1. component structure
For most practical applications, photovoltaic modules are the basic unit of a complete photovoltaic system. Understanding the design and integration process of photovoltaic modules is essential for understanding the design of the actual system. For this purpose, first consider the I-U characteristics of a single battery shown in Figure 1.
After connecting 3 identical batteries in series, their joint characteristics are shown in Figure 2.
The current does not change after the batteries are connected in series, and the voltage is the sum of the voltages of each battery. If batteries with different characteristics are connected in series, their I-U characteristics are shown in Figure 3.
It can be seen that the output voltages of different batteries in series are added together, but the current after series connection is equal to the smaller of the two currents. Therefore, the battery current in series does not change, and the voltage is the sum of the voltage of each battery.
2. Photovoltaic modules
When photovoltaic cells are connected physically and electrically to form a photovoltaic module, photovoltaic modules are connected together to form a photovoltaic array. At first, most commercial photovoltaic modules were able to generate an open circuit voltage of 20V, with a nominal charging voltage of 14V, so that they could charge a 12V battery. It usually consists of 36 batteries connected in series and is called a 12V module. Figure 4 shows a module with 36 batteries connected in series.
In recent years, with the further development of the grid-connected market, manufacturers have also produced components with higher voltages. The most common one is composed of 72 photovoltaic cells with a rated voltage of 24V, but the components are not suitable for charging the battery (for example, a component composed of 100 cells), which is only used to adapt to the voltage window of the grid inverter after being connected in series. .
With the development of maximum power point trackers (MPPTs) and other electronic devices, it is now possible to more energy-efficiently use high-voltage arrays to charge low-voltage batteries (for example, use an array with a rated voltage of 120V to charge 48V batteries). This makes it possible for most of the components on the market to be used in independent power supply systems.
At present, most photovoltaic modules have a rated voltage of 12V or 24V, which is easy to apply in independent power supply systems.
3. self-regulating module
The self-regulating module has a limit on the number of batteries connected in series, usually 32 batteries. The small number of batteries allows the module to generate only a maximum voltage of 14.5V, thereby avoiding battery overcharging. The use of self-regulating modules cannot automatically ensure that the photovoltaic system can self-regulate. The battery capacity and the temperature of the load must also be considered. In general, when the battery capacity is large, the use of the self-regulating module is safe. The battery capacity (unit: w*h) is usually 30 times the output power of solar energy. If the capacity is small, there is still the possibility of overcharging the battery.
Note: Most of the components in the independent power supply system are non-self-regulating, and most of the manufactured components are also non-self-regulating.
4. Commercial components
In recent years, the capacity of photovoltaic modules produced has increased from 39MWp in 1994 to more than 1,900MWp in 2006.
The growth of the industry has caused more manufacturers to start producing photovoltaic modules. It is very important that only components that meet the quality requirements can be installed and implement existing standards. The commonly used standards for photovoltaic modules are as follows:
(1) IEC61215-2016 “Design Appraisal and Finalization of Crystalline Silicon Photovoltaic Modules for Ground Use”.
(2) IEC61646-2008 “Design Appraisal and Finalization of Thin-Film Photovoltaic Modules for Ground Use”.
Different types of photovoltaic modules are shown in Figure 5.
The power of photovoltaic modules on the market ranges from 2W to more than 300W. Common module manufacturers include: BP Solar, SCHOTT Solar, Isofitel, Solar World, Kaneka, Sharp, Kyocera, Suntech, Mitsubishi Electric, Uni Solar, Photowatt.
A qualified manufacturer should provide at least information in the specification list including: rated power (Pmax), warranty, power tolerance, maximum system voltage, rated power point voltage Ump, open circuit voltage Uoc, rated power point current Imp , Short-circuit current lsc.
If the standard manual is not provided, qualified manufacturers also need to provide the following information as required: power temperature coefficient, photovoltaic cell nominal operating temperature, open circuit voltage temperature coefficient, short-circuit current temperature coefficient.
The version of AS/NZS5033-2013 “Photovoltaic Array Installation and Safety Requirements” requires all modules installed in Australia to meet IEC61730.1.
IEC61730.1 requires components to be marked with the following: manufacturer’s trademark, type or model number and serial number, terminal polarity, maximum system voltage and safety level.
Other signs or content that should be included in the installation information are as follows: open-circuit voltage, short-circuit current, maximum overcurrent protection level, recommended maximum series/parallel module configuration, product application category.
5. Standard test conditions and nominal working temperature of photovoltaic cells
The data in the manufacturer’s specification list are all determined under standard test conditions. Because the characteristics of components will change with changes in conditions, such as temperature, radiation, etc., operating conditions must also be considered. Only under exactly the same conditions, the performance of different components can be compared with each other. Under international standard conditions, all components are tested in accordance with the following standard test conditions:
(1) The battery temperature is 25°C.
(2) The radiation intensity is 1kW/m².
(3) Air quality 1.5.
Testing under standard test conditions helps to rate the output power of the modules so that different modules can be compared and sold based on the values measured under standard test conditions. Nevertheless, under normal operating temperature conditions, when the components are in full sunlight, the battery temperature can be 25°C higher than the ambient temperature, and therefore higher than the standard test battery of 25°C.
Pool temperature. Therefore, many module manufacturers provide the nominal operating temperature of photovoltaic cells, that is, the cell temperature in the module under the following conditions.
(1) The atmospheric temperature is 20°C.
(2) The radiation intensity is 1kW/m².
(3) The wind speed is 1m/s.
(4) The circuit is open.
The difference between the nominal working temperature of the photovoltaic cell provided by the manufacturer and the ambient temperature of 20°C can be used to evaluate the actual temperature of the photovoltaic cell under the typical ambient temperature at the location where the module is installed. When considering the system design, assume that the difference is approximately 25°C.
Note: In the standard AS4509.2, the system design should adopt the power value when the ambient temperature is 25℃ or the current value when the ambient temperature is 25℃ and the voltage is 14V.