The raspberry pi super computer
The raspberry pi 2 comes with around 6 times the processing power of the original pi.
It is still fairly low powered, but it is interesting to compare the pi with some older computers that were considered powerful in their day.
The pi comes with an ARM 7, quad-core processor. It also has a Broadcom graphics processor that can potentially be used for compute intensive work.
It also comes with 1GB of memory.
For disk storage the nornal approach is to use a micro SD card. There are 16GB cards that give 45MB/s read/write capability.
For less than $1000 you can get 20 pi's, with fast SD cards. The result is a machine with 80 cores, 16GB of memory and 320GB of disk, with a combined bandwidth of 900MB/s read and write.
These things are low power, so a 20 pi machine would require just 100 watts of power.
It is reasonable to assume that more powerful devices will be available in he future.
In short, computing power is becoming very, very cheap.
Cray-XMP
To give some indication of how cheap compute power has become, this comparison to a Cray-XMP shows that the older Pi B+ has very similar compute power to the Cray-XMP.
When you compate the inflation adjusted price tags you see how cheap compute power has become.
The Pi 2 is roughly six times the power of the B+. Hence 20 of those gives you 120 Cray-XMP's. For $1000.
Renewable energy
One criticism often directed at renewable energy is that sun, wind and wave are not reliable. In order to reliably meet peak demand much it is necessary to have much more capacity than is needed on average.
One way to mitigate this cost is to have uses for the spare capacity.
One possible use is to have a compute farm to mop up spare capacity. This works particularly well if you have long running computations that can be used to mop up spare compute cycles.
Bitcoin mining is one potential application.