The solar water heater is composed of vacuum heat collecting pipe, heat preservation water tank, bracket, connecting pipe, etc.
The working principle of solar water heater can be divided into three steps:
1. Heat absorption process
The solar radiation passes through the glass cover plate, and is absorbed by the collector plate, and then transmitted to the water in the heat pipe along the rib and the pipe wall. After the water in the heat pipe is heated, the temperature increases, the specific gravity decreases and rises, forming an upward power and a thermosyphon system. With the continuous upward movement of hot water and storage in the upper part of the water storage tank, at the same time, the lower temperature water is continuously replenished through the lower circulation pipe. In this way, the whole tank of water will eventually rise to a certain temperature.
Most of the existing flat plate collectors use a good combination of multiple tubes, such as rolling or calendering, in which the thermal resistance between the drain tube and the heat absorbing plate can be almost ignored. The main factors that affect the performance of plate core of plate collector are structure design and surface absorption coating. The fin efficiency of a well designed collector should be over 93%. The fin efficiency of the collector is related to the core structure, surface treatment and the overall structure of the collector. The influence of the overall structure of the collector can be described by the total heat transfer coefficient, and its influence degree is the same as its own geometry size (fin thickness, material). That is to say, under the same efficiency, the plate core can be thinner when the heat loss of the collector is small. Selective absorption surface can improve the heat collection efficiency, but in order to improve the economic benefits, the fins of such products are often thin. When it is used in water heater, the actual heat collection effect of this kind of product is not so different from that of the collector with less selectivity (or even no selectivity), but with thicker fins.
The domestic solar water heater usually works in a natural circulation way without external power. A well-designed system can circulate well as long as it has a temperature difference of more than 5 ~ 6 ℃. The rationality of the diameter and distribution of the water circulation pipe directly affects the heat exchange efficiency of the collector. In most cases, the flow pattern in the pipeline of natural circulation domestic water heater system can be regarded as laminar flow. The resistance of the pipeline system in the collector mainly comes from the resistance along the path, and the influence of the local resistance is much smaller, and the resistance along the path of the branch pipe is much larger than that of the main pipe. When the water temperature increases, because of the decrease of kinematic viscosity, the resistance along the path becomes smaller, and the influence of local resistance becomes larger. In a certain range, when the main pipe diameter is constant, increasing the branch pipe diameter will not only reduce the resistance along the way, but also reduce the local resistance. Generally, the hydraulic radius of branch pipe shall be more than 10 mm. When the main pipe diameter reaches a certain value, increasing the main pipe diameter is not significant to reduce the system resistance.
3.Top water use process
The way of using water for domestic solar water heater can be divided into the down water type and the top water type.
The use mode of falling water is not affected by the tap water supply. Its disadvantage is that the water temperature is low first and then high in the use process. If not mastered well, it is easy to cause the embarrassment of sudden water shortage. The top water mode is that the water temperature is high first and then low, which is easy to master, and the user is easy to adapt, but the tap water is required to maintain the water supply capacity. When the tap water is guaranteed, the top water is recommended. When the domestic solar water heater is designed as the top water mode, the internal structure of the water tank must be designed reasonably to ensure the uniform water output and avoid the formation of "short circuit" or dead angle of the water path. The best way to use the pipeline is to convert it into a falling water connection, which should be urgently used when the tap water pressure is insufficient or the water supply is stopped.