Study suggests OEMs should use a modular design for PHEV and EREV vehicle battery packs to offer capacity choices to customers
|TCO in € cents/km of EREV as a function of battery size for users with different annual mileages. Redelbach et al. Click to enlarge.
Car manufacturers should develop a modular design for plug-in hybrid and extended range electric vehicles (PHEVs and EREVs), allowing them to offer a choice of storage capacity to meet individual customer requirements rather than forcing a “one size fits all” approach, according to the results of a German-market-specific TCO study by a team from the Institute of Vehicle Concepts, German Aerospace Center (DLR).
The authors of the study, published in the journal Energy Policy, stress that they are not suggesting OEMs offer each customer an individual battery size, but rather than they offer, as an example, three different battery sizes dedicated to drivers with low, average and high mileage. The development of a modular design for battery packs could help OEMs to change the size with less effort and few implications on the rest of the vehicle, they suggested. (This is analogous to the approach taken by Tesla Motors with its two—originally three—pack capacity sizes offered in the Model S.)
In their study, the DLR team analyzed the impact of different driving profiles on the optimal battery setup from the total cost of ownership (TCO) perspective. Results showed that the battery size has a significant effect on the TCO.
For the average German driver who drives 15,000 km/year (9,321 miles), battery capacities of 4 kWh (PHEV) and 6 kWh (EREV) would be cost optimal by 2020. However, the researchers found, the values vary strongly with the driving profile of the user. For example, for an EREV, the optimal size changes to 2 kWh or 13 kWh is the annual mileage is halved or doubled, respectively.
Moreover, the optimal battery size is also affected by external factors, e.g. electricity and fuel prices or battery production cost.
One of the most promising powertrain technologies are [sic] plug-in hybrid electric vehicle (PHEV) and extended range electric vehicles (EREV). They combine emission free driving of battery electric vehicles with the unrestricted driving range of conventional cars powered by gasoline or diesel. However, the battery is still a very critical component due to the high production cost and heavy weight. Therefore, the right sizing of the battery is the key for electric powertrains to meet customer expectations and become cost competitive against conventional technologies.—Redelbach et al.
While numerous studies have already explored the impact of PHEV battery size on costs and greenhouse gas emissions, existing studies neglect some significant aspects in this context, the DLR researchers said:
The studies do not account for the heterogeneity across different driver types;
They do not consider that drivers with higher annual mileage typically spend more time on motorways with a higher average velocity than drivers with lower annual mileage—thus affecting energy consumption and the share of electric driving;
None consider the technical differences between hybrid architectures such as parallel (PHEV) and serial (EREV) configurations; and
Some do not take into account that batteries are subject to degradation and aging processes which require a substantial oversizing of the initial energy capacity.
… this paper aims to close these gaps by introducing a holistic approach for the optimization of the battery size of PHEVs and EREVs under German market conditions by considering the battery degradation and secondary effects of additional mass on energy consumption. The assessment puts special focus on the heterogeneity across drivers, by analyzing the impact of different driving profiles on the optimal battery setup from total cost of ownership perspective for the year 2020 in Germany. Furthermore, specific CO2 emissions (tank to wheel — TTW and well to wheel — WTW) for grid connected cars are analyzed as a function of battery size. The most relevant data for this analysis, e.g., energy consumption or battery costs, is based on own vehicle simulations and cost models.—Redelbach et al.
|Structure of the battery-size optimization problem. CD = charge depleting; CS = charge-sustaining; TTW = tank-to-wheel; WTW = well-to-wheel. Redelbach et al. Click to enlarge.
For their analysis, they used three representative driving behaviors, based on national data: user A, with 7,500 km (4,660 miles) per year; user B, with the average 7,500 km/year; and user C, with 30,000 km (18,641 miles) per year.
Among their findings were:
The energy consumption of the PHEV is significantly higher compared to the EREV especially for higher battery sizes.
The cost of energy decreased with growing battery size due to the higher share of electric driving.
For a given battery size, specific energy costs rise with the mileage of the user.
For EREV users A, B and C, last costs are achieved with battery sizes of 2.0, 6.0, and 13.0 kWh, respectively, corresponding to 8, 24 and 51 km of electrical range.
For PHEV users A, B and C, least costs are achieved with battery sizes of 1.5, 3.5 an 5.0 kWh.
The results of this paper imply that higher battery capacities would reduce the overall WTW GHG emissions. If the political target is to reduce the GHG emissions even further by encouraging OEMs to design high battery sizes for PHEVs and EREVs, the public authorities may influence the results of TCO by different measure.
This can happen in basically two ways: On customer side financial incentives could be provided in the form of a direct purchase bonus or tax benefits which are linked to the battery size of the electric driving range of the new car. Furthermore, policy makers may increase the petrol price by increasing the taxes on it. On OEM side, the legal CO2regulations could contain norms that reward manufacturer with additional credits depending on the electric driving range of their partial zero emission vehicles (as realized in the ZEV legislation in California). Finally, the electricity to operate plug-in hybrid vehicles in electric mode should be provided from renewable energy sources to reach their full environmental benefit.
It should be noted that this paper assumes a rational customer who has objective to minimize the total cost of ownership. In reality, the behavior of consumers may not be fully rational. Consumers may prefer high electric range due to several reasons. This phenomenon may be analyzed in the future and the factors that may distort a rational choice may be identified.—Redelbach et al.
Martin Redelbach, Enver Doruk Özdemir, Horst E. Friedrich (2014) “Optimizing battery sizes of plug-in hybrid and extended range electric vehicles for different user types,” Energy Policy, Volume 73, Pages 158-168 doi: 10.1016/j.enpol.2014.05.052