3 ms·

My understanding is that bulk power generation is probably more efficient than having the motive power in the locomotive itself. There are also isolated use ca

My understanding is that bulk power generation is probably more efficient than having the motive power in the locomotive itself. There are also isolated use cases that the US railroads have been electrified for in the past (a lot of it was de-electrified to artificially boost revenues with asset sales in the midcentury). Namely * mountain tunnels; a fair amount of Western mountain tunnels can only accommodate one diesel train at a time due to ventilation, and the crews carry respirators and oxygen in case the train stalls inside * any sort of long sustained slope; downhill trains using regenerative braking can return electricity for those going uphill

mlyle 5y ago
> My understanding is that bulk power generation is probably more efficient than having the motive power in the locomotive itself. Almost certainly. But this probably evaporates once you need to pull significant amount of battery.
- ErikCorry 5y ago
  Trains are very efficient at carrying weight, especially if they can dump power into the grid (or battery) when braking. That metal-on-metal wheel has super low rolling resistance.
  - mlyle 5y ago
    > Trains are very efficient at carrying weight Sure. But you also need a whole lot of them to provide sustained power to move a large train for a decent trip length. > especially if they can dump power into the grid (or battery) when braking Smaller benefit here to regen. Sure, a 10000 tonne train has a lot of kinetic energy-- maybe 3 gigajoules or something. But compare to > 50MW to keep that train moving at a decent clip. So a complete stop with 100% of the energy recaptured is only about 60 seconds of rolling resistance energy use.
    - ErikCorry 5y ago
      I think the cost of batteries is going to be a bigger issue than the weight. Which is why hybrid (battery combined with overhead power lines) looks so attractive to me.
      - mlyle 5y ago
        > Which is why hybrid (battery combined with overhead power lines) looks so attractive to me. It may be good for some segments, but consider the number of charge cycles and charge/discharge rate, along with the peak charging currents on the overhead lines. For, say, 10000 tonne trains, 15MW sustained is a good very optimistic target for how much power you need to put out to keep rolling (before you make this any worse with adding battery mass and before you consider e.g. slopes, braking, etc). If your goal is a discharge rate of C/1, you're carrying 75 tonnes of batteries-so it masses about 1% more. Then, what duty cycle are you charging at? 100% coverage from the overhead power lines means you need to bring 15MW into the train. If the goal is a big saving in the amount of overhead power lines needed, 10% coverage from the overhead power lines means you'll instead need to bring 150MW into the train during those spans (and, this is a really aggressive 10C charge rate...) I think C/1 is too aggressive for good cycle service, 10C is waaaay too aggressive for charging-- and these are best case scenario (flat ground, etc). If you multiply the amount of batteries by a big amount, now the mass change to the train and the rolling resistance contribution to the train from batteries becomes significant). edit: in the end, I think you're usually better off just electrifying the portions of the train route where it's easiest, and burning diesel the rest of the way, instead of trying to somehow dotted-line-overhead-electrify things and limp through with batteries.
        ErikCorry 5y ago
        Agree that 10% coverage is too low, but for 50% coverage charging speed falls to 1C, which is totally doable (and as you say, for diesel-electric hybrid it could start making sense at even lower percentages). The Pareto principle means that even if battery capacity means you need 80% or 95% coverage you could still see large savings over a 100% electrification. Remember that you can also coast. If it's just about a low bridge or short tunnel you might lower the pantograph a few 100 metres on either side of the bridge. No need to wear the battery at all for such a short stretch - even with the brakes fully applied the train takes about a mile to stop. Trains don't currently do this because they don't have sophisticated position-based pantograph control and if they had to do an emergency stop under the bridge they would be stuck, but with more advanced control computers and battery backup both those problems can be solved. Where I live they are electrifying 150 miles of track. This means 85 bridges need to be raised, demolished or rebuilt, and it's a big part of the cost. And Denmark is pretty flat - there are no tunnels as far as I remember.

mlyle 5y ago

> My understanding is that bulk power generation is probably more efficient than having the motive power in the locomotive itself. Almost certainly. But this probably evaporates once you need to pull significant amount of battery.

ErikCorry 5y ago
Trains are very efficient at carrying weight, especially if they can dump power into the grid (or battery) when braking. That metal-on-metal wheel has super low rolling resistance.
- mlyle 5y ago
  > Trains are very efficient at carrying weight Sure. But you also need a whole lot of them to provide sustained power to move a large train for a decent trip length. > especially if they can dump power into the grid (or battery) when braking Smaller benefit here to regen. Sure, a 10000 tonne train has a lot of kinetic energy-- maybe 3 gigajoules or something. But compare to > 50MW to keep that train moving at a decent clip. So a complete stop with 100% of the energy recaptured is only about 60 seconds of rolling resistance energy use.
  - ErikCorry 5y ago
    I think the cost of batteries is going to be a bigger issue than the weight. Which is why hybrid (battery combined with overhead power lines) looks so attractive to me.
    - mlyle 5y ago
      > Which is why hybrid (battery combined with overhead power lines) looks so attractive to me. It may be good for some segments, but consider the number of charge cycles and charge/discharge rate, along with the peak charging currents on the overhead lines. For, say, 10000 tonne trains, 15MW sustained is a good very optimistic target for how much power you need to put out to keep rolling (before you make this any worse with adding battery mass and before you consider e.g. slopes, braking, etc). If your goal is a discharge rate of C/1, you're carrying 75 tonnes of batteries-so it masses about 1% more. Then, what duty cycle are you charging at? 100% coverage from the overhead power lines means you need to bring 15MW into the train. If the goal is a big saving in the amount of overhead power lines needed, 10% coverage from the overhead power lines means you'll instead need to bring 150MW into the train during those spans (and, this is a really aggressive 10C charge rate...) I think C/1 is too aggressive for good cycle service, 10C is waaaay too aggressive for charging-- and these are best case scenario (flat ground, etc). If you multiply the amount of batteries by a big amount, now the mass change to the train and the rolling resistance contribution to the train from batteries becomes significant). edit: in the end, I think you're usually better off just electrifying the portions of the train route where it's easiest, and burning diesel the rest of the way, instead of trying to somehow dotted-line-overhead-electrify things and limp through with batteries.
      - ErikCorry 5y ago
        Agree that 10% coverage is too low, but for 50% coverage charging speed falls to 1C, which is totally doable (and as you say, for diesel-electric hybrid it could start making sense at even lower percentages). The Pareto principle means that even if battery capacity means you need 80% or 95% coverage you could still see large savings over a 100% electrification. Remember that you can also coast. If it's just about a low bridge or short tunnel you might lower the pantograph a few 100 metres on either side of the bridge. No need to wear the battery at all for such a short stretch - even with the brakes fully applied the train takes about a mile to stop. Trains don't currently do this because they don't have sophisticated position-based pantograph control and if they had to do an emergency stop under the bridge they would be stuck, but with more advanced control computers and battery backup both those problems can be solved. Where I live they are electrifying 150 miles of track. This means 85 bridges need to be raised, demolished or rebuilt, and it's a big part of the cost. And Denmark is pretty flat - there are no tunnels as far as I remember.