Taiyounetsu-onsuiki (太陽熱温水器) – a passive solar water heater. In its simplest form, a passive solar water heater is little more than a water container that doubles as a heat sink. By having the heat sink portion of the container located toward the bottom and the storage portion at the top, the propensity for heat to convect upward will ensure the coldest water component always replaces the heated water in the heat sink.
The climate range in which these water heaters are practical however, are limited. Hot water is generally in demand in colder climates and seasons, while these are the conditions in which the sun will have less effect. Even if the days are warm, if the temperature drops at night, the water will quickly turn cold as the heat will leave through the same uninsulated heat sink it came in from.