|PEV battery life cycle with second use. Source: NREL. Click to enlarge.|
Researchers at the US Department of Energy (DOE) National Renewable Energy Laboratory (NREL) are identifying battery second use (B2U) strategies capable of offsetting vehicle expenses while improving utility grid stability.
Second-use options for automotive “end-of-life” Li-ion battery packs support a broad spectrum of sustainable energy strategies, as they increase the potential for widespread PEV adoption by eliminating end-of-life automotive service costs, in addition to helping utilities support peak electricity demands while building a cleaner, more flexible electricity grid. NREL research confirms that after being used to power a car, a Li-ion battery retains approximately 70% of its initial capacity—making its reuse a valuable energy storage option for electric utilities, before battery materials are recycled.
Per the Energy Department’s request, we set out to identify the benefits and viability of B2U strategies, as well as potential roadblocks. We not only confirmed that reusing batteries this way is feasible, we found that it has potential to deliver great benefits to automakers and utilities, as well as car owners.—Ahmad Pesaran, NREL Energy Storage Group Manager
NREL paired techno-economic analyses with laboratory and field testing to better understand battery degradation issues, the ever-evolving world of energy storage, and the true potential costs and benefits of reuse. This research revealed utility energy storage as the most promising of possible applications, with a relatively low expense and abundant battery supply.
The most promising application identified for second use batteries is to replace grid-connected combustion turbine peaker plants and provide peak-shaving services. In comparison to automotive service, use in this application will entail relatively benign duty cycles, generally much less than one cycle per day with discharge durations of greater than one hour. Under these conditions, it is anticipated that second use battery lifetimes will be on the order of 10 years.
While the value to the original automotive battery owner is restricted primarily to the elimination of end of service costs (battery extraction, disposal, recycling, etc.), the value to the broader community could be significant: decreased cost of peaker plant operation on the order of 10% to 20%, reduction of greenhouse gas emissions and fossil fuel consumption, and deferral of battery recycling.—“Identifying and Overcoming Critical Barriers to Widespread Second Use of PEV Batteries”
Most batteries will become available for second use at the end of the expected PEV service life of approximately 15 years. NREL studies show that these batteries—with as much as 70% of their initial capacity—potentially can continue to operate for another 10 years in second use as energy storage for utilities, translating into a total service life of up to 25 years. The 25-year estimate is based on a simulation of 15 years of automotive service, then another 10 years in second use with a daily 50% depth of discharge duty cycle using NREL’s battery degradation life model.
|B2U Repurposing Cost Calculator|
|For B2U, NREL’s repurposing cost calculator is available for free to download, enabling users to explore the effects of different repurposing strategies and assumptions on economics.|
NREL analysis showed regional repurposing facilities specializing in a single model of PEV could harvest and retrofit Li-ion batteries for second use at relatively low costs (less than $500 for today’s typical PEV battery), while avoiding transportation expenses associated with nationwide battery collection.
Although the supply of second-use batteries has the potential to overwhelm many high-value markets for energy storage by an order of magnitude or more, battery reuse by utilities would call for a large-scale supply capable of absorbing a significant portion of these components and diverting them from premature recycling or the waste stream.
Just as PEV batteries are expensive, the utility infrastructure has finite capacity to meet ever-growing electricity needs, and the majority of this system is powered by coal and natural gas generators. Energy storage systems based on reused automotive batteries can increase the availability of peak-period electricity and store energy during periods of low demand. These capabilities are expected to become even more valuable in future facilitation of higher penetrations of solar- and wind-based generation.
To field test its analysis findings, NREL and DOE supported establishing the world’s first fully integrated energy storage system consisting of multiple heterogeneous reused vehicle batteries on the campus of project partner University of California – San Diego (UCSD). The system is operated by the Center for Sustainable Energy under both demand charge management and frequency regulation control algorithms, responding in real time to solar power production and facility load to reduce peak loads, or to the microgrid frequency to improve stability.
The tests have confirmed that B2U systems can successfully accomplish both goals. Additional tests of the nature of battery wear under similar conditions in NREL laboratories suggest that B2U system degradation will be slow, indicative of a long second-use service life.
We used batteries from two different manufacturers with distinct designs, characteristics, management systems, voltages, and capacities. Even with these variables, the system has been operating successfully since late 2013.—Mike Ferry, Center for Sustainable Energy
Other NREL B2U research partners include utilities (San Diego Gas & Electric and Southern California Edison), universities (UC Davis and UC Berkeley), hardware developers (AeroVironment), energy agencies (California Energy Commission), and automakers (BMW).
It is estimated that a critical mass of used PEV batteries will become available beginning in 2030. That gives automakers and utility operators 15 years to develop new strategies to ensure reliability, compatibility, cost, and market acceptance.
The NREL team made a number of recommendations to support the future viability of widespread B2U practices based on their findings:
Automotive and Battery OEMs. Automotive and battery OEMs should include onboard diagnostic capabilities that accurately track the capacity of individual cells (or parallel groups thereof) and pledge to share this data with repurposers. This will enable accurate identification of a battery’s value and its viability for second use service. In addition, degradation and related statistics from automotive service should be quantified and shared.
Systems Integrators and Installers. Systems integrators and installers should work to develop large megawatt-scale ESS solutions for repurposed PEV batteries that minimize integration, BOS, and installation costs. These systems should monitor the health of individual modules and enable efficient replacement of individual faulty or end-of-life modules in the field.
Utilities and Regulators. Utilities and regulators should develop policies that encourage the use of ESS, particularly as peaker plant replacements, and that will support access to sufficiently large markets for repurposed batteries. Guidelines should be defined for minimum required system durations in these roles. Enabling the use of assets with durations as little as 1 hour would also be helpful. Demonstrating both new and repurposed batteries in these roles will be critical.
Laboratories, Universities, Future Repurposers, and other Third Parties. Battery degradation in both the automotive and second use environments is a critical uncertainty in the analysis of B2U strategies. Quantifying degradation of battery packs in first and second uses and developing tools to assess SOH and predict future battery degradation are therefore of great value to the field. Repurposing processed batteries should be demonstrated to confirm that a product of adequate reliability can be provided at low cost. Life cycle analyses that show the overall benefit to society of B2U strategies are important to demonstrate value that may not be captured in economic calculations.
If these recommendations can be implemented, it is quite possible that B2U could become an important part of both the automotive and electricity industries. While the analyses herein suggest that B2U has little ability to reduce the upfront cost of PEVs, it can eliminate end-of- service costs for the automotive battery owner and provide low- to zero-emission peaking services to electric utilities, reducing cost, use of fossil fuels, and greenhouse gas emissions. Thus, the overall benefit to society can be quite large. The authors are hopeful that government, industry, and academia will recognize these benefits and continue to push this important research area forward.—“Identifying and Overcoming Critical Barriers to Widespread Second Use of PEV Batteries”
This project was funded by the Vehicle Technologies Office of the Energy Department’s Office of Energy Efficiency and Renewable Energy.