An electric vehicle charging at the greenway buildingThe Great Plains Institute’s pilot project to study managed electric vehicle (EV) charging with on-site solar generation shows how a solar-synchronized EV charging system can function throughout the year—even in wintry Minnesota—and meet the needs of EV drivers at a workplace. This post describes the impacts of the solar-synchronized system on EVs at GPI’s headquarters in Minneapolis. The pilot project findings provide data that can contribute to discussions about building managed charging infrastructure, grid impacts, and factors to consider for future system designs.  

Three takeaways:

  • Behind the electric meter, managed charging with solar synchronization shows the potential for delivering sufficient charge for EV drivers in a workplace setting.
  • More analysis is needed to understand the implications such programs at a broader, grid-level scale.
  • Even low solar months in Minnesota typically see enough solar energy to sustain the EV chargers.

This is the fourth post in a series sharing GPI’s results and insights from the pilot project at our headquarters to synchronize on-site solar energy production with electric vehicle charging using a managed charging system. Here’s what to expect:

  • Part 1. Results from our analysis of how seasonal and daily variation in rooftop solar generation levels impacts their ability to provide electricity year-round, using EV charging as an end-use test case.
  • Part 2. Intro to how we’re deploying a managed EV charging system to synchronize rooftop solar production with the varying energy demand set by EV charging levels.
  • Part 3. Describes the benefits of managed charging and details how it has worked in practice, using specific charging scenarios to illustrate the data we’ve analyzed.
  • Part 4. Shares data on the impacts of managed charging on EV drivers during the pilot project and how additional analysis can increase understanding of the broader implications of managed charging.

Impacts of seasonal variation on charging levels

GPI’s pilot project uses managed charging to adjust the level of power used in an EV charger depending on the level of solar generated throughout the day. Data gathered from our building’s rooftop solar array show varying levels of solar generation throughout the year. In winter months like January and February, the level of solar energy generation at our building is quite low, about 10 kilowatts (kW), enough to charge one EV at full/high power or up to four EVs at minimum power levels. This is less than half of the solar energy generation in peak months like July and August, which typically see solar generation as high as 30 kW, enough to charge three vehicles at full power.

The minimum power level our chargers output is approximately 1,500 watts or 1.5 kW. On our chargers, EVs will typically use a maximum charging level of approximately 6,000 to 6,500 watts, or 6-6.5 kW. While variation in available solar between seasons is high, even low solar months typically see enough solar energy to sustain the chargers. In the gif below, you can see the minute by minute levels of solar energy generation in each month throughout the year.

Monthly Average Trends of Greenway Building Solar Array Production


Impact on EV drivers charging from the solar-synchronized system

There are exceptions to the overall monthly trends in solar generation. Some days have no captured solar at all, or days with both low solar and a high charging demand. Fortunately, this doesn’t necessarily result in an uncharged vehicle.

On days with no solar, the chargers will still provide a minimum charging level. On days with even a small amount of solar, the chargers should be able to fully charge a vehicle at a workplace. That’s because vehicles at a workplace spend significant time plugged into the chargers, often most of a business day.

In the morning when a vehicle first plugs in, it will typically demand charge for a few hours. However, vehicles typically begin to reach a full charge in the afternoon, which we can see in the time vehicles spend plugged into the chargers, but not idle (or not drawing power). The ratio of plugged and idle minutes to charging minutes suggests that the duration of a typical workday at GPI is more than long enough to fully charge a vehicle, with plenty of extra time to account for variations in charging levels. EV drivers may need more or less charging on a given day based on the vehicle charge level when they arrive at the office. Below, you can see the charger use for July 2019.

Greenway Building Electric Vehicle Charging Activity, July 2019
Greenway Building Electric Vehicle Charging Activity, July 2019

Source: Maggie Kristian, GPI

The GPI managed charging program shows that behind the electric meter, managed charging is a promising tool for meeting the needs of EV drivers charging at their workplace. More analysis can improve understanding of the implications of programs like this at a broader, grid-level scale and help answer questions such as:

  • Where is it viable to install managed infrastructure, both physically and economically?
  • How might solar-synchronized chargers impact load? How can we reliably predict that impact, given factors such as snow cover, snowmelt, and passing clouds?

We’ve begun to dive into some of these questions in partnership with the Solar Energy Innovation Network hosted by the National Renewable Energy Lab, which will be explored in an upcoming blog series.

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