Solar cell modules need to be equipped with a control system to form a practical solar cell system (Figure 1). The design of the solar cell application system needs to consider the following factors: ①The solar radiation of the solar cell system; ②The load power of the solar cell system; ③The output voltage of the solar cell system, the difference between direct and AC; ④The time that the solar battery system needs to work every day; ⑤The length of continuous rainy weather without sunlight; ⑥The load of the solar battery (including the starting current and the nature of the load resistance, etc.); ⑦The number of system requirements, etc. In the design of a photovoltaic system, every decision will affect its cost. If the system is too large due to unrealistic requirements, the initial cost of the system will increase unnecessarily. If parts that are not durable are used, maintenance and replacement costs will increase. If the selection is incorrect in the system design process, it is easy to increase the life cycle cost of the entire system. Therefore, the design of the photovoltaic system must conform to the practical and flexible principles.
According to different operating modes, the system is divided into independent type, grid-connected type and hybrid type. According to its scale, it can be divided into three categories: large, medium and small, of which large-scale refers to independent photovoltaic power plants; medium-scale refers to application systems; small-scale refers to types that are smaller than household systems, such as solar street lights (Figure 2). Small-scale systems are generally independent systems.
The basic composition of an independent solar cell system
Figure 3 is a schematic diagram of a common solar independent photovoltaic power generation system structure. The system consists of a solar cell array, a DC-DC converter, a battery pack, a DC-AC inverter and an AC/DC load. If the load is DC, the DC-AC inverter is not required. The DC-DC converter transfers the electric energy converted by the solar cell array to the battery pack for storage for use when the sun is insufficient. The energy of the battery pack directly supplies power to the DC load or supplies power to the AC load through a DC-AC converter.
The main components of an independent solar power system are: solar cell components and brackets, maintenance-free lead-acid batteries, charge and discharge controllers, inverters (used when AC loads are used), various special AC and DC lamps, power distribution cabinets and cables, etc. The control box should be made of good material, beautiful and durable; a maintenance-free lead-acid battery and charge and discharge controller should be placed in the control box. Valve-regulated sealed lead-acid batteries are also called “maintenance-free batteries” because of their low maintenance. Using it is conducive to the reduction of system maintenance costs; the charge and discharge controller is designed with functions such as light control, time control, overcharge protection, overdischarge protection and reverse connection protection. For example, for an independent solar street light system, its working principle is: The solar panel receives solar radiation energy during the day and converts it into electrical energy, which is stored in the battery through the charge and discharge controller; at night, when the external illuminance gradually decreases to a certain value, the open circuit voltage of the solar panel reaches the corresponding value, and the charge and discharge controller detects this voltage value and acts, and the battery supplies power to the lamp. After the battery is discharged for the set time, the charge and discharge controller will act, and the battery discharge ends. The main function of the charge and discharge controller is to protect the battery. The charge and discharge conditions and the lighting time of the street lamps can be set by the controller according to the user’s needs. The storage battery discharges into direct current. If alternating current is needed, an inverter that converts direct current into alternating current needs to be added.