RechargeIT Blog

Vehicle to Grid (V2G) Overview

Tuesday, September 30, 2008 at 8:48 AM


Current concerns about climate change, energy security and record high oil prices have generated a lot of enthusiasm for plug-in vehicles -- both plug-in hybrids and pure battery electric vehicles. Widespread adoption of plug-in vehicles would result in significant reductions in CO2 emissions from transportation. It would also reduce our dependence on fossil fuels by replacing petroleum-sourced energy with renewable, domestically produced electricity. But the benefits of plug-in vehicles extend beyond just those well known areas. These vehicles are also enablers for new technology that could offer significant benefits to the electric grid and to plug-in vehicle owners. In particular, Vehicle to Grid (V2G) technology - in the form of vehicles capable of full bi-directional power flow (true V2G) and those capable of uni-directional "smart charging" - allows these grid-connected vehicles to provide grid stability and load management services in near real time. This gives the grid operators additional grid stability headroom, allowing even adoption of renewable energy on the grid. Plug-in vehicle owners would benefit from cheaper electricity rates and could even profitably contract with their utilities to have their vehicle provide grid ancillary services such as grid frequency, regulation and spinning reserves.

So how does V2G work "under the hood"? Historically, plug-in vehicles have simply connected to the grid and charged. There was no communication or control between the vehicle and the grid, and power flowed only from the grid to the vehicle. A fully V2G-capable plug-in vehicle is equipped with a communications interface that receives signals from the grid as well as an intelligent charging system/battery management system (BMS) that allows bi-directional power flow both to and from the vehicle. When this vehicle is connected to the grid, control signals are sent from the grid operator to manage the flow of energy between the vehicle and the grid. In the simplest case, the grid might just turn the vehicle charger on and off in response to grid load. The grid could also tell the vehicle to defer charging until off-peak hours, or possibly have the vehicle charge only when the electricity rates are at their lowest. In the most complex scenario, the grid might send a constant flow of messages to the vehicle, changing the charging rate or even reversing the flow of energy to feed back to the grid depending on a variety of factors including the current grid load, the current amount of renewable generation, the state of charge of the vehicle, and real-time energy pricing.

The benefits of V2G extend past grid load management. An NREL study shows that plug-in vehicles acting as a storage resource on the grid would help the penetration of intermittent renewable energy generation resources such as solar and wind. The energy storage offered by the plug-in vehicles would help smooth the peaks and valleys of renewable energy. This is particularly beneficial in the case of wind energy, which in many geographic locations often produces the greatest amount of energy during low-demand hours, such as late in the evening or at night.

Though V2G has many benefits to offer, it is not a near-term solution. Deploying V2G will require significant investment to evolve the existing grid into the Smart Grid of the future - the existing grid simply does not have the infrastructure in place to communicate with vehicles. Additionally, the V2G services depend on a fairly large scale deployment of plug-in vehicles to be of any value to utilities and grid operators. Several promising pilot projects have demonstrated the capabilities of V2G, but no large scale implementations are in the foreseeable future. There is also significant work to do to formalize standards for how the vehicles will connect and communicate with the grid, and the Society of Automotive Engineers (SAE) has committees working specifically in these areas - J1772 for connections and J2293 for communications. Lastly, there are issues to work through concerning the impact of constant cycling of the vehicle's battery and the effect this will have on battery life.

The good news is that many of the benefits of grid-connected vehicles don't require that they be deployed with full V2G functionality right from the start. An excellent starting point is just manufacturing vehicles with industry-standard connections that can do "smart charging" directed by the grid. A PNNL study shows that even if plug-in vehicles comprised three-quarters of the American passenger car fleet, the existing grid has sufficient power generation capabilities to handle the charging needs of these vehicles if they are charged during off-peak hours -- all without requiring any new power plants. In addition, the ability to control the charge time and energy flow rate of plug-in vehicles represents value to utilities and grid operators for grid ancillary services and for the ability to dispatch load to match up with real-time renewable generation even if the vehicles are not capable of sending power back to the grid. Finally, even if the grid itself is not capable of handling bi-directional power flow from vehicles, full V2G capabilities could be deployed as part of a "smart garage" at a home or a business, sometimes referred to as V2H (vehicle to home). In this scenario, the plug-in vehicles would be treated as a power generation resource along with solar or wind power, and controlled directly by an energy management system which controls the energy load at the home or business.

Though V2G is still in its infancy and many questions remain -- Who should own the batteries in the vehicles? What communications technology should be used? How would we handle the intermittent connectivity & mobility of vehicles? -- it has a great deal of potential and will be an integral part of the next generation power grid fueled significantly by clean, renewable energy resources.