As stated before, there are six water tanks (depth 2.8cm,length 240cm, width 100cm) painted black on the solar room floor which provide buoyancy after sunset. Especially in summer, hot water in the tanks should store heat to maintain buoyancy until sunrise. It can also be used for shower and dish washing.
The total amount of water in the tanks is 310 liters. The tanks are aligned with the sun as much as possible, and the highest end is adjusted to be lower than the water level of the header tank outside. From there the water flows to a stainless iron cylindrical pipe at the focus of a large two-dimensional reflector in the room. The black-painted pipe has the dimensions:length 4.8m, diameter 18cm and thickness 1mm. It is covered with a sheet of transparent plastic with an air gap of about 4cm. The opening of the reflector is about 1.5m high and 4.8m long. At the end of the pipe, the water flows to the hot water cylinder beneath the solar room floor or directly to the shower. If the water is colder than that in the tanks, it is returned by natural circulation to the water inlet of the first tank. The temperature change of the water at the cylindrical pipe on a winter's day is shown in the next figure.
The temperature is not quite warm enough for a shower. A few improvements, such as painting the floor black, making use of top part of the solar room where a warm layer of air is 10 degrees higher than on the floor, and peeling off the protecting film on the reflector surface to get a higher reflection coefficient, will raise the water temperature a few degrees. And as long as the reflector can track the sun, it is not difficult to obtain warm water.