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Solar thermal protector - temperature control switch specific application

Article Source:thermal protectorAuthor:Yaxun electronicPopularity:Published time:2019-11-23 08:25

In the summer when solar energy is better, the solar system will cause the system temperature to be too high and the system pressure to increase if the water consumption continues to be small or not. Under such conditions, the solar fluid (heat transfer medium) has poor stability and has a certain influence on the life of the solar collector itself. Solar thermal protectors will play a huge role.

Solar thermal protector

I. Design and selection of solar anti-heat protection device
In order to avoid this phenomenon and to ensure the normal operation of the system, an overheat protector is installed in the system. When the temperature in the three water tanks exceeds 80 ° C and the temperature of the solar collector exceeds 90 ° C, the normally open solenoid valve DCF 5 is closed, and the solar thermal protector is connected in series in the heat collecting system for heat dissipation; When one of the three water tanks has a water temperature lower than 80 ° C or the temperature in the heat collecting system is lower than 90 ° C, the solenoid valve is closed. The most common and simplest of these is the thermal protection of the KSD301 temperature control switch, which is small in size, simple in action and stable in quality. It can be combined with an overheat protection of the system introduced below for you to lock up again for safety. If you need to prevent the solar radiant panel from freezing, you can use this type of switch as an antifreeze temperature control switch.

Calculation of solar overheat protector
Statistics of solar radiation intensity data

Solar radiation intensity (w/㎡) is the basic data collected by the solar radiation meter. The data provided by the National Weather Service is the amount of solar radiation (MJ/㎡). There is a lack of basic data on solar radiation intensity in Beijing, as solar energy resources in Beijing and Shandong are basically the same. To this end, the company's self-test data in Texas as the basis for calculation. According to the above statistics, the solar radiation intensity is 1000w/m2, the system heat collection area is 178.4㎡, and the radiation intensity on the lighting surface is 178.4KW. The heat collection efficiency is 0.5, and the heat collected by the solar collector system is 89.7 KW. Solar water heating system overheat protector, borrowing an air conditioner outdoor unit. The TCL air conditioner outdoor unit was initially selected, the model number was DZR-280W/A, the heat dissipation was 84KW, the operating condition was liquid temperature 38~42°C, and the outdoor air temperature was 35°C. When the solar collector system is overheated, the operating conditions are that the liquid temperature in the tube is 90 ° C, and the outdoor air temperature is about 35 ° C. This type of heat sink is used to ensure overheat protection of the system.

II. Principle of solar water heating system, calculation of heat collecting area

1, solar collector schematic
Operating principle:
Collector temperature probe T1
Three water tank temperature probes T2-1, T2-2, T2-3
(1) Temperature difference cycle: The three water tanks are cycled in sequence. The temperature difference cycle between the water storage tank C and the heat collector is first, and when the difference between the temperature T1-1 of the heat collector and the temperature T2-3 of the water storage tank C is greater than 10 degrees, That is, (T1-1-T2-3 ≥ 10 ° C) the circulation pump P1 is started. When the difference between the two temperatures is less than 5 degrees (T1-1-T2-3 ≤ 5 ° C), the circulation pump stops circulating; When the temperature in the water storage tank C reaches 50 ° C, the system automatically switches to the heat collecting cycle of the water tank B water tank, and sequentially circulates.

(2) Water supply: The frequency conversion water supply is adopted, and the water storage tank C is used as the water supply water tank, and the water temperature in the water storage tank C is maintained at not lower than 45 ° C, and the water volume in the water tank is used as needed.

(3) Auxiliary energy (electric boiler B): When the water temperature in the water tank C is lower than 40 ° C, the boiler is started, stopped at 45 ° C, and the temperature of the water in the water tank C is maintained. The system monitors the temperature in tank C before 4:00 am. If it is lower than 35 ° C, the water temperature in the water tank C is heated to 40 ° C by the low-valley electricity to meet the water requirement in the morning.

(4) Overheat protection: When the temperature of the internal medium of the collector is higher than 90 ° C, the heat sink starts to work to reduce the pipe temperature to 80 ° C. The temperature difference circulating pump P and Pb are used one by one, and two circulating pumps are used alternately to avoid the pump working time is too long and the service life is reduced.

(5) System design will heat the water to 60 °C every week to prevent the growth of Legionella.

(6) In order to prevent high temperature scaling of water in the water tank, the system is designed to install a purified water device in the cold water.

(7) Pipeline cycle: When the pipe temperature is lower than 30 ° C, the solenoid valve DCF3 is opened, and the circulation pump P4 is started to perform the pipe circulation by the pipe pressure change. When the pipe temperature reaches 35 ° C, the solenoid valve closes and the pipe cycle is stopped.


2, the basic conditions:
1), basic water temperature: 15 ° C
2) Solar radiation data
According to the National Meteorological Radiation Data Yearbook (2001) provided by the National Meteorological Center. Monthly average daily and annual total radiation data (unit: MJ/m2) in Beijing (district station number: 54511; east longitude: 116°28'; north latitude: 39o48'; observation point altitude: 31.3m):

The annual average daily radiation on the horizontal plane is: 14.46MJ/㎡, considering the angle between the lighting area of the collector and the horizontal plane is 37°, the slope coefficient is 1.05, and the daily average radiation amount on the lighting surface is 14.46×1.05=15.183MJ/㎡.

3, the total area of the system collector
1) Total area of direct system collector:
2) Total area of indirect system collector:
Calculate the total area of the collector of the indirect system is 178.2㎡, the lighting area of a single collector is 2.23m2, then 79.9 units need to be installed, and the total heat collection area of 80 sets of U-tube collectors is: 178.4㎡. Rainy weather and winter are provided by auxiliary energy electric boilers.

III. Automatic liquid filling device for solar collector system
The heat collecting system adopts closed indirect heat exchange mode, and the normal operating pressure of the system is 0.2-0.3Mpa (roof collector system). In order to meet the normal pressure of the system and ensure the normal operation of the system, a voltage regulator is installed in the heat collecting system.
The automatic liquid injection is composed of a solar liquid storage tank, an infusion pump, a pressure controller, a pressure tank and the like.

Operation mode: When the pressure in the solar heat collecting pipeline is less than 0.15 MPa (the liquid injection equipment is on the roof), the liquid injection pump is started to perform liquid injection, and the pressure is increased. When the pressure in the pipeline is greater than or equal to 0.25 MPa (the injection device is on the roof), the injection pump stops. When filling for the first time, start the infusion pump and open the valve F1 at the same time, and close the valve F2. When the system pressure reaches the set value, first close the valve F1, then open the valve F2, and finally stop the injection pump.

IV. Selection of auxiliary electric heating equipment
Calculate hourly heat consumption based on water appliances in the building

V. Plate heat exchanger selection
In the past engineering design, the calculation of the plate heat exchanger was calculated by hand. There are two ways to do this:


Simple algorithm: Assuming the theoretical heat transfer coefficient, the heat exchange area is determined, the manufacturer and the heat exchanger model are selected, and the flow rate between the plates is calculated. The actual heat transfer coefficient and flow resistance were found by the heat transfer characteristic curve and flow resistance characteristic curve provided by the manufacturer's sample. After repeated verification, the results of the process conditions are met, and the heat exchanger model and heat exchange area are finally determined. The advantage of this algorithm is that the calculation is simple, the steps are few, and the time is short; The downside is that the results are not accurate. The reason for the inaccuracy of the results is mainly because the heat transfer characteristic curve and the flow resistance characteristic curve provided by the sample are curves under certain working conditions, and the design conditions may not match.

Standard algorithm:

The selected manufacturer determines the heat exchanger model according to the angular flow velocity, and finds various physical parameters of the cold and heat medium under the design conditions from the manual. According to the heat transfer empirical formula and the flow resistance empirical formula provided by the manufacturer's sample, the thermal calculation is performed to determine the heat transfer coefficient and flow resistance. After repeated verification, the results of the process conditions are met, and the heat exchanger model and heat exchange area are finally determined. The advantage of this algorithm is that the calculation results are accurate; The disadvantage is that the calculation is complicated, the steps are many, and the time is long.

Using the computer to design and calculate the plate heat exchanger, the computer operation speed is fully utilized. One calculation can be completed in a few seconds on the microcomputer, and the accuracy of the result is difficult to achieve by hand calculation. Another main feature is that the program stores various physical parameters of the water at different water temperatures required for calculation and all parameters of the plate heat exchanger setting device. Designers can calculate the results immediately by inputting process conditions (such as water volume, water temperature, flow resistance, etc.) on the computer, which provides great convenience for designers. The calculation personnel can also input different process conditions (such as water volume, water temperature, flow resistance, etc.) to obtain different calculation results, or replace the heat exchanger model to obtain different calculation results. Through the comparison and optimization of the results, the plate heat exchangers with reasonable economic performance and reliable performance were finally selected.

The heat exchange capacity is 95.2KW (slightly greater than the calculated heat dissipation power)
Heat medium inlet and outlet temperature: 60 ° C, the outlet temperature is 50 ° C. The medium is a 50% aqueous solution of ethylene glycol
Heating hot water inlet and outlet temperature: 40°C, outlet temperature 45°C, treated tap water