Steam Heating on Ceres

06 Jul 2018 at 19:37 // under Chemical Engineering

Yesterday I watched the latest video from the Isaac Arthur Science and Futurism youtube channel on Colonizing Ceres.

I have been enjoying these videos recently which offer hard sci-fi without any (or very minimal) plot. They look at what would be possible in the future assuming known science and only expected improvements in technology.

There was one section that had me doubting what was being suggested:

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So they use fission and solar, and have big pipes running around the crater carrying steam off from the reactors and running back through when it condenses back to water. This helps warm the domes.

Sure, in the past we may have used steam as a heating device for habitable areas (whether a house, office or a factory) but I don't think that would be the most sensible thing to do today (or whenever we colonize ceres).

My prefered alternative would be to pass ALL the generated steam through a total condensing steam turbine to generate as much electricity as possible from the steam¹. Then use the cooling water that was used to condense the steam as a hot water energy source and pump that around the crater to the various habitats. Once this water has done its job and heated the habitats, it can be reused in the condenser.

So why do I think this is a better option?

Safety: Stored energy

Steam is a compressed vapour. It wants to escape. If you get a leak, it will expand into the space around the leak point. Outside this is probably not too much of a concern (though you wouldn't want to be in the line of fire), but in a confined space this could be deadly. The escaping vapour would push out the oxygen from the environment leaving any victims only hot steam to breath.

Hot water on the other hand will just spray out, without any expansion.

Therefore you would want to minimise the number of areas exposed to a steam hazard (a boiler house and turbine room) rather than the whole site.

Safety: 2 Phase flow

When you transport steam vapour around in a pipework system, you will always get some condensation on the edges of the pipes due to heat loss. This water condensate must be removed from the vapour system or you risk steam hammer. This is when the condensate gets cooled, then disturbed and contacts with the steam. The cold water rapidly condenses more steam and the volume of that steam collapses to a tiny fraction of the original volume. This causes a low pressure region and accelerates the water into this region, which then needs to get stopped by some other pipework.

This can lead to damage and leaks from the system.

Two phase flows are also more abrasive and can erode the pipework faster. When the system is fully filled with water, there is no 2 phase flow problem to deal with.

There is a phenomenon known as water hammer however it is much easier to mitigate (slowly closing valves).

So again, you would want to minimise the amount of steam piping.

Heat Losses

This is simple enough. Heat losses are proportional to the temperature difference between the substance and the surroundings. The closer these two are together, the less of a driving force for the energy to escape.

If you pump water around at 80°C, it will lose less energy than pressurized steam at 150°C.

Another factor would be the size of the pipes. Water is much more dense than steam and so generally you use smaller pipes to transport it. This would be cheaper to construct and also would provide less surface area for steam to escape.

Final thoughts

So those were the things that sprung to mind when I saw the video. I look forward to having this discussion with the chief engineer designing the base one day.

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1. Though how you condense steam in ultra low gravity is an interesting problem itself. ↩

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