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Capacitor-Based Semis and Highway

Replace diesel based semi-tractors with capacitor powered ones
  [vote for,

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.

Fris85, Sep 12 2012

(?) "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]


       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.
rcarty, Sep 12 2012

       I like it. Infrastructure installation and upkeep would be a bitch, but it's still a very viable concept. [+]
Alterother, Sep 12 2012

       // Infrastructure installation and upkeep would be a bitch   

       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.   

       The trucking companies get the benefit of all the infrastructure and cheaper operating costs. The road owner gets the benefit of increasing their revenues.
Fris85, Sep 12 2012

       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.
FlyingToaster, Sep 12 2012

       Isn't there a similar system to this, but using two continouous metal bars place a set distance apart instead of asphalt or concrete?   

       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.
8th of 7, Sep 12 2012

       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.   

       [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.   

       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.
Alterother, Sep 12 2012

       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.   

       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.   

       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.
ytk, Sep 12 2012

       Oh, and welcome to the Halfbakery.
normzone, Sep 12 2012

       Oh yes, that too. Welcome...   

       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.   

       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.
8th of 7, Sep 12 2012

       Oh yeah, that too! Welcome!   

       (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.)   

       EDIT: Hmm. We're still having problems with that echo in here, it seems.
ytk, Sep 12 2012

       The emphasis being on "scientifically" - the idea has attracted ruthless, unsparing analysis, devoid of sentiment or malice.
8th of 7, Sep 12 2012

       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.   

       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.
MechE, Sep 12 2012

       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.   

       Kind of disappointed, actually.
normzone, Sep 12 2012

       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.   

       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.
MechE, Sep 13 2012

       //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//   

       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.
ytk, Sep 13 2012

       Graphene Capacitors appear to be more in the range of 1000x electrolytic capacitors at this point.
MechE, Sep 13 2012

       how are their longevity ?
FlyingToaster, Sep 13 2012

       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.   

       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.   

       Anyway, this idea gets my bun, even though I still think it's probably infeasible.
ytk, Sep 13 2012

       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.
rcarty, Sep 13 2012

       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.   

       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.   

       Naturally, you can't regenerate your diesel fuel by slowing down.   

       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.
Carmi, Sep 13 2012

       That's quite the welcome!   

       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.   

       // Isn't there a similar system to this, but using two continouous metal bars place a set distance apart instead of asphalt or concrete? //   

       This had me me rolling on the floor!   

       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.   

       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.   

       // 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. //   

       Are you an EE ytk?   

       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.   

       // Some trucks do run 24 hours a day //   

       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!
Fris85, Sep 13 2012

       //Spending time recharging a truck is time it's not making money on the road.//   

       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.   

       //Are you an EE ytk?//   

       No, I just like typing numbers every once in a while, to spread out the wear and tear on my keyboard.
ytk, Sep 13 2012

       ///That's a lot of downtime right there. //   

       I suppose that's right. Although there is a difference between a truck not being ready to roll, and just being parked.
rcarty, Sep 13 2012


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