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.