Electric cars can become electricity banksFri, 23 Sep 2011
Ever try to store electricity? It won't stay in a bottle, and you can't keep it in your sock drawer. So when you have a lot of it--say, when it's windy and your wind turbines are really spinning, or when it's sunny and your gallium arsenide photovoltaics are lighting up, or at night when the utility's generators can run unhindered--you have to store it somewhere.
How about in the excess battery capacity of thousands of electric cars?
That's the idea behind bidirectional charging, also known as Vehicle-To-Grid or V2G. Dave Sivertsen of AC Propulsion (ACP) presented the case for V2G at a recent IEEE meeting in Southern California and we went along. AC Propulsion, whose power electronics units have been in many EVs the world over and will no doubt be in many more, has been developing V2G-capable systems for almost 20 years.
"Nineteen years ago, when Alan Cocconi founded ACP, he said, 'We might as well do everything we can in one box.' "
With V2G, Sivertsen said, electric power can flow to or from an electric car. You can use it to recharge the battery of a stranded electric car--the interface puts out alternating current (AC) at up to 18 kilowatts, which can be fed to the charge port of another vehicle capable of charging off AC. That provides more than a mile's range per minute of charging.
An EV can be used as a power source at a remote work site--as much as 80 amps single phase at 220 volts. And you can even power the refrigerator and air conditioning at your house during a blackout.
But the main reason to consider and implement V2G is that it can feed electricity back into the grid when needed.
"V2G vehicles keep the grid healthy with moment-by-moment responses to a corrective signal from the grid operator," said Sivertsen.
Fleets of EVs could even buffer peak demand on hot afternoons when everybody's powering their air conditioners full speed. This is known as "peak shaving." Often the difference between a blackout and full power is a small amount of juice. Drawing the needed power from fleets of EVs can easily make the difference. One hundred electric cars can equal about 1 megawatt, or 1 million watts.
And before you start howling, "the government ain't telling me what to do with my electricity," consider that when the utility draws the electricity from your car, it will pay you for it. In a 2002 V2G demo project presented to the California Air Resources Board, AC Propulsion found that an electric-car owner could earn up to $300 a month selling power back to the grid.
Many EV owners will not need to use the full capacity of their car's battery every day--most EVs will be parked most of the time, during which time they could be either charging, say late at night when electricity is cheaper, or discharging, like late afternoon when peak demand is hitting. A V2G infrastructure would know your driving profile and location so it would know how many kilowatt-hours you need to get home and wouldn't take all your juice and leave you stranded. It would take what you could spare and no more. You would simply be renting storage space in your battery pack, for which you'd be paid either directly or in reduced battery/vehicle cost upfront.
So far, V2G is still in the development stages. The University of Delaware has a fleet of seven AC Propulsion E-Box electric cars with V2G capability and has been working with them for three years. The Technical University of Denmark has one car to study V2G, the goal being to help Denmark's goal of using 100 percent wind power for its electricity needs. The U.S. Postal Service has an AC Propulsion electric mail truck and will begin V2G evaluations on it this year.
AC Propulsion is also converting a small fleet of Ford E-150 vans to electric power and V2G capability. And the fleet of 620 BMW Mini Es built by AC Propulsion are all V2G-capable, with some small changes to software and charge interface required to bring them online.
How would all this affect battery life? Good question.
"Batteries have a calendar life and an operational (cycle) life," said Sivertsen. "V2G will manage the power flow to get the best throughput--using the battery in a way that minimizes wear out and maximizes the economic benefit."
Ultimately, bidirectional charging is going to be necessary when the world warms up to electric cars. Now's the time for utilities to plan accordingly.
By Mark Vaughn