Note: because of the anticipated length of this blog I have broken it up into two parts
Part 1 of 2: the geothermal context and power generation
Geothermal energy is heat from the earth and has been used by mankind for bathing and heating for centuries. The first power plant to use geothermal heat dates from 1904 in Larderello, Italy and it is still operating. Geothermal energy has an extensive and growing literature and my purpose in this blog is not to ‘reinvent the wheel’ but to add to the history where I have personal information that is not widely known, and to speculate on geothermal’s future. It is a large energy resource, in many ways the largest on earth, and one that is just in the early stages of realizing its potential.
Many useful references can be found on geothermal energy and its various manifestations and applications. I list three web references below as good places to start:
– Wikipedia: http://Wikipedia.org/wiki/geothermal_energy
– Union of Concerned Scientists: http://www.ucsusa.org/clean_energy/our-energy-choices/renewable-energy/how-geothermal-energy-works.html
– Geothermal Energy Association: http://www.geo-energy.org
Geothermal energy derives largely, but not exclusively, from radioactive decay of uranium, thorium and potassium in the earth’s core. Lesser amounts of core heating derive from heat released when iron cools and solidifies at the earth’s central core, mineral phase changes, friction heating associated with earth’s tides, and even impact collisions with matter from space. This heat convects and conducts up to the earth’s thin crust (just one percent of the earth’s mass) through various pathways and manifests itself as hot water and steam, hot rock, warm earth, magma and volcanic eruptions. We can think of the crust as a blanket on the rest of the planet.
This heat has been flowing from the center of the earth for more than 4.5 billion years and will continue as long as the earth exists, about another 5 billion years. Since this flow is limitless geothermal may be considered a renewable energy source . It also is constantly available and thus a baseload energy source.
Temperatures close to the earth’s center are about as hot as the sun’s surface (5,500C or 9,900F), and geologists estimate that the rate at which energy flows from the earth’s interior is on the order of 44 terrawatts (TW, millions of megawatts). The replenishment rate from radioactive decay is estimated to be about 30 TW. To put this number in perspective, today’s global installed electrical generating capacity is just over 5 TW.
Initially the core of the earth was a hot liquid but it has cooled over geological time and the core is now seen as an anisotopic very high temperature core of solid iron created under conditions of extremely high pressure. Somewhat above the core some rock is still molten, creating magma that convects upward since it is lighter than rock. The magma heats rock and water in the crust, creating hot water and steam at various points on and near the earth’s surface. It is estimated that the amount of heat in hot rock and water within 6 miles of the earth’s surface is more than 50,000 times as much as all the energy stored in the planet’s oil and natural gas resources.
How has this heat been used used in the past, how is it being used today, and how might it be used in the future? Historically, hot springs have been used for bathing by humans since Stone Age times and since Roman times for space heating. These uses are still present and growing, and the first district heating system in the U.S., in Boise, Idaho, was powered by geothermal energy starting in 1892. In Iceland 90 percent of the households are heated by geothermal energy. Other applications include desalination, agricultural drying and industrial heating, for a total of about 30 GWt.
In modern times geothermal energy is best known for its application to power generation and ground source (aka ‘geothermal’) heat pumps. I will say just a few words on each of these applications, and then focus on geothermal’s potential, which is huge.
Today’s geothermal power plants (called ‘hydrogeothermal’) use geothermal heat in the form of dry steam issuing from the ground, hot water that flashes into steam, or the vapor of a volatile liquid (such as isobutane) heated by hot water, to drive a turbine generator. The U.S. currently leads the world in geothermal power generation (3,200 MWe), followed by the Philippines (1,900 MWe), Indonesia (1,200 MWe), Mexico (960 MWe), Italy (880 MWe), New Zealand (770 MWe), and Iceland (660MWe). As of May 2012 twenty four countries had geothermal power plants, for a total generating capacity of 11,400 MWe. Future geothermal power plants will use so-called ‘enhanced geothermal/EGS’ (previously called ‘hot dry rock’) systems in which deep wells are drilled into hot rock with no natural water and water is introduced from and returned, heated, to the surface. Estimated global potential varies from 0.04 to 2 TW, depending on the depth of drilling and level of investment. Wells as deep as 6 miles are now common in the petroleum industry.
One piece of history about EGS: this technology was pioneered at Sandia National Laboratory with U.S. DOE support for many years post the Arab Oil embargo of 1973-4. In the mid 1990’s, when geothermal was one of the renewable energy programs I managed, I met in San Francisco with the heads of all U.S. geothermal power companies to discuss the technology’s future. It was a time of financial difficulty for the companies, limited Congressional budgets for renewables, and hard decisions had to be made on how to support a broad range of emerging technologies with federal funds. All at the meeting agreed that while hydrogeothermal was the basis of their existing businesses geothermal’s future was in hot dry rock. Not willing or able to have DOE support geothermal development close to 100 percent into the future, as had been true for many years, and being a strong believer in cost sharing to advance commercialization, I offered to meet the industry half-way on further EGS development – 50 percent DOE funding, matched by 50 percent industry funding – and to issue an RFP (Request For Proposals) committing DOE to that arrangement. Unfortunately, not one company submitted a proposal in response to the RFP (times were tough and anticipated energy costs from EGS were high) and I was forced to terminate the hot dry rock program. Today EGS is looking to be much more commercially attractive.