In Part 1 we learned that the heat energy within the Earth’s core is seen in the hot springs, geysers, and fissures emitting steam at the surface of the planet. This hot water reaches the surface due to movements of the tectonic plates.
Deep circulation of this hot groundwater along fracture zones results in it collecting the heat flow from a broad area and concentrating it into shallow reservoirs containing hot water and/or steam or sometimes discharging as hot springs. By drilling into these resources, we can pipe the hot water and/or steam to the surface to power turbines for power generation.
With the right applications, this heat energy can be used mechanically, as in generating power for turbines, or used as a direct method of heating homes and businesses. And geothermal energy has already proven to be an economical, environmentally-friendly and sustainable energy source.
Geothermal resources are defined by enthalpy or temperature
The International Geothermal Association says there is no standard international terminology in use today that would facilitate a common understanding of geothermal sources. Suffice to say that these sources are based on the enthalpy or temperature of the geothermal fluids that transport heat from the Earth’s core to the surface.
Rather than go into the ambiguous nomenclature used by different sources, we can say that, basically, geothermal energy sources are divided into two categories: High-temperature resources and moderate/low-temperature resources. High-temperature resources are generally 220 degrees Celsius and up. These geothermal sources are usually found in volcanic regions and on island chains.
Moderate/low-temperature resources can be found on all our continents, and while high-temperature geothermal sources are almost always used to produce electricity, moderate/low-temperature sources are mostly used for direct heating or agriculture and aquaculture.
Harnessing geothermal energy
Drilling is required and the bore holes can go a mile or more to reach the geothermal resource. And basically, three forms are utilized, dry steam, flash steam and low-temperature steam. Determining which source is available requires doing a geochemical survey including isotope geochemistry, if necessary. This type of survey is less costly than more sophisticated methods, such as geophysical surveys.
To distinguish between dry, wet and superheated steam, let’s imagine a pot of water in which we can keep the pressure at a constant rate, say 1 atm, or a standard atmosphere. When we boil the water to 100 degrees Celsius (boiling temperature at a pressure of 1 atm), the water will convert to steam. The pot will contain both water and steam at this point. This is wet steam.
But let’s continue to heat the pot of water at 1 atm. It won’t be long before all the water has evaporated and only steam is left. This is now called dry steam. Both wet and dry steam are called “saturated steam.”
Now we will increase the temperature of the pot to 120 degrees Celsius but leave the pressure at 1 atm. We will then have what is called super-heated steam which is above the 100 degrees Celsius mark for creating steam. Variations of the temperature and pressure are what is happening below the Earth’s surface, creating the heat we can use as energy.
Three forms of geothermal power plants in use today
Dry steam power plants use very hot (>455 °F, or >235 °C) geothermal steam. The hot steam flowing out of geothermal deposits is used to heat geothermal water that turns the turbines that in turn, spin a generator to produce electricity. This method is the oldest type of power plant, first used in Lardarello, Italy, in 1904. And again, this is a form of mechanical energy production.
Flash Steam Power Stations use hot water (>360 ºF, or >182 ºC) from deep wells drawn up from the geothermal reservoir. When the water is raised up, the pressure the water was under drops suddenly, some of it converting to steam. The steam is then used to turn the turbines producing electricity.
Both dry steam and flash steam power plants emit small amounts of carbon dioxide, nitric oxide, and sulfur, but they have been found to emit 50 times less than traditional fossil-fueled power plants. Any hot water not used to generate power is put back into the geothermal reservoir using injection wells.
Binary Cycle Power Plants use moderate-temperature water (225 to 360 ºF, or 107 to 182 ºC) from the geothermal reservoir, exploiting the difference in boiling points based on density. In this method, the hot geothermal waters are passed through one side of a heat exchanger, heating fluid in a separate pipe that has a lower boiling point. That fluid vaporizes and turns a turbine to generate electrical power.
Organic compounds with a lower boiling point, such as Iso-butane or Iso-pentane are used in binary-cycle power stations. Another method, called the Kalina Cycle System uses ammonia-water as the working fluid in place of Iso-butane or Iso-pentane. The makers say the Kalina method boosts the efficiency of a binary cycle plant by 20-40 percent and in turn, lowers the cost of producing electricity.
It is worth noting that there is one other method, called geothermal heat pumps. Because the Earth’s surface layer is almost constant in temperature, this method uses geothermal heat pumps for heating and air-conditioning buildings. A system of buried pipes is connected to a heat exchanger and then to the duct-works in buildings.
Simply put, in the winter, relative warmth is transferred to the buildings, and in the hot summer months, the hot air is transferred back into the ground or sometimes used to heat water.
Is fracking for geothermal resources the same as fracking for oil or gas?
Many people are not going to like the answer, but yes, geothermal fracking is akin to fracking for oil and gas. To explain it better, the process that involves fracking is called an ]enhanced geothermal system (EGS). This method generates geothermal energy without the need for having natural geothermal convection sources.
This method is used in regions where naturally occurring heat, water, and rock permeability are absent, or in other words, it is used in dry, and impermeable rock using “hydraulic stimulation.” The methodology is very similar to hydraulic fracturing in that water under intense pressure is injected into the rock triggering shear events that enhance the system’s permeability. EGS proponents like to refer to this as hydro-shearing, perhaps to differentiate it from the fracking used by the oil and gas industry.
Regardless of what the system is called, it has been used with disastrous consequences. An EGS project in Basel, Switzerland — constructed on a known seismic fault was suspended in 2006 when it generated earthquakes that resulted in millions of euros in damage to the local infrastructure, according to Renewable Energy World in July 2013.
“It’s easy to generate a lot of fear. You can scare people about things without providing much solid information,” said David Stowe, communications director at AltaRock. “The Basel story is dredged up over and over again — but we have learned from it, and it is pretty easy to put safeguards in place that will severely minimize risk.”
And many people continue to be opponents of EGS technology, although the U.S. Department of Energy has improved the seismic technology and added enhanced safety standards to the methodology. Actually, the DOE has the only protocol in place for the sub-surface energy industry.
