For deep space misssions (such as the one currently on its way to Pluto), there is not enough sunlight available to power a spacecraft and recharge batteries. These missions already use RTGs (radio(active) thermal generators) I believe the one on its way to Pluto is a 24 Watt unit.
cool, everyone loves a good Scottish 1816 patented Stirling engine (air engine as it was known at the time)design and those MIB's http://www.youtube.com/watch?v=aZQBd2fv0wc&feature=player_embedded#!
its also rumored that the NASA american colonies will be using some of these designs at the end of this video to make far faster Rovers than what they sent out to the planets so far so its all good.
personally i prefer the many rotary engine Stirling engine design's and Quasiturbine Locomotives 50 kW with radio-isotope [AND] solar cogeneration etc combined