By 2050, "heat mining" of the earth's crust could produce as much of the nation's energy as is currently generated by nuclear power, according to a government-sponsored study (PDF) released on Monday. How do you mine for heat?
By pumping water through scalding underground rocks.Heat mining mimics the cycle that generates such natural phenomena as geysers and hot springs. Cool, dense water on the earth's surface descends below ground and encounters rocks that have been heated up by energy moving up from the earth's core and the breakdown of radioactive material. As the water heats up, it becomes less dense and ascends back to the surface, where it cools off again. Heat miners create an artificial cycle by digging a pair of deep wells, connected by an underground reservoir.Cool water rushes down one well, flows past hot, subterranean rock, and re-emerges from the second well, where a surface plant converts the heat into usable energy.
In most places, you'd have to dig pretty deep—and spend a lot of money—to find rocks that are hot enough for mining. However, it is possible to find unusually hot rocks close to the surface in areas of recent volcanic activity or along the fault line of a tectonic plate. Most of the200 or so American heat-mining facilities in operation today make use of these geological features.
Miners also prefer shallow sites because the rock becomes increasingly compact the farther down you go. That means there will be less porous material and fewer pockets of water than at shallower depths. Since heat mining depends on circulating and extracting fluid, the compact rock poses an obvious problem.
Because shallow heat reserves are few and far between, the process has never before been considered as a viable energy source for the entire nation. Heat mines have been more successful on a local scale: The most famous mine in the United States, called the Geysers, generates enough power for 1 million homes in northern California. (Heat mining has been more extensive elsewhere: In Iceland, for example, it provides 90 percent of the country's heat, as well as a good portion of its electricity.)
The study released Monday suggests that deeper wells could be more feasible than we thought. The compacted rock can still act as a reservoir if pressure from the well forces water into tiny pores and opens up natural fractures in the earth's crust. In rock this dense, a parcel of water could take more than a day to travel a single kilometer, but it would be piping hot when it reached the well on the other side—ready to heat a home or light a bulb.
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Explainer thanks Jeff Tester of the Massachusetts Institute of Technology.