h a l f b a k e r yI CAN HAZ CROISSANTZ?
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This idea would apply to semi-truck transport on the
Interstate highways. Obviously, battery electric vehicle
technology has a looonnnng way to go before the range,
weight, and recharge rates will be acceptable to start
replacing diesel semi-tractors. But why should the semis
have to carry
all their power with them?
Instead, imagine semi-tractors with a large bank of
super
capacitors. These would hold enough power to move the
truck for say 15 miles. The key would be to recharge the
trucks often enough to keep them on the move.
Something
similar to the bus in the video linked below. Obviously
though, a truck stopping for 30
seconds every 15 miles would not be very efficient
though.
So to solve that, include special charging strips 1/2 mile
long along designated "cap-routes" every 10 miles. The
charging would be overhead like a catenary wire. When
a
truck passes underneath, it would raise a pickup to touch
the wires. 1/2 mile would give 30 seconds of charging
at
60mph. Improved technology or lower speeds might
enable the charging sections to be shortened to 1/4 mile
(about 15 seconds). By placing the intervals at about 2/3
the max range capacity of the trucks, you would avoid a
less than perfect truck running out of juice.
The wires would be broken up into segments that could
power on and off individually. When the pickup first
made
contact with the wire, a low voltage signal could quickly
verify that it's a valid pickup touching the wires before
turning on the high voltage. This would be very similar
to
what Power over Ethernet does to avoid sending
damaging
voltages to non-PoE devices.
Power could be supplied by high voltage lines running
along the highway ROW. This would be used to provide
some redundancy to the national grid system.
Additional charge segments would be needed on feeder
roads between major distribution centers and the
highway.
Obviously this would not work for trucks that need to
leave
the highway and travel some distance to make local
deliveries, but the trailer could be switched to a
conventional diesel at an interchange pickup/dropoff
area.
Charge segments would also be needed in highway rest
areas or truck stops. A conventionally powered recovery
vehicle could carry a spare charge to trucks that don't
make it to the next charging segment but this should be
limited to trucks whose capacitors are malfunctioning.
As the technology continues to improve, the trucks would
weigh less, charging segments could be shortened
(reducing maintenance costs), and some charging
segments
could be eliminated altogether.
As far as private passenger vehicles go, these must often
go far beyond the highways. Their drivers are not
professionals who could be consistently counted on to use
the charging system in a reliable way. Also, they are
much
much shorter and would have to have either a
ridiculously
long pickup to reach the overhead wires, or would
require
a special, shorter lane.
(?) "Capabus"
http://www.youtube....atch? v=t3rg-SsPJuU
Video of existing capacitor powered bus [Fris85, Sep 12 2012]
ultra capacitor bus
http://m.youtube.co...oogle&v=LYL6NyU1g3k
5 min capacitor charge operates bus for 3 miles [rcarty, Sep 12 2012]
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I'm sort of a diesel technician even though I don't work in the trade. One thing I do know about the trade is that it is incredibly dependent on technicians, and electronics is not the strong point of most of the guys. It will be a big problem in terms of training to get everyone working well on electrical systems. Diesel engines have come a long way in satisfying environmental demands with DPF/ DOC emissions filters, very recently and costly. There is incentive to go electric with the cost of fuel and urea. But mechanical machines like the internal combustion engine can be repaired by not so bright guys. |
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I like it. Infrastructure installation and upkeep would be a
bitch, but it's still a very viable concept. [+] |
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// Infrastructure installation and upkeep would be
a bitch |
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True, but as a side benefit, the road owner
(federal, state, etc.) gets to bill the heaviest
users of the road very accurately for use. Think of
an electronic toll pass type device incorporated
into each charging segment. |
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The trucking companies get the benefit of all the
infrastructure and cheaper operating costs. The
road owner gets the benefit of increasing their
revenues. |
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The 10(?)% or so "brush loss" might make the overall system less efficient than the original. One or two strip outages in a row and you've effectively closed the highway until they're fixed. |
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Isn't there a similar system to this, but using
two continouous metal bars place a set
distance apart instead of asphalt or concrete? |
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If you did that, you could have a tensioned
cable overhead and you wouldn't need the
capacitors. You could hook lots and lots of
trailers togeter, too. |
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I too was planning to make a sarcastic comment about
electric trains, but then I actually finished reading the idea
and found it well-conceived and impressively detailed. |
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[Fris], the inspiration continues to flow with your last
anno. I question your statement of lower operating costs,
however; I think a great deal of research and modelling
would be required to prove that. |
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It also occurs to me that this sort of thing is far outside the
budgetary means of municipalities and even some states
(Maine, for example), effectively restricting this system to
use on interstate highways. This would limit the electric
trucks' delivery routes to distribution points within 30
miles of an interstate exit. Part of the infrastructure
would also have to entail a 24-hour emergency recharge
service for truckers who get detoured or lost and run out
of juice on some rural secondary route. |
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Taking the Nissan Leaf as our baseline for energy
usage,
we can extrapolate roughly what kind of capacitors
we'd need
to make this work. The Leaf has an operating weight
of about
3,500 lbs, and gets about 100 miles on a fully charged
345 volt
battery pack, which provides about 24kWh. So,
24kWh yields
roughly 350,000 lb. miles, or ~14583 lb. miles / kWh.
A
loaded tractor trailer rig has a gross weight of about
80,000
lbs., and needs to travel 10 miles, so 8,000,000 lb.
miles
total. Dividing by 14,583 gets us roughly 54kWh
consumed
over this distance, or 156.5 amp hours at 345 volts. |
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Capacitors are measured in farads, and the formula is
1 farad
= 1 amp second/volt. So for this application, we
need 1.633
kilofarad capacitors capable of handling 345 volts.
According
to a datasheet I found online from a company that
produces
capacitors for such applications, a 125V capacitor
rated at
63F weighs 60.5kg. Since those capacitors only
support 125V,
you'd need three in series to handle 345V, which
drops your
capacitance down to 21F for the package of three.
So you'd
need (1633 / 21) * 3 = ~233 capacitors total,
weighing 14,097
kg, or ~31,000 lbs. Yikes. For reference, a full tank
of diesel
fuel (200 gallons) weighs 1,430 lbs. All of this is in
addition to
the weight of the truck itself, including motor,
chassis, and
cargo. |
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Also, you'd probably need substantially more than
that, since,
unlike with batteries, capacitor voltage drops pretty
quickly as
it's discharged. Most likely, nearly the entire cargo
space of
the truck would need to be filled with capacitors for
this to
work, which sort of defeats the entire purpose. It's
not a bad
idea in theory, but barring any significant advances
in
capacitor technology I don't see it as being at all
practical. |
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Oh, and welcome to the Halfbakery. |
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Oh yes, that too. Welcome... |
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If you first idea is good enough that the
residents are prepared to provide a
mathematical analysis of its weakeness,
you've achieved something that few succeed
in. |
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The better the quality of the idea, the more
rigorous the challenges it will face, and the
more detailed and searching the critique;
here, that's actually a complement. |
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Oh yeah, that too! Welcome! |
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(Although, I can't think of a more appropriate welcome
here than having your idea scientifically torn apart by
some smug jerk who thinks he knows everything.) |
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EDIT: Hmm. We're still having problems with that echo
in
here, it seems. |
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The emphasis being on "scientifically" - the
idea has attracted ruthless, unsparing
analysis, devoid of sentiment or malice. |
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A Graphene super capacitor has about 1% the
energy density of diesel fuel, so (admittedly with
cutting edge technology) we can do much better
than [ytk]s numbers. |
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In addition an electric motor is significantly more
efficient than a diesel engine, and thus should
require less total energy for a given drive
distance,. Given that we are looking at driving a
relatively short distance between charges, I don't
think we are looking at numbers nearly as bad as
[ytk] thinks. |
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I personally was expecting something in the order of explosive discharge of the capacitor all at once, propelling the vehicle violently forward, then it sitting until it could be recharged again. |
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Kind of disappointed, actually. |
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A computer controlled semi convoy would have the
advantage of being able to disperse and re-combine
dynamically, and without a freight yard. That has
major advantages. |
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As best as I can tell, rail is still moving more than
trucking (40% of total ton miles) followed by truck,
then by water. But since rail can't handle last mile
pick up and delivery, everything not actually on a
rail spur ends up on a truck at some point. |
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//In addition an electric motor is significantly more efficient than a
diesel engine, and thus should require less total energy for a given
drive distance// |
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Perhaps, but the efficiency of an electric motor goes down rapidly as
voltage decreases (and below a certain voltage the motor simply won't
work at all), and as I mentioned earlier capacitors tend to experience
a rapid voltage drop as they discharge. A diesel engine, on the other
hand, can wrest the same amount of energy out of the last drop of
fuel as the first. So the above calculations are, in fact, way too low,
because you need to take into account this voltage drop. If you bump
the voltage up to 500 volts by adding an extra capacitor to each stack,
and assume that the motor will operate relatively efficiently at above
about 300 volts, you're looking at between 4 and 5.5 kilofarads. So,
even assuming the low number here, the actual number of capacitors
needed would be 4000/(63/4)*4, or 1016 capacitors, weighing in at a
whopping 134,400 lbs. Graphene capacitors are said to be about 20
times as powerful as the older style, so you might be able to get this
down as low as 6720 lbs. Perhaps not unfeasible, but consider that
enough diesel fuel to travel 10 miles weighs about 15 lbs. So, your
actual energy density is closer to 0.2% that of diesel fuel. |
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Graphene Capacitors appear to be more in the range
of 1000x electrolytic capacitors at this point. |
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how are their longevity ? |
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The best supercapacitors I can find commercially available have a specific energy of
around 6 watt hours/kg, meaning at you'd need about 9000 kilograms worth of them to
yield 54kWh. I've heard that you can buy them as high as 10Wh/kg, but even that
means a 5400kg payload. In the lab they're doing quite a bit better, with densities
reaching 85Wh/kg. That would yield a fairly manageable payload of only 635kg. But
remember that this is if you could get every bit of energy out of the capacitors, which
you can't. Going off my earlier estimate of 4-5.5kF minimum to travel ten miles
efficiently, you'd have to multiply that by at least 3. So, even under ideal
circumstances, you're looking at a minimum of 1900kg, or about 4200 lbs. Still pretty
hefty, even assuming the use of technology that, at the moment, only exists in the lab. |
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However, it occurs to me that this doesn't need to be an either/or proposition. Trucks
don't drive 24 hours a day, so presumably they could utilize both a capacitor and a
battery. The ideal proportion would be a battery that is small enough that it can be
charged to nearly full in about 12 hours, and a large enough capacitor to enable that
battery to get through the day. The capacitor would store small “top-up” charges that
it receives throughout the day. Immediately after the truck goes through a charger,
the motor would draw from the capacitors only until the voltage is below the threshold
to power the motor efficiently. It would then start drawing from the battery in
increasing amounts to keep the total voltage above the threshold. By the time the
capacitors drain completely it would be running solely on battery power. This would
also allow a significantly extended “no-charge” range, meaning you'd have far fewer
trucks stranded on the highway because they missed a charge for one reason or
another. |
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Anyway, this idea gets my bun, even though I still think it's probably infeasible. |
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Some trucks do run 24 hours a day. I've seen the holes cut
in floors and mounds of shit on the transmissions to
prove it. And ideally any truck in a fleet should be able to
run 24 hours a day. Down time costs money. Spending time
recharging a truck is time it's not making money on the
road. The entire industry is sensitive to economic issues
like that. It takes only a few minutes to fuel a truck for
hundreds of miles, and with three truckers in the cab each
taking 8 hr turns driving and taking dumps on the
transmission the current technology is hard to beat. |
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We should take into account the advantages of regenerative braking. The main problem with regenerative brakes in the Prius for example is that the battery can't handle the charge and a lot of the energy goes to waste. |
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Capacitors would avoid this issue, obviously. You can probably make about 20% of your energy expenditure back by charging the capacitors as you slow down or go down hill. Also, the closer you are to that last mile, the more you make off your regenerative brakes. |
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Naturally, you can't regenerate your diesel fuel by slowing down. |
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There are also advantages to electric motors that mean the gearbox can be simpler (and lighter), or indeed altogether absent. In-wheel motors even allow you to do away with the differential. They do leave a problem of unsprung weight though. |
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That's quite the welcome! |
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And I'm honestly very flattered at the detailed
critiques. At least it tells me that people are
taking the idea seriously enough to merit in depth
analysis. |
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// Isn't there a similar system to this, but using
two continouous metal bars place a set distance
apart instead of asphalt or concrete? // |
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This had me me rolling on the floor! |
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Don't get me wrong though, I'm a big proponent of
rail transport over all else. The reason I went to
this idea is that improving transport by rail in this
country is hindered by enormous economic and
regulatory hurdles. It takes a certain amount of
coordination and cooperation that I think is
unlikely from the current setup. For example, we
have a manufacturer in town that produces cast
iron bar stock. They have a rail spur off a well
used line that would be perfect for receiving bulk
scrap iron and delivering finished product.
However, they haven't used the spur in years
because of the costs of maintaining the switch
point and spur certification through the FRA and
limited delivery schedule offered by the line
operator. |
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Key point being, without major changes in the
way the rail industry is organized/regulated, I
think trucks will probably continue to be a major
player in transport. |
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// Going off my earlier estimate of 4-5.5kF
minimum to travel ten miles efficiently, you'd
have to multiply that by at least 3. So, even under
ideal circumstances, you're looking at a minimum
of 1900kg, or about 4200 lbs. Still pretty hefty,
even assuming the use of technology that, at the
moment, only exists in the lab. // |
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Anyways, let's be a little optimistic and say that in
the time required to gain support for this idea,
pass the legislation necessary to fund it, plan the
first routes and construct the system (maybe 5-10
years?) that we can bring that best case 85Wh/kg
out of the lab and into a production system. The
weight you mention (4200lbs) would be pretty well
offset by some of the items you could remove:
engine (3200lbs), transmission (1000lbs), fuel &
tank (1500lbs), exhaust system (???lbs). Obviously
you would have to add back in weight for a motor
(maybe 2000-2500lbs). Net change is 500-1000lbs
heavier for the capacitor version before taking the
exhaust system into account. |
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// Some trucks do run 24 hours a day // |
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This one certainly could. And if, like you
mention, there were three drivers, you could go
truly non-stop without even a fuel break. Plus
there wouldn't be a transmission for the shit to
get caught on on it's way through the hole in the
floor! Everyone wins! |
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//Spending time recharging a truck is time it's not
making money on the road.// |
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Truck drivers aren't allowed to drive more than 11
hours in a day (in the U.S.), and most long-haul
truckers drive alone. That's a lot of downtime right
there. |
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No, I just like typing numbers every once in a while,
to spread out the wear and tear on my keyboard. |
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///That's a lot of downtime right there. // |
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I suppose that's right. Although there is a difference between a truck not being ready to roll, and just being parked. |
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