Solar Energy can be used only in close proximity to Sun. Also, it requires the huge solar batteries that will be unable to provide enough electricity for great space platform. Nuclrear reactors are very heavy for space units and emitt the dangerous gamma-Radiation. Deadlock? No! There is a very-very huge energy in physical vacuum directly. This vacuum energy participates in numerous processes, but it puts on a mask of kinetic Energy. That is why we cannot use the Energy in our engines.
Physical vacuum is no a void. P-vacuum is very complex structure that acts in the strong correspondence with laws of classical mechanics, but it is perceived by uns as a void.
The density of vacuum energy (the quality of energy in 1 cubic meters) is calculated as E/V = c4 / (8pi G r2) where c - light velocity, pi = 3.14, G - gravitational constant, r - minimally possible radius of black hole. As radius r, we could use elektron-radius (then E/V = 2.45x1072 J/m3) or Planck length (E/V = 1.2x10112 J/m3). There are different ways for transformation of the energy to electricity and heat (vibrations, resonance, vacuum flows, gravitational fields and others). I prefer to work with vacuum flows: we form the electromagnetic field of corresponding space-time configuration and such field compels the vacuum to move as air or water. So, we create the vacuum flow that can rotate the rotor of electro-generator. Of course, the part of produced electricity goes to restoration of electromagnetic field.
Recently, I have published the book "Физика и магия вакуума. Древнее знание прошлых цивилизаций" ("Physics and magic of vacuum. Ancient knowledge of old civilizations"), where I describe these problems in detail. Language is Russian.