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Charge-packets will keep plug-in cars from crashing the grid


February 21, 2014 – According to the University of Vermont (UVM), the International Energy Agency forecasts there will be 20 million electric vehicles on the world’s roads by 2020, many of them plug-ins. So how do we keep them from crashing the grid when they plug in to recharge?

“How to manage all these cars seeking a socket at the same time—without crashing the grid or pushing rates to the roof—has some utilities wondering, if not downright worried,” reads the press release from UVM.

Yet a team of scientists from the university feel they have the answer.

“The key to our approach is to break up the request for power from each car into multiple small chunks—into packets,” explained Jeff Frolik, a UVM engineer and co-author on the new study.

By using the nation’s growing network of smart meters, the new approach would let a car charge for, say, five or 10 minutes at a time, then “get back into the line” and make another request for power. When demand is low, the vehicle would continue charging; when demand is high, the car has to wait.

“The vehicle doesn’t care. And, most of the time, as long as people get charged by morning, they won’t care either,” said UVM’s Paul Hines, co-author. “By charging cars in this way, it’s really easy to let everybody share the capacity that is available on the grid.”

Taking a page out of how radio and internet communications are distributed, the team’s strategy allows electric utilities to spread out the demand from plug-in cars over the whole day and night. The information from the smart meter prevents the grid from being overloaded.

At the same time, the Vermont team’s patent pending approach protects a car owner’s privacy: a charge management device could be located at the level of, for example, a neighbourhood substation, which would assess local strain on the grid. When demand isn’t too high, it would randomly distribute ‘charge-packets’ of power to those households putting in requests.

“Our solution is decentralized,” said Pooya Rezaei, a doctoral student working with Hines and the lead author on the paper. “The utility doesn’t know who is charging.” Instead, the power would be distributed by a computer algorithm that is the technical heart of the new approach. The algorithm is driven by rising and falling probabilities, which means everyone would eventually get a turn, but the utility wouldn’t know—or need to know—a person’s driving patterns or what house was receiving power, when.

“We assumed that drivers can decide to choose between urgent and non-urgent charging modes,” the scientists write. In the Urgent mode, the vehicle requests charge-packets regardless of the price of electricity. In this case, the system gives this car the best odds of getting to the front of the line, almost guaranteeing that it will be charged as soon as possible, but at full market rates instead of the discount rate that would be used as an incentive for those opting-in to the new approach.

Why put plug-in cars on ‘packetized’ demand instead of regular demand as with all other household loads? Because the new generation of car chargers are likely to be the biggest power load in a home, explain the scientists. “The load provided by an electric vehicle and the load provided by a house are basically equivalent,” said Frolik. “If someone gets an electric vehicle, it’s like adding another house to that neighbourhood.”