With the development of the economy and the progress of the society, people have put forward higher and higher requirements for energy, and the search for new energy has become an urgent task facing human beings. Solar power is considered to be the most important energy source in the 21st century because of its cleanliness, safety, extensiveness and adequacy of resources unmatched by thermal power, hydropower and nuclear power. The storage of solar energy is the key to the development of solar energy products. At present, various batteries are mainly used. However, the long charging time, short life and environmental protection of the batteries have always been the bottlenecks in the development of solar energy products. Environmentally friendly energy storage element, which brings new vitality to the development of solar energy products. This paper introduces the design and realization method of a supercapacitor solar lawn light in detail. The lawn light combines the advantages of solar energy and super capacitors. It can actively emit light without installing other power sources. It can also automatically control the switch of the light according to the intensity of the ambient light. It is easy to install, requires no wiring, and works stably and reliably. , maintenance-free, environmental protection and pollution-free, long service life, can be widely used in square green space, community lawn and other places.
1.1 The choice of light source
Because LED technology has achieved a key breakthrough, and the performance-price ratio has also been greatly improved. The current LED life can reach more than 100 000h, and the working voltage is low, which is very suitable for solar lawn lights. In addition, the LED is powered by low-voltage DC, and its light source control cost is low, the brightness can be adjusted, and it can be switched on and off frequently, and it will not adversely affect the performance of the LED. Therefore, considering the reliability, cost performance, color temperature and luminous efficiency, a super-bright LED with a rated voltage of 3.3 V and an operating current of 6 mA can be selected as the light source in the design. Because the lawn lamp not only has a decorative function, but also has a certain lighting function, so 8 LEDs can be selected for use.
1.2 Selection of solar cells
Solar cells are semiconductor devices that convert sunlight into electrical energy based on the photovoltaic effect principle of semiconductor PN junctions, and are the core devices of supercapacitor solar lawn lights. The performance of the solar cell directly determines the energy conversion efficiency and the stability of the output voltage, and also directly determines the performance of the supercapacitor solar lawn light. Therefore, single crystal silicon solar cells with better cost performance should be used in the design.
Since the annual total solar radiation and the average peak sunshine hours in various regions on the earth are different, the design of solar lawn lights and the geographical location of the lights are related, and the relationship between the rated output power of solar cell modules and the input power of lamps It is about 2 to 4:1, and the specific ratio depends on the daily working time of the lamp and the lighting requirements for continuous rainy days. The power of the solar cell of this system is 3.3V×0.006×8=0.1584W. Assuming that it works 12 hours a day, the efficiency of the solar cell is 40%, and the effective working time is 5 hours a day, 3 W/6 V can be selected. of solar cells.
1.3 Selection of super capacitors
Since the input energy of the solar cell is extremely unstable, and the lawn lamp only emits light when the surrounding light is weak, it is necessary to configure a power storage system to work effectively. At present, lead-acid batteries, Ni-Cd batteries or Ni-H batteries are generally used, but the use of lead-acid batteries as a power storage system has many defects: first, rechargeable batteries have a limited number of charging times (less than 1000 times), and their service life is short. ; Second, due to the influence of its chemical structure, it cannot be charged with high current; third, lead-acid batteries need to have control circuits such as anti-overcharge, anti-over-discharge, and temperature compensation, and the control circuit is relatively complex; fourth, rechargeable Type lead-acid batteries mainly use chemical reactions to charge and discharge, and the waste in the batteries will pollute the environment and are not environmentally friendly products.
Therefore, at present, most manufacturers choose lifepo4 battery as the energy storage element. The battery has a super-large capacity of Farad level, super charge retention ability, and a very small leakage current, and the voltage drop rate is less than 5% in 8 hours; no special charging circuit and control discharge circuit are required, the charging is fast, and it can be charged only in Charged at a state higher than its leakage current (typically about 1 mA), so the solar cell can charge the supercapacitor even on cloudy days; It does not have a negative impact on its life, high reliability and long service life (charge and discharge cycle life is more than 5,000 times, and the service life can reach 15 years); in addition, it has excellent temperature performance, and can be used at -40 ℃ ~ 75 ℃ It can be used normally at ambient temperature; it is a green power source without pollution; it can be welded, and there is no such problem as loose contact like traditional lead-acid batteries.
Solar lawn lights generally use a 5.0V/225 F capacitor module composed of six 2.5 V/150 F supercapacitors. The ESR (DC) of a single capacitor product is only 20mΩ, the diameter is 25 mm, and the height is 48 mm. The 5 V/225 F supercapacitor charging time is (at a charging current of 450mA):
In the formula: C-capacitor rated capacity; △U-capacitor working voltage change; I-capacitor charging current; t-capacitor charging time.
The supercapacitor discharge time is:
Where: ESR is the DC internal resistance of the capacitor. The 5 V/225 F supercapacitor can be discharged from 5 V to 0.6 V.
2 Design of the system control circuit
2.1 Charging circuit
The charging circuit in the system is composed of anti-overvoltage and anti-reverse charging circuits. In the charging circuit, when the voltage of the battery (lifepo4 battery) is too high, Q2 is turned on, and all the current of the lithium battery (lifepo4 battery) is consumed through the resistor Q2. When the voltage of (lifepo4 battery) drops, Q2 ends, and the lithium battery (lifepo4 battery) charges the super capacitor and protects the super capacitor at the same time.
The anti-reverse charging control circuit can ensure that when the input voltage of the solar cell is lower than the voltage of the super capacitor, the super capacitor will not reversely charge the solar cell, so as to avoid unnecessary energy loss.
2.2 Drive LED voltage regulator circuit
Since the rated voltage of the selected LED lamp is 3.3 V, while the voltage of the supercapacitor is 5.0 V, and the voltage of the supercapacitor will continue to decrease during the discharge process, so a voltage regulator circuit is required to drive the LED.
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