CMU team finds regional temperature differences have significant impact on EV efficiency, range and emissions
|Energy consumption per mile averaged across the LEAF fleet over a full year (Wh/mi). Credit: ACS, Yuksel and Michalek. Click to enlarge.|
An adage about batteries is that they are like humans in performing best at moderate (e.g., room) temperatures; extremes in either direction impact performance. Thus, the efficiency of battery electric vehicles can vary with ambient temperature due to battery performance—as well as the energy required for cabin climate control.
In a new paper accepted for publication in the ACS journal Environmental Science & Technology, Tugce Yuksel and Jeremy Michalek at Carnegie Mellon University have now characterized the effect of regional temperature differences on EV efficiency, range, and use-phase CO2 emissions in the US, based on aggregated real-world fleet data for the Nissan LEAF. Among their findings is that the resulting regional differences in efficiency, range and emissions are large enough to affect adoption patterns and the energy and environmental implications of battery EVs relative to alternatives.
Battery performance depends strongly on temperature. At cold temperatures, battery efficiency, discharge capability and available energy decreases. In addition, battery internal resistance increases, decreasing the power that can be drawn from the battery. Battery performance increases with temperature rise, but batteries also degrade faster at high temperatures, increasing thermal management requirements.
Ambient temperature determines initial battery temperature and thermal management loading (if the vehicle is parked outside, the battery is not thermally preconditioned, and solar radiation is negligible) as well as battery temperature and thermal management load during use. Weather conditions, therefore, have a direct impact on battery efficiency. Ambient temperature also drives use of cabin air conditioning to either heat or cool the cabin at cold and hot days respectively. The net effect of these factors causes customers to report up to 40% decrease in their driving range on cold winter and/or hot summer days compared to the maximum range they achieve. The cold temperature effect is generally larger for two main reasons: electric cabin heating consumes more power compared to cooling, and batteries have poorer performance at low temperatures.
Air conditioning (A/C) use during hot days is an important factor affecting the fuel economy in all types of vehicles, since A/C is the largest auxiliary load in many vehicles. Cold temperatures, on the other hand, are particularly disadvantageous for BEVs, since vehicles with internal combustion engines can use engine waste heat for cabin heating, whereas in BEVs heat must be generated using limited onboard stored electrical energy. Reduced efficiency results in increased energy consumption and increased emissions from the electricity grid when BEVs charge. The net effect on emissions varies across the country due to source of electricity generation as well as the regional differences in marginal electricity grid mix.—Yuksel and Michalek
Although earlier studies have explored regional differences in energy consumption and emissions of EVs, they assume constant vehicle efficiency and do not account for efficiency losses with temperature change, the CMU authors note. Nor have other studies focused on regional differences due to spatial and temporal ambient temperature differences.
To estimate the regional effects of temperature on electric vehicle efficiency, range and emissions, Yuksel and Michalek constructed models of vehicle energy consumption vs. temperature; US temporal and spatial temperature variation, vehicle driving and charging patterns; and US regional grid emission factors.
To establish the relationship between energy consumption and ambient temperature, the authors used publicly available data collected by Canadian company FleetCarma from Nissan Leaf users for more than 7,000 trips across North America, reported as average driving range versus ambient temperature. Thus, the CMU results are based on results experienced by real drivers in actual driving conditions instead of simulation models.
They used the Typical Meteorological Year (TMY) Database from the National Renewable Energy Laboratory (NREL) to obtain time- and location-dependent ambient temperature data, and the National Household Travel Survey (NHTS) 2009 dataset to obtain driving patterns. For grid emission factors, they used a recent analysis by Graff Zivin et al..
Among their findings were:
The average energy consumption per mile can increase by 15% from 273 Wh/mi (170 Wh/km) along Pacific Coast or at certain parts of South Florida to 315 Wh/mi (196 Wh/km) in the Upper Midwest.
Energy consumption can vary inside the same state because of the temperature differences of different locations. In Southeast California, the average energy consumption is 323 Wh/mi (201 Wh/km), 18% higher than the coast.
Greenhouse gas (GHG) emissions from EVs vary primarily with marginal regional grid mix, which has twice the GHG-intensity in the Upper Midwest (MRO region) as on the Pacific Coast (WECC region). However, even within a grid region, BEV emissions vary due to spatial and temporal ambient temperature variation and its implications for vehicle efficiency and charging duration and timing. Within the WECC region, for example, the emission rates can increase from 100 g/mi (62 g/km) up to 122 g/mi (76 g/km), a 22% increase.
The authors note that this increase in emission rates happens mainly because energy consumption changes with temperature, but also because as energy consumption changes so does the charging duration.
Cold climate regions also encounter days with substantial reduction in EV range: the average range of a Nissan LEAF on the coldest day of the year drops from 70 miles on the Pacific Coast to less than 45 miles in the Upper Midwest.
Yuksel and Michalek also ran two other cases: 1) with an increased battery capacity of 85 kWh and 2) with a lower charge rate of 3.3 kW. Both of these assumptions can change emissions estimates up to 4%.
In this study, we use data only for a particular electric vehicle, the Nissan Leaf. Other electric vehicles differ in vehicle efficiency, HVAC efficiency, battery technology, and thermal management and may therefore have different temperature-specific range and emissions implications. Nevertheless, the trends observed here are fairly general because 1) heater and A/C use increases BEV energy consumption, and 2) electrochemical reactions in batteries are temperature dependent. With improvements in battery technology and with the use of more energy efficient vehicle thermal conditioning systems, it might be possible to see a reduced effect of ambient temperature in the future.—Yuksel and Michalek
Tugce Yuksel and Jeremy J Michalek (2015) “Effects of Regional Temperature on Electric Vehicle Efficiency, Range, and Emissions in the United States” Environmental Science & Technology doi: 10.1021/es505621s
Graff Zivin, J. S.; Kotchen, M. J.; Mansur, E. T. (2014) “Spatial and temporal heterogeneity of marginal emissions: Implications for electric cars and other electricity-shifting policies.” J. Econ. Behav. Organ. 1–21; doi: 10.1016/j.jebo.2014.03.010