Empirical evaluation of V2G round-trip efficiency

The business case of vehicle-to-grid (V2G) technology and its potential to provide grid services is heavily dependent on the round-trip efficiency of this technology. Surprisingly, very little empirical research is conducted to determine the V2G round-trip efficiency of electric vehicles currently available in the market, resulting in a wide range of efficiency values used in V2G modelling studies. This study aims to create more insight in the current V2G round-trip efficiency to stimulate that more uniform and realistic efficiency values are used in other studies. A field experiment is executed to measure the round-trip energy efficiency of V2G for different dates, current rates and average state of charge. It was found that the average round-trip efficiency (i.e., combined inverter and battery efficiency) when charging between a state of charge 25% and 75% with 3x16 Ampere was 87.0%(±1%). However, various external factors could influence the measured efficiencies, which had a total range from 79.1% to 87.8%. Charging at lower ambient temperatures and lower current rates had a statistically significant adverse effect on the round-trip efficiency. Efficiency at high and low state of charge was found to be marginally lower than around medium state of charges. Two different electric vehicle + charging station models were tested, one with on-board AC/DC converter, which is a novel V2G setup, and one with external AC/DC converter, rendering no statistically different efficiency values.