Intelligent controller for solar lighting systems

(Department of Electrical Engineering and Applied Electronics, Tsinghua University, Beijing 100084, China) Solar cells, batteries, lighting fixtures and controllers: 2002-) 7-18. The operation includes charging, power supply lighting, and waiting for 3 working states. : Wu Libo (1980-), male (Han), Hubei, Ph.D.

â—ŽIn the existing solar lighting system, there is widespread efficiency. This communication contact Z1 person 18 Zhao Zhengming, 1 professor S Bo E-mail: you short, unstable operation, etc., proposed I A new type of intelligent controller for solar lighting systems. The controller realizes the working state control based on the single-chip microcomputer 89C51 and the intelligent management of the battery energy, which satisfies the requirements of stable operation and accurate switching of the solar lighting system under different working conditions. Solar cell maximum power point tracking and battery charging precise control are also implemented in this controller. The test results show that the solar lighting system using this intelligent controller has high efficiency and stable operation, and the battery life can be extended from 2a to more than 3a.

Discharge 615: As a new green energy source, A solar energy is rapidly being promoted and applied because of its advantages of never exhausting, no pollution, and no geographical resources. The discharge circuit primarily includes a high efficiency DC-) C converter that provides a matching voltage for the lighting load. In order to cooperate with light-efficient lighting fixtures, the converter uses a push-pull DC boost circuit to increase the output voltage. The energy management circuit based on single chip microcomputer determines the working state of the system by detecting the output of the solar cell and the battery power, and realizes the automatic control and intelligent management of the whole system.

1 Solar lighting system The structural block diagram of the solar lighting system using the proposed intelligent controller is as shown. The system consists of a photovoltaic array, a battery, an energy-saving lamp for lighting, and an intelligent controller. The part of the dotted line is the structural block diagram of the new intelligent controller, which consists of a charging circuit, a discharging circuit (a DC-DC converter with a push-pull structure) and a control circuit. The characteristics and cooperation of the various components of the system are described below.

Solar cells: Solar cells are the input to a solar lighting system that provides the entire system with the power needed to illuminate and control it. Under daylight conditions, the solar cell converts the received light energy into electrical energy, and charges the battery through the charging circuit; after dark, the solar cell stops working and the output opens.

Battery: As the energy storage link of the solar lighting system, the daytime battery converts the energy output from the solar cell into chemical energy and stores it back to the lighting load at night. The power of the intelligent controller is always supplied by the battery throughout the day.

Lighting fixtures: Lighting fixtures generally use energy-saving lamps with long life and high luminous efficiency to improve efficiency and system stability.

The selection of the capacity of each part of the system requires a combination of cost, efficiency and reliability. With the rapid development of the photovoltaic industry, the price of solar cells is gradually decreasing, but it is still the most expensive part of the entire system. Its capacity affects the cost of the entire system.

In comparison, the battery is relatively inexpensive, so you can choose a battery with a relatively large capacity to make the best use of the power generated by the solar cell. In addition, when working with lighting loads, consideration should be given to the situation of continuous cloudy days, leaving a certain margin for system capacity.

2 system operation state analysis system operation includes charging, power supply and waiting for three working states.

Each operating cycle of the system consists of these three operating states. The controller selects the appropriate working state according to the environment and system conditions, and performs battery energy management to optimize the system performance.

2.1 State of charge During the day, the solar panel charges the battery via the charging circuit.

The charging circuit selects different charging modes according to the state of the battery to perform phased charging. The charging circuit is connected to the solar cell, and the output is connected to the positive and negative terminals of the battery. Because the solar panel has current source characteristics at the low output voltage stage, it can be used as a power source regardless of the overcurrent problem of the load. In the actual circuit design, a diode is added to the main circuit to prevent reverse discharge of the battery to the solar cell.

2.2 After the power supply state is dark, the controller detects that the solar cell has no output, and automatically turns on the discharge circuit after the set time. This part of the circuit is equivalent to a smart switch, which is a push-pull amplifier circuit using pulse width modulation (PWM) control.

2.3 Waiting state On the premise that the solar panel output is zero, if the lighting setting time is up, or the battery power is insufficient, the power supply lighting is stopped, and the system enters a waiting state. The battery only supplies power to the controller. The reason is based on the control circuit of the single-chip 89C51. Its control signal is the solar cell output voltage. Under daylight conditions, the control circuit detects that the solar cell has a normal output, then turns on the charging circuit, turns off the discharge circuit, and the system operates in a charged state. After dark, the solar cell output voltage drops below 1V. At this time, the control circuit turns off the charging circuit, turns on the discharge circuit, and the system supplies illumination. The state control circuit avoids malfunction by state interlocking to ensure the stability of the lighting system. The control circuit has a timing function.

The energy management module prevents overcharging and deep discharge of the battery by monitoring the working state of the system and counting the working mode of the battery power selection system while meeting the lighting requirements as much as possible. The lighting method includes three modes: uninterrupted, timed and energy-saving. The mode of use is selected according to the battery power at the end of charging. According to different lighting loads, the power supply output mode can also be flexibly selected. When necessary, the inverter module can be added to supply power to the AC load.

3 battery charging strategy When the solar lighting system is working in the state of charge, on the one hand, it is hoped to maintain the maximum output power of the solar cell, and on the other hand, it must take into account the current receiving capability of the battery with different power. Therefore, depending on the state of the battery, the charging circuit takes two different strategies for charging control.

3.1 Solar maximum power point tracking Fast charge stage The battery's current acceptance capability is greater than the output capacity of the solar cell after the charging circuit. Therefore, it is possible to consider only how to achieve the maximum power output of the solar cell. The controller implements a first-order tracking of the maximum power point of the solar cell.

It is the output characteristic curve of a solar cell under different light intensities at a certain temperature. As can be seen from the power voltage curve shown, there is one maximum power output point for each curve, and this maximum power point is unique under current lighting conditions. The first-order MPPT used in the practical application system utilizes the characteristic that the maximum power point such as /dv is zero. First, the output voltage and current of the solar cell are continuously sampled, and a set of voltage and current data of each sample is multiplied and converted into a power value, and then the power value obtained by the previous sampling is subtracted, that is, the power difference value. When the power reaches the maximum value, it satisfies the state control and energy management fast charge phase after the end of the state. Because the output capacity of the solar cell* is controlled by the different state of the system and the battery energy ship has exceeded the acceptance capacity of the battery, the controller stops. The solar power is approximately considered to reach the maximum power point, thus forming a first-order difference algorithm for maximum power point tracking.

The MPPT control of the pool is converted to constant voltage control (CVT).

3.2 Battery Charging Method The use of the battery is, in the final analysis, how to use the charging and discharging characteristics of the battery. Effective and scientific use of batteries is critical to improving battery efficiency and extending battery life.

For a battery, choosing the right charging method can not only extend the battery life, but also improve the charging efficiency. This requires accurate determination of the state of charge of the battery to select the operating state of the charging circuit. The charging circuit used by the controller adopts a charging method of fast charging, overcharging, and floating charging: fast charging phase: in the fast charging phase, the output of the charging circuit is equivalent to the current source. The output current of the current source is determined according to the state of charge of the battery, and is the maximum acceptable current of the battery / M1AX. During the charging process, the circuit detects the voltage of the battery terminal. When the battery terminal voltage rises to the switching threshold, the charging circuit goes to the overcharge phase.

Overcharge phase: In the overcharge phase, the charging circuit provides a higher voltage Voc to the battery while detecting the charging current. When the charging current drops below the switching threshold/OCT, the battery is considered to be fully charged and the charging circuit is turned to the floating phase.

Float stage: In the floating charge stage, the circuit provides a precise floating charge voltage with temperature compensation function. 3.3 Floating charge voltage compensation After the battery is fully charged, the best way to maintain the charge is to add a constant voltage to On the battery. This puts forward a requirement for providing a suitable float voltage for the charging circuit. The float voltage value should be large enough to compensate the self-discharge current of the battery; it should not be too large, so as to avoid decomposition of chemical components inside the battery due to overcharge. In the proper floating state, the fully enclosed maintenance-free lead-acid battery can work stably 610a. Even if the float voltage is only 5% deviation, the battery life will be halved.

It must be considered that the voltage characteristics of the lead-acid battery have a significant negative temperature coefficient, and the 2V battery is about -4.0mV/*C. That is, a charger that can work normally at 25*C, at 0C It is not possible to provide and maintain sufficient power; in contrast, this charger can cause severe overcharge at 50C. Reasonable consideration of the temperature variation range, the charger should give some form of compensation according to the temperature coefficient of the battery. Actually take the formula (2) to determine the float voltage Vf, where Vf and T are the voltage and temperature values ​​of the reference point, respectively, c is the voltage temperature coefficient. In the controller, the charging circuit composed of the charging control chip UC3906 effectively satisfies the above. Claim. It simultaneously detects the charging voltage, charging current and battery temperature. According to the battery status, it can provide three kinds of charging states, including overcurrent in the charging state, overcharge protection, and temperature compensation in the floating state. The life of the battery can be maximized.

4 power supply circuit implementation method Discharge (power supply) circuit uses a push-pull structure switching power supply design without power frequency transformer. The power frequency-free transformer switching power supply utilizes a small-sized high-frequency transformer to replace the bulky power frequency transformer to realize voltage conversion and isolation, and has the advantages of small size, light weight, high efficiency and the like. The pulse width modulator is the core of this kind of switching voltage. It can generate the driving signal with fixed frequency and adjustable pulse width, control the on/off state of the switching power tube, and adjust the level of the output voltage to achieve the purpose of voltage regulation. The schematic diagram is as shown.

Where: q is the duty cycle, n is the transformer winding ratio, n=Ni*N2. The push-pull amplifier circuit implemented in the controller has a switching frequency of 50 kHz, an input DC voltage of 12 V, and an output DC voltage of 300 V. The measured data of the test device indicates that The power supply circuit adopting this design has high efficiency, stable output voltage, and an average efficiency of over 90%, thereby effectively improving the efficiency of the entire system.

5 Commissioning results and test waveforms The solar lighting system of the structure shown was built and tested. The system consists of: 12A DC fully enclosed maintenance-free lead-acid battery at 60A, 2W 46W solar panels, 12W dual U-shaped energy-saving lamps and lighting controllers.

Table 1 is the test data of the battery terminal voltage and charging current in the charging state fast charging phase. The initial battery capacity is 30% of the capacity. Table 1 Fast charging phase Battery voltage - ammeter time Battery terminal voltage / V charging current / A Note: Due to sunshine The intensity changes rapidly, and the corresponding charging current also has a certain fluctuation. The charging current at each time point is the average value within 10 minutes from the time point.

After the fast charge phase is over, the charge is transferred to the overcharge and float state, and the charging current is rapidly reduced.

Under the power supply state, the battery output current is about 1A, which varies with the battery terminal voltage. The power MOSFET in the push-pull circuit is verified by the transformer turns ratio and the input-output voltage of the push-pull circuit. The system operates with high efficiency in both charging and power supply states.

6 Conclusions This paper introduces a new intelligent controller for solar lighting systems. The controller consists of three parts: charging, discharging and energy management. The charging circuit realizes the charging control of the battery and the first-order maximum power point tracking of the solar cell; the power supply circuit adopts the push-pull structure of the power-free transformer switching power supply design, and the output is connected to the lighting load; finally, the energy management circuit based on the single chip 89C51 passes the detection. The external environment state and battery energy, select the system working state, and realize the automatic and stable operation of the whole system. The test and operation results show that the solar lighting system using this intelligent controller has the advantages of high efficiency and good stability, and can be automatically operated in the maintenance-free state for a long time, and has broad application prospects.

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