The earliest and most widespread application of solar energy was only for heating water, and today there are millions of solar water heating installations worldwide. The main components of a solar water heating system include a collector, a storage device and a circulation line. In addition, there may be auxiliary energy devices (such as electric heaters) for use when there is no sunlight, and there may also be forced circulation of water, devices to control the water level or control the electric part or temperature, and piping to the load. According to the circulation method solar water heating system can be divided into two types.
1、Natural circulation type:
This type of storage tank is placed above the collector. The water is heated by solar radiation in the collector, and the temperature rises, resulting in a difference in density between the collector and the storage tank, thus causing buoyancy, which is a thermal siphoning phenomenon, prompting the natural flow of water in the tank and collector. By the relationship with the density difference, the water flow is proportional to the amount of solar energy absorbed by the collector. This type does not need to circulate water, maintenance is very simple, so it has been widely used.
2、Forced circulation type:
The hot water system uses water to circulate between the collector and the storage tank. When the water temperature at the top of the collector is higher than the water temperature at the bottom of the storage tank a number of degrees, the control device will start the water to make the water flow. The water inlet is equipped with a check valve to prevent the backflow of water from the collector at night, causing heat loss. The flow rate of this type of hot water system can be known (because the flow rate from the water can be known), easy to predict the performance, can also be projected in a number of time to heat the water. As in the same design conditions, it has the advantage of higher water temperature than the natural circulation method, but because it must use water, so there are problems such as water power, maintenance (such as leaks) and control devices moving and stopping, easy to damage the water. Therefore, in addition to large hot water systems or the need for higher water temperature, only select the forced circulation type, most of the general use of natural circulation type water heaters.
