One occasionally sees, cut into the side of a hill or lying flat next to a home's foundation, a door in that ground that leads to a drab, potato-colored wonderland of vegetable storage: a root cellar.

Tucked in a grassy ridge cutting across the Small Family CSA Farm in La Farge, Wis., is a refrigerator so efficient it requires not a single watt of electricity, yet it can keep some crops chilly for ...