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1 Introduction
Energy is an important material foundation for the existence and development of human society. With the development of society, energy has been declining and environmental problems have become increasingly prominent. As a result, more and more countries have turned their attention to renewable energy. As one of the important energy sources, solar energy is rapidly developing with its advantages of being inexhaustible and pollution-free. However, due to the influence of the environment (mainly including sunlight intensity and temperature), solar cells have obvious non-linear characteristics in the process of their operation, resulting in a mismatch between the battery and the load, thus failing to maximize the conversion efficiency of solar energy. For power output. In order to maximize the power output of the photovoltaic power generation system, it is necessary to perform tracking control of the maximum power point of the photovoltaic cell, that is, MPPT (Maximum Power Point Tracking) control.
In the photovoltaic control technology, there are many kinds of MPPT control methods. Currently, the controller that uses the CVT (Constant Voltage Tracking) control technology is commonly used in the market, because the CVT method is relatively simple and manufacturing is relatively easy, but this control technology brings The relatively serious power loss, compared to the high price of photovoltaic cells and the growing development of power electronics technology, is not economical and practical.
Therefore, various photovoltaic controllers with MPPT function are gradually developed. The controller designed in this paper is a small-scale photovoltaic controller based on the “voltage perturbation method†using a high-performance single-chip microcomputer to control the charging and discharging of the super capacitor.
2 Basic principles and photovoltaic characteristics of photovoltaic cells
Photovoltaic cells are devices that use the photovoltaic effect to convert light energy into electrical energy. When sunlight strikes a semiconductor PN junction, photogenerated voltages are generated on both sides of the PN junction. When the load is connected, a current is generated. The current is proportional to the light intensity. When the received light intensity is constant, the photovoltaic cell can be considered as a constant current source. Photovoltaic cells have obvious nonlinearities due to the influence of the external environment (mainly including temperature and light intensity).
From the PV characteristics of the photovoltaic cell in Figure 1(a) and Figure 1(b) under the standard temperature and standard light intensity, it can be seen that the output characteristics of the photovoltaic cell are greatly affected by environmental changes, in which the light intensity mainly affects the photovoltaic cell current. The photovoltaic cell voltage is mainly affected by the temperature, so a simple CVT control technology can not meet the maximum power output requirements of the photovoltaic cell, thus making the MPPT control technology more applicable.
3 super capacitor energy storage principle and equivalent circuit model
3.1 principle of super capacitor energy storage
Super-capacitor is a new type of energy storage device that has appeared in recent years. Compared with conventional capacitors, its capacity can reach farads and even thousands of farads. It has the advantages of high power density of conventional capacitors, high energy density of ordinary batteries, and short charge and discharge times, good cycle performance, long service life, wide operating temperature range, and no pollution to the environment. Therefore, in a sense, supercapacitors have the dual functions of traditional capacitors and batteries, which make up for the gap between the two traditional technologies and therefore have great potential for development.
The energy of the super capacitor is stored inside the double layer and the electrode. When a supercapacitor cell is charged with a DC power source, the positive and negative ions in the electrolyte are oriented toward the surface of the solid electrode to form an "electrode/solution" double layer for storing charge.
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