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Could water (steam) be used as an effective power source for a manned spaceship headed to Mars?

The basic idea is simple... Heat the water on the sunny side of the spacecraft, allow the steam to turn a turbine to produce power (to run computers/electrical equipment). Cool the water on the dark side of the spacecraft and rinse/repeat.

Most spaceship designs/proposals utilize a form of solar power. Is there a reason steam power gets overlooked? (Is it feasible? Is the weight/complexity greater then a solar panel system?)

Given the importance of water, there likely is already enough water on the spacecraft to run a steam powered generator. The water is just sitting there, inert... It seems more effective to use the water as a renewable power resource too. Is there something obvious that I am overlooking?

The ISS, while in the sun, can receive temperatures of 250F+ -- enough to boil water.ISS temperature differences

Water can be used as a form of radiation shielding. (AKA: Assume the ship already has water on all sides of the ship) Water-Powered Spaceship

"Low" temperature steam power generators, on earth, have a similar amount of power efficiency (12%-20%) as solar panels have (11%-15%)

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    $\begingroup$ You aren't doing a fair efficiency comparison - getting 20% efficiency on a steam engine with a high burning temperature caused by combustion isn't the same as getting that much efficiency from solar power (which is what the solar panels do) $\endgroup$ – user2813274 Mar 31 '15 at 1:17
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    $\begingroup$ Solar panels have no moving parts and don't leak. $\endgroup$ – Organic Marble Mar 31 '15 at 1:42
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    $\begingroup$ Also, vacuum is a pretty good insulator, so the "datk side" doesn't cool all that effectively. (Most Earthbound steam plants use conduction (running a cooling loop into a river or lake), evaporation, or forced-air cooling. The ISS in fact has fairly large radiators to get rid of excess heat: en.wikipedia.org/wiki/External_Active_Thermal_Control_System $\endgroup$ – jamesqf Mar 31 '15 at 5:49
  • $\begingroup$ I think you could also use a Carnot cycle for an upper limit of the efficiency of the turbine, which is defined by the temperature difference. I am not sure though what the temperature range would be. $\endgroup$ – fibonatic Mar 31 '15 at 13:49
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    $\begingroup$ @fibonatic The answer is "not good". The most difficult thing on a space station to do is achieve sufficient cooling. Which is weird, because space is cold, right? Yes, space is cold, but it also happens to be a FRIGGING massive vacuum flask, so its pretty well insulated. This means the cold reservoir is not actually going to be very cold. This makes the carnot efficiency pretty pants. $\endgroup$ – Aron Mar 31 '15 at 15:03
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Very simple answer: You would need some kind of heating panels through which to let the water flow while it gets heated. Solar panels are thin slices of silicon. You will not be able to build the water heating panels lighter than the solar panels, and even if you somehow did, you would still have to carry a steam turbine.

As a commenter already pointed out, in your efficiency comparison you neglect the absorption efficiency of the panels themselves. Also, the value of 20% you quoted was for a high pressure turbine. If the panels have to withstand such a high pressure, that will make them even heavier.

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Very simple answer. You are looking at the wrong metric for efficiency.

The most expensive thing on a space station is not area, you don't pay per foot of "land" (and its associated sunlight) in space. The most expensive thing is mass, and getting it up there. A steam engine is heavy per unit power compared to a solar panel (especially when compared to the multi-junctioned solar panels that have ~30% efficiency that they use in space).

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  • $\begingroup$ There is the fact that any cool/heat exchanger is already (often) present on sattelites. It's often a requirement that measurement equipment needs to be in a constant temperature. The more advanced satellites might run active coolant systems. So with a heat exchanger already in place: the additional mass might be low. $\endgroup$ – paul23 Jun 8 '18 at 1:36
  • $\begingroup$ @paul23 actually. Simple thermodynamics would tell you that a steam engine would require increasing the heat exchanger capacity by a few multiples. Assuming most systems on the satellite use electrical power, ideal Carnot cycle would mean a huge increase of waste heat. $\endgroup$ – Aron Oct 12 '18 at 23:41
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A 1971 Nasa pdf says that silicon solar cell arrays [meaning photovoltaic] are established as the most reliable and economical generator of sustained power in space.

A key factor in reliability is that the cells themselves have no moving parts. However, the array must be mechanically deployed, and would typically be mounted on gimbals to face the sun. Failure in either of these systems are not uncommon.

Of course, a lot has changed since then - photovoltaic solar cells have become much more efficient. Two layer cells are widely available at 30% under one-sun illumination (meaning without concentrators).

A 2013 pdf gives comparable figures.

In summary, the answer is no. Photovoltaic solar cells are more efficient and more reliable for medium term (up to 10 year) uses.

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  • $\begingroup$ This is a really good answer, concise, well-sourced, and logical. $\endgroup$ – uhoh Oct 6 '18 at 3:56

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