A simple rotary heat engine that uses a small temperature differential to produce hundreds of foot pounds of mechanical torque for direct application or for electrical generation,can be built to operate from a hot spring or simple solar collector as heat source.
Utilizing only simple engineering and thermodynamics, to define a working fluid which readily changes state from liquid to gas and back with minimal thermal energy needed (Such as PROPANE as it transitions from liquid to gas at 75 Degrees F. and 125 psi. and back also.)
We build a “Wheel” with canisters around the perimeter having a surface texturing both inside and outside to enhance heat transfer, perhaps fins, or another, to increase surface areas both outside and inside of the canisters. The canisters are connected together in pairs diametrically opposite. By a piece of tubing that routes from near the inside of the outermost surface of the first canister thru the canister and the innermost surface of the same canister to the outside then straight across barely missing the central “Hub” of this wheel to the opposite canister penetrating the innermost surface, passing thru the inner space and ending near the inside of the outermost surface.
The wheel must be built with suitable structure of struts and spokes as good engineering form should dictate to mount to a suitable sized shafting and supported by bearings and structure as necessary. The wheel will rotate when the canisters at the bottom, filled with fluid are heated with hot water or hot air flow. Some of the working fluid vaporizes and the increase in pressure pushes the liquid in the same canister up the pipe to the canister near the top where canisters are being cooled by water spray and /or ambient air, the vapor of the working fluid in the top canisters condenses to liquid resulting in a partial vacuum assisting the pressure from the bottom canister transferring the liquid to the top canister. With most of the heavy liquid at the top, gravity pulls down rotating the wheel. As it rotates the working fluid movement repeats so the wheel continues to turn and drives the load with suitable linkages, belts, roller chain, or gears.
This may be scaled larger or smaller as needed, a great way to run a pump in a rural area with a lack of wind, pumps all day and fills a reservoir for continuous use. Or spin a generator for electricity, for solar or hot springs, does not need steam like a turbine so mild temperature sources may be utilized.
Also because temperature differential is the only requirement a source of “Cold” such as an “Ice cold” Spring can also be used.
The working fluid is never consumed, a few drops of lubricant on the wheel bearings is its only maintenance. The “Low tech” aspect assures long life and makes it a good choice in less developed areas