Pairing Solar with Managed EV Charging for Distribution Grid Benefit

As utilities and regulators anticipate increased electric vehicle (EV) adoption, they’re assessing how to prepare the electric distribution grid for the additional load. Recent work by the Great Plains Institute (GPI) points to the potential distribution grid benefits of pairing solar production with managed EV charging. Using a newly updated tool from the National Renewable Energy Laboratory (NREL), GPI explored this value proposition by examining EV charging scenarios alongside solar production.

Key takeaways:

• Pairing EV charging and solar production can produce benefits for the site owner, the distribution grid, and the bulk power system.
• Through managed charging, EV charging and solar production within the same distribution grid are aligned.
• Additional study is critical to capture the distribution grid value of solar production and EV charging.

In 2020, GPI published a report called Solar Power+Electric Vehicle Charging: Capturing Synergies in Minnesota. The report evaluated the market potential for pairing solar production with EV charging in the workplace setting and identified market transformation pathways for deploying solar+EV technology at scale.

The report found that unmanaged EV charging has very minimal overlap with solar production. However, the report proposed that when EV charging is managed properly, it aligns well with solar production, and even creates value for the host site that is not gained by separately deploying solar and EV charging.

The solar+EV study described several value propositions generated by synchronizing EV charging with solar production: (1) electric bill savings to the site owner; (2) distribution grid benefits by avoiding overloading transformers and conductors on the local grid; and (3) benefits to the bulk power grid by being able to use more inexpensive renewable energy. Synchronizing EV charging with solar production requires a managed EV charging system.

EV charging methods: Unmanaged and managed

There are two methods for charging an EV: unmanaged and managed charging. With unmanaged charging, there is no technology controlling the charging rate and the EV will be charged as quickly as possible. After charging is complete, the charger is turned off even if the vehicle is still plugged in. Managed charging, on the other hand, regulates the rate of charging to achieve a management goal, such as avoiding adding to peak demand or synchronizing with the availability of clean or renewable electricity. This type of charging is known as “smart” charging.

Testing the distribution grid benefits of pairing solar with managed EV charging

GPI is testing the proposition that solar+EV could create distribution grid benefits. We’re using a recently updated tool created by the National Renewable Energy Laboratory (NREL) called Electric Vehicle Infrastructure Projection Tool (EVI-Pro) Lite. The tool allows users to create regional EV charging load curves for urban areas across the nation, and to segregate the load profile for specific types of charging, such as workplace charging. This enables us to visualize EV charging load in various settings and to examine the potential grid impacts of managed charging.

To demonstrate the potential, GPI used two workplace charging load profiles in a metropolitan area fleet of 10,000 vehicles for the Minneapolis-St. Paul metro region:

• an unmanaged scenario (as shown in figure 1)
• a managed charging scenario (as shown in figure 2)

We compared the EV charging load curve with a solar production curve from a Minneapolis commercial building solar array on a cloudless day and an approximate temperature profile. The analysis compared the unmanaged load curve and a managed load curve to solar production to see whether local solar production that is deployed with EV charging can “cover” the load impact to the distribution grid.

EVI-Pro Lite allows users to create an overall (all types of charging) charging load profile based on assumptions selected by the user, and then provides the ability to segregate the workplace and public charging portions of that curve. To create the workplace charging load curves for figure 1 and figure 2, we operated under the following assumptions:

• Each of the 10,000 vehicles travel an average of 35 miles per day.
• The average ambient temperature is 68 degrees Fahrenheit.
• Of the 10,000 plug-in vehicles, 75 percent of them are all electric.
• Of the 10,000 plug-in vehicles, 80 percent of them are sedans.
• Most charging (about 80 percent) would occur at home, but 25 percent of EV owners don’t have access to home charging and rely on workplace and public charging facilities.
• 80 percent of chargers provided by the workplace are Level 2 chargers while the remaining 20 percent are Level 1 chargers.
• The managed charging scenario provides the same amount of charge to vehicles as the unmanaged charging, just spread out over the workday differently.

Figure 1. Unmanaged load curve for electric vehicle charging in the workplace setting compared to solar production

Figure 2. Managed load curve for electric vehicle charging in the workplace setting compared to solar production

Scenarios indicate benefits of managed charging to the distribution grid

In figure 1, the charging load profile’s peak occurs at 9 a.m. while the peak of the solar production is just after noon. Much of the load profile is located outside of the solar production curve. This confirms that if all workplace charging in the metro area is unmanaged, then solar production and EV charging on the distribution grid are not aligned. EV charging is therefore adding to the load already on the distribution system.

In figure 2, the managed charging software shifts peak load to 11 a.m., much closer to the peak solar production happening shortly after noon. In addition, much of the load profile is located inside of the solar production curve. This indicates that when workplace charging in the Minneapolis-St. Paul metro area is managed, then the EV charging and solar production within the same distribution grid are aligned. In these scenarios, the EV charging load profile is an estimation.

GPI examined a real-life scenario of solar+EV charging where charging load was constrained entirely within the solar production curve for a demonstration site (see 4 part of our blog series). A future analysis would look to see if this can be done across the distribution grid.

Why is it beneficial to synchronize EV charging with solar production?

As discussed in the solar+EV charging report, aligning EV charging and solar production to coincide has the potential to produce many benefits for “the site-owner, the distribution grid, and the bulk power system.”

From a site owner’s perspective, any EV charging that occurs outside of the solar production curve of an on-site solar array reduces the energy they will have to purchase from the grid. Any EV charging load outside of the solar production curve is potentially additional demand charges borne by the site owner.

Similarly, from the distribution grid perspective, any charging that occurs inside the solar production curve for distributed generation reduces the load on the distribution grid and doesn’t require pulling additional energy from the bulk power system. For the bulk power system, using managed charging allows for the increased grid deployment of renewable energy which has many benefits. As the report notes, these benefits can include “potentially supporting meeting carbon reduction goals, reducing costs (marginal costs of wind and solar energy are close to zero), or both.”

More research needed on solar+EV pairing to maximize potential benefits

As mentioned in the solar+EV charging report, the literature and research surrounding distribution grid benefits from distributed energy resources supports the concept that pairing solar production and managed EV charging has grid benefit. However, measuring the resulting benefits to the distribution grid proves to be much more difficult than measuring benefits at the individual site level.

The managed charging scenario provides another visualization of pairing solar with managed EV charging and further demonstrates the benefit it can provide as EV deployment ramps up. More research needs to be done around this topic to determine how pairing solar with EVs can be properly deployed to capitalize on the projected benefits. GPI hopes to look more closely into this topic in future work.