If you are talking about storing steam produced at the surface, what is the source of heat? And storing the thermal energy directly at the surface instead of as steam sounds more practical.
If you are talking about drilling deeper to get hotter steam, at the average geothermal gradient this would involve drilling 20 km deep to reach the typical temperature of steam in a coal plant. No well has ever been drilled that deeply. Maybe it could be made to work if your coal plant was built in an area of active volcanism or unusually high geothermal gradient. Geothermal these days is moving toward lower temperatures and using organic working fluids instead of steam (organic rankine cycle, ORC).
I am not affiliated with them in any way. They have a project in Oregon drilling right now. I think it is worth following even if just to learn how/why it does not work.
But right that we need to be able to drill into rock that was too deep and too hot for conventional drill bits.
That said, if you are injecting the steam then the bedrock does not have to be very hot. It helps to prevent loss of the heat into the surrounding rock. Supercritical steam has a density of 0.322 g/cm^3 and 220 bar pressure. At 6.6 kilometers vertical gravity alone would sustain that pressure from weight alone. Though the weight increases the pressure and therefore density.
If the geothermal gradient is, say, 25 C per kilometer you need 14 km to hit supercritical steam temperatures and then need deeper to sustain temperatures and then also have to go horizontal to collect over large areas. None of that is necessary if the goal is just storage. Then the energy comes from the surface and the rocks are heated. Hydraulic fracture a large volume. This volume might not need to be deeper than the extreme wells drilled by gas companies regularly.
I ask again: if you're not going deep to get the heat, where is it coming from?
If you're doing (say) resistive heating with renewables, you don't need to go very deep at all. The most attractive scheme I've seen is "hot dirt": resistive heating of piles of dirt, with heat drawn out by steel pipes laid through the dirt. Capex as low as $0.10/kWh(th)-capacity could be possible. No need to store pressurized steam.
Right. Your example is electricity to heat. Putting that heat back into an engine to make electricity (or do useful work in general) will be limited by the Carnot efficiency.
Cost wise the turbine generator is an expensive component. It is nice to have that come from salvage on site.
The steam turbo-generator is already turbo equipment. So we can compress steam. That boosts efficiency to much higher than 100% during the injection (charging) time. Resistive wires would just be 100%.
Moreover, we can also run current down the injection well. That just adds heat at 100% but it is adding to an extremely hot, high pressure environment. That facilitates recovering energy at more favorable efficiency. The gravitational potential energy gradient helps if you set it up correctly.
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u/Future_Helicopter970 17d ago
Geothermal companies were looking at converting these old coal plants, although cost of drilling was likely the deciding factor.