Model-Based Impact Analysis of Vehicle-Integrated Photovoltaics and Vehicle-to-Grid on Electric Vehicle Battery Life

Abstract

Electric vehicles (EVs) equipped with vehicle-integrated photovoltaics (VIPV) and EVs with vehicle-to-grid (V2G) technology can support in overcoming power grid challenges emerging from the energy transition. Despite the widespread benefits that VIPV and V2G have to offer, their potential impact on battery life governs their economic viability. Current studies on the impact of VIPV and V2G on battery life are often simplistic, use unrealistic battery data, and rarely investigate methods to reduce battery ageing. To fill this research gap, this study combines validated models to determine the impact of VIPV and V2G on EV battery life and investigates methods to reduce battery ageing.

First, an EV battery data generation model was developed to simulate EV load profiles with and without VIPV and V2G. Afterwards, a one-year mobility and charging profile was constructed based on EV driving data in the Netherlands and Germany. Subsequently, driving cycles were simulated using Lightyear’s Vehicle Performance Model (VPM) to generate realistic per-second EV battery data. Following this, VIPV power generation profiles for the Netherlands and Spain were modelled using Lightyear’s SolarSimulator tool. Thereafter, two load profiles of V2G services in the Netherlands were modelled, namely day-ahead electricity trading and automatic frequency restoration reserve (aFRR), both with a battery capacity retention limit during V2G of 50% state of charge (SoC) and 20% SoC. The VIPV and V2G load profiles were merged with EV battery data to generate eight EV battery datasets. Finally, the EV battery datasets were implemented in a semi-empirical NMC-based ageing model (NMC-AM) and a semi-empirical LFP-based ageing model (LFP-AM) to determine the impact of VIPV and V2G on battery life.

Results from the EV battery data generation model show that gradual VIPV charging can reduce the annual grid charging frequency by 23% in the Netherlands and 44% in Spain. Reduced grid charging frequency due to VIPV caused the battery to range at lower SoC, which is beneficial for battery calendar life. Consequently, NMC-AM suggests that VIPV could extend battery life by 6 months, while LFP-AM suggests a battery life extension of 2 months. However, additional irregularity in the load profile due to gradual VIPV charging is likely to have caused the ageing models to overestimate cycling ageing. Furthermore, the ageing models suggest that additional cycling due to V2G, with the aim of maximising profits for the EV owner, could shorten battery life by 7.8 to 12.5 years for NMC and by 1.2 to 3.9 years for LFP. The results of the ageing models indicate that LFP batteries are more resistant to additional cycling than NMC, which is in line with literature.

Additionally, simulated scenarios in which the SoC was kept at 50% or 100% for one month per year, showed that SoC regulation could extend battery life by up to 2 years, allowing for 38,000 km of additional driving range before the battery reaches its end of life (EoL). Furthermore, results suggest that VIPV could lower battery temperature by 10 °C within one sun hour and can keep the battery cool when parked in the sun, by 23 °C in the Netherlands and 35 °C in Spain. Ageing simulations in which VIPV was used to cool the average annual battery temperature by 5°C, suggest that VIPV can extend battery life by up to 4.6 years, allowing for 88,000 km of additional driving range before the battery reaches its EoL. SoC regulation can be performed by delayed VIPV charging, delayed grid charging, or V2G. Battery temperature regulation can be performed using VIPV or grid power. Taking electricity cost into account, grid-powered battery temperature regulation could prove to be a cost-effective method to extend battery life, especially for EVs experiencing extreme temperatures.

Furthermore, as semi-empirical ageing models often lack clarity regarding their implementation, are usually not based on ageing tests with irregular load profiles, do not consider path dependency, are based on accelerated ageing tests under limited operating conditions and on a particular battery cell chemistry and size, applying these ageing models on irregular load profiles or other cells may lead to ageing estimation errors. Consequently, to determine how VIPV impacts battery cycle life, it is recommended to conduct battery ageing tests under identical operating conditions, with and without VIPV. Further research on the impact of VIPV on battery life would help develop strategies that optimally balance VIPV power used for battery charging and for battery temperature regulation.

Concluding, this research shows promising initial findings on methods to reduce battery ageing using VIPV and V2G, which could further improve their business case, accelerating the transition to sustainable mobility.