Study Finds PHEV Li-ion Iron Phosphate Cells Show Little Capacity Fade Under Combined Driving and V2G Usage; Economic Model Suggests Incentives Will Be Required for Vehicle Owners to Participate in V2G
[This is a revision of an earlier post, which had been pulled due to the status of the referenced papers as working papers. Both have now been revised and accepted by the Journal of Power Sources and are in press.]
|Degradation as a function of (a) capacity (Ah) processed by cell or (b) energy (Wh) at different DoD. Different DoD did not have a large impact on capacity fade. Source: Peterson et al. (CEIC-09-02) Click to enlarge.|
Researchers at Carnegie Mellon Electricity Industry Center have concluded that a PHEV pack comprising lithium iron phosphate cells would incur little capacity loss from combining vehicle-to-grid (V2G) activities with regular driving. Statistical analyses indicated that rapid battery cycling incurred when driving degraded the cells more than slower, vehicle-to-grid galvanostatic cycling.
Scott Peterson, Jay Apt, and Jay Whitacre also found that the percent capacity lost in the cells (they used A123Systems 26650 M1 cells, which are used in the Hymotion PHEV conversion packs) per normalized Wh or Ah processed is quite low even based on just use in a dynamic driving cycle—more than 95% of the original cell capacity remained after thousands of driving days worth of use. However, in a companion paper assessing the economics of V2G for consumers, they also concluded that the maximum annual profit for a PHEV owner to engage in V2G (~$10-$120) would likely prove insufficient to encourage use of the battery pack for grid electricity storage and later off-vehicle use.
|Test current profile used to simulate driving day for cells showing all trips. The times after trips 3 and 4 when V2G discharge was simulated are indicated. Source: Peterson et al. (CEIC-09-02) Click to enlarge.|
Performance. The quantify the capacity degradation of the pack, the authors derived nominal urban driving and driving/V2G power profiles and correlated battery test regimes by combing several common data sets. To determine the quantity and rate of energy transferred to and from a battery during driving conditions, they used a simple physics model that computed the energy needed to propel a typical vehicle through the trip profile.
To calculate the power vs. time battery duty cycle needed to achieve this velocity/acceleration profile, the vehicle was assumed to have the physical characteristics of a 2008 Toyota Camry. The efficiency of power transfer from regenerative braking to batteries was assumed to be 40%, the efficiency from battery to wheels was assumed to be 80%. The battery pack energy capacity was assumed to be 16 kWh. An 800 watt constant load was added to account for the power needed for all activities unrelated to movement such as heater, air conditioner, radio, lights and other accessories.
Among their findings were that the cell depth of discharge (DoD) does not does not have nearly as great an effect on lifetime as previously reported values for other battery chemistries. In cells discharged to 95% DoD per cycle, their measurements predicted that 5,300 cycles will be needed before reaching 80% of initial capacity instead of around 1,500 cycles as indicated by other data. Daily cycles with shallower DoD values do not appear to increase cycle life.
They also found that there is a difference between driving energy withdrawn and constant discharge—i.e., for V2G. The low rate constant discharge for V2G resulted in roughly half the degradation of that of the dynamic discharge for driving: -2.7x10-3 percent capacity lost per normalized Wh or Ah processed for V2G support vs.-6.0x10-3 percent for dynamic driving support. These values show that several thousand driving/V2G driving days incur substantially less than 10% capacity loss regardless of the amount of V2G support used, they concluded.
This result implies that a LiFePO4/graphite–based PHEV battery pack with properly matched cells can be cycled though a very broad state of charge range without incurring any significant increase in capacity loss as a function of Ah or Wh processed. In principle, a PHEV can utilize a smaller battery and use a greater proportion of the battery, however doing so might make discharge rate and associated ohmic heating more of an issue.
...the cycle DoD and relative fraction of low-rate galvanostatic cycling vs. acceleration/regenerative braking current pulses are not important even over thousands of driving days. Rather, it is the integrated number of lithium ions that have been intercalated/de-intercalated into the electrodes, regardless of the DoD at which these events occur.
—Peterson et al. (CEIC-09-02)
Economic model. In the companion working paper, the authors examined the potential economic implications of using plug-in vehicle batteries to store grid electricity generated at off-peak hours for off-vehicle use during peak hours. They used hourly electricity prices in three US cities to arrive at daily profit values, while the economic losses associated with battery degradation were calculated based on data from the first study.
For a 16 kWh vehicle battery pack, the maximum annual profit with perfect market information and no battery degradation cost ranged from ~$140 to $250 in the three cities. If the measured battery degradation is applied, however, the maximum annual profit (if battery pack replacement costs fall to $5,000 for a 16 kWh battery) decreases to ~$10-$120. It appears unlikely that these profits alone will provide sufficient incentive to the vehicle owner to use the battery pack for electricity storage and later off-vehicle use.
—Peterson et al. (CEIC-09-03)
They also estimated grid net social welfare benefits from avoiding the construction and use of peaking generators that may accrue to the owner, and found that these are similar in magnitude to the energy arbitrage profit.
...the vehicle owner might be able to avoid ~$200 of peaking costs. In states with traditional regulated electricity, the public utility commission might elect to avoid paying the utility to install and run a peaker, instead giving some of the avoided cost to V2G owners. In restructured states, the ISO/RTO may pay an aggregator to provide V2G power instead of paying a generator a capacity payment; the aggregator would then pay some of their revenue to the vehicle owner. In the absence of such incentives, it is unlikely that large-scale grid energy storage in PHEVs will be attractive to a large number of vehicle owners.
—Peterson et al. (CEIC-09-03)
Scott Peterson, Jay Apt, and Jay Whitacre (2009) Lithium-Ion Battery Cell Degradation Resulting from Realistic Vehicle and Vehicle-to-Grid Utilization (CEIC-09-02)
Scott Peterson, Jay Whitacre, and Jay Apt (2009) The Economics of Using PHEV Battery Packs for Grid Storage (CEIC-09-03)