EPRI & APS: Unraveling Complexities of Distribution Systems dealing with Grid-scale Battery Storage

Co-Written by the Electric Power Research Institute (EPRI) and Arizona Public Service (APS)

Commercial and consumer adoption of distributed energy resources (DER) – rooftop solar, battery storage systems, electric vehicles and charging stations – has exponentially increased in the past decade, driven by more choices, improved technology, and new and ongoing customer incentives.

With more myriad options becoming available, DER are becoming more common within energy systems. According to the International Energy Agency (IEA), U.S. battery power capacity grew by 35 percent in 2020 alone and has tripled in the last five years. This increase in battery storage has also given rise to a number of applications for support and operation. As choice increases, utilities are looking for the best way to benefit communities with these new resources, proving a win-win for consumers and communities alike.  

Spearheading battery storage adoption and integration is Arizona Public Service Co. (APS). APS’s system currently includes both utility-owned and customer residential battery storage systems. The utility anticipates increasing customer adoption of residential battery storage as the technology becomes more accessible and plans to educate partners, including regulators and energy stakeholders, about the capabilities of widespread battery adoption and use by residential customers.

Ensuring a Bright Future for DER in Arizona

Arizona’s seemingly never-ending sunny days make a strong case for using solar in the state’s clean energy transition. APS’s goal is bold: to produce 100 percent clean energy by 2050, with an interim goal to serve customers with 65 percent clean electricity by 2035 using 45 percent renewable energy. APS currently has an energy mix that is 50 percent clean electricity, largely comprised of energy from nuclear power generated at by Palo Verde Generating Station and a combination of renewable resources and owned-solar facilities.

Solar generation typically peaks in the middle of the day when the sun is high and energy demand is low. Shifting that power to be available during peak evening hours when demand is high but solar generation is low is where battery energy storage systems come to play.  

“In order for us to achieve our goal of powering more than 1.3 million homes and businesses in Arizona with 100 percent carbon-free electricity, we’re actively researching how to make smarter, more innovative decisions about our clean energy resources,” said Dr. Daniel Haughton, APS director of customer technology. “An energy transition like this requires a full understanding of how utility-owned renewables can work together with customer technologies such as rooftop solar systems and home batteries. By combining efforts and learning together, we’re working with the Electric Power Research Institute (EPRI) to explore how widespread customer rooftop solar resources and batteries interact in the real world. We want to see how all the puzzle pieces can fit together in APS’s larger DER strategy.”

In 2017, EPRI and APS began collaborating to better understand the future of a more complex distribution system that includes a variety of both grid scale and residential battery energy storage. The team hoped to gain a better understanding of how the technologies interoperate, and how to manage grid performance with these new and diverse technologies.  

The study, known as the Roadrunner project (named after the most famous bird in the desert Southwest), would provide utility and researchers the opportunity to take theoretical ideas about DER impact on a distribution system from computer-based models and simulation to test them in a real-world environment.

“With demonstrations like this, we keep an industry view in mind. In this case, that means exploring a range of scenarios to interpret the customer and grid advantages that are informed by practical experience,” said Dr. Ben York, manager of DER integration applications at EPRI.

To accomplish this, EPRI and APS needed to identify a service area that reflected the typical mix of residential and commercial usage found in Phoenix. A substation named Roadrunner fit the bill. The circuit includes a mix of underground and overhead construction, serves roughly 2,000 residential and commercial customers and has high concentrations of rooftop solar.

After the Roadrunner circuit was selected, researchers focused on testing two lithium ion-based storage technologies within the APS system. They are:

·        Community scale, which represents the utility-owned batteries connected along powerlines or the feeder backbone (also known as ‘front-of-meter’). These systems work through supervisory control by a utility’s advanced distribution management system. Previously, community-scale DER have been positioned as the easier-to-implement solution with better economy of scale. F

·         Residential scale represents the many variations of behind-the-meter customer-owned battery storage. In the Roadrunner project, these units provided a standard seven kW and were aggregated through a vendor platform for scheduling and dispatch.

Over two years, multiple strategies were employed to compare the utility and customer impact of residential energy storage to centralized, utility-owned energy storage. Researchers collected data from field testing, simulations, interviews, and other sources to develop a comprehensive assessment of the similarities and differences between the energy storage strategies.

Comparing strategies

By design, community scale and residential scale batteries are built to serve very different needs. Community scale batteries have a much greater storage capacity but are not designed to provide direct customer benefits such as a reduction of a customer’s utility bill or individual backup power. Instead, community scale batteries are best at reducing the feeder peak load and increasing the grid’s capacity for variable energy sources like solar and wind.  

Residential scale batteries, on the other hand, are often controlled to provide individual customer benefits, including back-up power and utility bill reduction. Though they are designed to serve the needs of the individual more than the community, residential batteries can support the greater good if they are seamlessly connected and integrated onto a utility’s grid. To optimize these residential scale batteries, researchers considered three possible algorithms:

·         A ‘utility-centric’ algorithm that produced very minimal customer bill reductions but showed the strongest performance in feeder peak reduction – a benefit to the utility

·         A ‘customer-centric’ algorithm that focused on reducing the customer’s demand charge as much as possible by fully optimizing residential batteries to serve the customer. In the study, the average annual customer savings amounted to between $400 and $900.

·         A ‘hybrid algorithm’ that represents an off-the-shelf option. During periods of high energy demand, it uses a portion of the battery for customer load management, and reserves a portion for utility needs. Both utility and customer see reduced benefit (bill savings are reduced, and the utility has a smaller available resource to support the grid).

During the study, researchers explored whether there was a ‘sweet spot’ within the algorithms that could optimize both community and residential-scale batteries and needs at the same time.

“It was tough to find that single approach with the existing technologies and service algorithms we tested, but we do believe that a mutually beneficial model is possible in the future – perhaps as a hybrid based on time of day or year,” said York.

Key Themes and Findings

Among the findings of the study, two key themes emerged. The first was that efficiency matters and is a key driver to unlocking more widescale adoption. Improving the efficiency of battery installation reduced overall installation cost, making adoption of DER more economical. Improving product efficiency – specifically the energy transfer loss as energy enters and exits the unit – increases the situations in which battery usage provides a cost-effective energy solution.

After efficiency, the second key theme was the importance of flexibility. The more flexibility in algorithms, aggregations and customer programs provided for batteries and their controls, the more potential benefit for customers and the grid.

Beyond these key themes, the study revealed some distinct advantages and disadvantages between the current community scale and residential scale batteries in key categories, including

Backup power: Residential-scale batteries had a significant advantage over community units. This is because community-scale batteries do not have the capability of providing power to the entire feeder. Islanding would be possible, but it would require segmenting the feeder and reconfiguring grounding and protections systems as well as implementing additional grid-forming controls. Residential scale units, on the other hand, could provide power to support the individual customer load demand during an outage.

Land usage: This category was another win for residential scale due to community-scale units requiring more consideration for easements, fire hazard distancing, and proximity to medium-voltage feeder infrastructure. Within the Roadrunner site, which was about 70 percent residential, these challenges were amplified, as available space and location constraints further limited the options for community scale unit installation. In contrast, residential scale units are designed to be smaller and accommodate sides of houses or other similar footprint areas. 

Next Steps

With the Roadrunner study complete, researchers plan to continue to investigate models and technology development which may help provide that middle ground ‘sweet spot’ which serves both customer and community needs.

“Experience is truly the best teacher. This study tells us that we still have a long way to go in algorithm and control development for battery storage – but there’s also opportunity to co-optimize individual and grid-level benefits. We plan on incorporating these findings into our industry-wide research efforts as well as provide a launching point for future international technology demonstrations,” said York.

APS has used the findings of this study to help develop the APS Residential Battery Pilot, a residential customer program launched in 2021. The program will help the utility learn more about the capabilities of residential battery storage devices, while participating customers receive special incentives from APS for sharing data or enrolling in utility battery management. APS will use this information to enhance customer experience, learn how battery technology can be leveraged to strengthen grid reliability, and even optimize a circuit for high concentrations of rooftop solar.

“More and more, we’re seeing a need to move toward a modern, smart grid approach and we’ve already been successful at implementing the most resilient, smart home technologies to help us meet expanding customer needs in our growing service territory,” said Haughton. “We’re focused on learning more about cutting-edge technologies and applying the insight we gather to develop our customer programs with reliability, affordability and sustainability at the core of their design.”

More information about this program can be found on aps.com/battery. To learn more about EPRI research, visit epri.com.