Sunday, March 8, 2009

Would Vehicle to Grid battery storage work?

Wind advocates like Mark Z. Jacobson claim that a system of using car batteries as a backup to a wind powered grid would compensate for the intermittency of wind. Jacobson states,
One calculation shows that the storage of electricity in car batteries, not only to power cars but also to provide a source of electricity back to the grid (vehicle-to-grid, or V2G), could stabilize wind power if 50% of US electricity were powered by wind and 3% of vehicles were used to provide storage.
There are about 150 million cars registered in the United States. If each car contributed 10 kWh of battery storage capacity to the grid, that would give the grid 1.5 trillion watts, or enough electricity to provide the United States 100% of its average electrical demand for 4 hours. This would at first seem impressive, but would it work in practice? First we should note that the GM Volt's battery pack cam store in theory up to 16 kWh of electricity, and costs a reported $28,000. In practice the battery pack can be charged to 12.8 kWh (80%) of its rated capacity when plugged into the grid. Drawing out 10 kWh from the charged Volt battery would deeply discharge it. The Volt engine starts to recharge the battery when storage drops to 4.8 kWh. Thus it would be impossible to draw 10 kWh from the Volt battery pack without carbon emissions entering the picture.

(This information suggests why the claims of EEstor are so attractive. EEStor claims 52 kWh of storage for its EESU at 10% of the price of the Volt battery pack. )

There are several human behavior issues that need to be considered before we can assess the vehicle to grid scheme issue. First the Volt costs about twice the price of current autos, to replace all 150 million current cars with Volt technology cares would cost $6 trillion dollars. For the sake of argument I will assume that technological improvements in the Volt's battery pack would make 10 kWh available to the grid. How many cars could we count on to provide the full 10 kWh of electricity? First it is unlikely that all 150,000,000 cars would be replaced with electrical cars. We can assume that the doubling of auto price would decrease the car market. People are likely to choose lower cost transportation modes, rather than purchase such expensive cars. Low cost transportation options would include mass transit, and electrical bikes and scooters. For short trips, people will walk. Electrical bikes and scooters would not provide significant storage to the grid, but would be practical for short urban drives.

Let us assume that the 150,000,000 cars are replaced by 75,000,000 cars, and 75,000,000 electric bikes and scooters: that would cut our theoretically available auto battery storage capacity in half. Vehicle to grid advocates claim that only 8% of all vehicles are on the road at any one time. In fact they hold that this 8% figure is true in the middle of the night and at rush hour. We will overlook this issue for the time being. Assuming that people are not driving constantly during the six hours of morning and evening rush hour, we can assume that about 25% of cars are involved in rush hour commutes. That will give us about 37.5 million home to work commuters, a figure that is obviously too low. Assuming the same number of commuters in the electric car era, this would mean that half of all cars would be involved in commuting. Now the battery charge of commuting vehicles would not be available for grid back up, because the commuters need it to get to work and drive home. This would leave us with only 37.5 million cars available for battery back up. We cannot assume that the car owners will want to daily devote 100% of their battery charge to grid back up. People own cars for transportation purposes, not to back up the grid. Assume 50% battery storage is surplus, that would leave us with a little less that one hour of battery back up from the vehicle to grid system. Now Jacobson would have us believe that only 3% of the current American car fleet or 4.5 million cars could provide backup for the entire wind powered grid. This would mean that Jacobson believes the entire grid can be backed up with 45 billion Watt hours of electricity from back up batteries, assuming the batteries were on a 24 hour a cycle. A single nuclear plant could produce 24 billion watt hours of electricity in a day, or over half the electricity that Jacobson claims will back up a wind penetrated grid. To appreciate the magnitude of the backup problem it should be pointed out that on February 28, 2008, Texas wind electrical production dropped from 1,700 megawatts to about 300 megawatt in a 10 minute period. 1100 MW of backup capacity were brought on line during the wind outage. This outage would require the battery storage of 110,000 Texas cars if the wind outage continued for an hour. Clearly Jacobson has failed to conduct a serious analysis of the V2G idea, and this failure is consistent with Jacobson's generally shoddy standards of analysis.

It is clear that given Jacobson's account of V2G wind backup, the V2G system would clearly not provide enough backup to manage wind intermittency without resorting to other massive resources.


Anonymous said...

Nice debunking of yet another case of magic thinking :)

Jason Ribeiro said...

Interesting post Charles. As I read it, I was thinking not only would it take a significant new population of serial hybrid cars, but also the interface to connect those vehicles. Unless you have a suitable garage or carport and plan to stay at your residence for quite some time, many people might not partake in voluntarily cooperating to make this work, even if it could.

Also, wouldn't this grid backup be available mostly at night when demand is lower and wind speeds might be up?

I would like to think that a major component of the green movement would be to create more livable communities where people are less dependent on cars. Newer suburban communities plan with sprawl in mind. Denser communities where walking and bicycling can potentially reduce those numerous short trip car trips that the volt hopes to do with its battery. The essence of this car battery grid backup seems contrary to that ideal. I think creating more walkable/bicycle access is an easier sell than trying to reengineer the grid infrastructure.

lad said...

You didn't incorporate the power of pixie dust into your analysis. That is the magic ingredient that will make renewable energy replace nuclear and fossil fuels.

donb said...

There is yet another problem - the life subtracted from the battery pack by the extra cycling caused by V2G operation.

Let's make a generous assumption that the Chevy Volt battery is good for 10000 cycles, each cycle being 10 kWHr of energy. This is a lifetime total of 100,000 kWHrs of energy through the battery. If the battery costs $28,000 as reported by Charles Barton, we have a cost of $0.28/kWHr for the battery alone. Now add the cost of electricity to charge the battery in the first place, and we have some rather expensive electrical energy.

This sort of cost may be tolerable for a few hours per year when everybody is running their air conditioners. But with wind and solar, one would see these costs nearly daily.

Anonymous said...


We reached Peak Pixie Dust a long time ago. They aren't making anymore of it.

Bobcat said...

One problem that has not been taken into consideration is how the constant charging and discharging will effect the life of the battery. It seems that V2G along with normal wear and tear would reduce the life of a car battery quite a bit.

If that was the case then what sane car owner would allow his car to participate in this type of program, which means we would have a serious energy shortage.


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