High Times for Hot Rocks?
A recent article in The Economist reviewed some of the issues with geothermal power; this follows an Ars Technica article about geothermal power. These are both interesting reads suggesting there is a bit of a media initiative going on with geothermal power. So what is going on overall?
First, this isn’t all that new. The DOE’s history of geothermal power is interesting in part because it shows something of the diversity of uses: greenhouse warming in 1930, downhole heat exchanger to heat a house in 1930, residential space heating in Reno in 1940, electric power from the Geysers in California in 1960. Geothermal power generation originally was from steam-driven systems, then some hot water systems and, eventually, from hot dry rock. There have been numerous problems. Really hot natural fluids carry a lot of dissolved elements, including such interesting things as gold. If you cool that water or steam, those things tend to precipitate out, clogging pipes and turbines (this was a huge issue in the Salton Sea area as geothermal was getting underway; such dissolved minerals are now being considered a target for exploitation). (Cooling of hydrothermal fluids is how you make most economic mineral deposits). Binary systems allow power to be generated from lower temperature geothermal resources (which have fewer of the dissolved solids problems). Original systems were designed to simply allow overpressured steam and water to rise up and cool; now most systems try to pump the used water back down to recharge the deep aquifer being used.
Two aspects of geothermal have driven the recent interest: that it is a continuous power source (not subject to wind and sunlight variations), and the development of what the industry calls enhanced geothermal systems; what they clearly fear is that it will be called a fracked geothermal system. Why? Um, because that is what they are doing: they fracture the rock around the primary drill holes to improve hot water flow. We’ve already visited fracking for oil and gas and noted that the fracking itself is usually not the problem people are complaining about; for geothermal development, as the water itself is the resource and it is cycled back down, there are fewer issues with contaminating the biosphere than in oil and gas development. Since the public and the media confuse fracking with all the evils of oil and gas development, you can see why geothermal proponents want nothing to do with the term.
What there is relative to oil and gas fracking is an increased risk of earthquakes; this is because, in essence, the recharge system is adding water like a oil-and-gas wastewater injection well, but there can be larger volumes of rock fractured for geothermal applications (most current oil and gas plays are in narrow stratigraphic zones, so the extent of fracturing away from the drill hole might be fairly small). Now since the water being returned is only the water that was removed (usually; hot dry rock does add new water), you might hope that there wouldn’t be a change, but the water pressures have changed to allow the flow to the production well and away from the injection well. Such earthquakes killed a major project in 2006 in Basel, Switzerland, where a hot dry rock geothermal well caused earthquakes up to M3.4, which caused some minor damage and so was viewed as too risky to continue.
While these large geothermal projects continue to develop, there is a lot of room to do small stuff. Simple heat pumps can deal with both heating and cooling of buildings, and there is some growth in lower temperature geothermal that exploits, among other things, the wastewater associated with oil and gas development. We’ll have to see if these newer technologies will help amp geothermal up beyond its current status.