All-electric transportation is a major piece in the quest to achieve climate mitigation goals. Electric vehicles (EVs) are powered by an electric motor that draws electricity from a battery and is capable of being charged from an external source. Depending on the charging patterns of the electrified transportation system, the electricity grid that supplies the energy may reach peak capacity at the same time of distribution demands. This might lead to transformer blowouts, electricity shortages, reliance on expensive peaking plants, or perceived need to build new power plants. A new study from MIT offers an alternative, however. If strategic EV charging station placement were to take place, it could lessen or eliminate the need for new power plants.
Peak power usage often occurs on summer afternoons and evenings, when solar energy generation is falling. An over-supply of electricity during midday and then decline in the evening hours can result in curtailed solar electricity and an inefficient ramp-up of fossil-fuel-powered plants to meet the early evening peak, often called the “duck curve.” If drivers primarily charge vehicles at home during the night, that could lead to a 25% surge in peak net electricity demand when states reach 50% EV ownership and possibly surpass grid capacity at even higher levels of ownership.
The scenario of peak charging coinciding with peak residential electricity demand in the early evening during higher-demand summer months causes concern to many people. “Given that there’s a lot of public money going into expanding charging infrastructure,” the MIT co-author Jessica Trancik says, “how do you incentivize the location such that this is going to be efficiently and effectively integrated into the power grid without requiring a lot of additional capacity expansion?”
Indeed, what can be done to minimize EV charging impact on the grid?
Planned, Strategic EV Charging
Charging control and infrastructure build-out are critical factors shaping charging load. In the new study, MIT researchers determined that it’s possible to mitigate or eliminate EV charging problems without the need for advanced technological systems of connected devices and real-time communications, which could add to costs and energy consumption. Instead, they recommend encouraging strategic EV charging placement, rather than allowing EV chargers to be situated merely due to charging company convenience or preferences.
Where would such EV chargers be located? And why?
- Workplaces: Better availability of charging stations where job sites exist could help to absorb peak power being produced at midday from solar power installations. What’s more, overproduction of power from solar farms during the daytime can waste valuable electricity-generation capacity. Workplace charging reduces the evening peak load from EV charging and also makes use of the solar electricity output. “Slow workplace charging can be more preferable than faster charging technologies for enabling a higher utilization of midday solar resources,” co-author Wei Wei says.
- Home charging: This term refers broadly to charging equipment in individual garages or parking areas but also to charging stations available in on-street parking locations and in apartment building parking. Pre-programmed at delayed times, each EV charger could be accompanied by a simple app to estimate the time to begin its charging cycle so that it charges just before it is needed the next day. Unlike other proposals that require a centralized control of the charging cycle, such a system needs no interdevice communication of information. The reason it works so well, Trancik reveals, is because of the natural variability in driving behaviors across individuals in a population.
Combining the two measures — workplace charging and delayed home charging — would reduce peak electricity demand, store solar energy, and conveniently meet drivers’ charging needs on all days, according to study results.
To best use public funds so strategic EV charging takes place, Trancik says, “You can incentivize charging installations, which would go through ideally a competitive process — in the private sector, you would have companies bidding for different projects, but you can incentivize installing charging at workplaces, for example, to tap into both of these benefits.”
Pre-Programmed Charging, No Networked Devices
Constraining the solutions to ones that can be pre-programmed, and therefore do not require networked devices, is different from other ideas to relieve EV pressure on the grid. The MIT study solutions do not require behavioral change on the part of drivers in terms of where and when they stop in between trips, although other behaviors, such as locating chargers and plugging in vehicles, would have to change. Workplace charging emerges as a simple and effective solution for abating both the peak increase and the over-supply of PV. Schedules and locations of vehicles remain unchanged before and after implementing the time-shifting of charging, and the behaviors simulated, such as a preference for workplace charging or delaying home charging, may be achievable through levers such as appropriately designed pricing schemes.
Delayed home charging nearly eliminates the increase in the evening peak demand for electricity. In this case, drivers would pre-program charging to finish a fixed amount of time before they intend to leave in the morning. Variability in charging requirements and departure times mean that this solution is not expected to lead to the sharp ramp rates associated with some time-of-day-based charging schemes.
With substantial degrees of PV and BEV adoption but with primarily home charging, excess peak loads from charging and midday overgeneration of PV are concerns in both New York and Dallas. Commencing charging when drivers arrive at work reduces the BEV contribution to the evening peak by 70% in New York and 80% in Dallas, and the BEV contribution to the evening peak is practically eliminated when workplace charging is combined with delayed home charging.
Studying EV Daily Charging
The research was conducted in New York and Dallas. One vehicle analyzed was a Nissan Leaf with a 62 kWh battery, as a representative lower-cost BEV. The researchers deduced that the Leaf could meet the range needs of 93% and 91% of vehicles in Dallas and New York respectively on a given weekday with once-daily charging and 95% and 93% respectively when Level 1 workplace charging was also available. These estimates were based on the assumption that existing trip schedules do not change and charging takes place at locations where charging is available for the duration that the vehicle is parked.
On extreme heat days, battery EV charging caused the highest observed hourly demand to increase by approximately 5%–10% in both cities, an effect that is worsened because peak charging loads tend to coincide with peak existing loads in the early evening. The researchers concluded that midday overgeneration, on the other hand, takes place throughout the year and tends to be most extreme in spring and fall months, when up to 30% of PV generation on some days competes with baseload generation.
The data were gathered from, among other sources, anonymized records collected via onboard devices in vehicles and surveys that carefully sampled populations to cover variable travel behaviors. They showed the times of day cars are used and for how long, and how much time the vehicles spend at different kinds of locations — residential, workplace, shopping, entertainment, and so on.
The findings, Trancik told MIT News, “round out the picture on the question of where to strategically locate chargers to support EV adoption and also support the power grid.” This research offers guidance to policymakers on where to focus rules and incentives.
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