Some existing codes and standards facilitate the introduction of V2G power. Others are currently impediments that would have to be addressed for widespread adoption of theV2G concept. This section briefly addresses several such issues: site generation interconnection safety, net metering and other renewable energy tariffs, demand charges, interruptible tariffs, and the National Electrical Code (NEC).
Renewable energy systems that are grid-connected have approved
interconnections and have a tariff for running the meter backwards. Similarly, co- generation systems are currently set up to feed the grid. Site renewable energy is a good comparison with V2G power in safety interconnections, but is the wrong model for tariffs.
In California, Rule 21 (Public Utilities Commission of California 2000) governs small power production interconnections for the large investor-owned utilities. This rule
requires small generators to have facilities such as automatic lockout to prevent energizing utility lines that have been disconnected for service and automatic
disconnection when voltage or frequency drift outside specified ranges. Vehicle-to-grid systems, as we describe them here, appear to meet these requirements, although some requirements not surprisingly assume that the system is site-built and stationary. For example, one area of particular safety concern for line workers is lockout of site renewable energy when power lines are turned off for service. Without this, workers could cut off power from the substation, but still be electrocuted from power coming back up the line from a customer's solar or co-generation unit. A similar safety interlock would have to be employed for vehicles. In fact, such an interlock has already been demonstrated as a low-cost design addition to on-board vehicle electronics.5 When such controllers are built into vehicles, no additional external interlock would be needed in the house wiring.
Some concern exists regarding an article defined in the National Electrical Code and concerning the backfeeding of power from vehicle to grid. The current National Electrical Code, Article 625-25, includes the following language:
Means shall be provided such that upon loss of voltage from the utility or other electric system(s), energy cannot be backfed through the electric vehicle supply equipment to the premises wiring system. The electric vehicle shall not be permitted to serve as a standby power supply. (Earley, Caloggero and Sheehan 1996)
This would prevent EDVs from being used for site power when the grid fails. This does not appear to prohibit vehicles from producing power or backfeeding as long as the grid is on. This section of the NEC should be clarified prior to implementation of the vehicle- to-grid concept, or the NEC would have to be amended.
Regarding tariffs, renewable and co-generation systems produce power when the sun is shining, when the wind is blowing, or when the site co-generation system needs to produce heat. This is resource- or site-determined timing, so the tariffs specify that the local distribution company pays in energy units, regardless of the time of day it is provided. Under one type of tariff, called "net metering," the power company buys power at the same retail rate as it sells it. This is generally considered an incentive rate, as the local distribution company is buying power at retail rates, that is, they are not compensated for bearing the grid system costs. But net metering is not adequate payment for V2G power. Vehicle power, as we propose it be configured, is much more valuable to the grid because it can be provided exactly when power or ancillary services are needed. Vehicles would deliver power, or other electric grid services, when the value is highest, not when the sun is shining or the wind is blowing. To capture this value, tariffs for vehicle-to-grid power would require that vehicles provide power precisely when needed, in exchange for premium rates well above the net metering rates for site renewable energy.
There are two existing tariffs, both for commercial customers, which might allow for economical use of vehicle power without rate changes. Because these rates already
5
This capability is already proven, as it has been built into the AC Propulsion AC-150 Gen2 drivetrain power electronics.
exist, and because the customer does all the controlling, no rate tariffs or control technologies need be added. Thus, these tariffs represent an opportunity for early introduction of V2G power.
The first existing commercial rate is demand charge. Most utilities bill residential customers only by energy, that is, the total kWh consumed. Commercial customers are billed by "demand" as well as energy. Demand is metered as the largest power flow, measured in kW, typically measured as a maximum over any 15-minute time during the month. This "demand charge" compensates the distribution company for expenses incurred by having to upgrade lines and transformers to handle the maximum, and for adjusting to fluctuations in load. It is common for the demand charge to be 50% of a commercial customer's bill. As will be seen from our detailed quantitative analysis, V2G can make money via demand charges only for commercial customers who have short and infrequent peaks, and sufficient vehicles to predictably offset those peaks.
The second existing tariff is interruptible rates. Under this tariff, large users get a year-round discount off their energy bills (e.g., 15%) in exchange for agreeing to sharply curtail their consumption when asked by the grid operator. The role for V2G would be to allow on-site V2G to substitute for the curtailed grid power. Such commercial customers could achieve the 15% savings while not curtailing production or other business
functions.