Talking about the composition of off-grid photovoltaic power generation system
Off-grid photovoltaic power generation systems are mainly composed of solar photovoltaic power generation devices, energy storage devices, controllers, and inverters. A brief introduction to each part is given below.
Solar photovoltaic power generation device
It can be composed of single crystal silicon or polycrystalline silicon or amorphous silicon photovoltaic devices. A group of solar cells packaged with glass and special materials is called solar cell module; multiple solar cell modules are connected to form a solar cell array. The type and composition of solar cells are selected according to the power required by the system, the use environment, and the cost.
Energy storage device
Because photovoltaic power generation is directly affected by weather conditions, it is intermittent and unstable. In order to maintain a certain continuous power supply capacity of the system, an energy storage device must be provided to store part of the electrical energy and provide electrical energy when there is no sunlight. The current photovoltaic power generation system Mainly use batteries for energy storage.
Commonly used batteries include lead-acid batteries, nickel-cadmium batteries, and lithium-ion batteries.
Lead-acid battery: single battery voltage is 2 volts, cycle life is 500-1500 times
Nickel-cadmium battery: single battery voltage 1.0-1.3 volts, cycle life 2500 times
Lithium-ion battery (single cell voltage 3.7V), cycle life 1000-10000 times
Nickel-cadmium batteries and lithium-ion batteries are expensive, but they have a large capacity per unit volume and are mostly used in applications that require small size and large capacity, such as digital products such as mobile phones, cameras, and portable computers. Generally, off-grid photovoltaic systems mainly use sealed lead-acid batteries. The price of lead-acid battery is very cheap, and the sealed lead-acid battery is sealed and maintenance-free, and it is very convenient to use.
The most important parameters of a battery are voltage, capacity, and cycle life. The unit of voltage is volt (V), the unit of capacity is ampere hour (Ah), and the cycle life is the number of charge and discharge.
Controller
The battery has certain requirements for charging and discharging. Frequent overcharging and overdischarging will reduce the service life. The battery must be controlled for charging and discharging, which is the primary function of the controller. Choose the maximum power operating point of the solar battery according to the user's electricity, battery charging and discharging, and the solar battery's light conditions, and coordinate the charging and electricity current. The controller also needs to detect, protect, and display data on the system. The controller's conversion of voltage and current mainly adopts a DC-DC conversion circuit. For the principle of DC-DC conversion, see the section "Principle of DC-DC Converter".
Inverter
The equipment that converts direct current to alternating current mainly uses SPWM full-bridge inverter circuits to generate sine wave alternating current. For the principle of SPWM inverter, see the section "Principle of SPWM Inverter".
Electric load
Electrical equipment that uses DC or AC power.
Simple solar photovoltaic power generation devices have no inverters and only output direct current. Some of them can output several different direct current voltages. The composition is shown in Figure 2. The controller receives the DC power output from the solar panel and controls the charging or discharging current to the battery. A simple controller directly leads the battery to the output terminal, and the system output voltage is equal to the battery voltage. Some controllers have DC conversion circuits, which can output several different DC voltages, such as 5V, 9V, 12V, etc., which are suitable for different electrical appliances. The output power of this kind of system is generally small, and portable structure is mostly adopted.
