Greenhouse Heating

During the usually drier and sunnier fall season, solar heat can be stockpiled. A most efficient way to move air is with what's called a "reverse curved impellor".  They can be powered by a simple photovoltaic panel so it only operates when solar gain is active.  The huge thermal mass of water can be utilized to absorb heat.  It's protected and insulated against evaporation and excess humidity by floating bubble wrap on the surface.  The water is contained with a pond liner sized to fit  the crawl space.  A solar powered air compressor can send the warm air into the water via aeration stones which also keeps the water fresh, like a large aquarium.  None of this is rocket science, just obeying the axiom of getting while the getting is good.