I had an idea about solar powered pneumatics. What if you had black pvc pipes that heated in the sun? One way for air to get in to the pipes via double check valves, and the way out has double check valves going to a compressor tank or more pipes acting as storage. Make sure the storage is in the shade and the black pipes are in the sun. The sun heats the black pipes, air inside expands and can only go to the air storage tank until equilibrium is reached. At night, the pvc pipes cool and suck in air, and repeat the process the next day. It would also, somewhat repeat a cycle with a passing cloud. Also, having about 2 or 3 times more heated volume than the tank will mean the tank could be filled in one day.
I originally came up with this idea for a friend that needed a few psi air pressure to slowly push a drip irrigation system. Last night I decided to check the math and used an ideal gas law equation.
P2 = P1 (T2/T1) where P is pressure and T is temperature in kelvin.
It turns out that going from 50 F to about 120F on a sunny day (even though the outside temp may be 85 to 90F, the black pipes will easily heat up to over 100F.), would only produce about 2 psi of pressure. So, this has very limited applications. You might be able to use black metal pipes and use a giant Fresnel lens to heat it, but that would only produce about 50 to 60 psi. Not that efficient.
Probably a lot better to just use a small solar panel and a small air pump that charges a tank.