What is Geothermal Energy

The Earth is fine for our survival at the surface, but gets uncomfortably hot when we go too deep! The natural heat (thermo) from beneath the surface of the Earth’s crust (geo) shows promise as an endless and clean source of geothermal power. 

This massive amount of heat, from magma, can give us water for energy when seismic hot spots are close to the surface. The famous geysers at Yellowstone National Park, for example, eject water that is as hot as 430° F or 200° C. Yellowstone, of course, is at the very thin surface that lies over one of the super volcanoes of the world.

There are many other hot spots in the world, where the magma comes close to the Earth’s surface. But “close” is a tricky concept. At about 6 miles or 9.6 kilometers deep, the magma undergoes natural deterioration of radioactive substances and potassium which creates most of the heat.

This deeply sourced heat is estimated to be capable of producing more 50,000 times more energy than all of the gas and oil resources in the world.

On a scale of capacity factor, which is a ratio of comparison between the actual energy produced over time vs the energy that would be produced f the plant were constantly running for the same period of time, geothermal power capacity factors are comparable to both nuclear and coal based power.

Also, at 4 to 10 kilometers or 2.5 to 6.2 miles, dry rock formations are estimated to produce enough heat for producing electricity by Enhanced Geothermal Systems, which is a technology that is yet to be developed.

At a higher level, about  10 to 100 feet below the Earth’s surface, there is enough milder heat to provide heat for any structures in most of the world.  

Where are the “hot spots” of the world?

The major hot spots are at tectonic plate boundaries and in places where the Earth’s crust is so thin that magma and heat can escape to the surface or to near the surface. In places that have active and very young volcanoes the surface heat is also highest. The Pacific Rim’s “Ring of Fire”, Northern Nevada, Washington, Oregon, Alaska and California have hundreds of “hot spots”

How is geothermal energy produced?

There are four possible types of systems:

Electrical grade systems for power generation

Warm water direct use systems

Geopressured systems

Normal Temperature Reservoirs

“Dry” Geothermal environments

With Electrical Grade Systems, generally the use of existing water, heat and some form of steam is used to power turbines.  

There are three classifications for systems that depend on the water temperature or the presence of steam: Direct Source, Flash Steam and a binary process.

If the water is hot enough, the steam can be directly used to power the turbine. If the water is cooler, it is “flashed” into steam, or depressurized to create steam. In a binary system, the hot water is used to heat another substance that converts more easily to steam. Isobutane is an example of liquid that converts more easily to steam in a closed loop. After the steam has done it’s job and has condensed back into water or the secondary liquid, it is returned to the source to reheat and to cycle through again.

Direct heat systems use thermal water that is not hot enough to produce steam. These systems can heat soil for crops, pools and structures. In 2000 the worldwide figures were estimated to be over 11,000 megawatts or the equivilent of over 20 million barrells of oil in direct heat energy usage.

Geopressured systems are not currently in use. These involve sealed, pressurized combinations of water and methane and were studied in the 70s and 80s. They were found to be not quite economically exploitable.

Normal temperature reservoirs are not that common, and simply exchange the non volcanic or non magma related heat of normal soil in heat exchangers.

Where is geothermal energy being produced?

In the entire world, there are 24 countries producing an estimated 8,900 megawatts of electricity in large scale geothermal operations. These operations produce enough electricity to power about 12 million US households.

We know what combinations of both energy hog and energy saving innovators American households can be, so it can be difficult to apply US power usage averages to world usage averages. 

Who uses geothermal energy technology?

The Philippines, Iceland and El Salvador are said to get 25 percent of their electricity from geothermal sources. The United States is said to produce the most geothermal energy at 3000 megawatts over eight states. 80 percent of this power is produced in California, which has 40 geothermal plants that produce 5 percent of the state’s power.

What is the potential for geothermal energy?

The USGS estimates that, using conventional geothermal technologies, production could be as high 8,000–73,000 MW, with a mean estimate of 33,000 MW. This is, of course, in public and private lands of the 13 Westernmost states.

In addition, USGS estimates for hot dry rock resources could provide another 345,100–727,900 MW of capacity, with a mean estimate of 517,800 MW, if the technology can be developed. This gives the potential for meeting all of the energy needs in the US.


An excellent PDF document from the USGS for understanding geothermal processes and geothermal energy