BOSTON (Reuters) - Mining heat stored in rocks in the Earth's crust could meet a growing portion of U.S. electricity demand, replacing aging nuclear and coal plants with an environmentally friendly alternative, researchers say. A Massachusetts Institute of Technology study said the mining of thermal energy could be done on a far larger scale than conventionally known, reducing spiraling oil import bills and strengthening U.S. energy security. "This is a big resource that is perhaps undervalued by people who are thinking of options for the country," said Jefferson Tester, an MIT chemical engineering professor who led the 15-month study released on Monday. "We're running out of time here with our existing fleet of nuclear reactors and all the coal-fired plants that we have that are exceeding emission guidelines," he added. Geothermal power -- generated from drilling wells that allow hot water or steam to power turbines -- is already on the rise globally as expensive oil and gas make it increasingly competitive despite high capital costs. Top energy consumer the United States is leading the way, with 61 projects in the works to double its geothermal capacity to more than 5,000 megawatts, according to the Geothermal Energy Association, a trade group. MIT's study, described by the researchers as the most far-reaching on the subject in 30 years, said the United States as a first step could achieve capacity of 100,000 megawatts - enough to supply about 25 million homes -- in 50 years at an eventual cost of just $40 million a year. That would represent about 6 percent of the current U.S. electricity supply. Coal is now the leading source of U.S. electric power, supplying 49.7 percent. "It wouldn't take a lot of money. It's not like this requires billions of dollars to accomplish," said Tester, who helped develop thermal energy technology in the 1970s. The proposed program would require a combined public and private investment of $800 million to $1 billion in the first 15 years -- about the same money needed to build one new clean-coal power plant, the study said. 'HEAT-EXCHANGER' The study, sponsored by the U.S. Department of Energy, said drilling several wells down to hot rock and then connecting them to a region of fractured rock through which water can flow creates a "heat-exchanger." That produces large amounts of hot water or steam to run electric generators on the Earth's surface. "Unlike conventional fossil-fuel power plants that burn coal, natural gas or oil, no fuel would be required. Unlike wind and solar systems, a geothermal plant works night and day, offering a non-interruptible source of electric power," it said. Various barriers must be overcome. Along with high capital costs for land and deep drilling, geothermal faces environmental hurdles common to new fossil- fuel projects. But unlike fossil fuels, of which there is a finite supply, thermal energy is renewable. The heat and steam, generated in many countries at the point where tectonic plates collide, is nonpolluting. "It's the sub-surface engineering where the risk is and we think the risk has been greatly reduced because of the knowledge of the field work in Europe and in Australia right now," said Tester. "I don't want to see us be in a situation where we are importing a big fraction of our natural gas just to generate power, and I think that's the direction we are headed in if we don't start to produce some alternatives," he added. >> I bring this article to your attention. there is a pure play here from Israel - Ormat <ORA> I have been in and out over the years- the annual reports on the technology are so fascinating to read yet the stock is a serial disappointer. Now different? Maybe.