Engineered geothermal systems

by Milan on September 13, 2010

in Power plants, Renewables

The Economist‘s Technology Quarterly includes a good article on engineered geothermal systems (EGS) – a type of power plant where holes are drilled into the hot granite about 3-4km below the surface of the Earth and water is pumped through which is then used to drive turbines. The advantage, when compared to conventional geothermal systems, is that EGS can theoretically be used anywhere. The amount of energy available is enormous:

According to “The Future of Geothermal Energy”, a report issued by the Massachusetts Institute of Technology (MIT) in 2007, the thermal energy available in America in rocks 3-10km (1.9-6.2 miles) beneath the Earth’s surface is nearly 140,000 times greater than its annual energy consumption. Conservative estimates suggest just 2% of that energy could be tapped by EGS in practice, but even that would be far more than is needed to supply all of America’s electricity. Tapping it will, however, require both technical and economic hurdles to be overcome.

Right now, the total global geothermal capacity is 10.7 gigawatts, producing 67,250 gigawatt hours (GWh) per year. That’s equivalent to about ten large nuclear reactors. With the deployment of EGS, those figures could be increased enormously.

Engineered geothermal systems (sometimes called ‘enhanced’ geothermal systems) do have limitations. While conventional geothermal power in the United States costs about 10¢ per kilowatt-hour (kWh), comparable to oil and gas, EGS is more like 19¢ per kWh. It should be possible to bring that down somewhat with increased scale and experience. Also, whereas the 10¢ for coal power fails to take into account the climatic harm associated with that power source, EGS produces no greenhouse gas emissions and emits no atmospheric toxins. Another limitation of EGS is the production of earthquakes, though supporters argue that with proper management it can be ensured that these are always too small to cause harm.

EGS also has the major advantage that it provides a consistent baseload level of power output – night and day. That stands out particularly in comparison to options like wind power and solar, where energy output varies throughout the day and year. Because drilling for EGS requires much of the same equipment and expertise as drilling for oil and gas, it could also serve as a mechanism to shift toward a post-fossi-fuel global economy, without wasting the capital and expertise that we have already assembled. Doone Wyborn, chief scientist of Geodynamics, stresses this point: “There are thousands of wells being drilled for oil across the world every year. I imagine that in a couple of decades all of those drilling rigs that are now redundant, because we’ve run out of oil, will be drilling geothermal wells instead.” Hopefully, that can happen sooner than in a couple of decades, and well before humanity has burned up all the world’s fossil fuels.

Google.org – the charitable arm of the search giant – is one backer of EGS, including in Canada.

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Milan September 13, 2010 at 1:51 pm

David MacKay’s Sustainable Energy – Without the Hot Air includes a chapter on geothermal power:

[G]eothermal energy comes from two sources: from radioactive decay in the crust of the earth, and from heat trickling through the mantle from the earth’s core. In a typical continent, the heat flow from the centre coming through the mantle is about 10 mW/m^2. The heat flow at the surface is 50 mW/m^2. So the radioactive decay has added an extra 40 mW/m^2 to the heat flow from the centre.

So at a typical location, the maximum power we can get per unit area is 50 mW/m^2. But that power is not high-grade power, it’s low-grade heat that’s trickling through at the ambient temperature up here. We presumably want to make electricity, and that’s why we must drill down. Heat is useful only if it comes from a source at a higher temperature than the ambient temperature. The temperature increases with depth as shown in figure 16.4, reaching a temperature of about 500 °C at a depth of 40 km. Between depths of 0 km where the heat flow is biggest but the rock temperature is too low, and 40 km, where the rocks are hottest but the heat flow is 5 times smaller (because we’re missing out on all the heat generated from radioactive decay) there is an optimal depth at which we should suck. The exact optimal depth depends on what sort of sucking and power-station machinery we use.

We can bound the maximum sustainable power by finding the optimal depth assuming that we have an ideal engine for turning heat into electricity, and that drilling to any depth is free.

For the temperature profile shown in figure 16.4, I calculated that the optimal depth is about 15 km. Under these conditions, an ideal heat engine would deliver 17 mW/m^2. At the world population density of 43 people per square km, that’s 10 kWh per person per day, if all land area were used. In the UK, the population density is 5 times greater, so wide-scale geothermal power of this sustainable-forever variety could offer at most 2 kWh per person per day.

This is the sustainable-forever figure, ignoring hot spots, assuming perfect power stations, assuming every square metre of continent is exploited, and assuming that drilling is free. And that it is possible to drill 15-km deep holes.

He notes that the prospects are better if you don’t choose to take the quantity of heat that regenerates continuously. As an alternative, the Earth’s heat could be ‘mined’ in such a way that it would eventually be exhausted – at least buying us some time to deploy other zero-carbon options.

In the end, MacKay isn’t very optimistic about the prospects for EGS, at least for the U.K., claiming that: “for Britain, geothermal will only ever play a tiny part.”

. February 4, 2011 at 6:49 pm

Stanford’s Single-Well EGS Investigation

Next week Stanford University will host the 36th annual Stanford Geothermal Workshop, one of the world’s foremost geothermal technical gatherings.

Stanford’s Geothermal Laboratory, led by Professor Roland Horne, has been investigating novel single-well approaches to Enhanced Geothermal Systems (EGS), sponsored by a research grant from Google’s RE<C initiative.

Currently, for every well that sends cold water down to the hot rock (called injectors), there are one to three wells that bring heated water back up to generate electricity (called producers). But what if one well could act as both an injector and producer? It could dramatically lower the cost of EGS (example below).

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