Don't they already have some 30% efficient solar cells using gallium arsenide or something like that? I think they do exist somewhere - but they're only experimental, and extremely expensive.
I'm looking forward to when they find a way of making solar cells CHEAP. Even just the standard 10% efficient things would be great if they were really cheap. Look retail - a 55W panel runs about $300! It takes a hell of an investment to cover a rooftop with solar panels.
Battery technology also needs a boost - lead acid batteries have good rated capacities, but you can't actually USE the full capacity, or else you damage the things. Lithium ion is expensive, difficult to charge properly, and it can't endure as many charge cycles as most other rechargable cell types.
Maybe Nickel-Hydrogen would be nice. Of course, I've only seen them mentioned in articles about the Hubble - but I bring them up, because the things in Hubble have been working since 1990, taking multiple charge/discharge cycles every day.
According to Space.com, as of May 12th, 2004, the Hubble has gone through 75,000 orbits - one orbit every 95 minutes. "They provided 600 ampere-hours when the mission began in 1990. That's down to 300 now." I wonder if any other batteries can manage that?
