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The big issue with geothermal energy is that rock doesn't conduct heat very well. When you first bore a hole and pump some water down, it's easy to use the hot
by jjk166 4y ago
The big issue with geothermal energy is that rock doesn't conduct heat very well. When you first bore a hole and pump some water down, it's easy to use the hot rocks to generate steam which can in turn drive a turbine, but doing so cools down the rock the water comes in contact with. Digging deeper, the rock starts out hotter, maybe it takes twice as long to exhaust the initial heat, but it's still going to happen in a matter of hours. Long term, a geothermal well can't extract heat faster than heat flows into the well from lower in the Earth. This heat flow rate is in the milliwatts per square meter range. Now with horizontal boring technology, a single drilling rig might be able to plumb a large area, but even if they got all the energy in a 10km radius, which is about 3 times what current technology is capable of, you're still looking at around 30 MW of thermal heat flux, which would at best produce about 20 MW of electricity. That's like 8 on shore windmills or a 100 acre solar plant. It's a little better given that the heat flow is continuous rather than intermittent, and multiple borehole locations can be connected by pipeline to a single generator station to keep capital costs down, but still this is not a lot of power.
For context, to provide the electricity demand for kansas would require about 35% of the total geothermal flux into kansas. New Jersey's electricity demand is 7 times higher than what its geothermal heat flux could provide. For the whole US, electricity consumption is about 75% of geothermal heat flux if you ignore variations like the yellowstone hotspot.
Geothermal certainly makes sense in certain locations where the heat flux is high and other power sources are problematic - for example iceland is probably the most ideal spot on earth. The technology for extracting geothermal power may also be useful for future efforts to control volcanism (though at this point such plans are highly speculative), so research is warranted. But it is unlikely to ever be more than a minor slice of the world's energy supply, and certainly anyone claiming to solve the issue by just digging deeper is selling snake oil.
- yayr 4y ago> For context, to provide the electricity demand for kansas would require about 35% of the total geothermal flux into kansas. New Jersey's electricity demand is 7 times higher than what its geothermal heat flux could provide. For the whole US, electricity consumption is about 75% of geothermal heat flux if you ignore variations like the yellowstone hotspot. It would be great if you can provide a bit more details on those assumptions. Thanks
- jjk166 4y ago(Electric energy use per year * 1 year) / (Area * 100 mW/m^2 * 60% thermal to electric conversion efficiency) In reality electricity capacity needs to meet peak demand, not average demand, the heat flux in continental crust is more like 65 mW/m^2, and the thermal to electric conversion efficiency is going to be closer to 40%, so the situation is actually much worse for geothermal, but maybe with the right technological developments and implementation you could get better performance.
- nabakin 4y agoRelated comment thread. The OP is a former CTO of a Canadian geothermal company https://news.ycombinator.com/item?id=24854216 https://news.ycombinator.com/item?id=24854216
- hengheng 4y agoCan I cool my house into the well during the summer? With surface heat abundant during the summer, I'd expect storing heat downstairs should make sense even if the efficiency of the underground reservoir is not exactly stellar.
- btbuildem 4y agoFrom what I understand, geothermal works both ways, yes
- jjk166 4y agoThis works very well. The low heat conduction of rock also means that without going too deep you get to a temperature that is almost constant year round, so you can cool your house in summer and heat in winter so long as you're not dumping absurd amounts of heat.
- shagie 4y agoLook up Drake Landing Solar Community - https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community // https://www.dlsc.ca/reports/swc2017-0033-Mesquita.pdf https://www.dlsc.ca/reports/swc2017-0033-Mesquita.pdf It is mainly addressing winter heating rather than summer cooling (the cooling degree days are rather minimal - https://okotoks.weatherstats.ca/charts/cdd-weekly.html https://okotoks.weatherstats.ca/charts/cdd-weekly.html // https://weatherspark.com/y/2404/Average-Weather-in-Okotoks-Canada-Year-Round https://weatherspark.com/y/2404/Average-Weather-in-Okotoks-C... "Over the course of the year, the temperature typically varies from 13°F to 76°F and is rarely below -12°F or above 86°F.") That said, there's nothing saying that one can't use a heat pump to as a source of heat in the summer (which would also cool the house) in additional to other sources of thermal energy.
- majou 4y agoHow about inter-seasonal heat storage, a la Drake's Landing? https://dlsc.ca https://dlsc.ca
- JumpCrisscross 4y agoI think this was marketed as “geoplutonic” power about a decade ago.
- downrightmike 4y agoBore hole open cycle loops probably wont be viable in areas that fracking is happening. Super hot steam mixed with methane doesn't sound like a good idea.
- LargoLasskhyfv 4y agoWhy not? Sounds like https://en.wikipedia.org/wiki/Syngas https://en.wikipedia.org/wiki/Syngas
- Retric 4y agoDon't discount the actual energy stored in the rock or capacity factor. Granit has 790 J/kg per degree C and 1 cubic meter is ~2700 kg. So cooling 10 cubic kilometres of rock by say 50C releases 790J * 2700 * 1,000,000,000 * 50 * 10 = 10^18J or 600MW of heat over 50 years. At 33% efficiency your talking 200 MW of electricity for 50 years assuming 100% capacity factor and ignoring how fast it recharges. If it’s a backup for solar and wind at 50% capacity factor then you could double the power output or double the lifespan.
- londons_explore 4y agoI think your maths is off by 1000x... The specific heat capacity of Granite is 790 J/kg/C So replace megawatts with kilowatts in your answer... Still not nothing, but you're gonna have to have super cheap drilling to make it viable. EDIT: Ah - you made another mistake... there is 1e9 cubic meters in a cubic kilometer. So your original answer is correct again!
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- worik 4y agoBut how do you access the heat of a cubic km of rock three km below ground? It would take more than a well. A lot of wells. I love this idea in general but it seems to me it will only work if you find water underground to carry the heat. I am not a geologist
- Retric 4y agoSame basic idea as a well. When you extract water from underground it gets to your borehole through lot’s of tiny cracks greatly increasing the surface area. In some places you can tap into an underground very hot aquifer. In others with sufficient permeability need only need to supply your own cold water. Worst case you also need to crack the rock: https://en.wikipedia.org/wiki/Enhanced_geothermal_system https://en.wikipedia.org/wiki/Enhanced_geothermal_system
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- elil17 4y agoWhat makes the heat flux in iceland so high? Why doesn't drilling deeper get you to a similar heat flux?
- jjk166 4y agoIceland is over a volcanic hotspot where hot rock from deep in the mantle is physically rising up, so the heat conductivity of the rock doesn't matter as much. It's like the difference between standing in a pool of water and being sprayed by a hose.
- elil17 4y agoThat makes sense - thank you for helping me understand. If we drilled down low enough would convection in the mantel increase the availability of heat? I assume the issue is that that would be way too deep to dig, even with new tech like this?
- jseutter 4y agoCorrect. The mantle is on average about 35km thick. Most (all?) oil wells are less than 5km deep, so this is roughly an order of magnitude deeper. The world's deepest borehole https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole is 12km deep. But even before this, downhole temperatures reach 200C which apparently caused the rock to act like plastic, causing the drill stem to get stuck. A well with 200C temperatures would yield more energy, but you're still limited by how fast heat energy can migrate towards the borehole, followed by transporting it for 12km without it cooling off. A similar situation is apparently happening with the London tube, but in reverse. Heat from the trains is being absorbed by the rock underground, but it can't migrate away fast enough. As a result, the tube tunnels are heating up decade by decade, to the point where temperatures underground reach the mid-30s in summer. Eventually they will have to do something about it, but that probably won't happen until enough people get killed by heat exposure in a stuck train.
- theptip 4y agoWhile I think this first-principle analysis is helpful, I fear it may be holding Quaise to too high a standard; their goal (AFAICT) is not to provide 100% of the energy needs of anywhere; it's a huge win if coal's 10-20% contribution of total power generation (or even just a substantial fraction of that) can be in-place replaced with a geothermal generator. Maybe this tech won't be cost-effective in 50 years as solar continues to get cheaper, but as a transitional bridge technology it could be incredibly impactful in the 10-20 year timeframe. Also, you didn't specify a depth for your calculation; won't the flux be higher the deeper you go (i.e. be proportional to the delta-T)? They are talking about going to 20km which as I understand it is WAY deeper than most geothermal systems contemplate. Their whole bet is predicated on the idea that with the gyrotron they can drill deeper since they don't need to mess with high-temp drill bits.
- jjk166 4y ago> Also, you didn't specify a depth for your calculation; won't the flux be higher the deeper you go (i.e. be proportional to the delta-T)? They are talking about going to 20km which as I understand it is WAY deeper than most geothermal systems contemplate. Their whole bet is predicated on the idea that with the gyrotron they can drill deeper since they don't need to mess with high-temp drill bits. The depth doesn't matter at this scale. 20km deep the heat flux is about 0.3% higher. Things get complicated as you go down into the mantle, but at 20km you're still in the top part of the crust. The only thing drilling deep does is increase your max temperature, which increases efficiency, but eventually you hit the same limit as with any other steam generating plant where you can only handle steam that is so hot and so high pressure. With current technology, that limits the efficiency percentage to the low 40s. Maybe with some technological improvements this can go up a bit, but the carnot efficiency of a heat engine where the water is heated to the point it will decompose is 87%, and there is no way you're even going to get near that in a real plant, so increasing efficiency isn't really going to make a huge difference.
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- bequanna 4y ago
- kuprel 4y agoWhat if you dig down deep enough to molten rock? Then you're aren't limited by heat conduction of the solid rock, but the heat convection of the magma
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- ncmncm 4y agoHow would you keep the hole open? But most places that is way, way deeper than 20km.
- hypertele-Xii 4y agoWikipedia says the Earth's crust is on average 15-20km thick. Soviets drilled 12km deep with 1980's tech. It's difficult for sure, but might be doable. https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole
- ncmncm 4y ago"Doable" is very different from "economically justifiable". The thickness of the Earth's crust is strongly bimodal, making "average" meaningless: thinner under oceans, thicker on continents. Much thicker. Most of the planet is, of course, sea floor, so mostly thinner. But drilling the sea floor is unpleasant and expensive, and at the end, the hole is all the way down there.
- MichaelCollins 4y agoFrom what I understand, most of the earth's mantle is not molten and you can only find molten rock, magma, in some specific places like mid-ocean rifts and volcanic hotspots. The mantle has convection currents, but those flow on geological timescales. The Earth's outer core is liquid, but that's almost three thousand kilometers below the surface.
- ncmncm 4y agoFurthermore! Maintaining a steam turbine is an operational expense not matched by any for wind or solar. There are reasons we are not drilling new regular geo wells. Literally everything costs more than wind and solar. It is why nukes will soon be mothballed.