NREL Study Finds That A “Dynamic Plug-in” Vehicle Could Be A Promising Technology Pathway for Cost-Effective Vehicle Electrification
|Cost-effectiveness of vehicle electrification using today’s assumptions. The EHEV pathway is the dynamic, road-charged hybrid. Source: Brooker et al. Click to enlarge.|
A study by researchers at the US National Renewable Energy Laboratory has found that, under today’s battery assumptions, while plug-in hybrids and electric vehicles significantly decrease consumption, none of a variety of electrification pathways were cost-effective compared to conventional vehicles or hybrids, except for a “dynamic plug-in” hybrid that recharges while moving.
The study by Aaron Brooker, Matthew Thorton and John Rugh aimed to identify possible pathways to cost-effective vehicle electrification by evaluating a variety of scenarios and technology improvements. Using current battery performance and cost, they evaluated PHEVs and EVs across a range of scenarios and configurations—which including 10-, 20- or 40-mile all electric range, with low or high electric power, with or without battery replacement, and with or without opportunity charging. Electric vehicle’s cost-effectiveness improved with battery replacement and opportunity charging, but it was not enough to match the conventional platforms.
Among their general findings are:
Increasing battery power in plug-in hybrids (10-, 20- and 40-mile AER, from low to high power) had little effect on fuel consumption results because the battery power can provide most of the driving on the test cycles, so the fuel economy only differs slightly. For the electric-powered vehicles, the electricity cost is relatively low, reflecting the low cost of electricity and the high efficiency of batteries and motors. The gasoline, on the other hand, is a large expense, especially for the conventional vehicle. Even so, they found, extra battery costs in PHEVs and EVs outweighed the gasoline cost savings.
Battery replacement had minor overall improvements in cost-effectiveness. In these cases, they reduced the size of the battery but used it more aggressively to reduce upfront cost and weight and take advantage of lower future battery costs. The advantages, they found, were mostly balanced out by the increase in battery wear.
Battery life cycle curves showing the relationship of SOC swing to cycle life. Source: Brooker et al. Click to enlarge.
For a smaller battery to provide the same electric range and regenerative braking, it must use a greater portion of the battery energy, and thus have greater depths of discharge. Since battery wear increases non-linearly with depth of discharge, each battery has to be larger than half of the single battery case.
For example, in the high power PHEV10 case, a 5.9 kWh battery would last the life of the vehicle using 34% of the energy. Having one replacement, however, required more than half of a 5.9 kWh battery. It required purchasing two 3.7 kWh batteries using 54% of the energy to meet the life requirement. Although it was assumed that future batteries cost less and that there is a time value of money advantage to purchasing the second battery, these advantages did not make up for the total added cost of buying more total battery energy. The nonlinear wear trend balanced out the advantages for little overall gain.—Brooker et al.
Opportunity charging further decreased the gasoline consumption, and thus gasoline cost, of PHEVs, but at a greater increase in battery cost. Although the fuel cost went down, opportunity charging increased the use of the battery. As one example, in order to sustain the additional use and wear, the battery energy had to be increased from 5.9 kWh to 10 kWh in a PHEV10. Including the additional electricity cost, opportunity charging increased the total cost for the PHEV10 by $4,400.
On the other hand, opportunity charging decreased the EV cost. Opportunity charging increased the frequency of recharging, reducing the depth of discharges and the amount of wear, and thus reducing the amount that the battery has to be oversized to last the required life. Specifically, it reduced the battery size from 47 kWh to 32 kWh. The EV still exceeded the cost of all the other vehicle types.
Combining battery replacement and opportunity charging increased the use of the high cost battery to better leverage the investment, but little to nothing was gained by adding battery replacement to the opportunity charging cases under current battery life assumptions.
Their analysis found three potential approaches for improving the cost-effectiveness of vehicle electrification: what they called the electrified HEV (EHEV)—i.e., a vehicle that recharges while moving along the roadway—because of the resulting downsizing of the battery pack; significant battery improvements reducing the cost to the DOE target of $300/kWh (they used $700/kWh as the current level); or an improvement in battery life by a factor of 10.
If an acceptable method for plugging in while traveling along the roadway can be devised, it may provide a cost-effective pathway to vehicle electrification. This approach benefits from the low electric fuel cost of a large battery without the high cost, cycling wear, weight, and efficiency loss. Even with assuming a $1,000 price for the connection device, the cost to the consumer was still lower than for today’s conventional and hybrid vehicles. This pathway requires infrastructure, but only along a small fraction of heavily traveled roadways to gain the same gasoline saving benefits as battery PHEVs.—Brooker et al.
The fraction of infrastructure is small because most travel occurs on just a few roads. The interstate, for example, makes up 1% of the miles of roadway but carries 22% of the vehicle miles traveled. Their scenario assumed that 50% of the distance driven is connected dynamically. It also assumed an additional $1,000 cost to the consumer for the dynamic connection, the same fuel cost per mile as an HEV when not connected dynamically, and the charge depleting fuel cost per mile of a PHEV when connected.
A paper on their work, which will be presented at the SAE World Congress in Detroit in April, details the assumptions and approach for the study.
Brooker, A.; Thornton, M.; Rugh, J. (2010). Technology Improvement Pathways to Cost-Effective Vehicle Electrification: Preprint. 17 pp.; NREL Report No. CP-540-47454