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June 27, 2007

More on Geothermal

Category: Geothermal energy – Dan 10:52 am

I’ve ignored geothermal in the blog for some time, so it’s time to make up for that. I just read an article in Technology Review titled “Abundant Power from Universal Geothermal Energy“. The article is essentially an interview with Jefferson Tester, professor of chemical engineering at the MIT Laboratory for Energy and the Environment.

Here are some of the key points Tester touches on:

  • Plenty of energy – The available energy in theory exceeds 100 million quads (a quad is one quadrillion BTUs). Even if you could only tap 1/10th of 1%, that’s still 100,000 quads or 250 times current world energy use of about 400 quads.
  • Available almost everywhere – New oil-field stimulation technology should make it possible to tap this energy by creating artificial geothermal reservoirs many kilometers underground.
  • Key advantages – Because the resource is widely distributed, Tester talks about this as “universal geothermal” energy because the reservoirs could be created near high-demand locations. In addition, the power would be on tap 7×24, and work like current base power sources like coal and nuclear.
  • Technology getting there – Much of the required technology is coming from research into extending oil production and tapping oil shale deposits. Tester says “we know how to create the reservoirs”, and now “we need to connect them better, to stimulate them better than we have in the past using some of these hydraulic methods and diagnostics that are now available to us”.

The challenge here, of source, is more economic than technical. Current geothermal technology takes advantage of naturally occurring reservoirs of hot water in places llke Iceland or the Geysers (in California). It will take considerable investment to build the first few plants. As the price of oil rises, and when we (hopefully) get carbon trading or a carbon tax in place, these investments will start to make sense.

Hybrid Power Solutions

One challenge for solar and wind sources is that they are intermittent — power is only generated when the wind blows or the sun shines. Utilities prefer what they call “firm” power — essentially power that is comes with a future delivery commitment. Because wind and solar are intermittent and the available power may not be known in advance, utilities are sometimes reluctant to assign capacity values to these sources. The result is utilities often don’t consider solar or wind output as firm[1].

This is not a major technical problem today, since these intermittent sources constitute only a fraction of the total electricity provided to the grid. Indeed many experts believe that these sources could exceed 20% of all power on a grid and still be manageable[2]. It is unfortunately still a perception problem however, and sometimes and institutional problem.

There are at least three solutions that help mitigate the fact that renewable sources are often intermittent. I’ll classify all of these as hybrid solutions, although what is being combined is different in each case.

Matching source to use

One approach is to co-locate intermittent power sources with applications that don’t need a firm power source. Such applications include water treatment, water pumping, desalinization facilities, some energy-intensive industrial facilities, and so forth. Consider water pumping; indeed the dutch have using wind power to drain their polders since the 16th century. The point here is that not every energy consuming activity has to be done right now. One reason that a number of these hybrid solutions are associated with water is that the demand for that water is also intermittent, so storing water for later use is common, and nicely complements that fact that your energy supply is also intermittent.

Source/demand synergies

To some extent, this is similar to the above point on matching source to use. Given that peak demand for electricity in warm sunny regions (for air conditioning) coincides almost exactly with peak output from photovoltaic and concentrating solar power sources, these become perfect complements to provide incremental supply when it’s most needed.

Hybrid energy sourcing

Another approach to this issue is to match an intermittent source to a controllable alternative source. This is commonly proposed as an approach for concentrating solar power (CSP). By pairing up CSP with natural gas (which is a relatively simple technical extension as the gas could heat the same working fluid as the solar energy), you can convert CSP into a firm source that essentially behaves like an ultra efficient gas plant. Not, strictly speaking, a renewable energy source, but better than pure gas-fired electricity.

What hasn’t been explored to any extent is hybrid solutions using paired renewable sources, such as wind and geothermal. This likely requires geothermal power technology to move ahead further (it’s still relatively expensive for most locations), but such combinations would provide the best of all worlds, and serve as a long term power source with virtually no negative environmental impacts.

June 19, 2007

What about geothermal power?

Category: Geothermal energy – Dan 6:11 am

Geothermal is a seldom discussed alternative source of power. Yet there is a vast potential geothermal energy resource located as heat in water and rocks at drillable depths of about 2 to 6 miles[1]. Interest in geothermal comes and goes. Just this past week, U.S. Rep. Jerry McNerney, (D-Pleasanton CA), is sponsoring a bill that would support the development of geothermal power[2].

This figure shows the geographic distribution of geothermal energy potential across the continental US at a depth of 6 km. Geothermal distribution in the US The best potential is in the more geologically active intermountain west. Data from the ASES shows that there is theoretical potential to supply all of US energy needs (total US demand in 2003 was about 98 quads, whereas the potential geothermal resource storage is about 14 million quads). So the good news is that even low geothermal energy recovery could displace substantial fossil fuel use. Geothermal has a second advantage because, unlike the wind or the sun, the resource is always available. In effect, we don’t have to figure out how to store the energy resource, because it’s already stored within the earth.

“Unlike other environmentally friendly sources, geothermal energy does not depend on wind or sunshine. This system can provide an uninterrupted supply of electricity, day or night.”
Jeff Tester, professor of chemical engineering at MIT

The challenge is making productive use of this energy. As with wind and concentrating solar, much of the resource is “stranded”, that is the best geographic locations are far from the demand (not to mention that it’s several KM deep!). And while the heat reservoirs within the earth are vast, it mostly constitutes low-grade heat that is harder to utilize.

17 geothermal technology specialists recently performed a study of geothermal potential on behalf of DOE’s Geothermal Technologies Program. The resulting report estimates that 2% of the energy could be recovered as electricity with current stimulation, drilling, and energy conversion technologies, assuming that these technologies advanced further to cut costs. At this 2% level, the study estimates that as much as 2.4 terawatts might be generated over the long term – easily enough to retire all the coal plants in the US. In the mid term (40 or 50 years), the study concludes that 100 GW (about 200 500MW coal plant equivalents) is feasible. This estimate takes into account practical implementation problems with geothermal technology.

While geothermal has the advantage of being a constant (vs. intermittent) power source, it needs much more research and early investment to become a proven alternative to traditional sources. It’s encouraging to see that this is becoming part of the energy proposals in congress, and with luck and support we may yet see a comprehensive energy bill that includes this promising and complementary source as part of the package.