Monday, 1 September 2014

Another copied PatLogan post : Does Geothermal make sense in the UK?


No, I stated that 75% of ELECTRICITY comes from conventional hydro - which it does, and leaves geothermal generation as a minority contributor to a small overall total volume of generation. After al, your original question was about

drilling down deep enough to hit hot rock in order to power the steam turbines in a power station

You do understand the difference between total energy use and electricity usage, don't you?

So, let me explain the difference between the sorts of conditions that you need for generating power from geothermal, as opposed to just getting hot water.

Power station steam needs to be hot and dry - and chemically pure. If you have droplets of water in it the blades of the turbine get eroded, and if you've chemicals like sulphur in there, you'll get etching and corrosion. That latter means it's generally a very good idea not to take water/steam direct from the heat source, but to pass it through heat exchanger - which means a loss of temperature of some tens of degrees.

The minimum temperature you'll realistically get way with at the turbine inlet is maybe 200C (otherwise you get those droplets) - and ideally 250C+

Even in Iceland the number of sites with that sort of conditions is limited. But Iceland's geologically very unusual - sitting on the North Atlantic Ridge, with all it's volcanic activity

In the UK, the general temperature lapse rate is about 25C/Km. That is, to get 250C temperatures, you need maybe a 10Km bore. That's deep - a lot more than is routine in oil and gas exploration - but not infeasible.

More than that, you need to transfer a lot of heat from the rocks to whatever's the working fluid. That means you need much more surface area than you get from just the bore walls.

So, when this was tried at Cambourne School of Mines, they came up with a system where you drilled two bores some metres apart (one for the fluid to go down, one for it to come up). They then needed to "frack" between the two - the idea being the fractures give a path for the fluid to flow through and pick up heat on the way. When Cambourne did it, they used explosives - you can't use water at pressure as in fracking for gas, as the rocks are too hot.

They found a number of problems; first the cost of drilling. Second, the crack systems run in all directions from the bottom of the bore, and a lot of the fluid gets lost. Third, the amount of energy to pump the fluid through the crack system is high. Fourth, the crack system tends to close up, through a mixture of plasticity in the hot rock, and chemicals leached from the rock tend to deposit in restrictions in the cracks and blocking the flow.

End result, it's a phenomenally expensive way of getting small amounts of power. At current costs, it'd be north of even tidal/wave which gets CfD support of £305/MWh.

As to "Are you a woman ?"

One of the brightest people on my BSc course was a female. She finished up as technology director for one of the world's biggest bulk chemical manufacturers.

I suspect she's got the edge on you for clarity of thought and brains....

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