Why isn't geothermal energy more common?

Factual Questions
1

No matter where you are on Earth, if you dig deep enough you will reach molten rock or at least very hot rock. Why isn’t this energy used?

In my idea of a geothermal power plant you have a U shaped pipe that goes into a hole that is drilled down to where the crust becomes very hot. The U shaped pipe is insulated until it gets deep enough to extract energy from the hot rock. In that pipe you pump molten salt or oil so it can come up at hundreds of degrees. You run that hot salt/oil through a pool of water, the water boils, you reuse the salt/oil and you have a geothermal power plant.

Something tells me that if this idea were feasible or at least cost effective it would exist already. I know that in Iceland and other places where there are hot springs they do use geothermal energy, but it seems to me that this idea could be applied anywhere.

2

The problem is with the bottom of the “U”. We can dig two boreholes, but getting them to connect at the bottom properly has proven extremely difficult; we just end up with a “\ /” instead of a “U”. So, we are presently stuck with only using geothermal where nature has already done the job of creating the horizontal part of the hole for us and drilling into natural layers of heated water.

If we could connect the bottom properly then yes, we could make a geothermal plant pretty much anywhere.

3

When I originally imagined my idea I assumed the U part of the pipe would be assembled at the surface and it would be lowered down as section of pipes were attached one by one.

4

Yes eventually. The main issue there is you’d have to drill beyond what we are capable of doing in most places. Oil wells that represent to peak of our capability do not reach magma. Depths of 5-7 miles down.

The Russians had a super-bore project that took 20 years to drill 7 miles, that was eventually stopped due to the excessive heat. It’s estimated to be only 1/3 the way into the mantle. From what I’ve read that heat still wouldn’t be sufficient to drive any practical power plant.

Directional drilling isn’t much of a problem these days, it’s the depths needed that are at issue. We do just fine with geothermal power where it is close to the surface.

In most area’s geothermal energy can be used for supplemental systems. Geothermal heating/cooling systems are becoming more common. Many of these use two water wells, 500-1000 feet. Cycling water to increase or decrease the temperature as needed. These are certainly some of the lowest energy usage heating available but in order to make up the initial cost of the wells it’s 15-20 years not including the maintenance costs.

5

In the Cooper Basin of central Australia the company Geodynamics is using hot fractured rock at a depth of 3.5 - 4km to generate base load power.

The animation below explains the concept.
http://www.geodynamics.com.au/IRM/content/gdyanimforweb/GDY_AnimV03_webversion.html

6

A related, but slightly different answer: homes don’t use geothermal heating and cooling mostly because of high initial costs. It’s pretty easy to put some tubing 10 ft down and regulate temperature. It just takes a while to get the investment back.

7

This is different from geothermal power. A five hundred foot long pipe laid either vertically or, if you have enough land horizontally ten feet below grade, together with an electric heat pump will provide heating and air conditioning for the average house at much lower cost than traditional methods. Someone in my neighborhood has actually done this.

Why don’t more? Well, it is hard to find a contractor who is willing to face the learning curve, for one thing. They are notoriously conservative. Understandably since they will be held responsible if anything goes wrong. I have been unable to find a contractor who will install a whole house fan in my house, which would make air conditioning unnecessary, at least in the Montreal climate. At least today.

8

While whole house fans are nice (I’ve got one) if it’s really hot out they are less effective, and they do nothing about humidity.

9

And noisy.

I bought a house with a whole house fan. The first summer I installed central air and have never regretted it for a second. And I’m in upstate New York, so the climate isn’t that much different from yours.

10

It isn’t just a simple U shape that is the problem. A reservoir of water at a certain size has to be maintained to allow a continuous flow of water without cooling off. For efficiency, the water has to be heated above the boiling point at normal air pressure, and the whole loop must be pressurized. This is easy to do at certain locations where the mantle is thin or fractured. A plant in CA has been producing significant amounts of power for over 50 years now.

However, geothermal power could be produced less efficiently almost anywhere in the world. If the costs of other means of generating electricity gets high enough, geothermal will take over rapidly, because the energy source is free, available 24/7, everywhere on earth.

11

As I said in the one hydro power thread:

12

And just as in that thread, it’s utterly irrelevant to the actual question being asked.

13

I just read (well, skipped over the too technical parts) a brochure about geothermal energy by the Bavarian ministry for nature and discussed this with my friend the geologist.

The actual technique used in some parts of Bavaria is close to what you describe, but with some key differences:

instead of a U pipe, two parallel holes are drilled and pipes sunk down. Then water is pumped up at the lower pipe, used to drive a steam turbine at the top, and flows back down through the up-stream pipe. (upstream relative to the water-bearing layer). It’s easier to pump up hot water than hot rock.

In Bavaria, this means however drilling down 4 000m (4 400 yards), which makes this quite expensive. (It also is not a strictly renewable energy in the usual sense - it will take 10 to 20 times as long for the rock layer to heat the water up naturally again compared to the usage time). This is because Bavaria is just touched by the disruption zone.

Even then, and with pumping the water back quite quickly, there is a certain danger of triggerin g Earth movements and earthquakes by removing the hot water from deep down.

Going into other European regions with hotter disruption zones, like Chech republic, the places where hot water bubbles up from Earth by itself are already in use for hot baths (therapeutic uses) - Karlsbad, Marienbad (Karlovy vary, Marianske lasne) etc. Building big power plants there instead would be frowned upon by the locals and destroy an existing business.
Other places, like Italy near Nepal, faces the other obvious problem: the closer the heat energy from Earth is to the surface, the bigger is the risk that it will erupt. This tends to discourage investors, knowing that this mountain could blow up one day and tkae their plant with them, and that any activity that disrupts the mountain could trigger this disruption.

Similar proplems apply to most of the globe: the firebelt is too hot and liable to earthquakes already for power plants, the other places are too cold or too deep down to be worth it.

Yellowstone is both a national park and far too remote to be practical. Aside from the fact that Yellowstone is a huge problem that, on the day it will blow (we know it will, we just don’t know the date), the whole northern hemisphere will be fucked up big time.

14

Yes. This agrees with what I said.

15

A very good animation. It covers the subject well. There are a couple of points to emphasise that could easily be missed as they go past very quickly in the commentary.

The rock is not heated by the earth’s core, and is not connected to a geologically active region. The source is commonly called a “hot rock” and there are two critical things that have to right for it to work. The rock is granite - granite is naturally radioactive, and thus self heating. The target rock is hotter than the structures beneath it, until you get very deep. The rock is insulated from its surrounds by lighter poor conducting strata, usually limestone. It is this insulation that allows the hot rock to heat up over time, from self heating. Not heat flow from the depths.

There are a few limitations to the technique. As the animation described, the rock is cooled, and eventually the region becomes too cool to be useful. The expected technique is to drill a ring of wells around a power generator and to rotate between the wells, initially drilling a new well when the old one goes cold, but with the expectation that eventually the very old one might regenerate enough heat to cycle back to it. (although the time needed is so long that technology will have probably advanced past the whole idea.) The other limitation is that at these temperatures and pressures, water is very reactive, and cheerfully reacts with the rock and leads to choking of the flow pathways, limiting the life of the well.

The big hope here in Oz with hot rocks, is that due to our geology, we are blessed with a large number of such rocks in the centre of the country. In this we are unusual. So, this isn’t really an example of the OP’s scenario, rather it is another special case of local geology making a geological thermal resource viable. The example constanze gave in Bavaria is another lucky bit of geology.

16

Well as a share of renewable electricity generation it is not insignificant. More than 22% as much of wind’s generating contribution and 4x as much as solar’s. “The United States leads the world in installed geothermal” and its costs are on the same range as other renewables. Like wind however it may be everywhere but the resource is better in some places than others. There is a lot of planned increase in capacity. It just flies under most people’s radars.

17

Your comment is?

We are not using geothermal more because as I said the start up cost is too great in most places.

18

I think the point is that in most places it is beyond us technologically as well as financially unfeasible.

19

Except that there are funded plans already in progress to increase geothermal from just over 3,086.6 MW capacity to between 5,256 and 7,877MW - roughly doubling it. The cost of geothermal turns out to be about 5 to 10 cents a kWh, pretty much the same as nuclear and wind. And like nuclear it is baseload power, 24/7. The Western United States have ample areas in which it is technically easy to exploit the resource. (see links above)

Hard to compete with coal or even natural gas though, until the carbon is monetized. And that’s why we don’t use it more.

20

Not all that long. I have a “geothermal” system. I put that in quotes because it’s only geothermal in the lay sense. No energy is being produced. It’s a simple heat pump. It’s more efficient than an ambient air heat pump because the temperature two meters down is pretty much constant. I.e., it’s always warmer than cold air, and always cooler than hot air. I prefer to use the term “ground source heat pump.”