If we find cheap materials that freeze at lets say 40 degrees will be a god send for a solar greenhouse! When a liquid freezes it gives off a 100 times as much energy as a one degree drop in tempreture of a equal volume of the liquid! This is very confusing in the fact that freezing seems to mean cold but just think in reverse!
The temp in a greenhouse during the day is usually very warm and cold at night! If you had a substance that melted at 40 degrees it would need 100 times as much energy to melt to a liquid during the day but when it freezes as the temp drops it would give this energy back to the greenouse as it freezes! Get my point? Suggestions on liquids or waxes that will freeze between 40 to 60? jello is sugar water and freezes in the frig at 40 degrees!