Design of Adaptive Pure Solar Street Light Controller

Solar street lights have been widely recognized for their advantages such as no need to lay cables and no consumption of conventional energy. However, there are still some problems with solar street lights, which cause their high cost and unstable reliability. For example, batteries often need to be replaced in less than a year, which not only increases the cost of later maintenance, but also increases the consumption cost of customers, and also causes waste of resources. Secondly, solar energy is an unstable energy source, and the energy distribution is uneven. In summer, the energy is sufficient, but the street lights are used for a short time. In winter, the effective lighting time is short, but the street lights are used for a long time, which greatly reduces the reliability of operation. The main reason is the performance of the solar street light controller. The solar controller is the core part of the solar photovoltaic system. It mainly completes the charging, discharging, dimming of the battery and the opening and closing control of the street lights, as well as timely and effective protection of the system when overcharging, over-discharging, overloading, etc. occur, to ensure the lighting time, reliability, effectively extend the battery life, and reduce costs.

1 Main design requirements and development stages of solar street light controllers

The main technical and quality requirements for solar street light controllers are:

1) The power supply system should be designed to have a constant current output according to the characteristics of the solar street light battery panel:

2) Overcharge and over-discharge protection;

3) It has the function of system power regulation;

4) Establish a network control system;

5) Product modularization according to market requirements.

The development of solar street light controllers has gone through three stages so far: the first generation had relatively simple functions, and the switch control required an external photosensor. The timing time could not be set, there was no battery protection circuit, and the system life was very short, so it was quickly eliminated by the market: the second generation, based on the first generation, set up a battery protection circuit, collected photosensitive data through solar street light battery components, and set the timing through switches or programs. The technology had a step-by-step development and was gradually accepted by the market: the third generation street light controller is that most merchants use PWM charging control function to trickle charge the battery, effectively extending the battery life and reducing the cost of use, thereby further expanding market share.

A good controller can make up for or even solve many problems of pure solar street lights and improve their reliability. The fourth generation of controllers is needed for daytime adaptive solar powered street lights. Its characteristics are power adjustment function of daytime adaptive lights, power detection and remaining power calculation are essential: it also has networking function, so that the brightness of street lights in the whole street can be kept consistent and communication can be carried out.

Abstract: To improve the reliability of seasonal load photovoltaic solar street lights, a new generation of adaptive solar-powered street light controller design method is adopted. Through power regulation, power detection and remaining power calculation, networking functions, etc., the charging and discharging of batteries and the on and off of street lights, maximum power tracking and other intelligent controls are performed, which improves the conversion efficiency of solar cells, extends the service life of batteries, and reduces product costs. The conclusion is that the adaptive solar-powered street light controller is to improve the reliability of seasonal load photovoltaic solar street systems.

Solar street lights have been widely recognized for their advantages such as no need to lay cables and no consumption of conventional energy. However, there are still some problems with solar street lights, which cause their high cost and unstable reliability. For example, batteries often need to be replaced in less than a year, which not only increases the cost of later maintenance, but also increases the consumption cost of customers, and also causes waste of resources. Secondly, solar energy is an unstable energy source, and the energy distribution is uneven. In summer, the energy is sufficient, but the street lights are used for a short time. In winter, the effective lighting time is short, but the street lights are used for a long time, which greatly reduces the reliability of operation. The main reason is the performance of the solar street light controller. The solar controller is the core part of the solar photovoltaic system. It mainly completes the charging, discharging, dimming of the battery and the opening and closing control of the street lights, as well as timely and effective protection of the system when overcharging, over-discharging, overloading, etc. occur, to ensure the lighting time, reliability, effectively extend the battery life, and reduce costs.

1 Main design requirements and development stages of solar street light controllers

The main technical and quality requirements for solar street light controllers are:

1) The power supply system should be designed to have a constant current output according to the characteristics of the solar street light battery panel:

2) Overcharge and over-discharge protection;

3) It has the function of system power regulation;

4) Establish a network control system;

5) Product modularization according to market requirements.

The development of solar street light controllers has gone through three stages so far: the first generation had relatively simple functions, and the switch control required an external photosensor. The timing time could not be set, there was no battery protection circuit, and the system life was very short, so it was quickly eliminated by the market: the second generation, based on the first generation, set up a battery protection circuit, collected photosensitive data through solar street light battery components, and set the timing through switches or programs. The technology had a step-by-step development and was gradually accepted by the market: the third generation street light controller is that most merchants use PWM charging control function to trickle charge the battery, effectively extending the battery life and reducing the cost of use, thereby further expanding market share.

A good controller can make up for or even solve many problems of pure solar street lights and improve their reliability. The fourth generation of controllers is needed for daytime adaptive solar powered street lights. Its characteristics are power adjustment function of daytime adaptive lights, power detection and remaining power calculation are essential: it also has networking function, so that the brightness of street lights in the whole street can be kept consistent and communication can be carried out.

2 Design of Adaptive Solar Powered Street Light Controller

At present, various modern control theories, such as adaptive control, self-learning control, fuzzy logic control, neural network control and other advanced control theories and algorithms are also widely used in photovoltaic power generation systems. Among them, the design of adaptive control solar power supply street light controller is a technology worth promoting.

2.1 Design Goals

The design goal of the street lamps powered by solar energy is mainly for branch roads and residential roads and sidewalks for pedestrians and non-motor vehicles. This design is also suitable for street lamp systems powered by south wind energy or wind-solar complementary. Due to the unreliability of solar energy and the strict lighting design standards for main roads, the design of street lamp controllers powered by solar energy alone is more complicated than that of street lamps powered by city electricity. If the system control requires switching between solar energy and city electricity, it can be simplified based on this design. The target location is in Beijing.

2.2 Design features and functions of adaptive solar-powered street light controller

The purpose of the design scheme of the pure solar power street light controller is to reduce costs and improve reliability through precise control. It mainly has the following features and functions (taking the solar street light energy storage device as a lead-acid battery as an example):

1) MPPT circuit

According to the characteristics of solar street light battery panels, if the output voltage of the solar street light battery array is controlled near a certain constant voltage value, the solar cell is approximately at the maximum power point during the entire working process, and the energy conversion efficiency of the solar cell module is the highest. The PWM technology is used to achieve constant current to the LED by stabilizing the load voltage, thereby ensuring the reliable use of the LED. The MPPT dedicated chip SPV1020 from STMicroelectronics is used. The tracking efficiency can reach 98% and the energy conversion efficiency is 95%. In theory, the use of MPPT technology will increase the efficiency by 50% compared to traditional methods. In actual tests, due to the influence of the surrounding environment and the energy loss of various gods, the final efficiency can also be increased by 20%-30%.

2) Overcharge and over-discharge protection

The charging voltage limit and battery temperature rise detection strategies are adopted, such as the battery power is 36 V, the charging cut-off voltage is 42.5-43 V, the charging cut-off temperature is 80℃, and the charging cut-off temperature rise is 30℃. However, the battery is basically in an undercharged state most of the time. At the same time, through the real-time acquisition of battery voltage data, the battery is protected by voltage limit using software control: the battery power is calculated in real time to prevent overcharge and over-discharge protection, and charging is stopped when the power is 100%, and discharging is stopped when the power is 20%. In order to extend its life, a second line of defense is made. Figure 1 is a flow chart of battery overcharge protection.

Figure 1 Battery overcharge protection flow chart

3) Intelligent control switch, real-time monitoring, early warning function

The solar street light panel current detection, battery voltage detection, battery power monitoring, and ambient temperature detection are carried out. The light is turned on and off, and the working environment and status data are uploaded in real time to warn of faults and ensure the reliability of the system. Figure 2 shows the on and off control process network of the solar street light.

Figure 2 Street light on and off control flow chart

4) Adaptive adjustment of brightness

Usually, solar street light manufacturers only increase the battery capacity to ensure normal operation on continuous rainy days. Generally, the battery capacity can reach 5 times the capacity of the solar panel. In fact, this does not solve the problem. Because the reliability of operation on rainy days does not depend on the battery capacity, but on the balance of many factors. According to the current geographical location, season, time, meteorological conditions, light radiation, dust concentration, working environment and remaining power, the brightness of the light is adjusted adaptively and energy is distributed reasonably. Since the design is purely solar-powered and dual power supply is not considered, the only solution to improve system reliability is to sacrifice the brightness of the light.

According to the remaining power before the day's electricity consumption and the charging amount of the day, the daytime adaptation adjustment is carried out to ensure normal lighting while keeping the battery's working point at a high potential for a long time and keeping the depth of charge and discharge below 30%. According to the battery cycle life curve, the battery life can be extended by 4-5 times, effectively reducing the cost of solar street lights and improving reliability. The following will explain the calculation process of the remaining power and charging amount respectively.

2.2.1 Battery Level Detection

1) Algorithm for power detection

A large amount of experimental data shows that when the battery is aged, there is a high correlation (about 0.88) between the internal resistance of the battery and the charge. The internal resistance of the battery when fully charged and fully discharged differs by 2-4 times, so the battery charge can be detected more accurately by measuring the internal resistance of the battery.

2) Establish the relationship curve between internal resistance, power and cycle period

In order to obtain the real-time remaining power value, a database of the relationship between power and internal resistance should be established.

Using time as the standard, we can establish a curve of internal resistance, power and cycle period, and then use Matlab's curve fitting function to get the relationship between internal resistance, power and cycle period.

As shown in Figure 3, the remaining capacity decreases exponentially as the internal resistance increases.

Figure 3 Relationship curve between battery internal resistance and remaining capacity

3) Online power detection

Before the solar street light starts working, the remaining power is detected, and the internal resistance value is measured by using the AC voltage drop internal resistance measurement method. By checking the prepared data table and performing data correction, the corresponding power value is obtained.

A fixed frequency and a fixed current are applied to the battery (generally 1 kHz frequency and 50 mA current are used today), and then the voltage is sampled. After a series of processes such as rectification and filtering, the internal resistance of the battery is calculated through the operational amplifier circuit. Figure 4 is a hardware block diagram for online measurement of remaining power.

Figure 4 Hardware block diagram for online measurement of remaining power

2.2.2 Calculation of charging capacity

The charging capacity is calculated by the radiation intensity received by the solar panel and the area of ​​the panel. The radiation intensity received by the solar panel is the product of the radiation intensity of a single day and sin a, where a is the average angle between the solar radiation at noon and the panel. The area of ​​the panel can be referred to the content of the configuration calculation part and is obtained after optimization.

2.2.3 Calculation of remaining power

By calculating the integral of the current in the time domain, the power change value can be obtained. The battery power detected before the street light works is taken as the initial power, and the remaining power is the initial power minus the power change value. At the same time, by integrating the output current of the MPPT circuit as the correction value of the power change, a more accurate remaining power value can be obtained. Figure 5 is a flowchart for the remaining power calculation.

Figure 5 Remaining power calculation flow chart

1) Zighee wireless communication system connection

Ensure that the on and off time of the street lights on the entire road is consistent, the road brightness is uniform, ensure driving safety, and avoid visual fatigue of the driver; transmit data in real time, conduct remote monitoring and control: online software upgrade, reduce maintenance and debugging costs; standby sleep, reduce system power consumption. Applying Zigbee wireless sensor network technology to the charge and discharge parameter detection in the battery production process will greatly improve the flexibility and reliability of product testing, which is of great significance to improving the quality and efficiency of battery production.

2) Modular scalability

The power supply system of the designed controller can be modular, and the design adopts a constant current charging method, so the battery panel can be expanded and the LED module can be expanded in parallel according to the system power.

According to the above calculation, the specific design block diagram is shown in Figure 6, which is the hardware framework of the solar street light control system. Figure 7 is the circuit schematic diagram of the solar street light control system.

Figure 6 Solar street light control system hardware framework

Figure 7 Schematic diagram of solar street light control system circuit

3. Simulation of the design scheme of adaptive solar-powered street light controller

The time of switching on and off lights is determined by the time of raising and lowering the flag at Tiananmen Square. As shown in Table 1, the longest lighting time of the year is 14.52 hours in December, and the shortest is 9.13 hours. The lighting time is divided into three periods. The first period starts from the time of lowering the flag at Tiananmen Square on the same day and lasts for 5 hours. The second period lasts until 5 am, and the third period lasts from 5 am to the time of raising the flag at Tiananmen Square. The weight ratio of the light brightness in each period is 5:2:3. If the design standard is 100 W light source, the maximum power consumption of the light source is 1.068 5 kW-h.

Table 1 Lighting strategy benchmark parameters

Figure 8 shows that the monthly solar panel area of ​​m is arranged in a column according to the data in Table 1, so the area of ​​the panel can be selected as the plot value of February at the inflection point in the column, and the solar panel area is 2.2 IT12. The battery is 115 Ah. The reason for this choice is that it can ensure 85% of the lighting time throughout the year, and the remaining 15% is over-discharge. However, an adjustment margin must be left for daytime adaptation adjustment, so the value of the solar area calculated based on the February data is 2.216 2 m2, the over-discharge situation is 3 months, and the over-discharge ratio is 25%, so there is an adjustment space of 100/e.

Figure 8 Column chart of solar panels area arrangement in each month

4 Conclusion

Baishishi designed a pure solar power supply street light controller and realized an intelligent solar street light controller with MPPT circuit as the control core. It has the characteristics of simple peripheral circuit and high reliability, realizes the maximum power point tracking of solar cells, adopts a reasonable battery charging and discharging strategy, and has a simple algorithm. It not only improves the utilization efficiency of solar panels, but also prolongs the service life of batteries. For individual under-charged and over-charged lamps, the area of ​​the battery panel is increased or reduced according to the problem, and the battery or lamp beads are replaced. The configuration of each street lamp is flexibly adjusted according to its actual situation, so that each lamp can work in the best state, which not only ensures normal lighting, but also avoids waste of resources and reduces product cost. It has certain reference and promotion and application value.