With the bulk of the energy issues centered on transport fuels sourced from oil, excepting the pollution and CO2 matter that coal offers, the pressure has mounted on the major oil use – worldwide personal transport vehicles, the dream realized by many and desired by many many more. When someone critically views the impacts that certain technologies might bring to the market consumers, financiers, manufacturers and policy types better be paying attention.  The guy I’ve been watching with both respect and a touch of amusement is Andrew F. Burke at the University of California Davis.

Burke’s latest paper is a study on ultracapacitors as energy storage devices for vehicles. In the paper he evaluates the current state of the art in ultra capacitors and just how suitable they might be to use in the next generation of power sets and drivelines.  Burke doesn’t waste much effort with obtuse perspectives but looks squarely at the applicability in usable energy and the power requirements.  He sort of “prewrings” the topic out so one can see quickly what might be fluff and hype and what one better give some serious attention.

The paper of interest is “Ultracapacitor Technologies and Applications in Hybrid and Electric Vehicles.” For hybrids and electric vehicles useful energy stores and the maximum power pulse matters most as that defines the maximum engineered output and what is needed to service it.  That’s the answer to the steep onramp matter that frightens most drivers – can I get on here with this thing –question.

What really seized my attention is that Burke is using carbon based capacitors in part of the research.  Until EEStor gets to market and likely for a long time to come the carbon guys with Reticle for a new example have great futures.  Burke explains with comparisons using ultracapacitors and advanced (mostly lithium) batteries with applications chosen where the ultra capacitors can be used in place of, or with batteries to cut the fuel use.  He even goes far enough to compare costs, naming the manufacturers he is informed about.

Burke's Simulation Results. Click image for the largest view.

Burke's Simulation Results. Click image for the largest view.

Now before we go all off with acclaim Burke is using simulations, with each comparison framed from the same internal combustion midsize vehicle, the Ford Focus 2 liter 4 cylinder engine.  Thus with a real world map the simulations get more real of course, but are simulation nevertheless. In the paper the Burke allows for a “micro-hybrid” with only 30 Wh 6kW peak power ultracapacitor.  More simulations include higher power charging motors, with up to 12 kW (that’s under 20 hp) and larger storage up to 50 Wh.  One fast lesson is charging motors can be too small, a 3 kW motor reduced fuel economy by more than 50%.

Another lesson is that straight carbon ultracapacitors are more economical that hybrid type carbon ultracapacitors. “hybrid carbon devices had higher energy density, but even though their power density for 95% efficiency was relatively high (1050 W/kg), it was not proportionally higher—that is twice as high—as the carbon/carbon devices with lower energy density. These results show clearly that it is essential to develop high energy density ultracapacitors with proportionally higher power density; otherwise their use in vehicle applications will be compromised.” EEStor beware.

On the bright side is the observation that the minimum energy in watt hours required to operate a series type vehicle in real world driving considering the energy density of ultra capacitors “are such that the power and cycle life requirement will be met in most cases if the unit is large enough to meet the energy storage requirement.”  Now that’s reassuring.  It seems from these simulations that low power charge motor requirements in the 20 hp range with adequate storage would provide quite nice performance.  Very little more would mean major improvements in performance such as quicker acceleration and more power accessories.  Series hybrids with ultracapacitors look more welcoming and carbon ultracapacitors could get some major market share if pricing can be driven down far enough quickly enough.

In the plugin EV simulation Burke learns using a high energy density battery (200 Wh/kg) and ultracapacitors would provide two-thirds of the power to a 70 kW electric motor.  Burke’s plug in EV would be the type needing a charge sustaining motor too, but adds more storage.  In this simulation, “The combined weight of the cells in the battery and hybrid carbon ultracapacitors would be 78 kg for a plug-in hybrid with an all-electric range of about 40 miles. The combined weight using the carbon/carbon ultracapacitors would be 100 kg. Using a high power lithium-ion battery with an energy density of 100 Wh/kg without ultracapacitors, the weight of the battery cells alone would be 120 kg. Hence for plug-in hybrids combining a battery with ultracapacitors is an attractive design option.”

In a simulation that confuses this author, Burke considers plug-in series hybrids (extended range electric vehicles) where the batteries are unable to provide the peak power to the electric motor. This would be most likely, Burke wrote, if the plug-in range was relatively short and/or high energy density batteries of modest power density were being used in the vehicle. In those cases, the ultracapacitors would greatly reduce the power demands on the battery and lead to less stress on the battery and longer cycle life.  That must be the urban transport kind of vehicle that seems of dubious value in the U.S.  But Burke learns “The simulation results indicate that the fuel economy of the series hybrid is slightly higher than that of the parallel charge sustaining hybrid when both are operated in the charge sustaining mode. The engine/generator was sized such that the series hybrids were full-function vehicles. Thus all the hybrid vehicles including the series hybrids in the all-electric mode have performance equivalent to the conventional ICE vehicle.”

Burke even goes so far as to include fuel cell power.  Now fuel cells are still out there in the future but offer substantial efficiency advantages over an internal combustion engine as the charging source.  Fuel cells offer a power handling opportunity to simplify their incorporation into power sets with ultracapacitors.  As capacitors drain down the voltage drops as well.  A fuel cell could be wired in parallel as done by Honda.  The fuel cell charges as the cap drains so self-controlling the system to a large extent.  But fuel cells are still – out there for a while.

Burke doesn’t come out and say explicitly, but the research study suggests that should carbon based capacitors drive to higher volumes and lower prices high power drives to the pavement supported by high efficiency charging motors can be market viable very soon.  Maybe that 200 wheel horse power on demand making an average well over 50 mpg isn’t such a far out dream after all.


9 Comments so far

  1. Twitter Trackbacks for Simulating Carbon Ultracapacitor Performance in Cars | New Energy and Fuel [newenergyandfuel.com] on Topsy.com on August 21, 2009 7:08 PM

    […] Simulating Carbon Ultracapacitor Performance in Cars | New Energy and Fuel newenergyandfuel.com/http:/newenergyandfuel/com/2009/08/21/simulating-carbon-ultracapacitor-performance-in-cars – view page – cached Andrew Burke at UC Davis stimulates ultracapacitor performance in midsized cars learning that the options are already better than realized today. — From the page […]

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