New semiconductor-based devices for managing power on the grid could make the "smart grid" even smarter. They would allow electric vehicles to be charged fast and let utilities incorporate large amounts of solar and wind power without blackouts or power surges. These devices are being developed by a number of groups, including those that recently received funding from the new Advanced Research Projects Agency for Energy (ARPA-E) and the National Science Foundation.
Smart Transformer: A prototype of a smart solid-state transformer from the Electric Power Research Institute. It’s smaller and more versatile than today’s transformers. The module on the left converts high-voltage alternating current from the grid to direct current. On the right is an inverter that converts that power to the 120-volt AC that comes out of standard wall outlets. To the right of the outlets are two more power interfaces, one for 240-volt AC power and one for 400-volt DC.
As utilities start to roll out the smart grid, they are focused on gathering information, such as up-to-the-minute measurements of electricity use from smart meters installed at homes and businesses. But as the smart grid progresses, they'll be adding devices, such as smart solid-state transformers, that will strengthen their control over how power flows through their lines, says Alex Huang, director of a National Research Foundation research center that's developing such devices. "If smart meters are the brains of the smart grid," he says, "devices such as solid-state transformers are the muscle." These devices could help change the grid from a system in which power flows just one way—from the power station to consumers—to one in which homeowners and businesses commonly produce power as well.
Today's transformers are single-function devices. They change the voltage of electricity from one level to another, such as stepping it down from the high voltages at which power is distributed to the 120- and 240-volt levels used in homes. The new solid-state transformers are much more flexible. They use transistors and diodes and other semiconductor-based devices that, unlike the transistors used in computer chips, are engineered to handle high power levels and very fast switching. In response to signals from a utility or a home, they can change the voltage and other characteristics of the power they produce. They can put out either AC or DC power, or take in AC and DC power from wind turbines and solar panels and change the frequency and voltage to what's needed for the grid. They have processors and communications hardware built in, allowing them to communicate with utility operators, other smart transformers, and consumers.
The devices are so flexible that researchers are still working out how to make the best use of them. There are several possibilities. Today, charging an electric vehicle at home takes many hours, even if it's plugged into a special charger with 220/240-volt circuits rather than more common 110/120-volt outlets. Direct-current chargers can cut the time for charging a 24-kilowatt-hour pack like the one in the new Nissan Leaf from eight hours to just 30 minutes, but they're inefficient, wasting about 10 to 12 percent of the power that comes in to them. The new transformers could replace these special chargers, and they're more efficient, wasting only about 4 percent of the power, says Arindam Maitra, a senior project manager at the Electric Power Research Institute, which is developing smart transformers. What's more, because the transformers have communications and processing capability, if several neighbors plug in their cars to charge at the same time, the transformers can prevent circuits from being overloaded by slowing or postponing charging based on consumer preferences and price signals from the utility. The same devices can also be used to send DC power from solar panels to the grid, eliminating the need for some equipment currently used to convert the power from solar panels and leveling out fluctuations in their voltage that could otherwise cause the panels to trip off and stop producing electricity.
As power consumers such as big-box stores start to install more solar panels and energy-storage devices, smart transformers could be key to integrating power from these sources and the grid, Maitra says. Storage systems and distributed energy can allow stores to decide when to draw power from the grid and when to send power back to it, depending on the price of electricity at a given moment. Smart transformers could coordinate this potentially rapid change from buying to selling power, while keeping the grid stable and preventing neighbors' lights from dimming. They could even allow people to buy electricity from their neighbors, Huang says. "If you plug in your electric car at night, you could charge it by negotiating with those in your neighborhood who have excess power," he says. "You actually pay him. You don't pay the utility."
Other kinds of devices can do many of the same things, but the idea of coordinating a large number and variety of consumer-owned devices makes utilities nervous about their ability to keep the grid stable. The new transformers would simplify the system and be utility-owned, making it easier for grid operators to keep the lights on, Maitra says.
Another potential benefit of smart transformers—or what the Electric Power Research Institute is starting to call smart-grid interfaces—is saving energy. For one thing, they can set the voltage of electricity at any given time so that it is at the minimum level appliances need to perform properly. One recent study suggested that doing this could reduce power consumption in the United States by up to 3 percent, which is equivalent to several times as much power as is now generated by all solar panels in the U.S. Even larger energy savings could be seen if smart transformers supplied DC power rather than AC to servers in data centers. Ordinarily, the servers convert the AC to DC themselves—and they do it inefficiently. (Other inefficient conversions, too, are involved in the uninterruptable power supply.) A recent demonstration of such a system by Duke Energy, a large utility company, and the Electric Power Research Institute found that supplying DC could cut power consumption at data centers by about 15 percent.
Smart solid-state transformers are still in the development stage and likely are a few years away from being ready for market—researchers are still working on their efficiency and cost, for example. Taking advantage of their DC capability will require developing new construction standards for homes and businesses. Mark Wyatt, the vice president of smart-grid and energy systems at Duke Energy, cautions that solid-state transformers will need to be supplemented with other devices for controlling power on the grid, and they may not prove cost-effective in many areas. "It's not one size fits all," he says.
Yet in the long term, Huang says, smart transformers and other smart solid-state devices could enable an unprecedented amount of two-way power flow. "It could be revolutionary to how we construct the grid," he says.
By Kevin Bullis
From Technology Review