Vehicle-to-grid (V2G) describes a system in which plug-in electric vehicles, such as electric cars (BEVs) and plug-in hybrids (PHEVs), communicate with the power grid to sell demand response services by either delivering electricity into the grid or by throttling their charging rate.
Vehicle-to-grid can be used with such gridable vehicles, that is, plug-in electric vehicles (BEVs and PHEVs), with grid capacity. Since most vehicles are parked an average of 95 percent of the time, their batteries could be used to let electricity flow from the car to the power lines and back, with a value to the utilities of up to $4,000 per year per car.
One notable V2G project in the United States is at the University of Delaware, where a V2G team headed by Dr. Willett Kempton has been conducting on-going research. Their goals are to educate about the environmental and economic benefits of V2G and enhance the product market. Other investigators are the Pacific Gas and Electric Company, Xcel Energy, the National Renewable Energy Laboratory, and, in the United Kingdom, the University of Warwick.
V2G is a version of battery-to-grid power applied to vehicles. There are three different versions of the vehicle-to-grid concept:
- A hybrid or Fuel cell vehicle, which generates power from storable fuel, uses its generator to produce power for a utility at peak electricity usage times. Here the vehicles serve as a distributed generation system, producing power from conventional fossil fuels, biofuels or hydrogen.
- A battery-powered or plug-in hybrid vehicle which uses its excess rechargeable battery capacity to provide power to the electric grid in response to peak load demands. These vehicles can then be recharged during off-peak hours at cheaper rates while helping to absorb excess night time generation. Here the vehicles serve as a distributed battery storage system to buffer power.
- A solar vehicle which uses its excess charging capacity to provide power to the electric grid when the battery is fully charged. Here the vehicle effectively becomes a small renewable energy power station. Such systems have been in use since the 1990s and are routinely used in the case of large vehicles, especially solar-powered boats.
Peak load leveling
The concept allows V2G vehicles to provide power to help balance loads by "valley filling" (charging at night when demand is low) and "peak shaving" (sending power back to the grid when demand is high). It can enable utilities new ways to provide regulation services (keeping voltage and frequency stable) and provide spinning reserves (meet sudden demands for power). In future development, it has been proposed that such use of electric vehicles could buffer renewable power sources such as wind power, for example, by storing excess energy produced during windy periods and providing it back to the grid during high load periods, thus effectively stabilizing the intermittency of wind power. Some see this application of vehicle-to-grid technology as a renewable energy approach that can penetrate the baseline electric market.
It has been proposed that public utilities would not have to build as many natural gas or coal-fired power plants to meet peak demand or as an insurance policy against blackouts Since demand can be measured locally by a simple frequency measurement, dynamic load leveling can be provided as needed.
carbitrage: This is a portmanteau of 'car' and 'arbitrage'. When the electric utility would like to buy power from the V2G network, it holds an auction. The car owners are able to define the parameters under which they will sell energy from their battery pack. Many factors would be considered when setting minimum sale price including the cost of the secondary fuel in a PHEV and battery cycle wear. When this minimum price is satisfied, it is deemed as meeting carbitrage.
Backup power solutions
Future battery developments may change the economic equation, making it advantageous to use newer high capacity and longer-lived batteries in BEV/PHEVs and in grid load balancing and as a large energy cache for renewable grid resources. Even if cycled daily, such batteries would only require replacement/recycling every 55 years or so. Since BEVs can have up to 50 kWh worth of battery storage they represent somewhat more than the average home's daily energy demand. Even without a PHEV's gas generation capabilities such a vehicle could be used for emergency power for several days (for example, lighting, refrigerators etc. with combined load of 1 kW could be powered for 50 hours). This would be an example of V2H or Vehicle-to-home transmission. As such they may be seen as a complementary technology for intermittent renewable power resources such as wind or solar electric.
These utilities currently have V2G technology trials:
- PG&E, USA, converting a number of company-owned Toyota Prius to be V2G PHEVs at Google's campus
- Xcel Energy, USA, converting six Ford Escape Hybrids to PHEVs with V2G 
University of Delaware
Dr. Willett Kempton, Dr. Suresh Advani, and Dr. Ajay Prasad are the researchers at the US University of Delaware who are currently conducting research on the V2G technology, with Dr. Kempton being the lead on the project. Dr. Kempton has published a number of articles on the technology and the concept, many of which can be found on the V2G project page. The group is involved in researching the technology itself as well as its performance when used on the grid. In addition to the technical research, the team has worked with Dr. Meryl Gardner, a Marketing professor in the Alfred Lerner College of Business and Economic at the University of Delaware, to develop marketing strategies for both consumer and corporate fleet adoption 
Denmark's Edison project, an abbreviation for 'Electric vehicles in a Distributed and Integrated market using Sustainable energy and Open Networks' is an ongoing partially state funded research project on the island of Bornholm in Eastern Denmark. The consortium of IBM, Siemens the hardware and software developer EURISCO, Denmark's largest energy company DONG Energy, the regional energy company Østkraft, the Technical University of Denmark and the Danish Energy Association, is currently exploring how to balance the unpredictable electricity loads generated by Denmark's many wind farms, currently generating ~20% of the country's total electricity production, by using electric vehicles (EV) and their accumulators. The aim of the project is to develop infrastructure that enables EVs to intelligently communicate with the grid to determine when charging, and ultimately discharging, can take place. At least one rebuild V2G capable Toyota Scion will be used in the project. The project is key in Denmark's ambitions to expand its wind-power generation to 50% by 2020. According to a source of British newspaper The Guardian 'It's never been tried at this scale' previously.
There is some skepticism among experts about the feasibility of V2G. As the New York Times states:
|“||An analyst at the Minneapolis-based utility Xcel Energy, [explained] a "pie-in-the-sky vision" for V2G in which a company would offer incentives to its employees to buy plug-in hybrids. The parking lot would be equipped with recharging stations, which could also return power to the grid from the vehicles.||”|
An Environmental Defense representative stated: "It’s hard to take seriously the promises made for plug-in hybrids with 30 miles (48 km) all-electric range or any serious V2G application any time soon. It’s still in the science project stage (in 2007)."
The Vehicle-to-grid potential of Honda’s full hybrid vehicles is unexplored, but Honda is doubtful of using them to power homes. "We would not like to see stresses on the battery pack caused by putting it through cycles it wasn’t designed for," said a Honda spokesman. "Instead, they should buy a Honda generator that was made for that purpose." However, in December 2013, Honda announced a partnership with the University of Delaware where they delivered an Accord Hybrid with on-board bidirectional charger to enter into the PJM Interconnection's frequency regulation market.
Poor net efficiency
Charging a fairly efficient battery system from the grid is at best 70 to 80% efficient. Returning that energy from the battery to the grid, which includes "inverting" the DC power back to AC with efficiencies of about 90% yields 63 - 72% energy return to the system. This needs to be factored against potential cost savings as well as the additional wear and tear on the batteries (current batteries last a few thousand cycles at maximum) and especially increased emissions if the original source of power is fossil based. The cycle of energy efficiency needs to compared with pumped-storage hydroelectricity which is less efficient. In addition, it is possible to take a small amount of energy from a large number of batteries if there are enough PHEV's distributed in society. 1 kW from 1000 vehicles is 1 megawatt of power and the energy is already distributed so it will not tax the existing powerlines if properly managed.
Not all skepticism is warranted. Jon Wellinghoff, from the US Federal Energy Regulatory Commission, points out that partial grid regulation (absorbing excess surges, but not supplying peak power) can be done without decreasing the life of the battery. This can be done "without affecting the charge whatsoever." 
The REV 300 ACX vehicle includes a V2G system.
- Charge Control
- Charging station
- Distributed generation
- Electrical substation
- Electricity meter
- Energy demand management
- Feed-in Tariff
- Grid energy storage
- Grid-tied electrical system
- Inverter (electrical)
- List of energy storage projects
- Load profile
- Load balancing (electrical power)
- Operating reserve
- Peaking power plant
- Power outage
- Small is Profitable
- Smart meter
- Unified Smart Grid
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- In Two-Way Charging, Electric Cars Begin to Earn Money From the Grid April 25, 2013 NYT