Approximately 20 geothermal plants in the United States have a combined nameplate rating of 3,421 MW or 0.3% (three tenths of one percent) of total US power generation capacity (1,121,686 MW).
It’s believed that geothermal resources currently identified in the United States could provide a total of 20,000 MW of capacity. Federal government grants and loan guarantees over the past three years have amounted to over $300 million.
Realistically, US geothermal generating capacity in 2020 could reach 5,000 MW.
There is another type of geothermal that has extreme environmentalists excited. It’s Enhanced Geothermal, commonly referred to as “Hot Rocks”, which is a futuristic proposal where water is pumped deep into the earth to create steam from very hot rocks located well below the earth’s surface. Most people see this as a fantasy rather than a real possibility.
An Australian company has been trying for over a dozen years to no avail to drill two wells to a depth of 14,000 feet where rocks are hot enough to boil water, so that water could be injected down one well and steam could be extracted from the other for use in a steam turbine generator. They use fracking to create splits in the rock to allow water and steam to migrate.
Traditionally, geothermal generates electricity using three methods.
- Direct Steam
Direct steam uses high temperature steam as it emerges naturally from the earth to drive a turbine generator. These are the most cost-effective plants, but sites with steam are rare.
- Flash Steam
Flash steam systems take high temperature brine (above 400 °F) from the earth and injects it into a low-pressure chamber where the brine flashes directly into steam. The steam then drives the turbine generator.
- Binary cycle
The binary cycle method passes moderate temperature brine (below 400 °F) through a heat exchanger where its heat is transferred to another fluid which vaporizes. The vaporized fluid drives the turbine generator.
In the binary cycle, the fluid from the geothermal source never passes through the turbine and the rest of the plant. Instead, the brine is contained in a separate loop from the time it leaves the geothermal source, to where it passes through the heat exchanger and then returned to the earth. The fluid that is converted to a vapor in the heat exchanger travels through the turbine in another loop. The two fluids never come in contact.
Moderate temperature brine is the most common geothermal resource so Binary cycle plants tend to be the most common.
This diagram from the U.S. Idaho National Laboratory shows a Binary system.
A discouraging aspect of geothermal is that the amount of energy available from a geothermal source gradually declines, though reinjection of fluids can help preserve the fluid volume of the reservoir. The reservoir should outlive the useful life of the equipment so the investment is worthwhile, but each location has a finite life, just as any other man-made endeavor.
The cost of producing geothermal electricity is the lowest of all renewables, execpt possibly for hydro, but it’s still more expensive than electricity generated by natural gas or coal-fired power plants. It will be difficult to significantly lower the cost of geothermally generated electricity, since these installations use established technologies (heat exchangers, turbines and electric generators) in traditional ways. Drilling and exploration represents 24% to 50% of the cost, so new drilling technologies may help lower the cost of new plants.
Though we mostly welcome geothermal when it is competitive with other methods for generating electricity, not everyone in the world does.
Even in Hawaii, people are afraid of disturbing Pele, the goddess of fire. The same is true in Indonesia, as was recently reported in the Wall Street Journal.
I learned on my trip to New Zealand that there is considerable concern about using geothermal resources, of which New Zealand is blessed, for generating electricity. Many of the geothermal areas are sacred and are a part of Maori history.
In the final analysis, geothermal can economically produce small amounts of electricity, but it shouldn’t be showered with federal grants and loan guarantees since it has such a small impact on our economy and our ability to generate electricity, and is irrelevant with respect to becoming energy independent.
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