How many electric cars can the grid take? Depends on your neighborhood

How many electric cars can the grid take? Depends on your neighborhood

High concentration of EVs can lead to equipment trouble later.

We all know what it’s like to ask for more electricity than a system can give. Throw some soup in the microwave, put a poptart in the toaster, plug in your hair straightener, and pop! All of a sudden, you’re in the dark, searching for the switchbox to reset the blown fuse.

Though that problem takes place on a small scale, utilities work hard to make sure surges in demand don’t affect normal grid operations on a large scale. And as more and more electric cars get plugged in to residential garage systems, some experts have wondered whether the various grids that serve our homes can handle the extra demand for energy.

Matteo Muratori, a researcher at the National Renewable Energy Laboratory in Colorado, tried to take a granular look at how electric vehicle (EV) market share can affect grid operations. He found that when certain communities adopt electric vehicles more quickly than others and drivers charge their vehicles in an uncoordinated manner, EV adoption can strain certain areas of the grid, even if aggregate market share is low.

In his research, Muratori estimated the effect of EV market share at different levels, from 3 percent to 100 percent. For context, EV market share in the US in 2017 was just 0.9 percent according to the International Energy Agency (PDF).

But even if the total number of EVs in the US remains low, a high concentration of adoption in specific neighborhoods can “significantly increase the peak demand seen by distribution transformers” and can “require upgrades to the electricity distribution infrastructure,” the paper notes.

The problem compounds when residential drivers use Level 2 (240 volt) charging to fill up their cars. Although Level 2 charging is faster than Level 1 (120 volt) charging (which means cars won’t need to be plugged in for as long), the power demand slope for Level 2 charging is steeper.

Muratori simulated the changes in electrical demand using data from the US Department of Energy’s Residential Energy Consumption Survey. He selected 200 representative houses with 348 passenger vehicles in the Midwest. In aggregate, increased electricity demand was sustainable up to 25-percent EV penetration, as long as you were only counting in terms of kilowatt hours of electricity consumed and assumed all charging was happening at night.

But when Muratori studied more local scenarios, heavy demand was more concerning. The researcher simulated “a residential distribution transformer connected to six households” with 11 vehicles total. The transformer could handle up to six electric cars charging with Level 1 charging, but the simulated transformer saw demand in excess of its nominal capacity as soon as one EV with Level 2 charging was added to the neighborhood.

While electrical transformers are built to withstand such temporary surges in electrical demand, Muratori cites research that shows the expected life of transformer equipment can decrease “by two orders of magnitude when a transformer hits ’50 percent above its nominal capacity.’” While you wouldn’t necessarily have a blown-fuse scenario, the research shows that utilities may soon face a choice of either upgrading the equipment or having to replace existing hardware more often.

Coordinated charging

Muratori’s paper only considered uncoordinated charging, but the researcher notes that one way to ameliorate this problem is with smart charging. Smart charging would use an energy management system that can time when an EV charges and modulate charging power accordingly. Within a demand-response program, utilities might offer cheaper charging for customers willing to be part of such a program, which would reduce the total cost of ownership of the vehicle.

The growing number of EVs on the road could also present an opportunity for utilities as much as a problem. For one, increased electricity demand is good for a utility’s business. Small studies have also been done to model vehicle-to-grid (V2G) setups in which plugged-in vehicles can supply electricity back to the grid, either to stabilize it or offer back-up power if part of the grid goes down.

Making charging more widely available at non-residential locations could also help this situation. If you can charge up a little bit at work or at the store, you’ll need to charge up less in your garage.

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