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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

assumptions regarding battery state of charge and the duration of the dispatch. The values in Table 1 are calculated simply by multiplying the available energy by the state of charge and then dividing by the dispatch duration. For simplicity, we do not account for the slight losses associated with the inverter to convert the DC battery power to AC grid power. These results are based on a PHEV with 10 kWh of available energy, and thus would scale down for smaller battery packs and scale up for PHEVs with larger battery storage systems.

The data in Table 1 suggests that, as expected, the power capacity per vehicle is highest with a full battery and shorter dispatch durations. In these cases, the capacity of the plug circuit presents the limiting constraint on available power per vehicle. In all but two cases presented in Table 1, a conventional wall outlet allowing 2 kW of reverse power flow sets the limit on the power being exported to the grid and thus what a vehicle can sell. A higher power circuit allowing 10 kW of reverse power flow readily is available for most homes, but may need to be installed at a location that would allow grid charging of a parked PHEV. With these higher power circuits, about half of the possible situations presented in Table 1 would be limited by the 10-kW plug circuit limit.

We provide some basic calculations on the potential annual revenue assuming a 2-kW and 10-kW limit on reverse power flow from a parked PHEV. Assuming 50 percent state of charge and that the regulatory authority specifies run times of 30 minutes for spinning reserves and 15 minutes for resources providing regulation services, the PHEV would be allowed to sell 10 kW and 20 kW of power into the market for spinning reserves and regulation respectively. In both cases the limiting factor for reverse power flow is the rating of the plug circuit—2 kW and 10 kW.

Kempton and Tomic (2005) 12 provide detailed equations for calculating revenue and costs for various vehicle configurations. Here, we take average market clearing prices from two control areas—PJM and ERCOT (the Electric Reliability Council of Texas)—to estimate the annual revenue to a PHEV owner from providing regulation and spinning-reserve services. Table 2 provides for the average 2005 market-clearing prices from both regions and both grid services.

Further, we assume that the vehicle is plugged in and ready to provide grid services for 75 percent of the available hours in a given year. We consider only capacity payments, however most rules include compensation to resources for energy delivered. We found that this generally nets out when one considers the energy that must be purchased to charge the vehicle. Fig. 4 presents the annual potential revenue to a vehicle owner assuming 2 kW and 10 kW power availabilities.

The results in Table 3 suggest that PHEVs could generate significant revenue to a PHEV owner, from a low of $184/year to a high of $3,285/year. Relaxing the run-time requirement for resources providing spinning reserves from 60 minutes