3 simple DIY "Sand Battery" Air Heaters! all w/non-electric "heat powered" fans! all 3 units run totally off-grid! the hot sand in the 'sand battery' heats the air AND runs the stove fans! no electricity is needed! the hot air rises off the sand and the fans blow that hot air into the living space of a room. hot sand heat storage is a new and exciting technology. sand is great for storing heat *and can store it at very high temperatures. once the sand is heated, the unit (and it's fan) can run for hours! 3 different sand batteries are shown (along with 3 different size stove fans). the on-grid options for heating the sand are stove top or oven. off-grid options are numerous and include campfires, rocket stoves coal stoves, propane stoves, alcohol stoves. (a fresnel lens, a parabolic dish or a solar oven can also be used). a fresnel lens would probably heat the sand very fast