The solution to "Africa has unreliable energy" isn't to waste what little they have, it's to help them invest in more things like solar panels. Bitcoin doesn't help anything but itself.
As for the useful computations you propose that ostensibly pay as well as cryptocurrency mining... which ones? If you're referring to something like cloud computing, keep in mind that the bandwidth requirements for cryptocurrency mining are basically negligible - which is an important factor to consider even in rural America, let alone rural Africa.