The idea is to establish a macroscopic temperature difference, so the rankine cycle is still applicable. One would take all the microscopic temperature differences and pool them to create a macroscopic one. Here is a thought- can one concentrate sunlight to a temperature hotter than the sun? This would mean that energy is flowing from cold to hot. The answer is yes. We produce temperatures hotter than the sun in fusion experiments (much, much hotter than the surface of the sun). We could power these experiments with solar panels. So, we can draw energy from a colder source (the sun) and put it in a warmer source (the fusion experiment).