Geothermal energy; specifics--whats the holdup?

Question

If we drill a hole several miles deep and inject water (anywhere in the world), we can extract almost unlimited geothermal energy.

So why are we not developing this energy source more?

I'm sure drilling a hole miles deep is darn expensive, but here's my questions;

(1) Specifically, what needs to be improved in the drilling technology?

(2) I understand (thank you, eiscubes) that ground water precipitates muck up the heat exchanger, but that just does'nt sound like a show-stopper; just design a big, inefficient (but easy to clean) heat exchanger, right?

Answer 1

Geothermal is practical and you don't have to go much deeper than 4 feet to harness the energy.

Heat pumps have been around for years and they exchange the temperatures between two bodies using compressors. Which means they can both heat and cool homes from the same source and without burning fossil fuels to do so.

Most heat pumps are air exchange heat pumps and suffer from a loss of productivity when they are needed most, extreme heat or cold, because they operate off of a temperature differential. By creating a ground link which really only need to be a few feet deep you can have a constant temperature of around 60 degrees F on your heating and cooling loop. Of course this is relatively small scale but viable.

As for commercial generation you can drill the deep holes and insert a closed loop in the hole. By using a closed loop intermediary you would completely bypass your muck issue. Drilling is a problem these are expensive holes but you know there are dry oil wells or wells that never struck oil, why not start with one of them?

Answer 2

It works brilliantly - but only at first.

Then the hot rock around the deepest part of the borehole gets cooled by the cold water that you pump down to get heated up, so less heat starts coming through. It never stops, but it gets very slow, because rock is such a poor conductor. You need a good mathematical physicist and/or computer programmer to set up the equations, and work out how much heat you can get out altogether before the rate drops to something completely uneconomical. I think it doesn't pay for the drilling.

Iceland uses geothermal heat, so it should be interesting to find out what they've got that improves the economics. Better conductivity? Ready-made deep fissures? Higher temperature difference to start with?

Answer 3

One big problem with geothermal is the risk of a volcano eruption. Drilling through the earth can affect magama flow, and therefore produce a high risk of eruption.
And referring back to your question, we should not really drill everywhere possible, especially along plate boundaries, as those are the areas most vulnerable to earthquakes and possible volcanoes.