When mom couldn't recall the tank, Gram told her it was not a big round 50+ gallon barrel, it was a tank that measured just about 12" deep and almost four' by six' across it's face. You hardly noticed it against the green shingles of the roof as it weathered to a verde gris color. She thought it was just some bit of salvage he rescued from the early days of the railroads in Florida but, she wasn't sure. What she did know for sure was the water was very warm and never very hot. It was perfect for bathing but needed heat to make coffee, even her favorite instant, and to rinse the dishes, but with a supply of very warm water, it took less time to come to a boil for any reason.
A tank that both gathers the heat, stores the water and delivers the water through gravity is probably the most Passive Solar water heating system. However, a Passive system just means basically, there is no circulating pump for the water.
Any tank that holds water in close proximity to a heat source is a 'system'. I go to a powwow with the family that is held in a huge hay field after the hay is cut. The promoters provide a solar shower for the campers. It is just two large copper tanks (drum shaped 100 gallon tanks). The tanks were perched on top of what used to be an old horse shed. A garden hose is connected to the spigot. It fills one tank that is connected to another with a copper pipe. When tank one is full, it over flows into the other tank. When the two tanks are full, they turn off the hose, there is NO high end plumbing fill valves or cut off float valves used here!
The tanks are exposed to the sun, the wooden shed frame is roofed with tin. The pipes that deliver the hot water to the shower are copper and are attached to the under side of the roof. The cold water is delivered through a system of garden hoses and zip ties! It is crazy! There are two shower heads that allow the water to overlap as it rains down on you. You adjust the cold and hot, then get in and good luck! The best time to shower is mid afternoon, the water is boiling by then and the tourists have had their cool morning showers!
This coil of hot fluid replaces the gas heated or electric coil that heats your water now.
I have included two graphics here from energy.gov. Just to show the next steps in a simplified form.