As the world struggles to meet the challenge of preventing catastrophic climate change, the power generation industry has been recognized as the most important area for reducing carbon emissions. Therefore, more than half of the states in the United States have enacted renewable electricity directives, including some of the largest states such as California, Texas, and New York, and the European Union’s Renewable Energy Directive has set similar targets. Especially due to the intermittency and variability of wind energy and solar energy, the integration of renewable energy resources is an increasingly serious challenge for public utilities.
In the past ten years, the cost of wind and solar energy has dropped sharply. In many cases they have a competitive advantage over fossil fuels, especially when deployed on a grid scale. Commercial and industrial-scale installations are also extremely economical, as evidenced by the fact that companies such as Wal-Mart, Target and Amazon have placed a large number of solar panels in warehouses and retail locations. With the continuous development of offshore wind energy and non-fixed solar panel technology, the applicable points of renewable energy are also expanding.
Coupled with the ever-expanding residential solar capacity, another challenge for utilities is that the integration of distributed energy is not under their control. Some states impose mandatory regulations on the net metering or feed charges behind the electricity generated by the meters, which adds complexity and affects utility revenue.
Another major challenge is also related to climate change: the safety and reliability of grid infrastructure. The recent wildfires in California and the bankruptcy of PG&E are early signs of how extreme weather and climate change will affect the power grid. PG&E even now conducts preventive large-scale power outages to protect equipment, customers and forests.
Another resource added to this combination is energy storage. Energy storage can take many forms, including pumped storage, large flywheels, subsea pressurized airbags, and even cranes lifting huge concrete blocks. Many of these options require large-scale construction to save costs, or require very specific geographic features.
The most prominent and fastest-growing energy storage technology is batteries. Batteries are highly scalable and can be used from household scale to power plant scale. They can also be deployed in almost any location without the need for extensive environmental assessment, infrastructure construction and consideration of local regulations like traditional power plants. In the end, companies have confirmed that they can install large batteries in just six months, which is in stark contrast to the decades needed to plan and support fossil fuel power generation.
Energy storage brings many benefits, especially when combined with intermittent renewable energy. The most obvious use of energy storage is energy arbitrage. When electricity prices are low, energy is stored and then sent back to the grid when electricity prices are high. On a sunny day, when photovoltaic (PV) sources generate excessive power, electricity can flow into the storage element, so that these "must-consume" resources can be utilized to the greatest extent. At night, when the solar power generation declines, the battery will supply the lost power, and the base load power generation will rise. Therefore, many large-scale battery facilities are deployed in the same location as solar farms.
If PG&E shuts down customers when the risk of fire is high, batteries and solar panels will protect homes and businesses from power outages, thereby keeping critical processes running and preventing food from spoiling. In addition, power operators are now coordinating and controlling distributed energy sources as "virtual power plants" that generate, store, and transmit electricity according to demand. In some cases, this includes demand response, where the electrical load is shifted to off-peak hours.
The key interface for connecting wind, photovoltaic and battery sources to the grid is the inverter. Simply put, the inverter converts DC power to AC power and synchronizes it to the 60Hz electrical frequency of the grid. Figure 1 shows a simplified diagram of a solar panel connected to the grid, focusing on the structure of the inverter. There are many styles of inverters, including unidirectional and bidirectional and multi-level inverter multi-topology structures. Each topology has its own advantages and disadvantages under specific circumstances. The key component of the inverter is the power switch, shown in the figure as an insulated gate bipolar transistor (IGBT).
Inverter: inverter
ACGrid: AC grid
The inverter uses a microprocessor, appropriate detection and feedback, and correct algorithms to provide various services to the grid, not just storing and releasing electrical energy. One example is voltage support, frequency regulation, and harmonic reduction to maintain power quality. Distributed energy can reduce the load on power transmission and distribution networks because electrical energy is used close to the power generation. This can reduce the strain and congestion of the power grid, and even postpone the upgrade of the power line.
When a large amount of power passes through the inverter, the conversion between AC and DC power must be very efficient. In fact, the peak efficiency of commercial inverters is 96-98%. But grid operators want higher energy efficiency, especially on the scale of utilities, because small changes in energy efficiency still mean a lot of electricity.
In order to achieve these energy efficiency levels, power devices must have very low losses. Today, IGBT has become the main switch for these applications. However, the conduction current of an IGBT is several hundred amperes, blocking voltages of several thousand volts. It is made of silicon using a process similar to that used to manufacture high-performance computing chips for mobile phones and data centers.
However, new materials are expected to achieve higher performance, higher energy efficiency and higher reliability. Specifically, silicon carbide (SiC) is the material of the future. SiC power electronic devices have lower conduction and switching losses than similar silicon devices. The first stage of the transition involves a low-level diode, as shown in Figure 1, which is connected to the IGBT in anti-parallel. Replacing silicon diodes with SiC diodes can reduce losses and reduce overshoot during switching, thereby reducing stress on the inverter. Although SiC diodes are more expensive than silicon diodes, a smaller heat sink and system size can reduce overall system cost.
SiCMOSFET is the next stage of the transition. The switching speed of SiC MOSFETs is much faster than that of silicon IGBTs, so their use in the boost stage of solar power generation systems brings greater advantages. Generally, a DC-DC converter is used to increase the output voltage of the solar panel. SiC MOSFETs can switch faster, thereby reducing the size of expensive passive components such as inductors in the boost stage and improving efficiency.
ON Semiconductor provides a variety of IGBTs, SiC diodes and SiC MOSFETs to meet the voltage and current requirements of various inverters. The most popular is the power module, which packs many different power switches and diodes together to achieve small size, easy design and efficient heat dissipation. In addition to the main power electronic devices, ON Semiconductor also provides gate drivers, galvanic isolation and high-performance operational amplifiers to complete the system.
With the improvement of renewable energy and energy storage technology and the decrease in cost, the "reverse change" of the power grid continues to proceed at an ever faster rate. In addition to reducing carbon emissions and pollution, inverters also support a more flexible and participatory power grid, blurring the boundaries between consumers and producers. The correct control and coordination of power companies can improve power quality, reduce upgrade costs, and provide users with more reliable services. Power electronics is the key enabling technology that enables our critical infrastructure to be updated.