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June 29, 2012


As electric vehicles become more widely deployed the public is having to deal with many new acronyms; BEV (Battery Electric Vehicles), PHEV (Plug-in Hybrid Electric Vehicles), EREV (Extended Range Electric Vehicles) and V2G (Vehicle-to-Grid). Many of these acronyms are fairly easy to understand, but V2G is a very complex term that can have many meanings depending on the context.

When one sees the term G2V it is understood to refer to the normal process of using the electric grid to charge the battery of an electric vehicle. V2G however is better understood as using the battery as a source of electric energy. The discharge of the battery to generate electricity may or may not have a connection to the grid. There are a variety of applications for the technology; Vehicle-to-House (V2H), Vehicle-to-Building (V2B), and, of course, V2G.

V2H is not a new concept. The use of an automobile battery as a source of electric energy is independent of the deployment of grid enabled electric vehicles. For example, there are press releases of a Florida resident plugging his Toyota Prius into the backup uninterruptible power supply unit in his house -- to run his refrigerator and lights following a hurricane  -- that caused a wide spread power outage. Mitsubishi (News - Alert) sent 80 vehicles to the Tsunami area to enable homeless persons to have electricity to cook. This battery usage has been at hoc since cars were not designed to allow bidirectional electric energy flow.

In August 2011, Nissan announced they would design the Leaf to enable bidirectional energy flow. The Leaf battery can provide enough electric power to run an average Japanese home for up to two days. This would be very useful to accommodate the rolling black-outs resulting from the loss of power generating facilities due to the Tsunami. Toyota also announced that they would conduct V2H tests in Toyota City.

In the United States this capability could help alleviate power outages from weather related events such as hurricanes, tornados and heavy snowfall. However, the average American home uses 4 times more electricity than the Japanese home. This could require enabling electricity from the battery to run a subset of in-home demands for electricity, e.g. lights and refrigerator rather than to full household.

An extension of the concept of V2H is V2B. In this application, an employer with several employees could aggregate the electric energy from several vehicle batteries to run his/her building. This could mean paying employees for supplying electric power from their vehicle while they are working. This application is a possibility and some research indicates that it could be available in three to five years.

More extensive applications of V2G will depend on larger numbers of BEVs. Pike Research (News - Alert) that by 2017 approximately 90,000 light-duty vehicles and an additional 1,500 medium/heavy duty trucks will be enabled with V2G technologies. The global revenue for PEVs participating in ancillary services related to the electric grid would grow from less than $100,000 in 2011 to more than $18 million by 2017.

The most likely early adopters of V2G will be fleets of light-duty vehicles and medium- and heavy duty trucks. Fleets will be more easily integrated into the grid than individual consumers and trucks that have larger battery packs will generate greater revenue per vehicle than light-duty vehicles.

When it becomes more common to integrate vehicles with the grid and V2G becomes a reality, it may become viable for individual consumers to sell energy back to the grid during high demand periods. This is often what is thought of as when V2G is mentioned. The process is called “peak shaving”. A related process is “valley filling” when the battery is charged during times of low demand. A consumer will be motivated to charge at night with low charging rates and allow discharge during the day with higher charging rates. This will result in net revenue for the consumer.

However, the value to the consumer is unclear. Some research has indicated that when used exclusively in a peak shaving/valley filling role the net return to the owner can be so small that there will be little financial incentive for him/her to resell surplus power to the grid especially when the potential for life shortening of the battery due to excessive charge/recharge cycles is taken into account.

Many researchers have concluded that the value of V2G as a grid resource will be very important. V2G has been proposed as a source to provide ancillary services such as frequency and voltage regulation and spinning reserve. The concept of frequency and voltage regulation could be very important. As renewable energy sources become a larger fraction of electricity generation, the distributed energy sources and the uncertainty associated with the sun and the wind will cause stability issues. Using V2G to stabilize the grid is attractive because it can deal with the instabilities close to the source and it is always available.

As noted above, peak shaving and valley filling will impact the grid to some extent but other ancillary services could be more important. The highest value ancillary service is frequency stabilization. This application matches generation to load demand. It is under direct real-time control of the grid operator and must respond within a minute or less by increasing or decreasing the output. Increase is the use of V2G. Decrease is the normal G2V battery charging. It is controlled automatically by the grid operation and may be called several hundred times a day. It must respond within a minute and it only runs a few minutes at a time.  

In 2008 a report by Kempton, et al documented a practical demonstration of V2G providing real-time frequency regulation from an electric car. V2G presents a mechanism to meet key requirements of the grid, using electric vehicles when they are parked and underutilized. This approach to frequency stabilization can have average values of $30-$45/MW per hour, with hourly rates fluctuating widely around that average.

V2G can also address a second market, spinning reserves, i.e. additional generating capacity necessary to meet high demand. This has value in the range of $10/MW per hour, but with much less frequent usage. The primary revenue in both of these markets is for capacity rather than energy, and both markets are well suited for batteries as a storage resource because they are always available and can meet quick response times yet the actual total energy demand is low.

A later application, when parked V2G-capable cars are connected and aggregated in large numbers, would be to use them as dispersed energy storage for intermittent renewable resources such as wind and solar. The results of the Kempton demonstration show that V2G, in addition to providing valuable grid services, could also prove to be a prominent application in the global transition to the emerging green and sustainable energy economy.

The proposed ancillary services and energy storage are many years away from deployment. They depend on large numbers of V2G equipped vehicles being available. It is likely that fleet operations will be used in limited arrangements in the near future. However, the ability of V2G to address grid related issues has been demonstrated and will very likely be implemented as BEVs come on line.

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Edited by Amanda Ciccatelli

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