The development and utilization of solar water heaters in China began in the 1950s and 1960s. This kind of water heater is a flat cylindrical solar water heater composed of a shell, an insulating layer, a transparent cover plate and a water collecting tank. The upper and lower sides of the water tank are flat, and the left and right sides are arc-shaped flat barrel structures. Support ribs are arranged in the water tank to improve the pressure bearing capacity of the water tank. The utility model has the advantages of simple structure, high thermal efficiency and convenient use, and is an ideal domestic solar water heater, especially suitable for the vast rural households. In the 1970s and 1980s, it developed into flat-panel and straight-tube water heaters. Their prices are relatively low, but ordinary flat-panel water heaters have low thermal efficiency and cannot be used normally in winter. At present, China has developed vacuum tube solar water heaters and heat pipe vacuum tube water heaters. They use excellent sunlight selective absorption film and high vacuum insulation technology, and have the advantages of high thermal efficiency and good stability. Various new types of vacuum heat collector products required by the market, such as heat storage type, direct current type, and curved heat absorption plate type, have also come out one after another.
But vacuum tube water heaters have some disadvantages: First, the cost is high. According to statistics, the production and sales of solar water heater products in China are increasing at a rate of 20% to 30% every year. However, due to the high cost, the current product sales areas are mainly in economically developed areas. If a more economical water heater can be introduced and used nationwide, its energy saving potential and environmental benefits will be huge. Second, the vacuum tube water heater will be frozen and cracked at a temperature of about -18 °C. In areas with higher latitudes, it does not operate normally in winter. Third, the vacuum tube water heater has serious scaling problems, and the thermal efficiency will be reduced. Fourth, the temperature rise is limited. At present, the maximum working temperature of the vacuum tube water heater is 70~80℃, which cannot meet the demand for slightly higher temperature.
Worldwide, solar water heater technology has matured. One of the current product development directions is to focus on improving the efficiency of the collector, such as applying transparent insulating material to the interlayer between the collector cover and the heat sink to reduce heat loss, which has been mass-produced in Germany and Israel; the integrated heat storage collector developed in Greece combines the collector with the hot water storage tank to reduce heat transfer losses; Turkey has developed a conical collector, etc.