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this post was submitted on 17 Jul 2023
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You are aware that electric trains also use electric motors, just like electric cars do, right? And you are aware that electric cars rely on an electric battery while electric trains rely primarily on overhead electric power lines, are you?
That means cars require one extra component and an extra conversation of energy which trains don't need. Every conversation of energy reduces efficiency of the final outcome. The more conversations, the less efficiency.
Trains use: power lines -> electric motor
Cars use: power lines -> electric battery -> electric motor
Furthermore, bigger machines can be built to be more efficient than smaller ones. So bigger motors can use (electric) fuel more efficiently than smaller motors.
Well, tbf, both trains and cars require converters (i.e. inverters like variable frequency drives or VFDs; or rectifiers) to match power between the local electric supply and traction motors, in the case of trains, or between the battery and traction motors, in the case of cars.
You need to be able to ramp up or down voltage or current (or both) depending on the drawing load that the motor sees at each and every moment of a trip (cars and trains). Then there is the possibility of your train jumping between different electric systems along its route, and so you need to have a way to accommodate those difference if you want to serve the most amount of passengers.
There are Battery Electric Multiple Units (BEMUs), too, out in the wild today that incorporate batteries in addition to electric service on trains (or just batteries alone), mostly in Japan and some in Europe. These are in the minority though compared to electric-supplied units.
Interestingly, there are some projects, most notably in Germany, where overhead lines are being introduced to trucks, fuzzying the differences in transportation modes even more.
I still get your point about the conversions, though. Batteries don't have 100% Coulombic/Faraday efficiencies, meaning that they don't charge up from 0-100% every charge cycle: you might start at 0-100%, but the next charge cycle might be 0-99.9999%, then 0-99.99%, then 0-99%, etc. This efficiency loss isn't as great as the other losses you might find in the converters previously mentioned, or other resistive losses such as via Eddy currents in the motors/axles, demagnetization of the motors, etc.
A better description of these processes would be:
Non-BEMU Trains: power lines -> converter -> electric motor (acceleration)
Non-BEMU Trains: electric motor -> converter -> power lines (deceleration)
Cars/BEMUs: power lines -> converter -> [battery -> converter -> electric motor] (charging [acceleration])
Cars/BEMUs: [electric motor -> converter -> battery] -> converter -> power lines ([deceleration] discharging)
Totally. And trains that add batteries onboard can reduce the advantage that non-battery EMUs have, moreso resembling locomotives with big diesel engines and fuel tanks. I still find BEMUs better though because you can run the trains as married units, just like EMUs (and I suppose DMUs), but batteries can also be distributed along the rolling stock to allow for greater weight balancing. Idk if the major manufacturers like Siemens or General Electric have plans to design systems this way, but greater adoption may lead to more varied designs.
Hope this helps the discussion!
Totally! My idea was that -> arrows represent the converters and to put it simply more arrows = more efficiency loss. But right, since power can also be injected back into the network, which is a good thing, there could be <-> arrows, or maybe <=> to better hightlight the bi-directionality:
power lines <=> electric motor
Since you mentioned putting power back into the grid:
I heard another potential use for car batteries would be using them to balance out local power fluctuations in the grid to make it more stable. Since cars stand still most of their life anyway, they might as well be connected to the grid whenever they're parked. Not as a big energy reserve, since that wouldn't be very efficient and capacity would be too low, but just to keep things more balanced which is a healthy thing for the power network. I suppose that also applies for train batteries.
Nissan has already started rolling out Vehicle-to-Grid (V2G) or Vehicle-to-X (V2X) chargers for its offering of vehicles since 2022, so it's already happening.
Chris Nelder, who runs the Energy Transition Show podcast and who is a member of the Rocky Mountain Institute, published a paper even as far back as 2016 arguing how the potential for the US consumer rolling stock of BEVs (Battery Electric Vehicles; grid + batteries only) and PHEVs (Plug-in Hybrid Electric Vehicles; grid + batteries + gas) offering Demand Response services to utilities is enormous.
I'm not sure about the V2G compatibility of BEMUs feeding energy back to the grid to serve Demand Response is where the industry is going currently, instead favoring the implementation of overhead line islands as compared to extensive grid rollouts, but that reality is 100% feasible. The island approach I believe is also what Siemens is aiming for with the overhead-fed trucking solution I shared earlier.
Still exciting nonetheless!!!
Not to say trains are not more efficient than cars, because they are for a myriad of reasons. But electric motors scale relatively linear to my extent of knowledge, so usually it just ends up being that trains use many motors instead of one big motor.
Thanks for the info, I didn't know how exactly this works, but I was aware that this factor is different for each device.
Thinking about it I guess that explains why small electric motor powered devices exist often while small combustion powered devices are rare? The only items I can come up with are forestry/gardening devices, tools for cooking and I guess lanterns. With the latter only using the heat/light and not actually moving anything.
I was responding to your assertion about EVs not being much better than ICEs.
I did not make any prior assertions. The post above was my first comment here.
To clarify, when you say ICE you are talking about trains, right? As in intercity express. And when you say EV you mean an electric car, correct?
I don't understand why you argue that cars are more efficient than trains in this aspect. My argument is that since both machines use electric motors the motor efficiency can be nearly equal. Other factors probably favor a train more than a car if anything.
I don't make a claim, but assume that even diesel locomotives might be a better choice than cars using only renewable energy, since the latter are idle most of the time, take up a lot of space, and require a lot of resources both in the car itself and in infrastructure. Surely something worht looking into.