We need more steam engines. Ah, but we can’t burn our fuel, so let’s dig for it. The heat of the Earth itself would be more than enough.
And let’s not use the steam in the old fasioned way. Let’s use it in an old fasioned new way : It’s called a Sterling Engine.
And oh how we’ve neglected it. There are so many ways to power this wonderful device, only our imagination draws its boundaries.
The function is so simple, you could probably build one with an empty can… Oh, low and behold someone already has!
So we can build “farms” of these things, we can dig holes and use the difference in temprature of the Earth and sky (it doesn’t matter which side is colder or hotter, Sterlings can be designed to function with either side as “hot”) and all it will need is some difference in temprature between the sides.
What of Outer Space?
If I had a penny for each time I heard “the difference in temperature in outer space can be hundreds of degrees”, I would have had a jar full of pennies.
There’s your solution for long term energy supplies in space. Would you want an array of delicate solar panels exposed to the danger of space suppling power, or an array of hardy heat-exchangers exposed to space?
There’s also our own little trouble with fuel.
Well that’s my own gross over-simplification for the day… Now go forth and build!