Benefit to Harm (B2H) Ratio for Bulk Vehicle to Grid (V2G) Services Using Lifetime Degradation Digital Twin: Emulating Various Dominant Degradation Mechanism
Authors: Movahedi Hamidreza , Pannala Sravan , Siegel Jason , Stefanopoulou Anna 
Published: August 2024

Vehicle-to-grid (V2G) services, utilizing electric vehicle (EV) batteries for grid support, enhance reliability and reduce peak energy demand. We present a physics-based model tuned for three different cell chemistry families and examine the influence of bulk V2G services on EV battery life. We consider various duty cycles, cell chemistries with nickel content (NMCxyz and various anode graphites), and associated dominant degradation mechanisms. To quantify the impact of offering V2G services, we introduce the V2G benefit-to-harm ratio (B2H). We define the benefit as the Ah gained with V2G and the harm as the life lost in days due to V2G, both compared to the baseline noV2G.

B2H=(normalized Ah gained by 70% capacity fade) divided by (time in days reduction by the time that capacity fade has reached 70%). Note that the 70% capacity fade can be changed for various vehicle models depending on the warranty definition of the % capacity or usable battery energy (UBE) at the end of the warranty.

Our findings indicate that the dominant degradation mechanism plays a crucial role in the benefit-to-harm (B2H) ratio of V2G or vehicle-to-building (V2B) [1]. Specifically, we clarify that the relative contribution of calendar aging to overall capacity loss is pivotal in determining the degradation due to V2G and possible associated benefits.

Our model takes into account four degradation mechanisms based on a single-particle model. SEI growth and cathode transition metal dissolution are part of the calendar aging loss of lithium inventory (LLICal). The remaining LLI is attributed to Li-plating and mechanical degradation caused by particle cracking.

We calibrate the model for three families of cell chemistries and conditions. The primary degradation mechanism for one cell family is anode mechanical degradation (LAMNeg). The second family of cells predominantly ages due to SEI layer growth, and the third family is degrading due to both mechanisms [2].

We show that, in cases with a higher contribution of calendar aging to degradation (LLICal/LLI), V2G can be potentially more beneficial (in Ah gained by 70% capacity fade) than harmful (lifetime reduction in days by the time that capacity fade has reached 70%). Furthermore, we have evidence that the V2G charging pattern can have a secondary effect on the B2H ratio. For example, being a risk taker and charging the battery late or being cautious and charging it as soon as possible can lead to different degrees of degradation depending on the chemistry and conditions of the battery [3].

With our multiphysics reduced order model of battery lifetime degradation, we fully explain why previous studies [4] have shown that the impact on EV battery degradation varies from inconsequential [5] to projecting a need for early battery replacement [6]. Our results clarify that battery chemistry and usage patterns are important factors in determining whether or not to utilize a vehicle for grid support, as well as the overall financial impact on the owner and OEM warranty.

References:

Nazari, Shima, Francesco Borrelli, and Anna Stefanopoulou. "Electric vehicles for smart buildings: A survey on applications, energy management methods, and battery degradation." Proceedings of the IEEE 109.6 (2020): 1128-1144.

Movahedi, Hamidreza, Jason Siegel, and Anna Stefanopoulou. “Predictive Lifetime Battery Simulations of Intra- and Inter-Cycle Degradation for V2G Use: Final report for CRC project SM-4/8”, Coordinated Research Council (under review).

Movahedi, Hamidreza, Sravan Pannala, Jason Siegel, and Anna Stefanopoulou. “Assessing the Viability of Bulk V2G Operations for Different Battery Conditions Using Physics-based Degradation Models”, (manuscript submitted).

Petit, Martin, Eric Prada, and Valérie Sauvant-Moynot. "Development of an empirical aging model for Li-ion batteries and application to assess the impact of Vehicle-to-Grid strategies on battery lifetime." Applied Energy 172 (2016): 398-407.

Uddin, Kotub, et al. "On the possibility of extending the lifetime of lithium-ion batteries through optimal V2G facilitated by an integrated vehicle and smart-grid system." Energy 133 (2017): 710-722.

Dubarry, Matthieu, Arnaud Devie, and Katherine McKenzie. "Durability and reliability of electric vehicle batteries under electric utility grid operations: Bidirectional charging impact analysis." Journal of Power Sources 358 (2017): 39-49.

Figure 1. Capacity retention wrt.(a) days (b) normalized Ah throughput for different scenarios and operational conditions. Discharging to the grid reduces the battery life substantially and increases Ah throughput minimally in NMC111 cells. NMC622-45 case reacts the opposite, and NMC622-25C is somewhere between these two cases. (c) Comparison of the V2G benefit-to-harm ratio wrt. the portion of LLI that is caused by SEI growth.

Figure 1