Power Electronics for Renewable Energy Systems

Hybrid renewable energy power systems are positioned to become the long-term power solution for portable, transportation and stationary system applications. Hybrid power systems are virtually limitless in possible setups and configurations to produce the desired power for a particular system. A hybrid system can consist of solar panels, wind power, fuel cells, electrolyzers, batteries, capacitors, and other types of power devices. Hybrid systems can be setup with power electronics to handle low, high, and variable power requirements. For example, solar panels can be used to convert solar energy into electrical energy when sunlight is directly hitting the PV panels for maximum efficiency, and then power from wind turbines can be used when wind speed and direction is ideal. The energy from these devices can be stored in batteries and used for electrolysis to produce hydrogen. The hydrogen can then be fed to fuel cells to provide power for long periods of time or portable or transportation applications. Power electronics provides a key element in stabilizing, boosting and managing the power when necessary.

The electrical output of a specific power system may not provide the input needed for a certain device. Many applications, such as grid or residential power, require AC power. Other devices such as cell phones require DC power. The output of fuel cells and batteries, however, is DC voltage with an intensity that depends on the number of cells stacked in series. An inverter can be used to change the output from DC to AC power when needed. Also, many renewable energy systems can have slow startup times and can be slow to respond to higher power needs. Therefore, systems usually have to be designed to compensate for high or intermittent power requirements. Power converters can be used to regulate the amount of power flowing through a circuit. Figure 1 shows a general schematic with a fuel cell that illustrates the power electronics component as a key element in the fuel cell system.

Figure 1. Power electronics as a key element.

Most renewable energy technologies only provide a certain voltage and current density (depending upon the load) to the power converter. The power converter must then adjust the voltage available from the fuel cell to a voltage high enough to operate the load. As shown in Figure 2, a DC-DC boost converter is required to boost the voltage level for the inverter. This boost converter, in addition to boosting the fuel cell voltage, also regulates the inverter input voltage and isolates the low and high voltage circuits.

Figure 2. Fuel cell power electronics interface diagram.

An example of a hybrid power system is shown in Figure 3. This fuel cell/lithium-ion battery charger system includes the following major components: the fuel cell, the lithium-ion battery, a constant voltage regulation system, and a smart battery charger. A rechargeable lithium-ion battery can be located inside the fuel cell unit to maintain the microcontroller in a low-power standby or programmed-timer sleep state for several days. The battery will also enable immediate system startup and power during system shutdown. The battery will be automatically charged whenever the fuel cell is running. The internal battery charging circuit will stop charging the Li-ion battery once it has reached a certain voltage or has been charged for a specific amount of time.

Figure 3. A diagram of the overall fuel cell / Li-ion charger system.

Converters for Power Systems

The two basic power electronics areas that need to be addressed in renewable energy applications are power regulation and inverters. The electrical power output of fuel cells, solar cells, and wind turbines are not constant. The fuel cell voltage is typically controlled by voltage regulators, DC/DC converters, and other circuits at a constant value that can be higher or lower than the fuel cell operating voltage.

Multilevel converters are of interest in the distributed energy resources area because several batteries, fuel cells, solar cells, and wind turbines can be connected through a multilevel converter to feed a load or grid without voltage-balancing issues. The general function of the multilevel inverter is to create a desired AC voltage from several levels of DC voltages. For this reason, multilevel inverters are ideal for connecting an AC grid either in series or parallel with renewable energy sources such as photovoltaics or fuel cells or with energy storage devices such as capacitors or batteries. Multilevel converters also have lower switching frequencies than traditional converters, which results in reduced switching losses and increased efficiency.

Advances in fuel cell technology require similar advances in power converter technology. By considering power conversion design parameters early in the overall system design, a small, inexpensive converter can be built to accompany a reasonably sized solar panel, wind turbine or fuel cell for high system power and energy density.

DC-to-DC Converters

A DC-to-DC converter is used to regulate the voltage because the output of a renewable energy system varies with the load current. Many fuel cell and solar cell systems are designed for a lower voltage; therefore, a DC-DC boost converter is often used to increase the voltage to higher levels. A converter is required for these renewable energy systems because the voltage varies with the power that is required. A typical fuel cell drops from 1.23 V DC (no-load) to below 0.5 V DC at full load. Consequently, a converter will have to work with a wide range of input voltages.

DC-to-DC converters are important in portable electronic devices such as cellular phones and laptop computers where batteries are used. These types of electronic devices often contain several subcircuits, that each has its voltage level requirement that is different than supplied by the battery or an external supply. As the battery’s stored power is drained, a DC-to-DC converter offers a method to increase voltage from a partially-lowered battery voltage which saves space instead of using multiple batteries to accomplish the same task. Figure 4 shows an example of a DC-to-DC converter device.

Figure 4. Example DC-DC Converter.

Inverters

Renewable energy can be used in both homes and businesses as the main power source. These energy systems will have to connect to the AC grid. The renewable energy system output will also need to be converted to AC in some grid-independent systems. An inverter can be used to accomplish this. The resulting AC current can be at the required voltage and frequency for use with the appropriate transformers and control circuits. Inverters are used in many applications from switching power supplies in computers to high voltage direct current applications that supply bulk power. Inverters are commonly used to apply AC power from DC sources such as fuel cells, solar panels, and batteries. Figure 5 shows an image of an inverter.

Figure 5. Example inverter.

Conclusion

Electronics are an important part of the devices that we use every day and a critical part of hybrid energy systems. These components help to transform direct current (DC) into alternating current (AC), help to increase the voltage of an energy system, regulate the power that a system provides, and/or creates the proper waveforms and timing that a motor requires. Without integrating these electronics into the system, the voltage and power produced by an energy system would not be very useful. Therefore, power electronics is an essential part of every hybrid energy system.

Dr. Colleen Spiegel Posted by Dr. Colleen Spiegel

Dr. Colleen Spiegel is a mathematical modeling and technical writing consultant (President of SEMSCIO) and Professor holding a Ph.D. and an MSc degree in Engineering. She has seventeen years of experience in engineering, statistics, data science, research & technical writing work for many companies as a consultant, employee, and independent business owner. She is the author of ‘Designing and Building Fuel Cells’ (McGraw-Hill, 2007) and ‘PEM Fuel Cell Modeling and Simulation Using MATLAB’ (Elsevier Science, 2008). She previously owned Clean Fuel Cell Energy, LLC, which was a fuel cell organization that served scientists, engineers, and professors world-wide.

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