EVs, HEVs and PHEVs: Plug-in Vehicles and the Electric Grid
by Monica A. Mallini, P.E., Chair, National Capital Area Consultants' Network,
and Wally Lee, Washington Section Editor
The electric car is an idea whose time has come—again. Battery powered cars were ready to take off in 1912. Then Cadillac invented the electric starter, and internal combustion engines (ICE) have ruled ever since. Until now.
IEEE-USA sponsored a symposium, "Plug-in Hybrids: Accelerating Progress 2007," in September in Washington, D.C. The one-day conference presented a broad range of technological advances that support the current commercialization of hybrid vehicles and future challenges and opportunities.
Today’s electric vehicles (EVs) are adaptations of the traditional gasoline-powered car. Hybrid electric vehicles (HEVs) use an electric motor drive to supplement the ICE. Plug-in hybrids (PHEVs) are the special case of hybrid electric vehicles that have the means to charge the batteries externally and are capable of all-electric operation. Pure electric vehicles are powered solely by an electrically driven motor. All types of electric vehicles and hybrids offer the advantage of high efficiency compared to pure ICE automobiles. The efficiency advantage is due partly to regenerative braking, which recaptures energy from deceleration to charge the batteries.
The potential uses for EVs do not stop at efficient transportation. Electric car batteries are a valuable energy storage resource to displace power produced by a utility’s generators. This is called “vehicle to grid” power, or V2G. For example, an EV used for daily commuting consumes some fraction of its total battery capacity for the round trip. In the evening, the car is garaged with a partial charge remaining. The owner plugs it in and sells energy back to the electric utility, then recharges the battery overnight. In this way, the energy storage capacity of the car’s batteries benefits the electrical grid.
What are the benefits? First, there is an immediate benefit to the electric grid, provided the electric utility or an intermediary can control the dispatch of the battery-stored energy. Early evening is a time when electrical load ramps up to a peak. The utility serves the peak load with relatively costly “peaking units,” high-cost generators designed for intermittent service. If the utility can serve the peak load partly through stored energy, the use of peaking units is reduced. The batteries in EVs can function as a power system energy storage resource if they are connected to the grid and made available to the utility to dispatch in response to electrical load. As the number of hybrid vehicles grows, the aggregate storage capacity of the batteries can markedly change the shape of the daily load curve, smoothing out the peaks and valleys and avoiding excessive electricity demand that may lead to blackouts. This will improve the economics of the power system operation, resulting in cost savings that can be passed on to the consumer. Moreover, the purchase of additional peaking units can be avoided. The utility’s cost of meeting its peak load demand is reduced, and the capital cost of new generation is avoided. Customers ultimately benefit from both of these savings.
Greater awareness of energy consumption motivates individuals to take an active role in energy conservation. A coming trend is the variable pricing of electricity, which is implemented in its simplest form as fixed time-of-use rates. Variable pricing smooths out peaks in consumption that may lead to blackouts. Variable pricing will enable EV owners to exploit the price differential by recharging their vehicle batteries with inexpensive power off-peak and selling it back to the utility at a higher peak price. A more advanced scheme is real-time pricing, whereby the price of electricity varies throughout the day in response to current load demand.
Effective use of real-time pricing requires some degree of information, control and coordination between the utility and the customer. This coordination may take the form of a comprehensive energy management scheme, including a customer energy meter, to capture energy consumption and report current and historical energy use, switches to operate individual appliances, and a control scheme with preferences that can be updated by the user. Consumers will receive the current price information, allowing them to tailor their energy use to the price of electricity. For example, the EV may be set up to offer power back to the grid whenever the price exceeds some threshold and to charge its batteries overnight or whenever the price is low.
As EVs enter the mainstream, there is concern over what market penetration the present electric power grid can support without an extensive infrastructure rebuild. This question was answered at the IEEE-USA symposium. The consensus is that the present electrical grid can support about 70 percent of the current number of automobiles in the U.S. if they are replaced by EVs. Hence, overloading the electrical grid is not a near-term concern. However, there are some issues that must be addressed before V2G becomes widespread. For example, distribution transformers are designed to operate with a daily loading cycle. It is not known how the equipment will perform when the base load is increased.
The high point in the symposium for the authors was the opportunity to actually ride in one of the cars. The first thing we noticed is the quiet operation. I first noticed this when someone had to tell me that I was standing in the path of a car that was backing up. You also notice the quietness when you are riding in the EV. There is no change in engine noise during acceleration like there is in a conventional vehicle; the car has constant acceleration from the constant torque. You do feel the pressure on your back when the EV accelerates, which was impressively demonstrated on a steep incline. These cars are amazingly quick and will only get better as the technology matures.
The EV and hybrid vehicle revolution will involve electrical engineers in many ways. In recognition of these changes, the IEEE Power Engineering Society is modifying its name to become the Power and Energy Society.
EVs are not the exclusive domain of the automobile industry and the power industry. We must design new power stations that function as charging facilities. There must be methods of connecting EVs to charging stations. Automotive mechanics must learn new repair techniques.
Our cars will need IP addresses and software. Engineers will develop power electronics circuitry, user friendly interfaces, communications schemes and software to drive us to the future.
Hybrids and electric cars will influence us in ways we have only begun to dream.
The authors wish to convey their thanks to IEEE-USA for sponsoring their attendance at the symposium.
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Updated 12/29/07
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