« on: 16/10/2016 15:02:27 »
Quote from: Wolfhart WillimczikAt a maximal deepness of about 20 kmI understand that the typical temperature rise with depth is about 25C per km.
So as you say, a very deep tunnel is needed to obtain high temperatures for electricity generation (ie expensive).
So it is usually more economical to find locations where there is a magma body at shallower depths, so the thermal gradient is greater (eg Iceland or New Zealand).
Thank you for your replay. I will try to answer everything. 20 km is indeed the worst case scenario. I choose this to show that every country on earth can get a strong and cheap energy source – to avoid political tensions as occur with oil.
I think in most cases are 10 km good enough. The simple rule is: One has to dig until it is hot enough. Some places are better – some worse.
Then the steam rises up in the sloped tunnel as in a chimney, cools down, condensate to water and starts the entire process again.The amount of energy you can extract from a heat source (like geothermal energy) is determined by the difference in temperature of the heat source and a cold sink. -right
Shallower rocks start off at a lower temperature than deep rocks. But rocks have fairly poor thermal conductivity, so as soon as you try and put heat into them, they heat up; when the temperature matches the heat source, you generate no more power.There is practical a cooling tower (10 km high) with a heat exchanger on the surface. Do you see?
- It is more efficient to have your cold sink on the surface - eg evaporation towers or the ocean
To enlarge the surface of the hot rocks many smaller tunnels and Fracking is employed as it is made already. Don’t forget the water has already at 10 km 1000 bar – free of charge! This opens up all the cracks in the rocks. (Today are used about only 100 bar)
runs down again a long pipe and feeds first the water turbines againIn theory, Electricity output really doesn't matter where you put your generators - on the surface or deep underground right
But operating costs are radically different between the surface and deep underground.This is a great thing that electricity don’t loose power up or down, but the steam looses energy.
- At elevated temperatures and dripping water deep underground, the generators will always be breaking down
- the electrical conductors will be arcing over
- Engineers and Technicians can't easily reach the equipment to service it, because the access tunnel is full of hot water.
- It's much better to put the active equipment on the surface, where it can be air-cooled, and accessible for maintenance
You are perhaps an engineer and you want everything on the surface – understandable, but there are great disadvantages also.
In Scientific America there is a strong argument against deep geothermic power plants:
„…This looses at least a third of the available power.”
For efficient power generation, you need several hundred degrees C of temperature difference. up to 600 or 800 degrees C is good.
Ergo, the turbines must be there where the hot steam is.
There are not many workers (intranauts) down and the control station is on the surface, but a second one down there in case the one on the surface is bombed or the like.
I would like also a second tunnel for the intranauts, the Gottard has actually 2 tunnels.
Down there the machinery has the same climate as on the surface – or better. In south Africa a 5 km deep shaft is cooled with ice – 15 t per hour. I see even better ways. Think on the free water pressure of 1000 bar. There is a lot, what you can do with it, for instance build a huge cooling system…
Do you know that in Germany there are protestors on the fence to geothermal power stations?
The operating company would love it, if everything could disappear on the surface and only a cable comes out of the earth – even if it cost money.