guest blogger john hare
One of the big attractions many advocates see in space development is energy. SPS is the green unlimited power of the future, Lunar helium 3 will power fusion here on the ground, and Lunar platinum is the basis for the fuel cell technology of the future. Technically, any or all of these could be true. Practically, none of them will make it to market in the time frame that terrestrial alternatives could if market conditions drive them. This is one alternative that could help derail long term plans for energy from space.
Terrestrial solar and wind have a problem with energy storage. The sun doesn’t shine at night and the wind is not reliable, not to mention weather and seasonal effects on both. Means of storing energy for night, inclement weather, and low wind tend to be more expensive than the power generation system itself unless a really good reservoir is available in exactly the right place for pumped storage. Not all locations have the possibility of a large convenient reservoir for power storage and almost all the other storage options are expensive and less efficient.
As a thought experiment, why not have a deep underground reservoir for power storage instead of artificial lakes or water towers? Mines and wells have been operated far deeper than the tallest buildings for years. The difference is that the reservoir would be emptied during excess power generation and filled through hydraulic turbo generators when more power is required by the grid. By having the underground reservoir 2 kilometers or more deep, it would have the 40 times the head pressure of a dam with 50 meters of head pressure. This means that each ton of water through the turbines would generate 40 times as much power through the deep storage as the dam storage turbines.
 
With the underground reservoir being relatively small and man made, it can be placed near the ocean for an unlimited supply of water on the high end. The limit is the volume underground. The volume underground being 1/40 of the volume required for a low height surface reservoir, has far less disruptive effects than a man made lake for the same purpose. It can be emptied by putting a light vacuum on top of the exit shaft. the water in the reservoir boils at lower pressure and rises as steam or water vapor, depending on the viewpoint and operating temperatures. The condenser at the top of the shaft collects the distilled seawater for municipal consumption. This becomes a power storage system that has clean water as a by product.
Efficient storage makes locally renewable sources viable in many cases. The by product of large quantities of fresh water would be a major selling point. Even transportation could be vastly improved if market conditions favor it.
In some locations, geothermal effects will provide the power to distill the water and return it to the surface. When the steam has energy left when it reaches the surface, an ammonia heat exchanger can extract the heat for more power production.
While this is just a thought experiment, I mean it more as a warning to people that base their plans on a many decades out return. Space system have to find returns on investment that are relatively soon, and not vulnerable to terrestrial alternatives that can beat them to market in the way that Iridium was beat by cellular and fiber optic alternatives. Tourism in space cannot be reproduced on the ground, which makes it unique.
Space business plans must be based on fairly rapid response to market opportunities. An excessive focus on one possible market many decades out is very likely to be a bad investment. I did this post after reading another SPS proposal in another forum.
johnhare
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I am still waiting for extremely low cost solar power to be demonstrated in space for the powering of satellites, space stations and what not. I am not sure what the current cost of electricity produced in space is, but I would expect it to still be on the order of at least tens times that on Earth.
Once low cost electricity in space is demonstrated, then we will have all the more reason to go there and use it – at the source where it is cheapest. An obvious first use would be much higher power satellites.
I’m afraid the kinetic energy needed to get the vapor out of the hole will rob you of most of the energy you think you’d get out of the pressure head down your hole. You’d also fill up your underground chamber with evaporated salt in short order.
If you drilled down from the sea and up to somewhere on land that was below sea level, it might work. The coast of California to death valley, for instance. Or from the Mediterranean sea to the lowlands of the Sahara dessert.
This idea really seems to work better when viewed as a way of hydrating desert lowlands rather than a power generation scheme.
This is an energy storage proposal for Earth based solar, wind, and other intermitent power sources. Due to efficiency losses, it will take more power to get the vapor out of the hole than you get from the water going in. The geothermal assist will help in some locations, won’t apply in some, and might be the primary power source in others.
The salt is a resource, along with other minerals evaporated out.
An article by Zweibel et al. in the Dec. 2007 Scientific American discusses the use of depleted natural-gas fields to store energy. During the day, excess solar-generated electricity is used to pump compressed air into the fields. At night, the air is released through turbines. Because it uses natural reservoirs, this scheme might be much cheaper than one requiring the construction of underground water storage spaces.
Any place that has natural features that support energy storage will use them. It seems probable that Japan doesn’t have too many natural reservours available.
A natural gas field is not somewhere I would want to pump oxygen. The combination is likely to catch fire or explode.
This could be a winner with geothermal sources hot enough to boil water, but not hot enough to produce much steam pressure. To maximize this you’d want the lower cave to have large area as opposed to volume. As for salt deposits, pull some of the water out in liquid form with the salt. Without geothermal assist pulling out the water as vapor would be a killer loss, I expect even for energy storage.
For the large surface area per volume in the lower cavern, a lateral drilling rig might be useful if it would operate underwater as well. One rig could continously expand storage capabilities. I think you are right on needing the geothermal assist, unless you have a location with no other available storage means that are economical.
It’s not exactly what you’ve proposed, but coal mine in Germany is going to be used for energy storage:
https://arstechnica.com/science/2017/03/german-coal-mine-may-be-prime-for-pumped-storage/
@jsuros “I’m afraid the kinetic energy needed to get the vapor out of the hole will rob you of most of the energy you think you’d get out of the pressure head down your hole.”
Steam at 100C and 1 atmosphere pressure has a density of 0.59 kg/m^3. If I did the math right, a column of such steam 2km high with 1 atmosphere pressure at the top would have about 1.12 atmospheres pressure at the bottom. This means the water would need to be raised to 103C to boil and provide positive pressure to the vapor column. Not a big increase in requirements for the geothermal resource.
And if collecting the water vapor winds up more expensive than just pumping the sea water back into the ocean then do that instead.