recover for heating and cooling with ground-source heat pumps, this form of energy comes from deep inside the earth.
To help us understand how this energy from inside the earth can be harnessed for electricity, a brief geology explanation is in order. Inside the earth, at its core, is a layer of hot and molten rock, called magma, which constantly produces heat from decayed uranium and potassium. It is this magma that we see in volcanic eruptions. Above the molten core at the center of the earth is the earth’s mantle: the mantle is that portion of the earth, about 1,800 miles thick, between the earth’s core and its crust.
The earth’s crust can be compared to a thick blanket of insulation, but it is a blanket that heat and magma can pierce.
The earth’s crust and mantle are called the lithosphere. The lithosphere is broken up into massive tectonic plates that cover the entire surface of earth – to include the continents and the oceans. These plates move in very slow motion against each other constantly, and produce earthquakes, volcanic eruptions, mountain building and oceanic trench formation at their plate boundaries, as a result of their motion.
At the plate boundaries is where the highest underground temperatures occur, and these “hot spots” is where we see active volcanoes, seismic activity, and hot water springs. Magma movement travels upward through fluid conduits in the earth’s mantle and up through the crust. In volcanic eruptions the magma lava is blown out of the center of the volcano with tremendous force; in places where hot spots are constant, heat is released in the form of hot water and steam. The temperature of a geyser’s water can be more than 430°F (200°C). These less powerful eruptions we call geysers, and the hot water and steam they emit are a form of untapped geothermal energy. That energy can be captured and used to power turbines to produce electricity. By drilling wells into the underground hot water reservoirs that feed geysers, we can bring up hot water or steam to the surface and use it to make electricity.
The first example of producing geothermal electric power occurred at a plant in Italy in the first decade of the 20th Century; additional plants were then built in Japan and in California. Today, geothermal electric power is generated by plants operating in over 20 countries.
Countries along what’s known as the “Ring of Fire” that circles the earth have the best ability to harness geothermal energy for electricity production because they lie atop tectonic plate boundaries. That is why countries like the Philippines, Indonesia and the United States are leading producers of geothermal electric power based on their installed megawatt (MW) capacities. And in a country like Iceland, which is a volcanic island, geothermal electric power provides most of the electricity and hot water used by Icelanders
It is estimated that the amount of heat within 33,000 feet of the earth’s surface contains 50,000 times more energy than all of the oil and natural gas resources in the world.
Hot Dry Rock (HDR): Plentiful But Deep
Conventional geothermal technology entails the production of useful energy from natural sources of steam or, much more commonly, hot water. These hydrothermal resources are found in a number of locations around the world, but they are the exception rather than the rule. In most places, the earth grows hotter with increasing depth, but mobile water is absent. The vast majority of the world’s accessible geothermal energy is found in rock that is hot but essentially dry -- the so-called hot dry rock (HDR) resource.
The total amount of heat contained in HDR at accessible depths has been estimated to be on the order of 10 billion quads (a quad is the energy equivalent of about 180 million barrels of oil and 90 quads represents the total US energy consumption in 2001). This is about 800 times greater than the estimated energy content of all hydrothermal resources and 300 times greater than the fossil fuel resource base that includes all petroleum, natural gas, and coal. (Tester, et al. 1989).
Deep Direct Use Geothermal (DDUG)
Deep Direct Use Geothermal is the use of deep reservoirs of heated water that are of sufficient temperature to be used directly for building heat and domestic hot water (DHW). Typically wells are drilled in pairs; one supply and another for return once heat has been extracted (called “doublets”). Direct Use Geothermal is less expensive than HDR, and is practical in many regions where the heated aquifer resources are available. However, there is an even greater abundance of lower temperature geothermal than can be tapped using Enhanced Geothermal Systems (EGS).
Enhanced Geothermal Systems (EGS) and Organic Rankine Cycle (ORC)
EGS are used when the temperature of the geothermal resource is not quite hot enough to make steam to drive a turbine. One of the methods or processes is the Organic Rankine Cycle (ORC). The Organic Rankine Cycle is a thermodynamic process where heat is transferred to a fluid at a constant pressure. The fluid is vaporized and then expanded in a vapor turbine that drives a generator, producing electricity. ORC’s are also used to generate electricity from waste heat such as exhaust gas from generators or other heating processes. This is called co-generation.
While HDR geothermal produces electricity efficiently and effectively, it also has the costliest capital investment, and is limited to specific regions where hot dry rock resources are accessible.
Deep Direct Use Geothermal and GSHP have the lowest capital cost, and can be effectively applied nearly everywhere.
For more Information on geothermal power generation, visit www.geo-energy.org.