Geothermal

1. Presentation

A geothermal power plant in California

Below the Earth's crust, there is a layer of hot and molten rock called magma. For commercial use, a geothermal reservoir capable of providing hydrothermal (hot water and steam) resources is necessary. Geothermal reservoirs are generally classified as being either low temperature (<150°C) or high temperature (>150°C). Generally speaking, the high temperature reservoirs are the ones suitable for, and sought out for, commercial production of electricity.
Geothermal reservoirs are found in “geothermal systems,” which are regionally localized geologic settings where the earth’s naturally occurring heat flow is near enough to the earth’s surface to bring steam or hot water, to the surface. These "hot spots" occur at plate boundaries or at places where the crust is thin enough to let the heat through (volcanoes, Seismically active region, etc.).

Geothermal power plants use steam produced from reservoirs of hot water found a few kilometers or more below the Earth's surface to produce electricity. The steam rotates a turbine that activates a generator, which produces electricity. There are three types of geothermal power plants: dry steam, flash steam, and binary cycle. 

- Dry Steam
Power plants using dry steam systems were the first type of geothermal power generation plants built. They use steam from the geothermal reservoir as it comes from wells and route it directly through turbine/generator units to produce electricity.

- Flash Steam
Flash steam plants are the most common type of geothermal power generation plants in operation today. They use water at temperatures greater than 182°C (360°F)  that is pumped under high pressure to the generation equipment at the surface. Upon reaching the generation equipment, the pressure is suddenly reduced, allowing some of the hot water to convert or “flash” into steam. This steam is then used to power the turbine/generator units to produce electricity.

- Binary Cycle
Binary cycle geothermal power generation plants differ from dry steam and flash steam systems because the water or steam from the geothermal reservoir never comes in contact with the turbine/generator units. In the binary system, the water from the geothermal reservoir is used to heat another “working fluid,” which is vaporized and used to turn the turbine/generator units. The geothermal water and the “working fluid” are each confined in separate circulating systems or “closed loops” and never come in contact with each other. The advantage of the binary cycle plant is that they can operate with lower temperature waters 107°-182°C (225°F to 360°F) by using working fluids that have an even lower boiling point than water. They also produce no air emissions.

Currently, two types of geothermal resources can be used in binary cycle power plants to generate electricity: enhanced geothermal systems (EGS) and low-temperature or co-produced resources:

• Enhanced Geothermal Systems: EGS provide geothermal power by tapping into the Earth's deep geothermal resources that are otherwise not economical due to lack of water, location, or rock type.
• Low-Temperature and Co-Produced Resources: Low-temperature and co-produced geothermal resources are typically found at temperatures of 300°F (150°C) or less. Some low-temperature resources can be harnessed to generate electricity using binary cycle technology. Co-produced hot water is a byproduct of oil and gas wells in the United States. This hot water is being examined for its potential to produce electricity, helping to lower greenhouse gas emissions and extend the life of oil and gas fields.

2. Market Analysis

By the beginning of 2011, geothermal power plants were operating in at least 24 countries, but the vast majority of global capacity was located in eight countries: the United States (3.1 GW), the Philippines (1.9 GW), Indonesia (1.2 GW), Mexico (just under 1 GW), Italy (0.9 GW), New Zealand (nearly 0.8 GW), Iceland (0.6 GW), and Japan (0.5 GW).

Although power development slowed in 2010, with global capacity reaching just over 11 GW, a significant acceleration in the rate of deployment is expected, with advanced technologies allowing for development of geothermal power projects in new countries. The International Geothermal Association (IGA) projects growth to 18,5 GW by 2015, due to the projects presently under consideration, often in areas previously assumed to have little exploitable resource. 

As of early 2011, nearly 0.8 GW of new capacity was in the drilling or construction phase in the United States and was expected to be generating by 2015; a total of 123 confirmed projects (accounting for up to 1.4 GW of resources) in 15 U.S. states were at some stage of development.
Iceland expects to add nearly 0.1 GW to an existing plant in 2011, and much more capacity is in project pipelines around the globe, with 46 countries forecast to have new geothermal capacity installed within the next five years. By late 2010, Germany had an estimated 150 projects in the pipeline, and projects were under development in Chile (0.2 GW), Costa Rica (0.4 GW), India (nearly 0.3 GW), and the U.K. (0.01 GW), among others.

The U.S. industry is the global leader, developing approximately one-third of the world’s new projects, all in its domestic market. Japanese firms Mitsubishi, Toshiba, and Fuji Electric supply 70% of the steam turbines at geothermal plants worldwide. Leading firms in conventional geothermal include Borealis Geopower, Calpine, CalEnergy, Chevron, Enel SpA, GeoGlobal, Gradient Resources, Magma Energy Corp., Mighty River Power, Nevada Geothermal Power, Ormat Technologies, Oski Energy, POWER Engineers, Ram Power, Terra-Gen Power, ThermaSource, and U.S. Geothermal. Leaders in EGS Geothermal include AltaRock Energy, EGS Energy, Geox, Geodynamics, and Potter Drilling.

3. Commercial Potential for the Carnot engine

As seen before, there is a lot of heat resources available underground. However, to be profitable, a geothermal reservoir must not be too deep and it has to produce either steam or hot water to be used by a power plant. Most of the traditional geothermal power plants (Dry steam and Flash steam facilities) are built above these types of reservoirs but such geological conditions are rare.

That’s exactly why new binary cycle facilities are now being developed. This technology allows cooler geothermal reservoirs to be used than with dry and flash steam plants. By doing so, it is opening a new range of available geothermal resources. Shallow and cold reservoirs that were once, not economically interesting, are now being potentially workable.

However, the thermal efficiency of a binary cycle power plant is lower than a conventional Dry or Flash steam plant because of the need of adding a heat exchanger in the system.

The Carnot engine has the capacity to exploit directly a low temperature flow from an Enhanced Geothermal Systems or any other low-temperature geothermal resource (old oil well, untapped geysers, etc.) without the need of adding a costly and less efficient heat exchanger.

4. Conclusion

The U.S. Geological Survey estimates that potentially 500 GW of EGS resource is available in the western U.S. (about half of the current installed electric power generating capacity in the United States). Another report by the Massachusetts Institute of Technology (MIT), that included the potential of enhanced geothermal systems, estimated that investing 1 billion US dollars in research and development over 15 years would allow the creation of 100 GW of electrical generating capacity by 2050 in the United States alone.

As we can see, the geothermal energy is already used in many countries and its potential is great. Our Carnot engine would allow to improve the overall efficiency of EGS power plants. Better efficiency means better profitability. It could also move the economic threshold of some geothermal projects toward the “bankable” side.