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How to Generate low pressure air 0.8 psi for pallet gardens? Stirlinge engines, fluidyne? How?

My pallet garden project is a big success. It works great and it is becoming simpler and cheaper to do over time. It uses aquarium bubble pumps to produce low pressure air and this air is used to airlift pump water that circulates to the plants in the garden  in an endless loop.   I measured a few days ago and my bubble pump is producing 22 inches water pressure (0.8 psi) at the pump and 60 litres per hour of air at this pressure and this drives all the pallet gardens at a great distance (100 ft ) from the pump. I use a 3.5 watt air pump. I would just  love it if it could be off grid, and driven by solar heat in a low tech way.    Anyone want to try to do this?  Because then it is of great value in poor countries.   Just to give you an idea of how little heat you need to make this pressure.  If you heat air from 20 degrees C to 36 degrees C  (68 F to 97 F)  in a closed container, you get 0.8 psi pressure increase in the container!  That is the bar, and it would be really hard to set it lower than that!    I do not yet understand stirling engines or fluidyne pumps but I think they could do this.  If I try it, I will have cylinders of air alternatively heating and cooling  in the sun and valved to pump the air in one direction.  But if you are clever, I am sure you can do better than that with fluidyne or flutterwing wind power. I would love to see some comments and interest in this.   It IS working and people are going to be very interested as gardening season gets into full swing.    Playlists are http://www.youtube.com/playlist?list=PL00C41C26C91A76BB&feature=view_all for the pallet gardening and http://www.youtube.com/playlist?list=PLkzXlmAwZTZcHUq4YX8__3qLEJIIzTduW&feature=view_all for some amazing alternative wind, water and solar machines. Thanks Brian