“Consumers, industries, and governments are taking various measures to increase the use of renewable energy. This is transforming the power generation and transmission and distribution system from a centralized grid to a more intelligent grid that supports local power generation. Topology, and smooth supply and demand through smart grid interconnection.
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Consumers, industries, and governments are taking various measures to increase the use of renewable energy. This is transforming the power generation and transmission and distribution system from a centralized grid to a more intelligent grid that supports local power generation. Topology, and smooth supply and demand through smart grid interconnection.
According to a report from the International Energy Agency (IEA) in October 2019, by 2024, renewable energy generation will increase by 50%. This means that the global renewable energy generation capacity will increase by 1200GW, which is equivalent to the current installed capacity of the United States. The report predicts that about 60% of renewable energy will be in the form of solar photovoltaic (PV).
The growth of renewable energy
Figure 1. Renewable energy capacity growth by technology in 2019 C 2024
The IEA report also emphasizes the importance of distributed photovoltaic power generation systems, as consumers, commercial buildings and industrial facilities begin to produce their own electricity. It predicts that by 2024, the total capacity of distributed photovoltaic power generation will more than double to more than 500GW. This means that distributed photovoltaic power generation will account for nearly half of the total growth of solar photovoltaic.
Figure 2. Growth of distributed photovoltaic production capacity in 2007 C 2024
Advantages of photovoltaics
Why is solar photovoltaic power generation so important in the growth of renewable energy capacity? One obvious reason is that solar energy is very easy to use directly, especially in remote areas or off-grid areas. Another obvious reason is that there is a lot of solar energy. According to calculations, at sea level, 1 kW of electricity per square meter per day can be generated. If factors such as day/night cycle, incident angle, seasonality and other factors are considered, 6 kWh of electricity per square meter per day can be generated. .
Solar power generation uses the photoelectric effect to convert incident light into electrical energy. Photons are absorbed by semiconductor materials (such as doped silicon), and their energy excites electrons out of their molecular or atomic orbitals. These electrons can then dissipate their excess energy as heat and return to their orbits, or spread to the electrodes and form an electric current.
As with all energy conversion processes, not all energy input to the solar cell is output in the preferred form of electricity. In fact, the efficiency of monocrystalline silicon solar cells has been hovering between 20% and 25% for many years. However, the opportunity for solar photovoltaic power generation is so huge that for decades, the research team has been trying to use increasingly complex structures and materials to improve cell conversion efficiency, as shown in this picture of NREL.
Figure 3. The progress of global research on the conversion efficiency of solar cells (NREL) from 1976 to 2020 (this figure is provided by the National Renewable Energy Laboratory of Colorado, USA)
Typically, the higher efficiencies shown are achieved at the expense of using multiple different materials and more complex and expensive manufacturing techniques.
Many solar photovoltaic devices rely on various forms of polysilicon or thin films of silicon, cadmium telluride or copper indium gallium selenide, with conversion efficiencies in the range of 20% to 30%. The units are built in modules, which are the basic units of solar photovoltaic power generation systems.
Efficiency challenge
20%-30% is an ideal state. In fact, the conversion efficiency may be reduced due to various reasons: rainfall, snow and dust deposition, material aging, and environmental changes, such as the growth of vegetation or the installation of new buildings. Increase the shadow.
Therefore, the actual reality is that although solar energy is free, the electricity generated by solar energy needs to be carefully optimized, including every stage of conversion and storage. One of the biggest technologies to improve efficiency is the design of inverters, which convert the DC output of the solar cell array (or its battery storage) into AC current for direct consumption or transmission through the grid.
The inverter works by switching the polarity of the DC input current to make it close to the AC output. The higher the switching frequency, the higher the conversion efficiency. A simple switch can generate a square wave output that can drive a load, but harmonics will lose more current. Therefore, the inverter needs to balance the switching frequency to improve efficiency, operating voltage, and power capacity. In addition, it also needs to balance the cost of auxiliary components for minimizing the square wave.
Advantages of SiC
Silicon carbide (SiC) has many advantages over silicon in solar power applications. Its breakdown voltage is more than ten times that of traditional silicon. SiC devices also have lower on-resistance, gate charge and reverse recovery charge than silicon. Characteristics, and higher thermal conductivity. These characteristics mean that SiC devices can switch at higher voltages, frequencies, and currents than silicon equivalent devices, while managing heat dissipation more effectively.
MOSFETs are favored in switching applications because they are unipolar devices, which means they do not use minority carriers. Silicon bipolar devices (IGBTs) that use both majority and minority carriers can operate at higher voltages than silicon MOSFETs, but they need to wait for electrons and holes to recombine and dissipate recombination during switching. Energy, so its switching speed becomes slower.
Silicon MOSFETs are widely used in switching applications up to 300V. Above this voltage, the on-resistance of the device rises and designers have to switch to slower bipolar devices. The high breakdown voltage of SiC means that it can be used to make MOSFETs with much higher voltages than possible in silicon, while retaining the fast switching speed advantages of low-voltage silicon devices. The switching performance is also relatively independent of temperature, so that stable performance can be achieved when the system heats up.
Since the power conversion efficiency is directly related to the switching frequency, SiC can not only handle higher voltages than silicon, but also ensure the ultra-high switching frequency required for high conversion efficiency, thus achieving a win-win situation.
The thermal conductivity of SiC is also three times that of silicon and can be operated at higher temperatures. Silicon cannot operate normally at around 175°C, and even directly becomes a conductor at 200°C. This happens to SiC until around 1000°C. The thermal properties of SiC can be used in two ways. First, it can be used to manufacture power converters, which require less cooling systems than equivalent silicon systems. In addition, the stable operation of SiC at higher temperatures can be used to manufacture dense power conversion systems where space is at a premium, such as vehicles and cellular base stations.
These advantages play an important role in power boost circuits with higher solar conversion efficiency. The circuit is designed to match the output impedance of the solar cell array (which varies with the level of incident light) with the input impedance required by the inverter to achieve the best conversion.
Figure 4: The introduction of SiC devices to improve the efficiency of solar boost circuits (ON semiconductor)
The image on the far left shows the lowest cost method, which uses silicon diodes and MOSFETs. As shown in the middle figure, the first optimization scheme is to replace silicon diodes with SiC versions, which will increase the power density and conversion efficiency of the circuit, thereby reducing system costs. As shown in the figure on the right, silicon MOSFETs can also be replaced with SiC equivalent, which provides designers with more switching frequency options, thereby further improving the conversion efficiency and power density of the circuit.
Using the familiar TO220 and TO247 packaged ON Semiconductor SiC Schottky diodes, rated voltage and current up to 1200V and 20A. It also provides bare chips for module manufacturers with rated voltages and currents up to 1200V and 50A.
There are also many 1200V SiC MOSFETs in the familiar D2PAK and TO247 formats, with typical RDSon as low as 20mW.
The company also sells hybrid modules that combine silicon IGBTs and SiC diodes, such as power integrated modules (PIM). It has dual boost characteristics, including two 40A / 1200V IGBTs, two 15A / 1200V SiC diodes and two 25A / 1600V anti-parallel diodes for IGBTs. Two other 25A/1600V bypass rectifiers can limit inrush current, and the module also has thermistor protection.
For those who want to use SiC in solar photovoltaic devices, ON Semiconductor has also developed a series of two-channel or three-channel SiC boost modules for solar inverters.
SiC power devices have many advantages over silicon alternatives, including their ability to switch high voltages and currents with high speed, low loss, and good thermal performance. Although they may be more expensive than silicon products on a similar basis (if silicon substitute products can be used), their good performance in the system can bring total cost savings, such as heat dissipation costs, area costs, and so on. Then there is the issue of efficiency. If the deployment of SiC can increase the efficiency by 2%, it will generate an additional 10GW of electricity.
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