Contributed by Philip T. Krein, University of Illinois at Urbana-Champaign and Zhejiang University, China
The impact of electric transportation on pollution and reduced carbon emissions and fossil-fuel consumption is complicated. On the one hand, well-designed electric vehicles (EVs) use much less energy per mile than conventionally-fueled vehicles. For passenger cars, good all-electric designs use roughly a quarter of the energy per mile of gasoline cars, tracing back to the source. On the other hand, the emissions impact tends to be much more local, and depends on how electricity is generated in a region.
How can utilities and customers work together to get the highest benefits and reduce fossil-fuel needs? “It depends” is not a useful answer, but this article seeks to offer some discussion and ideas that may help.
There are several different ways to answer the question, “What is the energy source for my electric car?” One is to assign fuel based on the local overall utility resource mix. The mix varies from hour to hour, but day-ahead planning and operation provide guidance and predictability. A second way is the “incremental fuel” perspective, in which the question changes to, “What fuel is being used for the next kilowatt-hour needed by my vehicle?” The reality of grid operation is that an added load at any moment might rely on a fossil plant to cover the extra need. Nuclear baseload generation might be supplemented by cycling coal plants, and so on. A third way assigns emerging loads (such as EVs) against emerging generation. For example, in the U.S. Midwest, wind energy is growing quickly. A March 2021 report by the National Academies of Sciences, Engineering, and Medicine offers a comprehensive discussion of the issues.
Any useful solution comes with three basic requirements. First, a driver (or fleet operator) needs convenient ways to charge up at home and at work. Second, it must be easy to program the vehicle or the charger to interact with the grid. And, third, there must be simple data exchange. Convenient charging is a long-term infrastructure development challenge. For workplaces, there is discussion by the U.S. Department of Energy on ways to plan workplace charging. Charge scheduling or programming is typical for EVs. Tesla, for example, continues to add programming features, and an owner can adjust schedules and actions separately for home or work. Data exchange can use public or private internet or cellular networks.
From these three requirements, broadly, there are passive and active strategies to set up incentive EV-charging rate structures that minimize fossil-fuel impact. A passive approach is carried out through hour-by-hour rate structures that make charging cheaper when fossil-fuel impact is low. Here are some examples:
- Special EV rates for workplace charging with low cost can be offered when solar energy is expected to be available. The most expensive energy is likely to occur between 4 and 9 p.m., as solar energy fades away and customer usage builds.
- Next-day EV rate structures can be adjusted to track expected wind contributions or timed for when hydroelectric energy will be flexible and available.
- The grid operator can set next-day, hour-by-hour EV rates with the lowest cost during times least likely to use fossil fuel. An EV charger programmed to gather the cheapest energy will favor those times.
Some of these ideas require intelligent chargers that gather hour-by-hour grid prices and figure out when to charge to get the lowest total cost or highest usage of renewable energy. This level of intelligence is not yet common, but growth is expected.
In downstate Illinois, for example, late-night baseload generation is mostly nuclear, even though the annual average is about two-thirds fossil fuel. If EV charging is assigned based on the hourly generation mix, the lowest prices and lowest fossil-fuel consumption fall between 2 and 5 a.m. on a typical day. The price can be less than half of the daytime cost—a strong incentive for a driver to program chargers for that time interval.
Passive EV rates that use some of these concepts are being tested or implemented in California and other U.S. states. Some are simple, based on time of day, such as active EV rates from PG&E. Comprehensive examples provide low energy cost for workplace connections between 10 a.m. and 2 p.m. local solar time, even lower late-night rates for home connections, and high prices from 6 to 10 a.m. and from 2 to 9 p.m. to discourage charging as load ramps up in the morning and late afternoon. In many regions, the concept of fossil-fuel reduction needs to add day-by-day adjustment of rates, with an extra penalty factor for fossil sources or a bonus factor for renewable energy.
In an active approach, the EV owner gives the grid operator some amount of control over EV charging in exchange for an additional discount. A charger can be assigned as a flexible load that mixes into next-day grid planning and operation. The EV owner still sets up a charge schedule but allows limited control of the process by the grid operator. Here are some examples:
- During hours with strong solar or wind resources, the operator can favor EV charging to take advantage of extra energy. The charging current might even be controlled to track fluctuations in solar or wind energy.
- If the grid needs to add in fossil energy for any reason, the operator can disfavor EV charging.
With an active approach, the challenge is to ensure the customer receives the needed energy by the needed time. Flexibility can adjust charger timing, but specified total energy still needs to flow to the customer.
Active approaches are in early study stages for consumer EVs. They are expected to be broadly similar to demand-response programs, such as those between Nest thermostats and various utility partners. There are examples already, including a rewards program in New England from Eversource. More comprehensive programs designed to minimize fossil-fuel consumption are likely to emerge in the next few years, as EVs become mainstream.
In summary, designated time-of-day EV rate structures can be used to nudge drivers toward favorable times—provided convenient charging is available at home and at work. The times can be set to minimize energy cost or fossil-fuel consumption. Active approaches in which an EV owner “sells” partial charger control to the utility grid are interesting since they can link into existing utility operation and planning to reduce costs and emissions. Ongoing and planned experiments with various rate structures will help us learn which strategies are effective and easy to implement.
About the author:
A research leader in the fields of power electronics and motor drives, Philip T. Krein’s transformative contributions to energy conversion have broadly impacted electric and hybrid vehicle technologies. Krein began working to improve battery management for electric vehicles at a time when few believed this was a technical necessity. He developed a battery equalization technique using switched capacitor circuits that helped reduce the size and cost of battery management systems. His method extends the lifetime and efficiency of energy storage systems, which are critical to the success of today’s electric and hybrid vehicles. His contributions to vehicle systems optimization include high-fidelity dynamic models of vehicle systems and their interactions, linking fuel cells, batteries, ultracapacitors, and motor drives. An IEEE Fellow, Krein leads the Grainger Center for Electric Machinery and Electromechanics at the University of Illinois at Urbana-Champaign and is a Distinguished Professor at Zhejiang University, Hangzhou, China. He was awarded the 2021 IEEE Transportation Technologies Award for contributions to electric vehicle battery management and hybrid system optimization.
