First you need to have an energy source to heat the water.
Next, water or steam will NOT combust. Steam is just water vapor.
Next, creating a vacuum will require an energy source.
So far, in your "solution" you need an outside energy source in 2 places, and you have produced steam.
You'd be better of running the water between 2 charged plates. One positive and one negative. This will split the water molecules into O2 and H2 components. It has been done on nuclear submarines since at least the 1950s.
Technically, if you heat it to plasma like temperatures you WILL unbond it and combust the hydrogen.. But just combusting the hydrogen is not gonna result in a gain in energy..
Got a better plan.. Take sketchy wind and solar OFF the grid.. Use it separate hydrogen from water and store it.. It's that "free energy" the greenies can't get out of wind/solar on the grid... Because they are NOT alternatives to RELIABLE generators.. THey are merely substitutes.
Using OFF grid wind/solar to produce hydrogen and ethanol and OTHER fuels is a no brainer.. There'd be lots of investments and interest... And the COST of those alternate fuels would go WAY down...
If you combust anything, you will gain power from it. As in the form of pressure. Pressure can be utilized. Also, when you combust anything, there is something else you will get from it. Heat. And when you are also talking about utilizing MHD, energy can also be produced from that.
Another thing is that I said earlier that 2% of water will combust at 3600 F. What if the steam you were injecting into the furnace was already at something like 3000F. Then getting it up to the proper combustion temperature wouldn't take all that much added heat.
You also bring up photovoltaic solar panels. Your anti alternative energy cult is one I can easily smash. Are you still paying attention? Good. I brought up earlier that even though they use the sun, solar panels are the closest thing we have to perpetual motion. in so much that that they create far more energy than was needed to create them. A lack of sunlight isn't a problem either. Because there is always (during the day) sunlight somewhere. And we transmit energy today, don't we. Another thing is that you use enough solar panels to create three times the energy you need. That way, with the use of batteries, you can always produce enough energy.
If you are thinking of banning me, I will tell you something else about using water to create energy. So that if you do ban me, hopefully this will make you feel bad for doing so. First of all, I called a car stereo place once and asked them how loud could you make a car stereo without having to alter your car's engine. Such as the alternator. The stereo person told me that it was basically unlimited. That 40,000 watts was achievable. Which means you could produce a LOT of sound from very little power.
Now all matter has a sonic resonant frequency that will disrupt it. That would have to include the H2O molecule. What if you shot such a frequency down a long tube filled with water or steam. You would then separate the molecules into hydrogen and oxygen. (At the very least it could make the amount of energy needed for electrolysis much less) This mixture might directly be used to cause ignition. I don't remember what the hydrogen and oxygen mixture used in the space shuttle was. But for proper ignition, the ratio of hydrogen and oxygen would need to be the same.
Getting back to electrolysis to separate the hydrogen and oxygen atoms. As that the atoms would already be freed from each other from sonic disruption, it wouldn't take all that much electrolysis to get the gasses to move in the direction you wanted them to go for collection. Not only that, you would likely be using DC. Which means a polarity. Strong neodymium magnets could be used to supplement this polarity. Maybe it would make any electrolysis unnecessary.