External characteristics of solar cells
From the application point of view, the main concern is the external characteristics of the solar cell. First of all, for a monolithic solar cell, it is a PN junction. In addition to generating electricity when sunlight shines on it, it also has all the characteristics of a PN junction. Under standard light conditions, its rated output voltage is 0.48V. The solar cell components used in solar lighting fixtures are all composed of multiple solar cells connected. It has a negative temperature coefficient. When the temperature rises by one degree, the voltage drops by 2mV. For solar cell modules composed of multiple solar cells, this is a problem that cannot be ignored. In use, the solar battery will not be damaged by open circuit or short circuit. In fact, this feature is used to control the charging and discharging of the system battery. The following issues must be paid attention to in the use of solar cells.
a. Selection of solar battery power. Generally speaking, the output power of solar cell Wp is the output power of the solar cell under standard sunlight 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’s light conditions around noon on a sunny day. This is not like some people imagine, as long as there is sunlight, there will be a rated output power.
It is even believed that solar cells can be used normally under fluorescent lights at night. That is to say, the output power of the solar cell is random, and the output power of the same solar cell is different at different times and in different places. The design of solar lamps is related to the area where the lamps are used. The relationship between the rated output power of the solar cell module and the input power of the lamps is about (2~4):1 in East China. The specific ratio depends on the daily working hours of the lamps and the lighting requirements for continuous cloudy and rainy days. In addition, the output power of the solar cell is about 120W/m2.
b. Selection of battery capacity. Since the input energy of the solar photovoltaic power generation system is extremely unstable, it generally needs to be equipped with a storage battery system to work, and solar lamps are no exception. They must be equipped with a storage computer to work. There are lead-acid batteries, Ni-Cd rich batteries, and Ni-H power storage sites. Their capacity selection directly affects the reliability of the system and the system price.
child. The battery capacity should generally be selected according to the following principles: First, on the premise that it can meet the lighting of the night courtyard, store the energy of the solar electrical components during the day as much as possible, and at the same time, it must be able to store the electrical energy that can meet the lighting needs of continuous cloudy southern days at night.
The recommended amount of the battery is too small to meet the needs of night lighting. The capacity of the battery is too large. On the one hand, the battery is always in a state of power loss, which affects the battery life and causes waste.
C, the choice of solar cell packaging. At present, there are two main types of solar cell packaging, lamination and glue. The lamination process can ensure the working life of the solar cell for more than 25 years. Although the glue is beautiful at the time, the working life of the solar cell is only 1 to 2 years. Therefore, for low-power solar LED systems below 1W, epoxy packaging can be used if there is no excessive life expectancy. For solar lamps with a specified service life, it is recommended to use laminated packaging. In addition, there is a kind of silicone gel used to encapsulate solar cells with glue. It is said that the working life can reach 10 years.
d. The choice of the inclination angle of the solar cell installation and the decorative cover. For the sake of beauty, many solar lamp factories place the solar cells horizontally. In this case, the output power of the solar cells will be reduced by 15%~20%. If a decorative cover is added to the solar cells, the output of the solar cells The power will be reduced by about 5%, and the solar battery is expensive. According to the data of foreign solar lamps, most of them choose energy-saving lamps between beautiful and energy-saving.
e. Heat island effect. Monolithic solar cells are generally unusable, and solar cell modules are actually used. The solar cell module is a combination of multiple solar cells to achieve the desired voltage value. During the use of the solar cell module, if a single solar cell is individually shaded, such as leaves, bird droppings, etc., the shaded solar cell will heat up and be damaged under strong sunlight, and the entire solar cell module will be damaged. This is the so-called heat island effect. In order to prevent the heat island effect, one is to place the solar cells obliquely so that leaves cannot be attached, and at the same time install anti-bird needles on the solar cell modules.