EV Growth Drives Workplace Charging

The EV market is growing rapidly throughout the United States, with more vehicle options and increasing EV adoption rates. EVs for personal transportation are most conveniently and economically charged from SAE J1772 Level 2 EVSE units. These units can fully recharge a typical EV in eight hours or less. Due to a vehicle’s dwell time of eight hours during a standard workday, workplace charging has increasingly become a common option for drivers looking to charge their EVs during the day.

The U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) offers a workplace charging program for employees through electric vehicle supply equipment (EVSE) units in its Golden, Colorado, campus parking garage. Due to a growth in interest over the years, NREL recently expanded the number of electric vehicle (EV) chargers by installing 72 additional EVSE units. This grew the program from 36 units to 108 Level 2 charging stations with a maximum output of 6.7 kilowatt (kW) each. If each charger were to operate at maximum capacity, the peak EVSE demand would reach approximately 720 kW.

Although the 108 EVSE parking spaces represent approximately 7% of the 1,630 spaces available in the NREL garage, the 720-kW peak load represents a significant majority of the garage’s electricity demand. If this peak demand occurred, it would overload the transformers that supply power to the garage and, depending on the time of day, could result in peak demand fees for NREL. In a typical scenario, the transformers would be upgraded to a rating greater than the peak demand; however, this upgrade would have been too costly for the budget of the expansion project. Therefore, the alternative to a new transformer is to limit the peak demand to a level below the rating of the existing transformer.

Managed charging solutions offer many financial benefits to facilities with a large number of EVSE. Managed charging solutions can limit a charger’s output or shift the time a vehicle charges to mitigate demand charges and take advantage of time-of-use pricing. Each time a car is plugged into an NREL EVSE, the driver inputs the amount of energy the car needs, as well as the duration the car will be parked. This information allows the charge management software to schedule EVSE loads throughout the day in a way that reduces the system peak while still supplying each driver the energy needed.

White electric vehicle being charged in the NREL parking garage.
A Nissan Leaf and Chevrolet Volt charging in the NREL garage.
Photo by Dennis Schroeder, NREL 26669

Chuck Kurnik, an EV driver at NREL, describes how this flexibility benefits both himself and NREL facilities: “The new load management system offers a transparent pricing structure where a flexible charging schedule receives a lower cost of electricity. If I ask for a charge over a longer period of time I pay a lower cost because I am helping to avoid demand charges. By knowing the price per kilowatt-hour of electricity, I’ve determined that it’s less expensive than at home, so I do most of my charging at work."

In the case of the NREL garage, the management system can also set a power ceiling by ensuring the overall peak demand of all the EVSE units remains below a set limit. To avoid exceeding the garage’s transformer ratings, the collective EVSE load is limited to a maximum demand of approximately 460 kW. This power ceiling appears to reduce the output of each EVSE unit down to 4.3 kW; however, that will rarely occur, because the scheduler shifts the EVSE loads to reduce the facility's overall coincidence peak. Scheduling these loads throughout the day allows EVSE to operate at or near their 6.7-kW capacity, while also abiding by the power ceiling set for the entire garage load.

"Selecting an EV charge management system like the one we did enables NREL to meet growing user needs while continuing to optimize campus operations and minimize costs," explained Lissa Myers, NREL's Intelligent Campus project leader. This program helps NREL provide EV drivers an economical and practical workplace charging solution to meet their commuting needs.