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Detailed analysis of solar lighting principles
- May 25, 2018 -

With the increasing global energy shortage, solar photovoltaic lighting has developed rapidly. In the development of solar lighting systems, people constantly analyze the control modes commonly used in lighting systems and design various practical working modes. At the same time, the light source technology is constantly being updated, and the charging mode of the battery is also under continuous research. In exploration, the effective utilization rate is getting higher and higher. In the development and coordination of solar energy components, the solar lighting system is continuously improving. Let's take a look at the related content with eeworld Xiaobian.


1 The principle and composition of solar lamps


The solar lighting system is a DC type independent photovoltaic system. Solar modules convert solar energy into electrical energy, which is controlled and protected by the controller. The electrical energy is converted into chemical energy and stored in the battery. When electricity is used, the battery then converts the chemical energy into electrical energy for use by the DC load or is reversed by the inverter to AC power for AC load use. The device stops working only when there is no light for a long time so that the power in the battery runs out.


Solar street lights consist of the following components: solar panels, solar controllers, battery packs, light sources, lamp posts, and lamp housings, and some also have inverters.


1) Solar panels


Solar panels are the core part of solar street lights, and they are also the most valuable part of solar street lights. Its role is to convert the sun's radiant energy into electrical energy, or send it to a storage battery. Solar cells are mainly made of monocrystalline silicon and polycrystalline silicon. The photoelectric conversion efficiency of the monocrystalline silicon is 13% to 15%, and the polysilicon is 11% to 13%. The latest technology also includes photovoltaic thin-film batteries.


2) Solar Controller


The most important part of the solar lighting system is the controller. Its performance directly affects the system lifetime, especially the battery life. The controller uses the industrial-grade MCU as the main controller. Through the measurement of the ambient temperature, the voltage and current parameters of the battery and the solar cell module are detected and judged, and the MOSFET device is controlled to be turned on and off to achieve various control and protection functions.


3) Battery


Since the input energy of the solar photovoltaic power generation system is extremely unstable, it is generally necessary to configure a battery system to work. Lead-acid batteries, Ni-Cd batteries, and Ni-H batteries are generally used. The choice of battery capacity generally follows the following principles: First, the energy of daylight solar modules should be stored as much as possible under the premise of meeting the requirements of night lighting. At the same time, it must be able to store the electrical energy needed to meet the lighting needs of continuous rainy days. The battery capacity is too small to meet the needs of night lighting, and the capacity is too large. On the one hand, the battery is always in a lossy state, which affects the battery life and causes waste. The storage battery should be matched with the solar battery and the electricity load (street lamp). There is a simple way to determine the relationship between them. The solar cell power must be more than 4 times higher than the load power to allow the system to work properly. The voltage of the solar battery must exceed 20% to 30% of the working voltage of the battery to ensure that the battery is normally negatively charged. The battery capacity must be 6 times higher than the daily load.


4) Light source


Which kind of light source is used for solar street light is an important indicator of whether the solar light fixture can be used normally. Generally, solar energy light fixtures use low-voltage energy-saving lamps, low-pressure sodium lamps, electrodeless lamps, and LED light sources.


LED lamp light source has long life, up to 1,000,000 hours, low working voltage, no need for inverter, high luminous efficiency, domestic 50lm/W, imported 80lm/W. As technology advances, the performance of LEDs will increase further.


5) The height of the light poles and lamp housing poles shall be determined according to the width of the road, the spacing of the lamps, and the illumination standard of the road. Lamp housing According to our collection of many foreign sun lamp data, between the beautiful and energy-saving, most choose to save energy, lamps and lanterns appearance requirements are not high, relatively practical on the line.


2 Solar Street Lighting Control System


1) System Structure


The solar street lamp microcomputer monitoring system is composed of microcomputer main control circuit, solar panel, battery charger and discharger, battery pack, LED light source driver and LED lamp. The system structure is shown in Figure 1:


2) Function control


(1) Basic requirements for solar street light controllers


The solar street lamp is made up of a series of LED lights. The street lamp lighting system not only consumes a large amount of electric energy, but also needs to invest a huge amount of daily maintenance expenses, which brings the dual pressure of electricity supply and financial expenditure to the city. The development of "on-demand lighting" power supply strategies can ease this conflict. Through programming, it is possible to realize the flexible and flexible control of street lamps distributed in the bustling sections of the city, and the switching control can be realized through PWM in any time period so as to achieve the purpose of both saving electricity and enhancing the city lighting atmosphere. The basic control requirements are as follows:


1 The brightness of the first and second half of the night is controlled, and the control ratio depends on the situation; 2 The single-side street light strategy is activated, that is, the existing battery power is only for one street light and the other street light is turned off; 3 The midnight light strategy, that is, the middle of the night Turn on the lights and turn off the lights in the middle of the night. The existing battery power is only used for the night lighting.


Solar street lights are controlled by natural light to control the lighting switch. The optimized design of these light-controlled solar lighting systems is the prerequisite for the long-term reliable operation of the system. The system capacity can be optimized according to the local geographical location, weather conditions and load conditions. However, due to seasonal factors, winter solar radiation is less than in summer. Solar arrays produce less electricity in the winter than in summer. However, the amount of electricity needed for lighting in winter is more than that in summer. As a result, the amount of electricity generated by the lighting system contrasts with the demand. Balance monthly power generation surplus and power consumption losses. In order to improve the utilization of the power generated by the lighting system and overcome the shortcomings caused by the lack of power in the system, in the development of solar lighting systems, people constantly analyze the control modes commonly used in lighting systems and design various practical working modes. The light source technology is also being continuously updated, and the charging mode of the battery is also being increasingly used for effective research and exploration. Therefore, in the development and coordination of various solar energy components, the solar lighting system is constantly improving. .


According to the characteristics of the solar street lighting system, the street lamp operation must take into account the influence of the remaining battery capacity. When the street lamp is normally turned on, the current battery capacity is obtained according to the battery residual capacity detection method. After the query, the battery supply time to be maintained by the battery is obtained, and the existing battery power is used on average. At the same time, the street lamp lighting mode is flexibly controlled according to the battery power that can be used that night. Use the battery's existing power wisely.


(2) Battery charge and discharge control function


Battery charge and discharge control is an important function of the entire system, it affects the operating efficiency of the entire solar street light system, and can also prevent overcharging and overdischarging of the battery pack. Overcharging or overdischarging of the battery has a serious effect on its performance and life. Charge and discharge control functions, according to the control mode can be divided into switch control (including single and multiple switch control) type and pulse width modulation (PWM) control (including maximum power tracking control) type. This system adopts the pulse width modulation controller way, and choose MOS transistor as the switch device. In the case of sunny days during the day, according to the remaining capacity of the battery, select the appropriate duty cycle to charge the battery, and strive to efficiently charge; at night according to the remaining capacity of the battery and future weather conditions, adjust the LED lamp by adjusting the duty cycle Brightness to ensure balanced use of batteries.


In addition, the system also has the protection function of the battery overcharge, that is, when the charging voltage is higher than the protection voltage (15V), automatically reduce the battery charging voltage; then when the voltage falls to the maintenance voltage (13.2V), the battery into the floating state When it is lower than 13.2V, the float charge closes and enters the equal charge state.


When the battery voltage is lower than the protection voltage (11V), the controller automatically turns off the load switch to protect the battery from damage. By using the PWM mode of charging, the solar cell panel can not only maximize its efficiency, but also increase the charging efficiency of the system. This design has corresponding protection measures for the reverse connection, overcharge, and overdischarge of the battery.


(3) Solar street lamp operation mode control function


High-brightness and high-current LED lamps have been widely used because of the same brightness, and have a power saving of about 90% compared to incandescent lamps. Nowadays, there has been a trend to gradually replace conventional lamps.


The solar street lamp is made up of a plurality of LED lamps connected in series, and the brightness is adjustable by the PWM method, that is, the current flowing through the LED is changed through the EN terminal, so as to adjust the brightness of the LED lamp. The current intensity can be from several mA to 1 amp, and finally the LED lamp Achieve the desired brightness.


The PWM signal can be generated by the microcontroller, or it can be generated by other pulse signals. The PWM signal can change the current through the LED lamp from 0 to the rated current, and the LED lamp can be changed from dark to normal brightness. The smaller the PWM duty cycle (longer for a long time), the higher the brightness. Using PWM to control the brightness of the LED is very convenient and flexible. It is the most commonly used dimming method. The PWM frequency can range from tens of Hz to several thousand Hz.


PWM dimming is achieved by controlling MOSFET transistors.


Because the voltage used by the street lamp unit of this system is generated by several batteries in series, when selecting the MOSFET transistor, the MOSFET's withstand voltage must be considered first. The system requires the MOSFET's withstand voltage to be higher than 40V. Secondly, according to the driving LED lamp current For the size, select the maximum current of the MOSFET's IDS. In the case of DC power supply, the first consideration is IDS maximum current value and RDS value. Under normal circumstances, the maximum current of the MOSFET IDS should be more than 5 times the driving current of the LED lamp; in addition, the internal resistance of the MOSFET must be selected; the larger the LED driving current, the smaller the RDS, and the smaller the RDS, the more the conversion efficiency. high.


Urban solar street light is a public facility closely related to people's lives. It reflects the prosperity and development level of the city to some extent. In the past, for a long period of time, the updates of street lamps were mostly limited to their lighting parts. With the development of urban and electronic technology, the city street lamp system has experienced the development process of manual control, automatic timing/photoelectric control, and computer program control. Using computers to realize the automatic control of urban solar street lighting systems has good economic and social benefits for improving the city's modern management level and saving manpower and material resources. By effectively adjusting the light switch time, the work quality and work efficiency of the street light system can be greatly improved, providing a comprehensive solution and strong technical support for the operation, maintenance, and expansion of the urban lighting system, and improving the urban lighting operation. Management level.


(1) Microcomputer main control circuit.


The main control circuit of the computer is the control core of the entire system and controls the normal operation of the entire solar street light system. Microcomputer main control circuit has a measurement function, through the determination of solar panel voltage, battery voltage and other parameters to determine, control the opening or closing of the corresponding line to achieve a variety of control and protection functions.


(2) Charging drive circuit


The charge drive circuit consists of a MOSFET driver module and a MOSFET. MOSFET driver module uses high-speed optocoupler isolation, emitter output, short circuit protection and fast shutdown. The selected MOSFET is an isolated, energy-saving single-chip switching power supply IC, driving the LED full voltage input range of 150 ~ 200V, the output current of 8 ~ 9A. Wide input voltage range, good voltage regulation and load regulation, strong anti-interference ability and low power consumption.


The system completes the charging of the solar battery pack to the battery through a charging drive circuit, and corresponding protection measures are provided in the circuit.


(3) LED drive circuit


It is composed of IGBT driver module and MOSFET to realize the adjustment of street light brightness and the switch of street light.


(4) Solar battery pack


The solar battery unit is connected by solar battery cells (operating voltage is about 0.5V, working current is about 20-25mA/cm2, area is 10cm & TImes; 10cm) in series and in parallel, and a standard component includes 36 chips. So that a solar cell module can produce about 17V voltage, a battery pack with a rated voltage of 12V. When the application system requires higher voltage and current components, multiple components can be combined into a solar cell array to obtain the required voltage and current.


(5) Battery pack


Since the energy obtained from the photovoltaic array does not always correspond to the demand of the electronic load, the battery can still operate the load when the photovoltaic array itself cannot provide enough power. If the electronic load needs to work at night or when it is cloudy or cloudy, it needs energy storage. The battery storage energy is designed to meet the average daily electronic load during autonomous operation. In general, it should be able to reserve 5 to 7 days of night lighting electricity consumption. In a stand-alone photovoltaic system, the power generated by the photovoltaic array is not always used at the same time as the power generation, so batteries are required in most stand-alone photovoltaic systems.


(6) Communication device


Composed of wireless data transmission modules. Wireless data transmission module supports GPRS, with RS-232 interface, communication distance of up to 100 meters, strong anti-interference, not interfere with radio and television, mobile communications, to achieve communication between adjacent street lamp terminals.