The recent combination of natural disasters, wars, and other terrible, international crises has impacted us in myriad ways, including our expectations of what to pay at the gas pump.
I had just finished refueling my car yesterday when someone at the pump behind mine scoffed and grumbled at the climbing price. “This’ll be $5 a gallon by end of summer!” I nodded. I wouldn’t be surprised.
We have all repeatedly heard someone mention that our need to reduce our dependency on oil and become less financially sensitive to devastating world events. Here at GE Global Research Center, we enjoy the challenge to improve and diversify the energy technology mix. Further developing a portfolio of affordable renewable energy technologies that take advantage of localized natural resources across the country is part of realizing this vision.
I am now part of the Renewable Energy Systems Laboratory, and one of my larger projects relates to geothermal power and is funded by the U.S. Department of Energy. The team is designing means to help make electricity from geothermal resources more efficient and more accessible, even in places like Upstate New York, an area well-known for outdoor activities, history and rural charm – but not geothermal resources suitable for power generation.
In this series of blog entries, I will describe how approximately 3 gigawatts (GW) are generated in the U.S. from natural and engineered geothermal resources. I aim to give you an understanding of where the industry is headed and why industry leaders so firmly believe that geothermal is an important part of our secure and sustainable energy future.
Resources in the U.S.
One of the main motivations to look into geothermal energy as a sustainable source of energy is the sheer size of the resource. Figure 1 is a map of the United States showing the estimated temperatures of the Earth’s crust at a depth of 6 kilometers (3.75 miles).
The red areas are up to 300°C and the deep blue areas are as cold as 80°C. Large geographic areas in the U.S. are cool, even at depths of 6 kilometers, and drilling deeper for higher temperatures can be cost prohibitive. An additional challenge facing geothermal power is in negotiating the difference between the geographic locations where geothermal energy is economically accessible and the geographic locations where people live. The large population in New York City, for example, cannot directly benefit from the economically high temperatures found in the Western States. One of the missions of the geothermal community is to develop the technology needed and, very importantly, a public awareness to make geothermal energy a ubiquitous resource. The idea is to mine for heat not only where temperatures are high, but necessarily where people live, even in the cooler areas where mining for heat is more challenging. Maybe we could build a small plant on a college campus. Maybe we could one day put a large plant in the middle of Washington, D.C. or power Times Square with the heat beneath our feet! Goals are lofty, but such lofty goals are opportunities to make strides toward a sustainable future.
In 2006, an 18-member assessment panel led by Jeff Tester, then at MIT, now at Cornell, released to the DOE the most thorough review of geothermal potential in 30 years. The panel used available information about the geology of the Earth’s crust in various locations across the U.S. and modeled the amount of heat in place using the thermal conductivity and heat capacity of rock among other physical factors. Figure 2 comes from the second chapter in that report and illustrates our entitlement to millions of exajoules (= 1018 J). In 2005, approximately 100 exajoules were consumed in the United States.
The horizontal axis is temperature at depth, or the temperature we could reach by drilling between 3.5 and 9.5 km into the Earth’s crust. The vertical axis is the amount of heat available at those temperatures and depths. The purple bars, non-existent at the higher temperatures, show how much energy is available at a depth of 3.5 kilometers. The bright red bars show how much energy is available at a depth of 9.5 kilometers. I like this bar graph because it shows how far I would need to drill to obtain a desired temperature. If my particular application required 200°C, for example, I can see that I would probably have to drill at least 4.5 km. However, if my goal were to gain access to thousands of centuries worth of energy, I would have to drill at least 5.5 km, and preferably as much as 7.5 km. For 200°C, I can see that there is not much to gain from drilling more than 7.5 km. A goal for heat mining is to find more economical means to drill to deeper depths. A goal for power generation is to improve energy conversion efficiencies at lower temperatures. Higher conversion efficiencies would make temperatures as low as 150°C more attractive for power generation in addition to district heating and other common direct thermal uses.
Figure 1 was taken from http://geothermal.inel.gov.
Figure 2 was taken from www1.eere.energy.gov.