Harnessing energy from hot rocks

Hot dry
rock energy is the natural heat stored in rock formations buried deep within the Earth. Their depths (a mile or more beneath the surface) account for both their heat and low water content. One way to extract energy from this rock is through heat mining, a concept proposed by researchers at Los Alamos National Laboratory in New Mexico in the 1970´s.
The concept of heat mining is quite simple. First, a deep injection well is drilled and highly pressurized water is pumped into it to fracture the hot rock. Then a second, or production, well is drilled to intersect this reservoir of frac-tured rock. Finally, pressurized water is pumped down the injection well, across the reservoir- where it becomes superheated as it flows through cracks forced open in the hot rock – and back up the production well. A heat exchanger at the surface extracts heat from the water, which is then recirculated through the hot rock reservoir in another heating cycle. The extracted heat can be used directly or converted to electricity.
The technical challenges of heat mining come from the depths at which the hot rock is buried, the difficulty of opening cracks in the rock when it is reached, the need to maintain pressurized water flow through these cracks and the problem of determining exactly what is occurring so far underground. However, when these challenges are met, we will be able to tap one of the largest energy sources available.
Estimates of the amount of potentially useful heat in hot dry rock vary, depending on the assumptions made about temperature requirements, practical drilling depths, subterranean geology and recovery efficiencies. A key variable is the geothermal gradient, or the rate at which temperature increases with depth. On a worldwide average, this gradient is about 80° Fahrenheit per mile. In the western United States, however, it can be greater than 200°F/mile. The higher the gradient, the shallower the depth at which hot dry rock energy can be tapped.
One estimate is that the United States has enough useful heat in hot dry rock at accessible drilling depths to supply the nation´s energy needs for more than 5,000 years. Put another way, the energy content of this rock is about 60,000 times the energy of proven U.S. reserves of crude oil.

Environmentally safe energy

Hot dry rock is an inherently clean energy source. A well-managed heat-mining extraction loop would not contaminate ground or surface waters, nor would it emit greenhouse gases, such as carbon dioxide, during normal operation. Thus, it will not add to the problem of global warming. No fly ash, radioactive waste, toxic byproducts or other long-term residues would be produced in extracting this energy.
Water consumption in not expected to be a problem with heat mining. Field tests at the Laboratory´s experimental heat-mining site have demonstrated that hot dry rock reservoirs may be operated with very low water losses (less than three percent). Such reservoirs could be created even in arid regions.
Finally, shutting down a hot dry rock power plant poses no environmental hazards. Shutdown procedures would be those already proven safe by the petroleum and geothermal industries.

Economic promise

Hot dry rock resources exist worldwide; only their quality varies. Economical recovery of even a small fraction of their heat would contribute significantly to meeting world energy needs. The more accessible the hot rock, the more economical it will be to extract its energy. There is an estimated 60,000 square miles of high grade resources (that is, those with high thermal gradients) in the western United States. Worldwide, that area is much larger.
Studies have shown that producing power from such high-grade resources can be competitive at today´s energy prices.
Given the wide geographic distribution of hot dry rock resources, heat mines might be developed where energy is needed, minimizing power transmission distances. Well heads and a power plant are the only surface structures required; no large energy collection or fuel storage structures must be built – they are underground. The combination of sitting flexibility and small space requirements will help keep power costs down.

Pioneering work at Los Alamos

The world´s first hot dry rock reservoir was completed at Fenton Hill, New Mexico, in 1977 and operated for more than a year. This initial work demonstrated the technical feasibility of extracting energy through heat mining. The quality of the recirculated water remained good, water losses decreased with time and adverse environmental effects were detected.
To explore the commercial feasibility of heat mining, we created a deeper and hotter
reservoir at Fenton Hill
in the mid – 1980s. This reservoir is at a depth of about 2.3 miles, where rock temperatures reach 460°F. It represents major advances in solving the scientific and engineering problems of extracting heat from hot dry rock. During a 30-day flow test in 1986, water was pumped through the reservoir at a flow rate of 220 gallons per minute and heated to 390°F to produce 10 megawatts of thermal power.
We have recently added a surface plant at Fenton Hill to stimulate a commercial power plant. A one-to two year flow test involving both the reservoir and the plant is scheduled to begin in late 1991. This test should answer critical questions concerning the productive lifetime of hot dry rock reservoirs, their operating parameters, their sustainable energy production and their water consumption rates.

Global perspective

The British have been conducting hot dry rock experiments since 1978. Japan and West Germany collaborated with the Laboratory in our early hot dry rock research; both nations now have their own domestic research programs. High – thermal – gradient resources are found in many parts of Japan and the Japanese are experimenting with multi-well heat-mining techniques.
Hot dry rock resources in Europe have lower thermal gradients, but the lack of other indigenous energy resources and mounting environmental problems have led to a greater interest in exploiting this clean and abundant energy. A consortium of German, French and British companies is working with the European Community to put together a $300 million, 10 – year program to demonstrate the feasibility of extracting energy from low-thermal-gradient rock.
In what was the Soviet Union, a hot dry rock heat mine is being developed in the Caucasus Mountains. It will provide heat for a nearby tungsten-mining complex and community. With the growing global interest in tapping hot dry rock energy, the primary question is no longer whether a commercial power plant can be built, but who will build it.