Desiccants: zeolites, a LiCl solution, and so on. Dry them in sunny or
windy times, and let them absorb water vapor for heating or cooling later,
possibly a lot later, eg dry all summer for backup winter heat storage.
Drying LiCl requires a higher temp (150-200 F) than solar house or water
heating. We might use some solar concentration or a wind "diversion load"
heater. The concentrated liquid can stay in an uninsulated sealed container
for a long time with no loss of energy. LiCl can absorb about 10X its
weight in water vapor, storing about 10,000 Btu/lb, vs a pound of water
cooling from 200 to 100 F and releasing 100 Btu of heat.
Desiccant systems have been used for air-conditioning German trains, for
camping and sailboat fridges, and for air-conditioning houses. One paper
described a Canadian house with seasonal heat storage in a 1 m zeolite cube.
How much heat can we store in a 4'x8'x8' tall LiCl shelfbox?
"Unglazed collector/regenerator performance for a solar assisted open cycle
absorption cooling system" by M. N. A. Hawlader, K. S. Novak, and B. D. Wood
of the Center for Energy System Research, College of Engineering and Applied
Sciences, Arizona State University, Tempe, AZ 85287-5806 USA, in Solar Energy,
Vol. 50, pp 59-73, 1993 describes: "An ordinary black shingled roof... used
as a collector/regenerator for the evaporation of water to obtain a strong
solution of [lithium chloride] absorbent... Experimental results [using a
36' x 36' roof] show a regeneration efficiency varying between 38 and 67%.
Cooling capacities ranged from 31 to 72 kW (8.8 to 20 tons)", ie about 1 ton
per 100 square feet of roof area...