Wednesday, August 13, 2014

Guest Post: Efficiency or Range? You Can’t Have Both.

The i3 is the most efficient production car available today

Every now and then I have a reader send me an article they wrote and ask if I'd like to post it here. Usually it's not exactly what I'm looking for and politely explain why I won't be posting it and thank them for sending it nonetheless.  Occasionally I'll get something interesting though, like the post below which was sent to me by Robert Kasper. I think it's particularly timely since just last week I posted the Tesla/BMW comparison piece and I think this is an interesting follow up to it.  I hope you enjoy:                    

  Efficiency or Range?  You Can’t Have Both.

…But Advanced Technology Can Help.

By Rob Kasper

In the world of electric vehicles, whether Battery Electric Vehicles (BEVs) or Plug-in Hybrid Electric Vehicles (PHEVs), there is a clear trade off between range and efficiency.  For a given technology, efficiency suffers as range increases due to the weight of not only additional battery capacity, but the increased structure and volume to haul that capacity around.  Now that there are a significant number of plug-in vehicles being manufactured, and a recognized standard to test them, we can identify trends.  Consider Table 1 and Figure 1, a plot of efficiency (as measured in EPA MPGe) vs. range in miles for 2014 plug-in electric vehicles measured by the EPA.  They are grouped into Battery Electric Vehicles (BEVs) and Plug-in Hybrid Electric Vehicles, and further identified as either conventional or advanced technology design and construction.  Conventional technology is generally characterized by a manufacturer’s use of an existing gasoline powered platform modified for battery electric drive, steel frame construction and cladding, and standard battery technology.  Advanced technology is generally characterized by a clean sheet, purpose built EV design, extensive use of aluminum or aluminum plus Carbon Fiber Reinforced Plastic (CFRP) for weight savings, higher energy density lithium ion battery packs, with the bonus of performance equivalent to or exceeding the best of conventional technology plug-in vehicles.

Figure 1: Efficiency vs. Range

Table 1: EPA Electric Range and MPGe

Beyond the obvious observation that the price of greater range is lower efficiency within a given technology, it is important to note the significance of advancing technology.  The ground-up EV design, significantly lighter weight construction, and advanced battery technology of the BMW i3 and Tesla Model S push the blue trend line significantly up and to the right of conventional BEVs’ green trend line.  As significant is the single data point (in purple) representing the only advanced technology PHEV currently available – The BMW i3 REx.  Not only is it capable of greater efficiency and far more range than any conventional PHEV (the red trend line), it is more efficient than all but two conventional BEVs, with only slightly less range than all but the most inefficient conventional BEVs.

It is this outlier of a data point, the BMW i3 REx that might best help illustrate why a smart means of increasing the range of an EV may not necessarily be to add more battery capacity.   Battery energy is clean and well suited for powering vehicles for relatively short-range transportation but due to its weight and lengthy charge times, inefficient and inconvenient for long distances.   On the other hand, the benefits of energy density and convenience make gasoline/diesel energy better suited for longer range transportation with the trade-off being greater well to wheel emissions in many parts of the world.  In the case of the BMW i3 REx, each mile of range requires either 0.15 pounds of gasoline, or 5.7 pounds of battery capacity.  At 37 times the mass specific energy density of battery power, very little gasoline is required to extend range for a given tank size, and that tank can be replenished in minutes nearly anywhere in the well developed fossil fuel infrastructure that currently exists worldwide.  This capability requires a 265 pound increase in the weight3 of the vehicle for the REx engine and associated systems, which imposes a 6% decrease in efficiency, but once set, that efficiency does not appreciably decrease as more energy in the form of gasoline is added to increase range.  Increasing battery capacity cannot increase range as efficiently, as not only must the weight of the battery increase by 37 times the weight of gasoline per mile in the first increment, but by the weight of increased structure and volume, as well as even greater battery capacity to offset the reduction in efficiency resulting from the weight increase.  There comes a point where the sacrifice in efficiency may no longer be worth the additional range to be gained.
See figure 2:

Figure 2: EV Energy Storage (and Generation) Weight vs Range for Advanced Technology EVs



1- EPA testing protocol does not account for approximately 4 miles of range remaining after REx fuel exhaustion when publishing a 72 mile battery powered electric range before REx activation, but does account for it in the total range calculation of 150 miles:  72 electric miles + 1.9 gal x 39 mpg + 4 electric miles = 150 EPA range (76 electric + 74 gasoline).  76 miles of range is also the result of dividing the EPA measured total i3 wall to wheel consumption of 22.0 kWh by the i3 REx EPA measured consumption rate of 0.288 kWh/mile.  This value is further corroborated by the CARB BEVx designation awarded to the i3 REx which requires the electric range not only be at least 75 miles, but that it must exceed the gasoline range, neither of which would be possible without accounting for the ~4 miles of range remaining after REx fuel exhaustion.

2- The EPA’s 95 MPGe rating of the Toyota Prius Plug-In Hybrid includes 0.2 gallons of gasoline operation plus 29 kWh of electric operation per 100 miles.  Subtracting the 10 mile of gasoline operation contribution to the total (0.2 gal X 50 mpg) yields 29 kWh per 90 miles, or 32.2 kWh per 100 miles, which results in 105 MPGe for electric only operation. (MPGe = 33,705 divided by watt hours per mile.)

3- While EPA rated at 87 miles of range in its base form, purchasers of the Mercedes-Benz B-Class can choose to pay an additional $600 for the Range Package, which makes an additional 17 miles of range available.  There is no difference in total battery capacity between the two configurations, only the percentage of SOC made available to the driver.

4- The 8 BMW battery pack modules weigh 55 lbs. each, for a total of 440 lbs.  Reference page 17 of the BMW i3 Service Managers Workshop Participant Guide at http://darrenortiz.com/website_pdfs/BMWi3PG.pdf.

5- 265 lbs for the REx engine and all associated equipment is the difference in weight between the i3 BEV and i3 REx as published on BMW’s spec pages:  http://www.bmwusa.com/Standard/Content/Vehicles/2014/i3/BMWi3/Features_and_Specs/BMWi3Specifications.aspx
http://www.bmwusa.com/Standard/Content/Vehicles/2014/i3/BMWi3RangeExtender/Features_and_Specs/BMWi3RangeExtenderSpecifications.aspx.  Adding the 440 lb. battery weight makes the total energy production and storage weight at 76 mile of range 705 lbs.  This increases by 11.4 lbs. of gasoline for every 74 miles driven beyond 76.

6- Widely quoted in other sources, Car and Driver claims the Telsa Model S 85 kWh battery pack weighs 1323 lbs: http://www.caranddriver.com/reviews/2013-tesla-model-s-test-review.  This is exactly 600 kg, making it appear to be an estimate, but it is the only number we have to work with, as Tesla does not publish the spec.

7- Weight of the 60 kWh Tesla Model S battery pack is estimated from the 85 kWh figure to be 60/85 X 1323 lbs. = 934 lbs.

3 comments:

  1. Great article, I totally agree (hence I got an i3 REX). Until battery technology makes a big technical leap, or fuel cell with hydrogen become common, REX is the best way to get efficiency and range. A removable unit would be the only improvement I see there in order to save weight when no REX is needed, maybe.

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  2. Very interesting analysis, job well done :)

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  3. Very interesting, thank you.
    But you are basically making the argument for Diesel ICE, as the most efficient fuel, for anything but short distance driving!
    " On the other hand, the benefits of energy density and convenience make gasoline/diesel energy better suited for longer range transportation"

    Unfortunately, current battery density being what it is, "longer range" means anything north of ~75 miles for an i3 REx.
    i.e., if you frequently travel more than that on a single trip, an i3 REx is probably not for you.

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