The difference between a solar water heater and an electric water heater or the largest gas water heater is that the energy it uses is clean and free. As long as the sun rises, the solar vacuum tube will automatically collect heat, but if you don't use it in time, the heat will accumulate and the temperature will rise. If the temperature rises above 100 degrees Celsius, the non-pressured solar water heater will “open the pot” and generate water vapor to dissipate heat; the water pressure of the pressure-bearing system will gradually increase with the increase of temperature, and the conventional pressure-bearing system generally passes. Drainage pressure relief and cooling. However, the system does not have water for various reasons. The temperature of the vacuum tube solar water heater will rise to an unacceptable state of the system and damage the system, especially the pressure-separated system. The system may also be controlled by sensors, sensors, and circulating pumps. If the fault fails, the heat can not be transferred into the water tank in time, and the temperature of the collector part can rise to a level that the system cannot bear in a short time. The following is a description of several solar collectors due to the high temperature of the vacuum tube:
Method 1: The system is not under pressure
The opening system, which is the structure of most of our traditional vacuum tube solar energy, when the solar energy is too much, that is, the solar energy exceeds 100 ° C, the water in the water tank is "opened", and the steam is discharged.
But the problems with this system are:
A. Reduce the comfort of the system. Non-pressured systems will cause users to have water temperature fluctuations caused by uneven water pressure;
B. Reduce the scope of application of the water heater: the water heater can only be installed at a higher place;
C. The problem of “overheating” has not been completely solved. When the water heater is out of water, the system will still have overheating problems.
Option 2: Pressure system using T/P valve
T/P valve, ie temperature/pressure safety valve, the function of the valve is that when the temperature exceeds or the pressure exceeds the set value, the valve opens. In the solar water heating system, the general domain temperature is 99 °C, the pressure is 0.6. MPa.
At present, most pressurized solar water heating systems are installed on the water tank as the main means of preventing "overheating" of the vacuum tube solar water heating system.
The T/P valve is a feasible means to achieve the low cost of the vacuum tube solar water heating system to prevent "overheating", but this feasibility has two prerequisites: First, the solar water heating system cannot be shut off, obviously this It is difficult to be absolutely reliable, and water shortage in the system is possible (the house is vacant for a long time to disconnect the waterway); second, the solar water heating system requires that the heat of the collector can be reliably transferred to the water tank.
At present, the pressure-type vacuum tube solar water heating system mainly has two types, one is a natural circulation type, and the other is a forced circulation type. The natural circulation system can transfer the heat of the collector to the water tank relatively reliably, but the installation and application of the system are very limited, and generally can only be installed on a roof or a balcony. The forced circulation of the vacuum tube solar water heating system is difficult to ensure that the heat of the collector is reliably transferred to the water tank. The faults of the sensor, the controller, the circulation pump and the like may cause the heat of the collector to be transferred to the water tank.
Option 3: Shading measures
When the system may overheat, go to artificial sunshade or passive automatic sunshade. The sunshade is to cover the sunlight with a covering on the surface of the collector to prevent the vacuum tube from continuing to absorb heat, so as to prevent the solar water heating system from "overheating", which is artificially shaded. When it is not used for a long time, artificial measures are taken to collect solar energy. The sun block on the device, this way brings insecurity and trouble to consumers.
Another passive automatic sunshade, when the temperature of the solar water heating system is too high, the sunshade mechanism automatically runs to cover the sunlight, the reliability of this method is not high, and the failure of the sunshade mechanism cannot be guaranteed.
Method 4: Improve solar vacuum glass tube to have high-efficiency heat absorption performance in low temperature zone, and poor heat absorption performance in high temperature zone
This method has certain difficulties in technical implementation, and has not been successful so far, mainly to reduce the performance of the high temperature zone, and the performance of the high temperature zone is subsequently reduced.
The researchers tried to apply a discolorable coating on the inner surface of the vacuum tube. At low temperatures, the film has good endothermic properties. At high temperatures, the film becomes lighter in color and the endothermic performance deteriorates. Unfortunately, the life of this type of coating is not guaranteed to last long.
Another method researchers try to add a substance in the double glass layer to have a very low vapor pressure at low temperatures, and a higher vapor pressure at higher temperatures, ie double glass in the high temperature region. The degree of vacuum in the tube is decreased, thereby increasing the heat dissipation effect of the vacuum tube in the high temperature region, but the difficulty of this method is that the first substance is relatively difficult to find, and when the second vacuum tube is in operation, the inner surface temperature is high and the outer tube temperature is low. After the substance evaporates, it will condense on the outer surface, causing the vacuum tube to fail.
Solve the road of "overheating" in the future
Think about the problem from another angle. We all know that there is a phenomenon: inland areas, especially in desert areas, the temperature difference between day and night is very great, just like Xinjiang in China, “wearing the skin in the morning and wearing the yarn around the stove”; while in the coastal areas, the temperature difference between day and night is relatively Smaller. The reason for this phenomenon is that the "heat capacity" of the surface is different. The "heat capacity" of the surface of the desert is small, the ability to store solar heat is poor, there is the sun, the temperature rises quickly, there is no sun, and the temperature drops quickly. The temperature difference between day and night is large; while in the sea area, the “heat capacity” of the surface is relatively large, the temperature of the rise and fall is relatively slow, and the temperature difference between day and night is relatively small. Therefore, according to the characteristics of the region, a solar water heater suitable for all places to prevent solar overheating should be developed, so that more heat can be stored, so that the clean products can be truly "once and for all" in the life cycle.