CMU team finds regional temperature differences have significant impact on EV efficiency, range and emissions
18 February 2015
|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
Not that surprising. Great write up!
Posted by: CarValetDublin | 18 February 2015 at 08:35 AM
OK, since we live in coastal CA, I guess it is time to consider an EV for the good of the planet. We just need about 150 miles range and a (not upper-)middle-class price point to make it practical for us. Here's hoping that we will have such an option within a year or two.
Posted by: Nick Lyons | 18 February 2015 at 01:02 PM
Good thinking; Range is the ticket to feasible EVs.
The range limitations of the current EVs takes away spontaneous decisions; one must be concerned at all times with the fuel reserve and carefully plan all trips, included in the planning is the speed you intend to drive and running the heat and air conditioning system. I drive my 2011 Leaf in the foothills and I plan never to drive more than 60 miles without planning a charge.
Posted by: Lad | 18 February 2015 at 03:38 PM
I'm right at the border of IL/WI, and it is bloody damned cold right now. I generally only drive about 25mi/40km a day, so I don't sweat range too often but on days like these a couple of added errands can get worrisome (and colder: the first action, of course, is to turn off heat). In the morning I preheat to 75degF just before leaving the house and can generally drive the whole way to work with heat off, with just an occasional burp of the defrost, and remain quite comfy. The trip home is different: this evening when I climbed in it was 0F/-18C and battery temp indication was NO bars. Regen is also almost non-existent. And you cannot rely on only the (excellent) seat and steering wheel heaters; you HAVE to run the heater. So I do, on recirc, and within a few minutes I'm OK.
During late spring and most of the summer I manage from 4.2-4.6 mi/kWh. Right now I'm clinging to 3.3 and dropping.
I love the car but it's definitely more of a mission than a choice to operate one in the Midwest winter.
And let it be known here that as an eternal Tesla pessimist I predict this will be a REAL problem for the Model X. If the vehicle is really aimed at affluent young family transport, the dramatically lower real-world range on normal winter days, keeping up with traffic and keeping the kids warm, will kill the appeal very quickly. The Leaf draws 3-4kW just to keep chubby old me from being too chilled in a small cabin driving urban/suburban traffic. On the Boston Turnpike at 70-75, heat being scrubbed away, big cabin and cranky kids in the back... try 10-15kW. Do the math with the taller, heavier vehicle and you're looking realistically at a 150-175mi max range. It's not going to take many ruined hockey tournament trips to blunt the theoretical demand.
And a 120kWh battery would break through the weight and cost tolerances of designers and buyers, respectively. Look out below, folks.
Posted by: Herman | 18 February 2015 at 06:40 PM
Great post & experiences Herman - this is the one reason I'd consider a Volt over a Leaf at this point, I live in the same area - just to be able to run the engine for the heat in the winter.
Definitely seems like we need a better more efficient electric heating device - makes me wonder how efficient those mini microwave water heaters are (used now for individual faucets etc. - since a full on microwave warms water prodiciously off 110 volts), would seem to be an ideal solution.
Posted by: Sasparilla | 19 February 2015 at 06:49 AM
Herman, the cabin heater in the Leaf (at least the early model I am renting) is quite inefficient. The Think City EV originally came with a similar bottle heater and conventional heater core with antifreeze, but a recall upgraded that to a direct heater element (positive temperature coefficient) which is insanely fast and hot while using much less power.
Of course a heat pump is the real deal, and I have heard that many newer EV models are using a heat pump. Toyota even had a heat pump in the old Rav4 back in 1999 or so, if I can believe what I am told.
But you are right, winter is a big hit to EV range unless you wear long wool underwear and keep a hot water bottle in your lap.
Posted by: Jim McLaughlin | 19 February 2015 at 07:29 PM
The solution that immediately comes to mind is why not design a heater based on ethanol or propane, even if its an option for cold regions. From a CO2 point of view it has to be as efficient as converting gas to heat to electricity and then back to heat. I suppose it must be a safety issue or a cost issue. I might be inclined to pack a small propane heater in my emergency kit anyways.
Posted by: Calgarygary | 20 February 2015 at 10:48 AM
Sasparilla, I'm with you on the switch to EREV. I briefly used a company-leased Volt for awhile, and I can say to the surprise of some (but not veterans of the Leaf/Volt internet wars) that I drove more e-miles with the Volt than the Leaf. The reason, of course, was that e-range was not a limiting factor for those times when you prudently leave the Leaf at home and take the burner. Jim, the '13 and later Leafs (mine is a '12) have a heat pump, which makes a really big difference in W-h consumed on moderately cold days. Most heat pumps give up when it gets Great Lakes deep winter cold, but for most of the N. American EV operating days it would still make a big difference.
Gary, I'm absolutely with you when it comes to hydrocarbon burned for heat. It's a rational use of the fuel. The net emission is far less than that caused by the string of energy conversion. My understanding is that the original Th!nk City EVs had a small kerosene heater, but I do not know this for sure. Certainly a small LPG bottle (2-3kg) could be installed optionally under the hood and plumbed to an efficient heating element to supplement resistance heating. Used judiciously a fill would last the average commuter 7-10 days (if EVSE-based preheat is used before morning departure). This would be a very modest inconvenience for the 3-4 weeks of deep winter.
Posted by: Herman | 20 February 2015 at 12:19 PM
It would be interesting to see the average American vehicle and it's CO2 emissions by region as well. Using the A/C in hot Southern states has a sizable impact on their emissions and they generally put out more emissions anyway.
And since they used the emissions from the grid for the EVs, they should also add the emissions from refining gasoline into the ICE calculations.
It would probably be much easier to stop at that point and call it as close to apples-to-apples as you're going to get. You can't really factor in all the true well-to-wheels data without getting into a huge debate about fracking vs deep water drilling and coal mining, etc.
This is great data, but gives us no context to work with in the big picture.
Posted by: DaveD | 23 February 2015 at 07:12 AM