Solar-powered LED lighting system

Energy shortage is a hot issue in today's society. It directly restricts the development of economy and society. The use of renewable energy has become one of the focuses of the world today. Solar energy is the most important basic energy among various renewable energy sources. Biomass energy, wind energy , ocean energy, hydropower, etc. all come from solar energy. Broadly speaking, solar energy includes all the above renewable energy sources. In recent years, the use of solar energy has received widespread attention from countries around the world. The United States, Japan, and Germany have successively proposed the "Sunshine Plan" and "Energy Saving Plan" to vigorously develop solar photovoltaic power generation technology. Since the "Sixth Five-Year Plan", the Chinese government has also included the research and development of solar energy and renewable energy technology in the national science and technology research plan, which has greatly promoted the development of solar energy and renewable energy technology and industry in China. At the same time, lighting, as an indispensable part of daily life, has become an important energy consumption in countries around the world. According to statistics, lighting electricity accounts for more than 10% of China's total power generation. The application of green energy-saving lighting has received more and more attention. China proposed the " Green Lighting Project" in 1996, mainly to solve the energy supply problem related to lighting. New lighting source LED luminous products have gradually attracted the attention of the world in the field of lighting and decoration.

Solar panels and LEDs are both made of semiconductor materials. As semiconductor material technology becomes more sophisticated, it will surely promote the further development of solar energy and LEDs. Combining solar energy and LEDs provides a new solution for energy-saving lighting technology.

1 Overall structure of the lighting system

This system consists of three parts: solar cells, batteries and LED lighting systems. Solar panels convert solar energy into electrical energy , part of which is used to power the DC load LED, and the other part is stored in the battery. When there is no sunlight or the light is dim, the insufficient energy required by the LED lighting system is provided by the battery. The LED lighting part can not only realize day and night lighting, but also adopts automatic dimming technology to keep the indoor light constant. As shown in Figure 1, the controller 1 uses a Boost circuit to realize the maximum power point tracking of the solar panel . Controller 2 uses a bidirectional DC-DC conversion circuit to realize the charge and discharge control of the battery. Controller 3 is the constant current drive and PWM automatic dimming control of the LED light . The working principle of each part will be introduced below.

2 MPPT algorithm for solar energy

The output power of a solar cell is a nonlinear function of the surrounding external environment (light, temperature) . Figure 2 shows the PV characteristic curve of a solar cell. When the light is constant, the output power of a solar cell has a unique maximum power point; when the light changes, the output power of the solar cell also changes, and the maximum power increases with the light intensity. The output power characteristic curve of a solar cell is also related to temperature, and the maximum power decreases with increasing temperature. Making photovoltaic cells work at the maximum power point is the most effective way to improve photovoltaic cells. The most common MPV F algorithms are: constant voltage control method, which is simple but not suitable for areas with large temperature changes. Perturbation observation method, when the light changes greatly, the perturbation observation method may have problems such as wrong tracking direction judgment. The disadvantage of the conductivity increment method is that it requires high sensitivity of the sensor and fast system response speed, resulting in high hardware costs.

In view of the fact that the perturbation observation method cannot accurately track when the light changes quickly, an improved perturbation observation method is adopted. Three points are randomly selected at the apex of the P-V characteristic curve of the photovoltaic cell, and five situations can be obtained as shown in Figure 3(a). Set a comparison symbol foot, initialized to 0, and compare the power of the three points. When the power of point C is greater than or equal to that of point B, K=K+1, and when C is smaller than B, K=K-1; similarly compare point B and A. After the comparison, if K=2, the working voltage perturbation moves to the right, if K=-2, the working voltage perturbation moves to the left, and K=0 means that the working voltage corresponds to the maximum power point at this time and remains unchanged. The relationship between the three points A, B, and C obtained when the light changes is shown in Figure 3(b). At this time, K=0 is calculated, so that when the light changes, the maximum power point is not tracked, and the maximum power point is tracked after the light stabilizes, thereby avoiding the misjudgment of the tracking direction of the perturbation observation method after the light changes. The program flow is shown in Figure 4.

The Boost circuit is used as the MPPT controller. Its principle is shown in Figure 5. The output voltage is determined by the load and is a fixed value. By changing the duty cycle of the switch tube, the voltage provided by the input solar panel can be changed. The output voltage and current of the photovoltaic cell are sampled in each sampling cycle. According to the MPPT algorithm proposed above, the maximum power point tracking can be performed by adjusting the MOS tube duty cycle.

3Battery Charging Strategy

The factors to be considered when charging the battery in the photovoltaic system are: ① Use MPPT as much as possible to make full use of the photovoltaic cells; ② The charging characteristic curve must meet the requirements of the battery, thereby extending the service life of the battery. Traditional charging methods include constant current charging, constant voltage charging, etc. "1. The disadvantage of constant current charging is that the current in the later stage of charging is relatively large relative to the battery.

In constant voltage charging, although the current decreases as the battery terminal voltage increases, the charging current is still relatively large at the beginning of charging. The most important factor restricting these two control methods is the charging current, so the battery charging can be effectively controlled from the perspective of controlling the current.

The control idea is M1: In the initial stage of charging, a relatively large charging limit current is set for the battery. As long as the charging current does not exceed this limit value, MPPT can be applied. At the same time, the terminal voltage of the battery is continuously detected to determine the charging state of the battery. When the terminal voltage reaches the overcharge voltage, the charging limit current is reduced. Repeat the above process until the charging current reaches the floating charge current (C/100), indicating that the battery is fully charged. After that, the battery is charged with the floating charge current to compensate for the energy loss caused by the reverse self-discharge of the battery. In this charging process, as long as the charging current is within the set range, the MPPT control method can be used, thereby greatly improving the utilization rate of solar cells.

The control of battery charging and discharging is performed through a bidirectional DC/DC converter, as shown in Figure 6. This circuit can achieve buck-boost by adjusting the duty cycle of the switch tube on and off. Therefore, there is no need to consider the voltage matching problem at the input and output ends, and the high and low voltage ends can be isolated from each other, with high efficiency and flexible control. When there is sufficient sunlight, switch Q2 is disconnected, and the battery is charged by controlling switch Q1. When the energy provided by solar energy is not enough for the LED lighting system, switch Q1 is disconnected, and switch Q2 is controlled to provide energy to the lighting system.

4 LED constant current and automatic dimming control

With the continuous development of semiconductor technology, white light LED is gradually entering the field of daily lighting. LED is a semiconductor device that can convert electrical energy into light energy. It relies on the combination of positive and negative charges in the material to emit light. In reality, there is no semiconductor material that can directly emit white light. The most commonly used method of realizing white light LED is to use RGB three kinds of LED to mix into white light. Therefore, the application of white light LED to lighting can be configured into various LED lights with different color temperatures and chromaticities according to needs, which will bring people great freedom. At the same time, the driving voltage of LED is greatly reduced, and energy saving is nearly 90%.

Figure 7 is the LED IV characteristic curve. It can be approximately considered that the forward current is proportional to the nth power of the forward voltage y. Small fluctuations in the working voltage of a white LED will lead to a sharp change in the working current. Improper control may cause the PN junction to burn out. In addition, the brightness and life of the LED are directly related to the current, so the LED must be driven with a constant current. The constant current drive circuit shown in Figure 8 is adopted. The op amps A and B in the figure form a closed-loop feedback circuit. The current flowing through the LED generates a sampling voltage on the sampling resistor R, which is sent to the forward channel op amp A through the feedback op amp B and compared with the reference voltage V to generate a control voltage to adjust the gate-source voltage of the MOS tube, thereby achieving the effect of constant current. Analyzing the circuit, the constant current can be obtained as: Io=R2×Vt／R3(R1+R2). Among them, E is the reference voltage; R1 and R2 are feedback resistors; and R is the sampling resistor.

Commonly used methods for adjusting the brightness of white light LEDs are: ① Changing the current to adjust; ② PWM (pulse width modulation) method. Determined by the characteristics of the LED itself, its brightness is related to the size of the current passing through it, that is, the brightness of the LED increases as the current passing through increases. However, the relationship between the two is not linear. Although the brightness can be adjusted by changing the current, when the current increases to a certain extent, the white light LED will change color, and excessive current will also affect the life of the LED. In view of this, the PWM method is used for dimming, that is, under the condition of constant current and constant frequency, the average brightness is adjusted by adjusting the conduction time of the MOS switch tube. This method not only makes the current passing through the LED constant, but also helps the heat dissipation of the LED.

5 Tests and results

It uses 2 15W/17V solar batteries, 1 12V7AH battery, 9 1W white LEDs (3 in parallel and 3 in series). The controller is TMS320LF2407A from 1rI. This DSP chip is mostly used in the control field. It can output multi-channel PWM and has 10-bit AD sampling. It has fast running speed and strong processing ability.

Figure 9 is the result curve of charging the battery using the charging control strategy introduced in this article. At the beginning, the maximum charging current is set to C/IO=0.7 A. When the battery terminal voltage reaches the overcharge voltage of 14.8 V, the maximum charging current is reduced to C/20=0.35 A. Repeat the above process until the charging current is the floating charge current, indicating that the battery is fully charged. In general, the charging link meets the design requirements of this system.

Figure 10 is the measured curve of LED constant current. The horizontal axis is the voltage added to the LED, and the vertical axis is the current flowing through the LED. It can be seen that the voltage added to the LED end changes from 12 V to 17 V, which can basically ensure that the current flowing through the LED is about 0.98 A. The constant current accuracy can be within 0.03, which fully meets the requirements of LED constant current.

6 Conclusion

The system designed in this paper fully considers the maximum power point tracking of photovoltaic cells and improves the utilization rate of solar panels. The LED lights are driven with constant current to ensure the life and luminous efficiency of LEDs . Combining the two truly achieves energy saving and environmental protection, and has great practical value.