I read that fuel cells in space shuttles use hydrogen and oxygen to create electricity, but there aren't any on the ISS. I was wondering exactly why the ISS doesn't use them.
Solar panels supply all the power needs of ISS. The space shuttle used fuel cells because they take up much less space than solar panels. Size isn't all that important with ISS.
Actually, shuttles used apu's, burning hydrazine as a fuel. On the Apollo missions, there were 3 fuel cells - converting hydrogen oxygen into water - generating electricity in the process. For short-term missions - a couple of weeks - this is a good solution. The ISS uses solar panels (I think they have about an acre and half) for generating electricity - they require no fuel, very little servicing, and last years. As long as the sunlight angle is correct, their output is very steady as well, reducing the strain on voltage regulators.
Because they'd have to keep hauling hydrogen and oxygen up there to refuel and/or haul up even more heavy equipment to split the water that results from fuel cells back into hydrogen and oxygen. This would be very wasteful and expensive. The expense of launching something into space is based largely on its mass. The mass of launching all that hydrogen and oxygen up there over and over and over would quickly become VERY expensive. It worked on the shuttle because it was a back and forth craft. The station isn't, it stays there. So it was more efficient to haul up all those solar cells once and be done with it. Whereas hauling all those solar cells back and forth on the shuttle every time would have been wasteful.
It would require constant refueling, and hauling mass into orbit is very expensive. Solar panels only need to be installed once.
