Luis Arnedo (left) and Fei (Fred) Wang in the High Power Laboratory at the Center for Power Electronics Systems. Behind them is a "plug-and-play" modular converter and beside them is a converter system testbed
Imagine a device that automatically detects demand for power and delivers processed electricity in the required form (AC or DC) at the correct voltage and frequency. Energy routers, analogous to the familiar data and communications routers, could improve reliability, efficiency, and safety at every level of the electrical power system.
Ships, aircraft, and land vehicles could be made smaller, lighter, and more fuel-efficient. Businesses, factories, and even homes could better use greener and more efficient energy sources and loads, such as different equipment and appliances with variable-speed motors. The power grid could employ intelligent energy routers to quickly isolate problems while still providing necessary power.
Energy routers are still a concept, and their true realizations may be years in the future, but they are a dream of many power electronics researchers today. According to Fei (Fred) Wang, an associate professor of electrical engineering, at the heart of the concept are the electronic power distribution systems promoted by the Center for Power Electronics (CPES), an NSF Engineering Research Center led by Virginia Tech.
"It's a long-term goal, but in the end, your power electronic converter will be an energy router," he says.
Smaller, lighter high-power
Wang's interest in the concept evolved from his work in high-power electronicsdeveloping electronic circuits to convert and control electricity in systems that range from tens of kW to megawatts of power.
The team led by him and his colleagues at CPES is currently focused on improving the power systems of jets, ships, land vehicles, mini-grids, and even oil rigs. The team is not only engineering high-density power converters, but also designing the architecture and concepts that will help control and operate the high-power electrical systems of the future.
In high-power applications, the motivation for developing higher density power converters springs from a drive toward greater fuel efficiency, a desire to use less materials, and a need for increased usable space, Wang says. For example, the team is working with Boeing on an advanced motor drive system for potential aircraft use. "The goal is that it be lightweight and compact," he says. "To make these systems smaller, we must use advanced technology, like better semiconductor and magnetic materials, better capacitors, better cooling and packaging, new circuits, and new controllers."
Part of the solution was using advanced silicon carbide (SiC) switches that are not yet commercially available. "SiC can operate at 300C, whereas silicon can only work up to 150C. That is a big difference and means we need less cooling and a smaller heat sink." SiC also has lower loss and allows very fast switching enabling the design to reduce other components, such as capacitors and inductors, he says. "Every component we eliminate equals a savings in space and weight."
CPES is working on a variety of projects with other aircraft firms, including Rolls Royce, General Electric (GE), and SAFRAN. Much of the focus involves technology for the more-electric aircraft. The aircraft retain their fuel engines, but from that point onward, the goal is for all the subsystems to be electrical. Power density and reliability are key requirements for such a system.
The U.S. Navy has similar goals for its ships. "The Office of Naval Research (ONR) has been funding research on the all-electric ship, for, among other things, the reduced size and weight," Wang says. "We are looking at everything after the generating plant. This means we are trying to get rid of bulky, inefficient mechanical and hydraulic equipment. To do that, however, we need a family of high density power converters as large as 100 MW." Because the goals are also to reduce size and weight, the CPES team is collaborating with industry and national labs and studying the potential use of SiC devices for these converters, he says.
Transforming the entire system
Developing the individual electronic componentsno matter how largeis only part of the story, Wang says. "When we put all these converters together, how can we make sure they work together in an optimal way"
Conventional high-power systems employ generators, motors, transformers, and mechanical devices to process electricity, he says. "Now, we are talking about systems with a high penetration of electronic power converters. Power converters have much faster dynamics than traditional equipment. They are also highly nonlinear. This is good and bad."
Electronic converters are intelligent and fast, but they generate a lot of noise. This can result in poor power quality and electromagnetic noise interference. "We are now studying these system issues. We need to understand the interactions for systems to work in an optimal way," Wang says. Although the research is still at an early stage, he is already talking about "getting rid of the bulky stuff," like mechanical protection devices and breakers.
CPES has conducted power architecture studies for server, data center, aircraft and ship power systems, and is currently investigating a power system for offshore oil drilling. "Traditionally, these oil rigs use pneumatic power, but they want to use electric. How do you design that"
Another project, for the NSF, involves designing a distributed generation microgrid for electric utilities. "Today the grid is slow and based on electromechanical inertia. There is so much inertia that it doesn't crash easily," Wang explains. "Once we have electronic converters, they will operate so fast that we will no longer have inertia in the system. We will maximize utilization, but we need to make sure that while working fast, our system can also react fast."
Whether for offshore drilling, vehicles, or power grids, the questions can be as basic as whether to use AC or DC power, and at what voltage, he says. "The problem is that everybody is hand waving, but nobody has an analytical tool." CPES wants to develop a fundamental tool that uses scientific evaluation and provides engineers with the ability to model, analyze, and control the power in the most efficient manner possible.
Once we have a raw form of electricity, how do we convert it to what we need Wang and his CPES colleagues believe that future systems will be electronic power distribution systems, consisting of three types of power converters: source converters tied to the generating source, distribution converters for power delivery, and load converters for the dedicated use. The ultimate answers to getting the right form of power when needed may transform the whole system. One of those changes may be energy routers. Imagine life without wall warts