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Comment by TheSpiceIsLife | Hacker News Reader

> Not quite … The cost of enhanced geothermal is roughly the exact same cost as nuclear … shares similar economics of a very very low OpEx to CapEx ratio.

It can’t be both not quire and the same.

All else being equal, a five billion dollar geothermal plant running at 50% has the same problem as a five billion dollar nuclear plant running at 50%: where’s my profit?

Sure, load following is trivial with geothermal, but nuclear generally isn’t trying to compete in that space, so we can discount the difference there.

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Unlike nuclear, that time running at 50% isn't thrown away capital expense, it is merely delayed power output that can be utilized with slightly higher turbine capacity, which is the cheaper part of the capex, and would be needed for higher power output anyway.

For nuclear, adding thermal storage for time shifting would be the most equivalent to what's happening with geothermal storage, but with geothermal there's no additional capex or engineering needed.

> that time running at 50% isn't thrown away capital expense

I don’t understand how you think capex works?

You outlay x on capital expenses to get the plant running, that’s all the plant and equipment. You run the plant at 50% it takes you twice as long to recoup your capex.

Always happy to be corrected.

Geothermal is limited by the flux* of what you can pull from the ground, which sort of scales with capital costs.

Reducing the draw from the well at time X, leaves more available at time Y. In this way, the capital cost is reasonably preserved and your correction is offered.

--It's not exactly the same thing as flux ---I haven't verified the relative losses here of delaying utilizing the available heat, but rather am assuming the OP is correct about what I believe to be their point*

I still don’t follow.

If the geothermal plant isn’t ran at or near capacity all the time, the capex takes longer to recoup.

You can’t just, say, run it at 50% for a while, and then at 150% later, because a) ya typically can’t run plant at 150%, and for power generators ya can only run them at whatever you’re contracted to supply, so you’d typically want to run geothermal at capacity all the time.

This is true of anything that requires capital expenditure.

Drilling the well is the expensive part. It heats at a relatively constant rate depending on the geometry. You could size the plant to be exactly matched to the well output--but the generators and such are relatively cheap. So instead you make that part of the plant slightly oversized, so you can run at over the well's capacity when electricity is expensive, and under the capacity when it's cheap. The thermal mass of the rocks allows you to average this out over time.

So there are two capacities, that of the well and that of generation (oversized with respect to the well). On average this varying scheme utilizes the well at 100% of capacity (the expensive part to increase), and the turbine generation at less than 100% capacity (not expensive to oversize).

Like someone below said:

> They just let pressure build up higher than normal for 6-8 hours and then the turbine generates more power than normal until the pressure falls back to normal levels again.

So they can run at more than 100%.

You can run it at 400% by building more generators and heat pumps. They aren't the expensive bit.

The expensive bit is drilling and that gives you roughly X heat per month. You can use that at a constant rate or all in one week.

Everyone is overthinking this. They just let pressure build up higher than normal for 6-8 hours and then the turbine generates more power than normal until the pressure falls back to normal levels again. This would not take 50% and make it 100% again, but it does give you something.

Many turbine are running in extreme high pressure (for efficiency). There are no much margin for "build up higher than normal".

Of course we can build something less efficient to allow "pressure build up", but that's another trade off.

It's not pressure on the turbine that builds up, it's pressure at the well head.

> Sure, load following is trivial with geothermal, but nuclear generally isn’t trying to compete in that space, so we can discount the difference there.

Isn't this logic flawed?

> Sure, Metric A is better with Option A, but Option B is so bad in that space they are avoiding it, therefore we can discount the difference there.

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