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The dull academic answer is: Anything requiring high power density and/or anything that still needs to operate at nighttime.
For example, it is a near-certainty we will never have a practical solar-powered car simply because the amount of surface area of solar panels required to generate the power to move even the most basic lightweight car is many times the surface area of a car. The best we could hope for is a battery-powered electric car where the battery is charged by electricity generated from some mix of renewable sources.
My first answer has to do with something that belongs in Adult Mug.
My second answer would be anything to do with flight. That could be a disaster. A solar powered airplane. I know this is pretend and that cannot happen anyway.
What is with that typewriter? ???
A flashlight
Good one, LOL.
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It's interesting that modern design reactors the coolant flow relies on normal heat convection ... they are designed such that you have to put energy into them to keep the process going ... it's a safety design ... you lose electrical input and the thing stops .. I don't know if any of these designs have been built yet... that was in an article I read ... and yes I apologise for going off topic
With current known technology ... ever hear of the solar races in Australia every year?
There is a solar powered plane :)
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A radio report that I just heard was the inspiration for this Q.
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Umbrellas
@ozgirl -- Yes, I am aware of solar vehicle racing.
However, there are two problems: one engineering, one physical.
The engineering problem is that solar-powered racers resemble road cars in about the same way F1 cars resemble road cars. Yes, the sort of look the same and do the same things, but they are VASTLY different in cost, safety, and utility. No one is going to commute to work in a fragile bundle of balsa wood and plastic film which can't move if it's cloudy or if the single passenger weighs more than 130 lbs (oh, and this balsa-plastic wonder machine costs $500,000).
The more fundamental problem is simply energy density. Cars have a useful surface area of about 15 square meters. Solar energy density is on the order of 1000 watts/square meter. Solar photovoltaic cells are about 50% efficient in converting sunlight to electricity. That means a car covered will solar panels will generate AT MOST about 7500 watts...or about 10 horsepower. That's simply not enough to motivate any realistic modern car.
