V2G: Recharging the Grid with Electric Cars  

Technology Review has a post on a V2G pilot program in Newark - Recharging the Grid with Electric Cars.

A utility in Delaware has taken a step toward a future in which electric cars store renewable energy to help make its use more widespread. The city of Newark has approved a system called vehicle-to-grid (V2G), in which the battery pack in a car serves as a place to temporarily store energy from the power grid.

A big problem with renewable sources of power like solar or wind is that they only operate intermittently. For now, renewables provide such a small part of the total electricity supply that other sources can easily make up for the hours, minutes, or days when the wind isn't blowing and the sun isn't shining. But if we're ever to rely on them for a large part of our power, we'll need a cheap way to store the energy that they produce for when it's needed most.

The vehicle-to-grid concept suggests a way to store energy cheaply, since the batteries in electric cars have already been paid for. Most of the time, a car is just sitting around doing nothing. For short-term storage--needed to smooth out fluctuations in power from a wind turbine, for example--a utility could quickly charge a car (or, ideally, distribute a little charge to hundreds or thousands of cars) when the wind is blowing and then take that electricity back a few minutes later when the wind dies down. The more cars that are available, the more energy can be stored. Longer-term storage might also be possible: a car owner could charge up for a discount at night, provided she agreed to keep the car plugged in at work to supply extra power during peak power demand in the afternoon.

Of course, there would have to be some sort of agreement so that energy companies couldn't take so much that the driver ends up stranded, and they'd have to recharge a car before the evening commute. But there are even bigger challenges. There simply aren't many electric cars out there right now. Two-way hookups to the grid would also need to be installed, and the grid may have to be upgraded in other ways. And lastly, all the charging and discharging could shorten the lifetime of the battery (not to mention void the warranty from automakers).

The race is on to find the ideal battery chemistry for plug-in hybrids and all-electric vehicles, but a startup in Indiana believes that a combination of different storage technologies might be the best way to improve vehicle performance and reduce cost. The company's technology allows vehicles to run on a combination of fuel cells, ultracapacitors, and old-fashioned lead-acid batteries.

Noblesville-based Indy Power Systems has developed an energy management system for vehicles that can quickly switch between two or more energy sources, even when their voltages are different. "It's basically a switch that directs energy in any amount and any direction," says Steve Tolen, chief executive officer and founder of Indy Power, which operates out of Purdue Research Park. "The hardware handles the switching, and the software handles the timing and amounts."

Tolen says that the power electronics package--called the Multi-Flex Energy Management System--is only slightly larger than a laptop computer. He describes it as a custom, software-controlled, DC-to-DC converter that's bidirectional and variable.

"Imagine adding hot and cold water to a tub. We can add a variable amount of hot and a variable amount of cold in different volumes to match the outflow of the drain, which can also be variable," Tolen explains. "In other words, the motor can ask for different amounts of current, and we can provide that, and in different ratios from the two (or more) power sources, regardless of the voltage of the power sources."

For example, an electric vehicle could have both lead-acid and lithium-ion battery packs. Advanced lead-acid batteries may be cheaper, but they are also heavier and deteriorate more quickly if subjected to regular depletion and recharging. Lithium-ion batteries are generally more robust and lighter but are far more expensive. Combining the two means that you can use less of each. And just as important, says Tolen, the two chemistries can be balanced against each other to optimize performance. For example, the lithium-ion battery can be used to relieve stress on the lead-acid battery and extend its life, and vice versa.

Reza Iravani, a professor of electrical and computer engineering at the University of Toronto, says that Indy Power's system is part of a trend toward greater emphasis on hybrid storage. For example, he says, Researchers in Australia have designed a car-battery system that combines lead-acid technology with supercapacitors, resulting in a fourfold increase in the life of the lead-acid batteries.