Keywords: temperature control configuration solar water heater measurement and control system
In recent years, with the rapid development of economy, energy and environment have begun to pose great challenges to mankind. The development and utilization of green renewable energy represented by solar energy, geothermal energy and wind energy will be an effective way to solve the energy crisis and environmental crisis. At present, solar water heater has become the most mature and widely used product of solar energy utilization in China. As one of the sub-projects of "Building Capacity of Accelerating the Commercialization of Renewable Energy in China" supported by the State Economic and Trade Commission and the Global Environment Fund of the United Nations Development Programme, the thermal performance testing system of domestic solar water heaters will make an authoritative, impartial and scientific judgment on the domestic solar water heater industry. In order to adapt to the competition in domestic and foreign markets, the test system should not only meet the new national standard "Thermal Performance Test Method of Household Solar Water Heating System" GB/T 18708-2002, but also meet the performance requirements of the international standard ISO 9459-2.
First, the process introduction
The test process is as follows: Before 8: 00 a.m., pump the water of 20 0.2℃ (the set temperature is related to the ambient temperature) into the water inlet of the water storage tank of the solar water heater, and close the inlet and outlet valves of the water storage tank of the solar water heater when the water temperature at the inlet and outlet of the water tank does not change more than 65438 0℃ within the specified time. The solar water heater stores heat under sunlight, and the temperature at this time is recorded as the initial temperature for calculating the heat gain of the solar water heater. After 8 hours, start the circulating water pump to make the temperature in the water storage tank of the solar water heater uniform. Record the temperature at this time and measure the volume of water in the hot water tank by weighing method. Calculate the heat gain of the solar water heater according to the following formula.
QS=PwCWVS(t2-t 1)
Where: QS refers to the system heat gain contained in the volume of water in hot water storage tank VS, and the unit is MJ; Pw-density of water, in kilograms per cubic meter (kg/m3); CW-specific heat capacity of water, in J/kg. ℃; VS—— liquid volume in hot water storage tank, m3; T2—— The end point of the heat collection mode is the water in the hot water storage tank, in℃; T1-the water temperature in the hot water storage tank at the beginning of the heat collection test, in℃;
The above experiments are independent all-day experiments, and the whole experiment consists of n such experiments. Finally, the performance curves of solar water heaters under different solar irradiance are given.
The test system can test the thermal performance of eight solar water heaters at the same time, of which six can be tested by water mixing method and the other two can be tested by drainage method. The above test method is mixed water method, and the principle of drainage method is omitted in this paper.
Second, the measurement and control system composition
According to the process, it is very important to control the inlet temperature of solar water heater at 20 0.2℃. Therefore, the computer measurement and control system is adopted. The measurement and control system consists of Advantech PCI, PCL series boards, ADAM4000 series data acquisition modules, communication modules and industrial computer. The display control and data recording functions of the upper computer are realized by Advantech configuration software Autoview and soft logic software Kingact. The measured parameters include temperature, solar radiation, wind speed and flow; The controlled objects include three-way valve, electric heater, frequency converter, water pump, solenoid valve, etc.
The structure of the control system is shown in Figure 2.
The operating system of the upper computer is Windows2000. By running the monitoring software, the real-time display, measurement, control and recording of field parameters can be realized, and the over-limit parameters can be alarmed and corresponding emergency measures can be taken. The industrial control configuration software has a good man-machine interface, and drivers of different hardware products can be loaded as needed, which shortens the development cycle of the project and avoids repeated and time-consuming hardware interface development work.
Due to the high precision requirement of the test system for temperature control, all temperature sensors are calibrated with Pt 100 when the whole system is running. According to the calibration data, the error after regression by least square method is less than 0.05℃. After calibration and regression, the accuracy of the flowmeter is better than the measured value 1% in a given quantitative range, and the measurement error of the one with the largest error among the eight electromagnetic flowmeters is 0.25%.
Third, the typical control link
1, temperature control of circulating water tank
The water chiller comes with a set of control system, which automatically starts and stops the air-cooled unit according to the set value of circulating water temperature, and the outlet water temperature of the water chiller fluctuates between 2℃. The primary side of the plate heat exchanger is provided with an electric three-way valve, and the opening of the electric three-way valve is adjusted according to the temperature of the circulating water tank to control the temperature of the circulating water tank within the set value of 65438 0℃.
2. Temperature control of main water inlet pipe
The water from the main water inlet pipe is heated by the primary electric heater from the circulating water tank to the water inlet of each detection unit, and the SCR power regulator of the primary electric heater is adjusted by PID to control the temperature of the main water inlet pipe within 0.5℃ of the set temperature.
3, measuring unit inlet temperature control
This test system * * * has eight test units, and the inlet water temperature of each test unit needs to be stabilized at 0.2℃ of the set temperature, and its set value is set as the set value of inlet header temperature plus 1℃, so as to reduce the influence of inlet header temperature fluctuation. The SCR controller of secondary electric heating at the inlet of each detection unit is adjusted by PID to stabilize the temperature within the required range.
As can be seen from the above introduction, water temperature control is a process of gradual convergence through four-level control. 1. The water cooler cools the normal temperature water, and automatically controls the outlet water temperature of the water cooler at16 2℃ (taking 16℃ as an example); By adjusting the electric three-way valve on the primary side of the plate heat exchanger, the water temperature of the circulating water tank is kept at181℃; The main electric heater raises the water temperature to19 0.5℃; Finally, the inlet water temperature of each water heater is increased from the secondary electric heater to 20 0.2℃.
Fourth, the test data
The data curve in Figure 4 is one of the data obtained by testing the thermal performance of four domestic solar water heaters at the same time under the conditions of solar radiation and wind speed required by the specification. Before the water heater is exposed to the sun, ensure that the water temperature fluctuation of each detection unit is within 0.2℃ and the flow fluctuation is within 50l/h. ..
Verb (abbreviation for verb) The main function of software.
1. Real-time monitoring man-machine interface
Advantech Autoview configuration software has developed a convenient, easy-to-use and friendly man-machine interface. According to different functions and processes, the screens are divided into the following categories: process screen, detection unit screen, alarm screen, report query screen, real-time trend screen, historical trend screen, PID control screen and environmental parameter screen, which can be easily switched and adjusted online in real time.
On the process screen, the relationship between each unit of the process and the running state of the equipment can be displayed, such as the start and stop of the water pump, the fault state, the frequency value set by the variable frequency governor of the water pump, the real-time temperature data of each control point, etc. On the PID control interface, the set value of control link, real-time temperature data, output percentage, PID parameters and temperature control process curve can be displayed. You can modify settings, PID parameters, etc. on this interface.
2. Alerts and events
Alarms and events mainly include variable alarm events, operation events and user login events. Through these alarms and events, users can conveniently monitor and check the alarm and operation of the system. When the parameter exceeds the limit value, the corresponding prompt will appear in the alarm screen, and when the parameter returns to the normal range, the alarm screen will prompt the alarm to resume. The alarm and event history can be displayed in real time in the upper computer, and the alarm and event can be written into the ACCESS database through ODBC, which is convenient for fault and event analysis.
3. Real-time trends and historical trends
When PID adjusts parameters, the difference between the set value and the actual value can be observed through the display of real-time trend screen, and then the proportional, integral and differential parameters can be adjusted. The historical trend screen can simultaneously query the change trend of eight parameters in a certain period of time. The historical trend screen can also freely select the parameters to view and the query time period.
Step 4 make a statement
In the report screen, the operator can select the parameters and corresponding time intervals required for the report as needed, and the query results are displayed on the screen, which can be directly printed by the printer.
5. Data recording
The historical data collected and controlled by the system are stored in the specified database path in rec format. Historical data can be viewed through report query and historical trend call. Due to the large amount of recorded data, important parameters are called SQL functions through script programming, and ODBC is set to transmit data to access the database.
6. Safety management of the system
In order to ensure the normal operation of the system, prevent the influence of misoperation on the whole system, and at the same time, consider the security of the software itself and prevent operators from modifying the source program by misoperation, different security zones are designed, and different operators are assigned different permissions to ensure the accurate transmission of control commands.
7. The combination of soft logic and configuration software
Using Kingact soft logic software to realize complex control functions. The soft logic controller is loaded into the configuration software as a control device. Set and assign the collected controlled parameters, parameters to be set and adjusted, and conditional parameters for control to the corresponding soft logic parameter registers, and then transmit them to the relevant functional modules of Kingact. Kingact is programmed with ladder diagram, then online debugging, dynamic editing and downloading are carried out, and the control function is realized with soft logic. Practice has proved that good control effect has been achieved.
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