this working model of solar heater , e.g. passive solar heating batterymade of beer cans (0.5L) . the construction is very simple 64 beer cans , and 6-8 peaces of wood, of course couple pieces of glass . it will heat your room during day hours even at cold days, temperatures on output can get to 50-C. (all this for zero money to the grid :) feel free to post updates or any questions