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Electric & Hybrid Cars: New Load, or New Resource?

The industry must join a growing chorus in calling for new technology.

Fortnightly Magazine - December 2006


The key parameter for resources providing regulation reserves is the generating unit’s ramp rate. 10 Most regions specify regulation as a five-minute service. Thus, a generator with a ramp rate of 3 MW/minute in a region that defines regulation as a five-minute service, would be approved to provide 15 MW of regulation. The rules do not specify the power output duration that a resource must maintain, and for which it was approved. PHEVs providing regulation would have ramp rates far superior than the incumbent technologies. Industry experts indicated that inaccurate response to AGC signals requires grid managers to carry greater amounts of regulation reserves than would be necessary if resources were responding precisely to an AGC signal. Again, PHEVs with a communication and control infrastructure would be capable of very accurate responses to signals received from a central grid operator.

For the moment we set aside the regulatory requirements for resources to qualify as providers of ancillary sources, and look at the infrastructure and vehicle constraints that dictate the reverse power flow potential from PHEVs. Kempton and Tomic (2005) 11 identify three key factors that limit the amount of power a grid-connected car can deliver back to the grid. These include the onboard vehicle electronics, capacity of the plug circuit, energy storage capacity, and state of charge when the vehicle is plugged in to provide grid services. The key question is which of these serves as the limiting factor to the reverse power flow potential from a PHEV?

We don’t anticipate that a PHEV vehicle’s power electronics would create a binding limit on the amount of power that can be exported to the grid. PHEVs require high-power components for acceleration and to optimize vehicle performance. An existing electric drive train developed and manufactured by AC Propulsion provides 80 amps in either direction, allowing 19.2 kW of power output.

Thus, the critical factors dictating the reverse power potential come down to the capacity of the plug circuit and the size and state of charge of the PHEV’s battery pack. We assume that PHEVs would plug in to conventional residential and commercial circuits with a 120-V 20-amp service allowing approximately 2 kW of reverse power flow. Most homes and commercial buildings contain higher capacity circuits like 240 V at 50 amps for large appliances like ovens and dryers: These circuits could accommodate about 10 kW of reverse power flow from a vehicle back to the electric grid.

The parameters to evaluate with the most potential for variability that limits the amount of power a PHEV could deliver to the grid are: 1) size of the onboard battery pack; and 2) state of charge when plugged in and ready to provide regulation or spinning reserves. For purposes of demonstration, we assume that the average available energy from a fleet of PHEVs is 10 kWh: This is consistent with the energy storage needs of vehicles designated as PHEV20 (larger, less efficient vehicles) and PHEV40 (smaller, compact cars with higher efficiencies). Table 1 illustrates the available V2G power from a PHEV based on a range of