One of the challenges in using other heat sources than a small propane flame is the restrictions imposed by where you must place the heat and pipe being heated. the pipe being heated musxt be located at a specific place relative to the large tank (resevoir) and the fluid oevel inside the tank. Absorption fridges are extremely sensitive to being tilted because it upsets where the fluid level is located inside.
Solar heating works because the sunlight can be focussed with mirrors. Heat from other sources require piping.
