Let's review quickly how geothermal energy works:
Down at the bottom of the picture is hot rock and hot water, about 3 km down into the ground. You drill the two holes shown, and hot water rises up through the "production well". At the top it heats a working fluid - a liquid that will absorb the heat from the hot water and boil, thus powering the generator. The working fluid, as you can see, is a completely closed loop. The spent water from the ground gets pumped back down the "injection well" on the right and starts a new cycle.
Now, geothermal plants like the one in this picture currently only work in places where there is a lot of hot rock and water pretty close to the surface (like in Wyoming, where said water erupts in geysers).
However, there is something new - Enhanced Geothermal Systems, or EGS for short. You see, it turns out that if you drill a little deeper - say 5-8 km down - you can find really hot rock all across the country (picture courtesy of this 2006 MIT report on EGS):
Therefore, one can simply drill two holes into that hot rock. Cold water gets pumped down one where it gets hot and rises up the second hole and generates electricity. The report claims that if we started building plants like these we could generate up to 10% of our electricity this way.
That's pretty exciting considering the fact that these plants hardly pollute, and are tapping an energy source (the inner heat of the Earth) that is virtually endless. But before we get carried away, let's break down the most important issues:
1. We need to do more CGS research
Right now, the technology isn't cost competitive with, say coal. But once engineers start working on drilling stronger holes and pumping water more efficiently, etc. the report suggests that it will be cost competitive.
People are often skeptical about whether research will really pay off in the end. For something like cold fusion, or superconducting transmission lines, that skepticism is well-founded. But this is different. Figuring out how to drill holes that are stronger and pump water faster is business-as-usual for geological engineers. The oil and mining industries have been doing research like this for decades. With research, the costs will go down.
2. Water
I'm curious - where is the water coming from? Surely no one would waste drinkable fresh water on such an application. But then what water will be used? Sea water? Dirty polluted water? Is that safe?
Also, I'm not a geologist, but I'm a little confused about how the water manages to stay within the loop. Here's a picture (again, from the same report) of a standard EGS system:
I get that the water is pumped down into that black cloudy area. And I get that, if there is enough water saturated in that area, it will rise up the other hole. But doesn't a lot of water just get lost in the process? If so, then a) where does it go and what does it do to the environment? and b) don't you have to keep adding water to the system to keep it running?
In a related point, the report mentioned that one of the challenges being worked on is actually getting the water to flow through the whole system fast enough.
3. Will people invest in it?
Well, it seems Google already has (thanks, Tal). And I think that if a few successful pilot plants are set up in different places, investors will be convinced that there is little risk.
One small step for geothermal power, one giant leap for clean energy.
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