Could steam be used as a efficient power source for a space craft?

Question

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%)

Answer 1

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.

Answer 2

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).

Answer 3

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.

Answer 4

Only if it's the 1940s, early 1950s, or earlier, and neither effective photovoltaics nor space nuclear reactors have been invented yet.

The question includes some mistaken premises.

First, modern solar cells get better efficiency than that. Commercial solar cells may have efficiencies in the mid to high 20s, and spaceflight obviously uses the best and most efficient flight-tested photovoltaics available.

Second, "efficiency" is usually not the most relevant consideration. For power in space, what is important is often output power per weight, or lifetime energy per weight (if considering things like fuel cells, gas turbines, or nuclear reactors where the amount of fuel is finite). Other extremely important things in space applications include simplicity/reliability (moving parts are typically to be avoided), and ability to be miniaturized (not that great for steam engines).

Third, efficient steam engines do not work at the boiling point of water. Instead, water in boilers is pressurized so that it boils only at a much higher pressure, and steam is superheated to 500 deg C or more.

Forth, steam engines are heat engines, which are fundamentally based on their ability to release heat, at a low temperature. This is easy to do on the Earth, where you can have air-cooled or water-cooled condensers. For a spacecraft, it can only be radiated, and radiating large amounts of power requires large, high temperature radiators. This makes heat engines in general prolematic in space.

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In the 1940s and 1950s, speculative design for spacecraft and space stations often included steam engines that used mercury metal rather than water, but otherwise worked much as you describe. Parabolic mirrors would focus sunlight onto a mercury boiler, and mercury vapor would pass through a turbine and then be condensed into the liquid in condenser-radiators. Mercury was favored because the condensation temperature would be much higher than with water, and therefore radiation would work vastly better.

As the 1950s wore on, it became much more apparent that nuclear reactors would be a far simpler and more practical option, and it was often assumed that all spacecraft would use nuclear power. Meanwhile, it became apparent that space exploration would be done with small capsules and tiny automated satellites, rather than the huge atomic rockets of Heinleinian science-fiction lore and Von-Braunian theorizing. For these applications, solar panels provided a vastly lighter weight (no big heavy turbines and pipes and valves) and more reliable (no moving parts is always a huge benefit) option. For nuclear power, power conversion is likely to use the all-gas Stirling or Brayton cycles rather than steam, or to use no-moving-parts options like thermionic or thermoelectric power.