Application of Supercapacitor Energy Storage in Independent Power Supply System

1. INTRODUCTION In recent years, due to the limited nature and uneven distribution of conventional energy sources, they cannot meet the needs of sustainable economic development. Therefore, humans have focused their attention on renewable energy sources. As a clean renewable energy source, solar energy is increasingly valued by independent power supply systems.

Since solar energy is affected by the sunshine and the weather, how to store the solar energy for rainy days and nights is the focus of this article. This article weighs various types of energy storage devices, compares their development prospects and advantages, and selects supercapacitors as energy storage devices.

2. Description of Supercapacitors Supercapacitors are passive energy storage devices with high energy density. They mainly use ion/electrolyte interfaces to form ions in the double layer and absorb and desorb them to achieve energy storage and release. In other words, a supercapacitor is obtained by using an electric double layer structure composed of an activated carbon porous electrode and an electrolyte. Most supercapacitors can be Farader grades, with a capacitance range of 1 to 5000F. When used, they can be connected in series or in parallel to increase the resistance. Pressure and capacity.

The maximum charge and discharge performance of a supercapacitor is determined by the ionic orientation and charge transfer rate on the surface of the active material. Its main features are: high power density, long cycle life, wide operating temperature range, maintenance-free, green environmental protection.

Supercapacitors as energy storage devices have different charge and discharge characteristics from rechargeable batteries. During charging and discharging, the battery has a relatively stable output voltage, and the output voltage of the super capacitor decreases as the capacity of the electric energy decreases. Must be DC / DC conversion to become constant voltage output. The relationship between the storage energy of an ultracapacitor and its output voltage is:. Therefore, it is easy to calculate the storage energy of the super capacitor by measuring the voltage.

Compared with conventional batteries, supercapacitors have no strict restrictions on charge/discharge currents and can better adapt to the characteristics of large fluctuations in the power generation capacity of solar cells.

3, the system's hardware part of the design of the ultra-capacitor set as a storage device, the C8051F320 control chip, and its reasonable design of charge and discharge circuits to ensure the stability of the entire system.

3.1 Overall structure of an independent power supply system The independent power supply system consists of a solar panel, a super capacitor, and a controller. The controller collects the voltage and current of the solar panel to realize the tracking of the maximum power point of the solar cell, the maximum efficiency of the stored solar energy, and the management of the safety of the super capacitor and the system; the super capacitor group serves as the energy storage device through the DC/ The DC power chip outputs a stable DC voltage and powers the controller itself. The system structure is shown in Figure 1.

Mainly designed for 12V DC load power supply, current sensor selection MAX472, FET Q1, Q2, Q3 use IRF540, driven by the driver chip TC4427EPA, voltage regulator U1, U2 use LM2576, DC/DC converter selection MAX668.

3.2 Overall Design of Charge and Discharge Design of Super-Capacitor Bank The general idea is to use the capture/compare module of the C8051F320 to generate a PWM-regulated charge current for maximum power tracking, charging the super-capacitor group with maximum efficiency; by controlling the MCU's I/O lines, The on-off of the discharge device is achieved to ensure that the system is powered under abnormal conditions.

In the design, to satisfy the demand for power consumption, the capacitance of the super capacitor group can be obtained from Equation 2. Based on the consideration of the system cost (the size of the super capacitor and the price), the MOSFETs Q1 and Q2 are specially arranged to manage the on-off of the load and the controller. The voltage and current of the super-capacitor group are sent to the single-chip microcomputer through a voltage divider resistor and a current sensor respectively, and compared with the internal preset voltages Uref1, Uref2 of the single-chip microcomputer, and the load and the controller are switched on and off under the action of Q1 and Q2. However, the operating voltage of the SCM is not enough to drive the FET, so it is necessary to add a driver chip, and the TC4427EPA peak output current can reach 1.5A. The input supply voltage range is -4.5V to 18V, which meets the design requirements.

The voltage of the ultracapacitor group is in an unstable state during charge and discharge, increases with charging, and decreases with discharge. The regulator U1 and U2 are designed to apply buck-boost voltage to the super capacitor voltage to power the load and the controller. Consider using the electric energy that the supercapacitor stores to the maximum extent, hope that the input voltage minimum voltage of the steady voltage chip is designed to be as small as possible, so design and choose the steady voltage chip LM2576 (the input voltage range is 1.23- 37V) . Design parameters: Super capacitor group 24V, 175F, = 1.6V, = 1.5V, monocrystalline silicon solar cells 2W, 18V.

4. The system's software design system uses solar energy as a power generation device. From PV output characteristics of PV cells, it can be seen that as the terminal voltage gradually increases from 0, the output power shows a trend of rising first and then decreasing, and therefore, the maximum efficiency of storage. Solar energy, how to adjust the operating point of the photovoltaic cell in real time, so that it always works near the voltage value corresponding to the maximum power, that is to achieve the maximum power tracking (MPPT) is the main problem solved by the system.

There are many ways to achieve maximum power tracking, and the system uses perturbation observation method. The principle is to increase or decrease the PV array output voltage at regular intervals and observe the direction of the output power change afterwards to determine the next control signal. That is, a disturbance output voltage signal (Upv+ΔU) is given first, and then the power change is measured. Compared with the power value before the disturbance, if the power value increases, the disturbance direction is correct, and the disturbance can continue to the same (+ΔU) direction; After the disturbance power value is less than the disturbance, it is perturbed in the reverse (-ΔU) direction until the solar cell output power tends to be maximum, ie.

This paper compares the current output power of the solar panel and the previous output power, adjusts the maximum power point according to the disturbance observation method, and charges the super capacitor. During the discharge process, the FET Q3 is set to manage the load on and off, and the FET is set. Q2 manages the power supply to the controller. The system flow is shown in Figure 2.

5. Design conclusions and analysis In line with the application of new energy supercapacitors in new areas, this design uses C8051F320 controller to complete the signal acquisition at the maximum solar power point, complete the super-capacitor charging and set the voltage with maximum efficiency. The threshold ensures continuous and stable power supply to the system. In particular, the supercapacitor as a new energy storage element, with its advantages and features, the development of a wider range of applications will make it have a better application prospects.