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Powerpack
II - The Sequel In pursuit of the perfect
electric-drive power system, preferably in a neat little package. Originally Published on www.evworld.com: September 04, 2008 As stated in my previous article,
the deployment of electric cars can be hastened by the standardization of
their energy sources into easily swappable powerpacks. This article examines a few powerpack types
that could be build today and their resulting characteristics. |
PHOTO
CAPTION: Renault Kangoo Compact Concept car is a follow-on to the popular
Kangoo, versions of which included an all-electric model and one of the first
modern 'range-extended' electric vehicles developed around a small,
integrated motor-engine generator package. |
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The powerpacks can contain
batteries, motors, fuel cells, or according to the driver’s needs at that
specific time. Some of the responses point out the challenges of
standardizing the battery form, voltage, and other parameters. These are far
from the ideally packaged perfect solutions but do highlight the benefits of
having a mixture of the various types available to the same vehicle. And as
any technology advances, the swappable powerpacks allow car owners to choose
the technology that meets their needs, including affordability and range. Each Powerpack technology has
strengths and weaknesses. To explore this consider a common footprint in feet
of 2 High x 2 Wide x 1 Deep that was proposed in the first article. With that
set of dimensions and an associated design for mounting and connecting (clamping
in), we can simply consider the resulting powerpack characteristics based
upon internal makeup from available components. Thus let’s consider:
a. 4 marine deep-cycle 12V battery (similar to a car
battery) b. 12 Wheel chair batteries (UB12350s)
How do they compare? A comparison is clearly subjective
and depends on the market force assumptions used to form the selection
criteria. Obviously a break-through could change the selection significantly
but we shouldn’t hold our breath because the same solution (powerpacks) can
also enable these break-through technologies to become available and will not
hamper them. First assume the EVs have mounting
provisions for auxiliary powerpacks that are 1x2x2 feet. With the powerpack
assumption, a shorter range becomes acceptable for most EVs because of the
option to plug in a powerpack for range extension only when it’s needed. Also
important is the option to increase acceleration power via a different type
of powerpack when desired. For many consumers, cost is an
important interest. The nearly $4/gallon gas price is much more expensive
than $0.20/KWH electricity. Investment cost also matters since people won’t
buy what they can’t afford. Technology is assumed to be progressing linearly.
For output voltage
standardization, 150VDC nominal with limits of 120V to 180V accommodates the
170V sinusoidal peak of AC single phase power in the US and Europe. This 20V
difference from the nominal voltage will be excellent for most chargers to
avoid having to do any more than rectify the house power. The D-Cells (Lithium-Ion, and
NiMH) and Gas Generator easily provide this voltage via serial/parallel bank
wiring. The LA powerpacks can meet this via composition from the smaller
wheel-chair sized batteries wired in serial (car batteries would only get to
48V in the assumed powerpack size). Table 1 represents a decision
criteria sequenced according to the assumptions in an engineering trade study
and displayed in a scorecard format. Table 1. Powerpack characteristics
by technology
So which one wins? Lead-Acid (for
now) • Power/Weight ratio • Cost • Weight • Charge/discharge time The rapidness of charge/discharge
possible is especially noteworthy because it compares favorably to
present-technology super-capacitors that weren’t included here. Reference:
http://peswiki.com/index.php/PowerPedia:Capacitors. However, this number is
estimated on CCA ratings and does not mean the LA batteries can be fully
charged or discharged in that amount of time. The low investment cost is
important for keeping the total EV cost reasonable for mass production (the
everybody market). And at $400 per powerpack the business model is workable
for swapping powerpacks. And note also that the electric motors can produce
allot more torque off the line than traditional gas motors with engaging
clutches. The primary drawback of LA
batteries is the low scores on Range based on energy storage. That’s where
the other choices come in very handy to some EV owners but not all of them.
Without the powerpack assumption, the range via the Energy/Weight
(WattHours/lbs) ratio would be much more important and would lead to different
results.
Two examples were used to
represent LA. The wheel chair batteries were slightly denser and heavier
providing more total power and KWH capacity. However, the marine batteries
had a better power to weight ratio. Which one looses? NiMH. NiMH is too heavy and normalized
scores like Power/weight do poorly. In household AAA, C, and D cells, NiMH is
a good selection because of the reasonably high per-cell capacity (where it
would be compared with alkaline cells) – but only in owner-selected
applications where the recharging advantage would be worth the cost. And
the others? Niches. Gas Generator Niches The gas generator is especially
good for longer trips. At over 100lbs it’s clearly a benefit to be able to
disconnect it from the vehicle and leave it at home (or locked like a bicycle
somewhere). Without mechanical coupling to the EV wheels, the gas motor is
working at an optimal RPM for it’s maximum efficiency. 2-stroke generators
are also becoming popular in the portable market. These have very light
weight and with constant RPM and slowly variable load, 2 stroke motors are
much easier to tune for clean burning and efficiency than otherwise, which is
why they’re not currently used in autos but are so popular in tools like
chain-saws and weed-whackers. In theory these efficiencies would cause the
gas mileage will be substantially improved from the traditional auto and even
from the early “parallel” type hybrids. However, without the direct
mechanical connection there is substantial loss in the energy storage and
delivery process. Therefore, the niche for these devices in a powerpack will
primarily be to extend range and even enable very long trips with gas-only
refills. These generator powerpacks will also be excellent to have for home
backup generators, especially once the home charging stations become
similarly modular with several plug-in stations (for charging, and home
backup). Lithium-Ion Niches Lithium-Ion is an expensive investment
but can extend range and power substantially (keeping the Lead-Acid charged).
The ones I looked at don’t discharge/charge very fast and therefore aren’t a
good option for excessively long journeys unless they can be swapped
on-route. They also don’t provide as much instantaneous power as LA
powerpacks. Thus their niche seems to be the medium range commuters and
day-trippers who have allot of money to spend on luxury items. It is
noteworthy that there are allot of investments going on with this and other
battery technologies. Hopefully these efforts will continue to drive the
price of this option down and make it a competitive selection outside of the
current niche. Alkaline Niches These have been common for a long
time and have a fairly decent capacity for their cost when using just a few
in a common item such as a flash-light. NiMH have been challenging their
niche-role for some time but haven’t taken over due to their higher price. Others Niches Other technologies not explored
here include capacitors, and fuel cells. Again specialized powerpacks will
have different characteristics including costs. Also not directly considered
are any fundamental break-through with any of the technologies. In general,
technology advancements tend to follow a Moore’s law type of pattern with an
occasional break-through. Thus we could estimate the year when a given
technology should become cost effective. Many car companies are doing just
that and we’re hearing which technology they’re banking on. One has selected Lead-Acid
and will get a jump into the market first. Another has selected Lithium and
is banking on power-delivery improvements, fire-safety, and cost improvements
to make their cars deliverable to customers. The Take-Away? Well first is a realization that
Lead-Acid is a Top-Gun good solution to power an EV. So much so that an EV
manufacturer may choose to build in a primary set of LA batteries that aren’t
powerpack swappable. LA batteries are already in wide-spread use for
wheel-chairs, scooters, and golf carts. In car applications range becomes a
much bigger issue and thus there is a serious need to supplement the charge
LA batteries can hold. And that’s one of the foundational premises of this
article series – which leads directly to the conclusion that powerpack
standardization can hasten the delivery of EVs to the mainstream market.
Similarly any EVs may be designed with a primary power source technology. But
regardless of the selection, until 5 minute recharges and low cost packages
are a reality the need for powerpack options remains. The secondary take-way is the
generator does belong in a Powerpack or other easily attach/detach
configuration. It is heavy for little surge power but useful for a continuous
recharge, especially when continuously running at maximum efficiency. Thus
it’s primarily a range extender (assuming no mechanical direct drive) and
quick-mounting puts it into proper usage. A third take-away is that the
batteries are so heavy that we should be figuring out how to adjust the
number of batteries we have on board according to how far we would need to go
in a normal day. For example, to/from work 20 miles plus 10 for the grocery
mart and another 10 for reserve. Thus 40 miles worth of batteries. A short
commuter could use 10 miles worth of batteries and attach the generator to go
further only when necessary. The most impressive aspect of this
study is the wide variation in the technology characteristics and how they
match up differently to various market segments. Short range low cost commuting
is readily served by Lead-Acid technology. Expensive EVs can take advantage
of the energy storage of Lithium-Ion for good range on a single charge.
Generators can provide low investment long range. Powerpacks also have
secondary benefit creating a home energy centers for distributed power
generation and storage across the grid. Powerpack standardization can give
the technology choice to the EV drivers in both business models of investment
and rental/swap-out. And best of all, powerpack standardization enables EVs
to use technology that’s available today without any waiting for a promised
technology solution that is still very expensive. And when great battery
solutions do come, the powerpack standardization will again enable EV owners
to adopt it without changing the whole car. |
Contact: info@EDANS.org, EDANS
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