Application of supercapacitors in smart community solar street light systems

　　Smart communities are not only high-tech applications, but also focus on energy conservation and environmental protection. This solar street light system is further optimized on the basis of general solar street lights. It uses LED light sources and super capacitors, which have higher charging efficiency and longer component life. It is more energy-saving and environmentally friendly than general solar street lights. This solar street light system is mainly composed of photovoltaic cell plates, energy storage batteries, super capacitors, lighting fixtures and controllers.

　　1 Design of lighting fixtures and control methods

　　1.1 Design of lighting fixtures

　　Traditional lighting fixtures are inefficient, such as incandescent lamps and halogen lamps, which are not suitable for the energy-saving concept of intelligent buildings. The performance comparison of various lighting sources is shown in Table 1. The white light ultra-high brightness LED lamp has a luminous efficiency of 45 lm/W. Although it is not high, the light emitted is within the visible light range and is suitable for lighting sources. LED lamps have a long life of more than 100,000 hours, which can effectively reduce lamp waste. In addition, LEDs are powered by low-voltage DC power supplies, which are safer and suitable for frequent switching on and off. Therefore, the solar lighting system selected a 12 V, 10 W white light ultra-bright high-power LED lamp, whose luminous efficiency can reach 450 lm/W, equivalent to a 50 W incandescent lamp, which can achieve a good lighting effect.

　　1.2 Control methods of lighting fixtures

　　There are two main ways to control the on and off of solar lighting fixtures: timing control and light control.

　　Timing control is to set the time of turning on and off the lights every day and then the system will automatically control it. However, the set time needs to be adjusted continuously with the change of seasons, otherwise the lights will not be turned on at night and will not be turned off at dawn, resulting in energy waste. Lighting control is that the system turns on and off the lighting fixtures by detecting the light intensity, such as turning on the lights when the light intensity is lower than 10 lx and turning off the lights when it is higher than 10 lx. This can not only meet the needs of users, but also save electricity, which is in line with the concept of intelligent buildings. Therefore, the system lighting fixtures are turned on and off by light control.

　　The working time of the illumination control method is related to the local latitude and the solar declination angle of the day. In addition, the residual light in the sky is sufficient for illumination half an hour before sunrise and half an hour before sunset, so the street lights can be turned off, which can reduce the lighting by 1 hour every day. The smart community is located at 116.84° east latitude and 38.31° north latitude. The illumination control street lights in this area have a maximum working time of 12 hours at the winter solstice and a minimum working time of 9 hours at the summer solstice. It can be seen that the working time of the illumination solar street lights does not change too much, which can be considered as a balanced load, and has little effect on the inclination of the solar panel.

　　2 Solar cells

　　2.1 Determination of the optimal tilt angle of solar panels

　　Solar panels should be installed towards the equator, usually facing due south or slightly west, and should have a certain inclination relative to the ground plane, that is, the inclination of the solar panel. Because the angle of sunlight changes with time, the amount of solar energy received by the solar panel at a fixed inclination also changes accordingly, so the determination of the inclination of the solar panel is crucial to the entire system. In the optimization design of the solar street light system, the optimal inclination of the solar panel should be determined according to the load conditions, local climate conditions and longitude and latitude to maximize the average amount of sunlight received throughout the year. The load of this system is approximately a balanced load, and the determination of the optimal inclination of the solar panel adopts the internationally popular photovoltaic system design principle of "balanced throughout the year and maximum in winter" to receive solar radiation. That is, under the premise of ensuring balanced sunlight on the panels throughout the year, the optimal inclination makes the winter sunlight as large as possible, so as to increase the system's power generation in months with weak solar radiation and meet the needs of balanced battery charging and load.

　　Based on the meteorological data of Cangzhou City in the past 10 to 20 years, the model of sky scattered radiation anisotropy can be used to calculate the solar radiation received by the solar panels at different inclination angles. Combined with the theory of "year-round balance and maximum in winter", it can be determined that the inclination angle of the solar panels can be 38° at the local latitude. Because the solar radiation received by the panels with a small inclination angle in summer is large, and the solar radiation received by the panels with a large inclination angle in winter is large, it can be appropriately increased by 5° to 10° on the basis of the 38° inclination angle, which will have a better effect, and is conducive to the sliding of snow and reduce the maintenance workload. The inclination angle of the solar panels in this system is 43°.

　　2.2 Determination of solar cell array capacity

　　The power of LED is 10 W, and it works for 12 hours a day. Assume that the power of solar cell is WS, the efficiency is 40%, and there is a 20% margin. The sunshine working time is 5 hours a day, then:

　　WS × 5 h × 40% ÷ 120% = 10 W × 12 h

　　The solution is WS = 72 W. In order to meet the energy storage requirements of the battery, the power of the solar battery array should be larger. The system chooses a 12 V, 100 W solar battery array.

　　3. Selection of battery packs and supercapacitors

　　At present, the energy storage device with large capacity and low price is still lead-acid battery. Although supercapacitor has many advantages, its storage capacity is still not easy to achieve for solar street light system to cope with continuous rainy days. However, supercapacitor can assist battery to work better. Supercapacitor and battery form energy storage components, which can improve charging efficiency, extend battery life and improve system power supply reliability. Its structure is shown in Figure 1.

　　Figure 1 Structure of solar street light system

　　3.1 Selection of battery capacity

　　In the solar street light system, the battery is an energy storage device, and its capacity is directly related to the length of lighting time. The selection of the battery pack is mainly based on the rated voltage and rated capacity. The calculation formula for the battery capacity is:

　　In the formula, C is the capacity of the battery pack, in A·h; D is the longest number of days without sunshine, which is 6 days; F is the correction coefficient of the battery pack discharge efficiency, which is usually 1.05; Q is the daily power consumption, in W·h, which is 120 W·h in this system; L is the charge and discharge efficiency of the battery pack, which is usually 0.9; U is the discharge depth of the battery pack, which is usually 0.6; Ka is the line loss, which is usually 0.98; Vt is the system operating voltage, which is 12 V.

　　According to the formula, the battery pack capacity C = 120 A·h, and a 120 A·h/12 V single cell battery can be selected.

　　The daily power consumption of the load is 10 W × 12 h = 120 W·h, that is, 10 A·h. A 120 A·h battery can provide 12 days of power consumption. According to the discharge depth of 0.6, it can be used for 7 days. In actual use, the solar street light can illuminate normally for 7 consecutive rainy days.

　　3.2 Selection of supercapacitors

　　Supercapacitor is a new type of energy storage component. It is based on the principle of double electric layer and uses porous carbon material as electrode. It has a capacitance of thousands of farads. Its performance is between traditional rechargeable batteries and ordinary capacitors. It can be fully charged in a very short time. At the same time, it can store a large amount of electrical energy like other rechargeable batteries. When discharging, it uses electrons between mobile conductors (rather than relying on chemical reactions) to release current, thereby providing power for lamps. However, at present, its price is too high, and large-capacity power supply is not easy to achieve, so it can only be used to assist batteries.

　　The output power of solar cells changes with the weather. This unstable charging current affects the battery life, which will invisibly increase the system cost and cause more environmental pollution. Therefore, the system is designed with supercapacitors, which are intermediate components that can charge and discharge quickly. Especially when the sunlight is not strong, the control system stores the unstable electric energy output by the solar cells in the supercapacitor, and then charges the battery with a constant current after it is fully charged. This can increase the battery life. At the same time, the energy storage of the supercapacitor can also provide more energy for street lights on consecutive rainy days, increasing the lighting time.

　　The supercapacitor charging time can be calculated using the following formula:

　　In the formula, C is the rated capacity of the capacitor; dv is the change in the operating voltage of the capacitor; I is the charging current of the capacitor; t is the charging time of the capacitor.

　　According to equation (1), the charging time of a 13.5 V, 480 F capacitor is (charging current is 10 A):

　　It can be seen that the charging time is very short, which facilitates fast charging of the system.

　　The discharge time of the supercapacitor is given by the formula:

　　get:

　　If the discharge cut-off voltage is 3.5V, the discharge time is:

　　It can be seen from formula (2) that the energy storage of the supercapacitor can be discharged to the load for up to 1.6 h, which prolongs the power supply time of the system.

　　4 Design of solar controller

　　As a small photovoltaic system, the solar street light system's controller's own loss current should be less than 1% of the rated working current. The system controller circuit design uses low-power components and uses a voltage comparator composed of an integrated operational amplifier as the control circuit. This circuit is simple and reliable, easy to maintain, low cost, and the circuit itself has extremely low power consumption. It is a well-matched circuit. The key to this circuit is to design a relatively good voltage hysteresis for the battery's charge and discharge characteristics. At the same time, the selection of components must be reliable. In addition, the charge and discharge status indication circuit composed of light-emitting diodes becomes a practical controller circuit that can prevent the battery from over-discharging and over-charging.

　　The control system adds a supercapacitor based on the photovoltaic controller and charging controller, which is connected between the DC bus and the ground wire to stabilize the voltage of the DC bus and buffer the excess energy provided by the photovoltaic cell, which is then discharged to the battery and then provided to the load.

　　Photovoltaic controllers are usually designed with a boost circuit to generate a higher voltage than the two ends of the photovoltaic panel, which is conducive to charging the battery. At the same time, it also overcomes the disadvantage of the anti-backflow diode in the traditional circuit that clamps the battery voltage at 12 V. However, when the light is insufficient, if the battery is to continue to charge, the control circuit will cause the working point of the photovoltaic cell to deviate from the maximum power output point, which will reduce the power generation efficiency of the photovoltaic street light system. Therefore, when designing the control system, it is necessary to preset the threshold of the weak light section to achieve the purpose of ensuring the normal charging of the battery through supercapacitor buffering in weak light.

　　If photovoltaic cells are used directly to charge the battery, the output voltage will be unstable when the light is weak and there are other interference factors, making it difficult for the photovoltaic cells to maintain the minimum charging voltage during charging, and finally causing the system to be unable to charge the battery normally within the light range. The system uses supercapacitors to accumulate the unstable output energy of solar cells on cloudy days, and when certain voltage conditions are met, the energy in the supercapacitor is released to the battery through the boost circuit. The boost circuit diagram is shown in Figure 2. This method of using supercapacitors can improve the power generation efficiency when the sunlight is not strong.

　　Figure 2 Charging boost circuit

　　The control circuit of LED is relatively simple, and DC drive is sufficient, and its life span can reach 100,000 hours. However, the size of the driving current greatly affects the life span of LED. If the current is too large, it may cause serious LED light decay and shorten the life span. Therefore, its driving circuit must be designed reasonably. As shown in Figure 3, it is an LED constant current control circuit implemented by BUCK circuit.

　　Figure 3 LED constant current control circuit

　　5 Lightning protection grounding design

　　The working voltage of LED street lights is 12 V, which is a safe voltage and does not require electrical protection grounding. However, the metal poles of LED street lights should be grounded for lightning protection, and the grounding resistance was tested to be 8 Ω, which meets the requirements.

　　Configuration of solar street lighting system The basic configuration of a solar street light in this smart community is shown in Table 2:

　　6 Conclusion

　　After the smart community solar street light system was put into operation, the 10 W new LED light source was sufficient for lighting. The application of supercapacitors can ensure that the battery is properly charged, improve the charging efficiency, and extend the life of energy storage components. Especially when the sunlight is not sufficient, the system can better store energy, and its energy storage can be used for lighting fixtures for 7 consecutive days. The design of the system always follows the concept of "energy saving and environmental protection" of smart buildings, and uses solar energy, long-life LED light sources and supercapacitors. If 25 more supercapacitors are added, the capacitor energy storage can supply street lights for one day. With the expansion of supercapacitor capacity and the reduction of prices, it is feasible to use it as an energy storage component. The solar street light system of the smart community is also an attempt to apply supercapacitors.