As a new energy source, solar cells are first applied to lighting fixtures in civilian applications. At present, in the western bright project, the application of non-main road solar street lights, solar garden lights and solar lawn lights, solar decorative lights, etc. has gradually formed scale. In the design of solar lighting fixtures, there are many factors involved in the control of the light source, the solar cell system, and the battery charge and discharge. Any problem in any one of the links will cause product defects.
In this paper, the external characteristics of solar cells, battery charge and discharge control, solar lighting fixtures often compare LEDs with three-color high-efficiency energy-saving lamps, analyze their advantages and disadvantages and use occasions. At the same time, an improved method is proposed for the problems existing in the design of solar lamp circuits on the market. Due to its unique advantages, solar lamps have developed rapidly in recent years. The lawn lamp has low power, mainly for decoration purposes, and has high requirements for mobility. In addition, the circuit is difficult to lay and the waterproof requirement is high. The above requirements make the lawn lamp powered by the solar battery show many unprecedented advantages.
Especially in foreign markets, the demand for solar lawn lamps is very large compared to other products. In 2002, only the solar cells consumed by Guangdong and Shenzhen for the manufacture of solar lawn lamps reached 2MW, equivalent to one-third of the domestic solar battery production that year, and this year still maintained a strong development momentum, which was unexpected. Solar garden lights are widely used in parks, living quarters and non-main roads. At the same time, due to the rapid development, some products are not mature enough in technology, there are many defects in the choice of light source and circuit design, which reduces the economy and reliability of the product and wastes a lot of resources.
In view of the above problems, this paper puts forward your own views for the reference of factories producing solar lamps.
1 The external characteristics of solar cells are discussed from the perspective of application. The main concern is the external characteristics of solar cells. First, for a single-chip solar cell, it is a PN junction, which has all the characteristics of a PN junction in addition to the ability to generate electricity when it is illuminated by sunlight. Its rated output voltage is 0.48V under standard lighting conditions. The solar modules used in the use of solar lighting fixtures are all composed of a plurality of solar cell connections. It has a negative temperature coefficient, and every time the temperature rises, it is a problem that cannot be ignored for a solar cell module composed of a plurality of solar cells. Its output characteristic curve is shown in Figure 1.
In Figure 1, Isc is the short circuit current, Im is the peak current, and Voc is the open circuit voltage. Vm is the peak voltage and Pm is the peak power. In use, the solar cell will not cause damage due to open circuit or short circuit. In fact, we use this feature to control the charging and discharging of the system battery.
2 Problems that must be paid attention to when using solar cells
2.1 Solar cell power selection We say that the solar cell output power Wp is under standard solar lighting conditions, namely: the 101 standard defined by the European Commission, the radiation intensity is 1000W/m2, the air quality is AM1.5, and the battery temperature is 25°C. The output power of the solar cell. This condition is about the same as the sun lighting conditions around noon on sunny days. (On the lower reaches of the Yangtze River, this value can only be approached.) This is not like some people think, as long as there is sunshine, there will be rated output power, and even think that solar cells are It can also be used normally under the fluorescent light at night. That is to say, the output power of the solar cells is random, and the output power of the same solar cell is different at different times and at different locations.
2.2 Selection of battery capacity Since the input energy of the solar photovoltaic system is extremely unstable, it is generally necessary to configure the battery system to work. The solar lamps are no exception, and the battery must be configured to work. Generally, there are lead-acid batteries, Ni-Cd batteries, and Ni-H batteries, and their capacity selection directly affects system reliability and system price. The choice of battery capacity generally follows the following principles: First, the energy of the solar module during the day is stored as much as possible while meeting the night illumination, and at the same time, it is necessary to store the electric energy required for the night and night illumination. If the battery capacity is too small, it can not meet the needs of night lighting. The battery capacity is too large. On the one hand, the battery is always in a deficient state, which affects the battery life and causes waste.
2.3 Choice of solar cell packaging form At present, there are two main types of solar cell packaging, lamination and epoxy, and the lamination process can ensure the working life of solar cells for more than 25 years. Although the epoxy is beautiful at the time, the working life of solar cells is only 1 ~2 years. Therefore, the low-power solar lawn lamp below 1W can be used in the form of epoxy dispensing without the requirement of high life. For the solar lamp with the specified service life, it is recommended to use the laminated package. In addition, there is a silicone gel for epoxy-packaged solar cells, which is said to have a working life of up to 10 years.
2.4 Solar cell installation tilt angle selection and decorative cover For the sake of beauty, many solar lighting factories will place solar cells horizontally. In this case, the output power of solar cells will be reduced by 15% to 20%, if in solar energy Adding a decorative cover to the battery, the output power of the solar cell will be reduced by about 5%. The solar cell is expensive. We have collected many foreign solar light materials. Most of them choose energy saving between aesthetics and energy saving. The most ideal tilt angle for solar cells in the lower reaches of the Yangtze River is about 40 degrees, and the direction is positive south.
2.5 Heat island effect Single-chip solar cells are generally not usable, and the actual application is solar cell modules. The solar cell module is composed of a plurality of solar cells to achieve a desired voltage value. During the use of the solar cell module, if a solar cell is individually blocked, such as bird guano, the solar cell that is blocked alone will be damaged by heat under strong sunlight, and the entire solar cell module is damaged. This is the so-called heat island effect. In order to prevent the heat island effect, the solar cell is generally placed obliquely so that the leaves and the like cannot be attached, and the bird-proof needle is mounted on the solar cell module.
3 Charge and discharge control of batteries in solar lamps Regardless of the size of solar lamps, a good performance charge and discharge control circuit is essential. In order to extend the service life of the battery, it must limit its charging and discharging conditions to prevent overcharging and deep discharge of the battery. In addition, since the input energy of the solar photovoltaic power generation system is extremely unstable, the control of charging the battery in the photovoltaic power generation system is required. It is more complicated than the control of ordinary battery charging. For the design of solar luminaires, success and failure often depend on the success and failure of the charge and discharge control circuit. Without a good charge and discharge control circuit, it is impossible to have a good performance solar luminaire.
3.1 Anti-recharge control Prevents the anti-charging function. Generally speaking, a diode is connected in series in the solar cell circuit. The diode prevents reverse charging. This diode should be a Schottky diode. The voltage drop of the Schottky diode is lower than that of the ordinary diode. In addition, the field effect transistor can also be used to control the anti-charge function. Its tube voltage drop is lower than that of the Schottky diode, but the control circuit is more complicated than the front.
3.2 Anti-overcharge control To prevent overcharge function, a bleeder transistor can be connected in series or in parallel in the input loop. The voltage discrimination circuit controls the switch of the transistor to discharge excess solar energy through the transistor to ensure that there is no excessive voltage to the battery. Charging. The key is to prevent the choice of overcharge voltage, the single-cell lead-acid battery is 2.2V.
3.3 Anti-over-discharge control In addition to Ni-Cd batteries, other batteries generally have the function of preventing over-discharge of the battery, because it will cause permanent damage to the battery over-discharge. It should be noted that the solar cell system generally discharges at a small rate relative to the battery, so the discharge cut-off voltage should not be too low.
3.4 Temperature compensation Temperature compensation, battery voltage control point varies with ambient temperature, so the solar light system should have a temperature-controlled reference voltage. For single-cell lead-acid batteries, it is -3~-7mV/°C, we usually choose -4mV/°C.
4 Solar light source selection
At present, most lawn lamps use LED as the light source. The LED has a long service life and can reach more than 100,000 hours. The working voltage is low, which is very suitable for application on solar lawn lamps. In particular, LED technology has achieved its key breakthroughs, and its characteristics have greatly improved in the past five years. At the same time, the performance price ratio has also been greatly improved.
In addition, the LED is powered by low-voltage DC, its light source control cost is low, the brightness is adjusted, frequent switching is possible, and it will not adversely affect the performance of the LED. It is possible to control color, change the distribution of light, and generate dynamic phantoms, so it is especially suitable for solar lawn lights. There are many inherent characteristics that can cause undesirable consequences if not used. However, LEDs currently sell LEDs in the market with luminous efficiency of only 15Lm/W, which can only reach 1/3 of the three-color primary color energy-saving lamps. The luminous efficiency of three-color primary color energy-saving lamps can reach 50 Lm/W ~ 60Lm/W. . From the price point of view, the current cost per Lm: three-color base color high-efficiency energy-saving lamps (including electronic ballasts) 0.022 yuan, the price of f5mm white LEDs in 2002 is 1.9~3.0 yuan, the current cost per Lm is high, the price difference Disparity. From the perspective of service life, the life of three-color primary color energy-saving lamps (including electronic ballasts) can reach 6000 hours, and LEDs can reach more than 100,000 hours. On the surface, LED life is a three-color primary color energy-saving lamp (including electronic Tens of times the ballast), but this is not the case. At present, most of the solar lawn lamps use ultra-bright white LEDs. At 20 mA, the ultra-bright white LEDs maintain a luminous intensity of 50% of the initial intensity (life) of less than 10,000 hours. The Fudan University electric light source has proved the above. argument. That is to say, in many cases, LED is not the best source of solar lawn light, unless it is a low-end, solar lawn lamp with a life of only 1 to 2 years, or a solar lawn lamp of 1w or less. For solar lawn lights above 1W, it is best to use three-color primary color energy-saving lamps.
At present, there are some solar lawn lamps with 30~40 super bright white LEDs, and the input power is more than 2W. In this case, if the three-color base color high-efficiency energy-saving lamp is used, the price is only 1/10 of the LED, and the luminous flux is the original. 4 times, now has successfully developed 2~10W low-voltage DC three-color primary color energy-saving lamps, the life can reach 6000 hours. According to the above analysis, we believe that there is a low-power solar lawn lamp below 1W that adjusts the brightness and brightness and frequent switching functions. Generally, LEDs should be used as the light source. However, when using ultra-bright white LEDs, pay special attention to the problem of lumen maintenance, otherwise it will easily lead to accidents. For solar lawn lamps with higher power, it is more reasonable to use three-color primary color energy-saving lamps. It should be emphasized here that the above conclusions are only the current analysis. When the level of LED technology is raised, the price drops, and the above conclusions need to be changed.
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