This is a cool space idea. 

Assume, that in a somewhat close future, we can build more spaceships than we are capable today. 

Build many ships to slingshot around, say, Jupiter. They would speed up and be sent back to Earth.

On Earth, there would be a long tunnel-like space station. The barges would go in, and through electromagnetic fields, magnets and coils, transfer the kinetic energy to the space station. 

The station would relay the energy to Earth through some wireless means, undiscovered at present. Maybe it would just power the station itself.