There is a vast amount of heat contained by the earth interior. This internal heat is mainly comprised of the residual heat of planetary accretion and radioactive heat (from radioactive element decay). The hottest part of the earth is the core, a big part of which is in molten state. Heat radiates and gets transferred from the core to the outer layers of the planet by interior fluids and melts. The general geothermal profile of the earth (Figure 8.6) provides an idea of the scale of the thermal resource and the gradual change of the earth temperature at different depths. Because the earth structure is not uniform, heat is more readily transferred in some zones than in others. High heat transfer is usually associated with fracture zones and major faults, which are often located at the boundaries of tectonic plates.
The temperature change rate with depth depends on the density and thermal conductivity of rocks. Subdivision of the earth structure into layers is made according to the rock composition and rheological properties, so we see that the thermal profile within each layer can be quite different. The drastic change in temperature pattern around the boundary between the lower mantle and outer core is apparently related to the transition of the molten state.
The heat flux within the crust (the thin top layer) is highly variable due to the existence of large unified fragments of the crust (plates) divided by plate boundaries, the more mobile zones, where plates collide, spread out, or move relative to one another. Increased mobility of the plate boundary zones may cause creation of faults of various depth, which favor heat transfer to the surface. If you want more background about plate tectonic theory, you will be able to find a lot of resource on the web. For example, An Introduction to Plate Tectonics provides a nice and concise illustrated introduction to this whole idea.
There are a number of technologies that help convert the thermal flux and hydrothermal waters to usable energy. Next, we will refer to the following reading to learn how these technologies work.
Reading Assignment:
“How Geothermal Energy Works”, UCS, 4/1/2014
The following short video (5 min) provides an additional illustration of a utility scale geothermal plant.
Energy 101: Geothermal Energy (3:47)
Transcript: Energy 101: Geothermal Energy (3:47)
You may have relaxed in a natural hot springs pool.
Or seen the Old Faithful geyser blasting hot water into the air in Yellowstone National Park. But have you ever thought of where all that heat comes from?
Well, it comes from deep beneath the surface of the Earth, and it's called geothermal energy...
And we can use it to generate clean renewable electricity. OK, here's how geothermal works.
Heat from the Earth's crust warms water that has seeped into underground reservoirs. When water becomes hot enough, it can break through the Earth's surface as steam or hot water. This usually happens where the Earth's crust or "plates" meet and shift.
In the past, taking advantage of geothermal energy was limited to areas where hot water flowed near the surface. But, as geothermal technologies advance, we can leverage even more of these natural renewable energy sources. Engineers have developed a few different ways to produce power from geothermal wells drilled into the ground.
Have a look at this. It's a dry steam geothermal power plant, and it's the most common type of geothermal technology used today. Underground steam flows directly to a turbine to drive a generator that produces electricity. Pretty straightforward.
Another geothermal technology is called a flash steam power plant. A pump pushes hot fluid into a tank at the surface, where it cools. As it cools, the fluid quickly turns into vapor, or "flash" vaporizes. The vapor then drives a turbine and powers a generator.
A binary cycle plant works differently.
It uses two types of fluid. Hot fluid from underground heats a second fluid, called a heat transfer fluid, in a giant heat exchanger. The second fluid has a much lower boiling point than the first fluid, and so it "flashes" into vapor at a lower temperature. When the second fluid flashes, it spins a turbine that drives a generator.
The environmental benefits of this clean, round-the-clock renewable energy source are substantial: low emissions, small physical footprint, and minimal environmental impact. The few byproducts that can come up are often re-injected underground.
Geothermal energy can also help recycle wastewater. In California, wastewater from the city of Santa Rosa is injected into the ground to generate more geothermal energy.
Some plants do produce solid waste, but that solid waste may contain minerals that we can remove and sell, which lowers the cost of this energy source.
The U.S. Geological Survey estimates that untapped geothermal resources in the United States, if developed, could supply the equivalent of 10% of today's energy needs. In fact, electricity generated by geothermal energy already provides about 60% of the power along the northern California coast...
From the Golden Gate Bridge to the Oregon state line.
Geothermal energy... helping to push America toward energy independence, and a clean, renewable way to meet our growing energy demands.
While geothermal energy seems to be another unlimited and “free” energy resource, effective conversion of that energy and power distribution incur substantial costs. From economic evaluations, utility scale geothermal and natural gas power plants are comparable in overall cost, but only in the long term. Significant up-front expenditures for construction of the energy facility are much higher for the geothermal plant.