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When waiting for the underground, I've always been wondering why we could not absorb the kinetic energy of the train to slow it down at the station and then to release this energy to help it accelerating during the starting up. This would also reduce air pollution due to braking at underground stations.

Indeed, in frequently-stopping, trains such as underground trains, most of the energy is used to accelerate, so I am wondering whether the train could be slowed down by a large spring that would be compressed while arriving a stop station, then blocked when the train is stopped for the travelers to get out/in, then released to help the train accelerate to continue its route.

Of course, the train should not go to the opposite direction when releasing the spring, so a system should be designed so that the spring can be released in the correct direction.

This idea looks simple and I'm sure engineers has already thought about it, however, I've not found any discussion on this on the web. I found that on some Spanish lines, the kinetic energy during braking is converted to electricity and used by other trains from the same line (research paper), but why not keeping the mechanical energy to release it directly with a spring method?

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    $\begingroup$ Check out flywheel storage. $\endgroup$
    – Solar Mike
    May 31, 2022 at 10:10
  • $\begingroup$ There is a so-called "Federspeicher", exactly what you propose, in place in the subway in Bonn, Germany and probably in Cologne, since they share rolling stock. Whether it's still in use, I cannot say. $\endgroup$
    – arne
    Jun 1, 2022 at 7:01
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    $\begingroup$ I don't have any details, but afaik some metro trains raise the tracks in the platform area, which is another form of energy storage. $\endgroup$
    – jaskij
    Jun 1, 2022 at 10:21
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    $\begingroup$ Related: sustainability.stackexchange.com/questions/10444/… $\endgroup$
    – jpa
    Jun 1, 2022 at 18:24
  • $\begingroup$ Another thing to consider is that maintenance bases (including tooling, staffing, etc) for metro systems are one of the most important pieces of the puzzle. Your maintenance bases now need to be able to work on a brand-new system, unlike any others they're used to using. No other metro system uses it either, so you're on your own to figure out the best way to do this. That's expensive and risky. $\endgroup$ Jun 2, 2022 at 0:17

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I found that on some Spanish lines, the kinetic energy during braking is converted to electricity and used by other trains from the same line (research paper), but why not keeping the mechanical energy to release it directly with a spring method?

Because there is already a motor on the axle and there is no room for your spring. This and all the other problems that would be associated with controlled collection and release of huge amounts of energy. The KE of a 28 tonne London Underground car doing 20 m/s (72 kph) would be $ \frac 1 2 mv^2 = \frac 1 2 28000 \times 20^2 = 5.6 MJ $. Your wind-up train would require engage / disengage mechanisms, have to allow running in both directions, have to allow free-wheeling during shunting and require some means of discharging any stored energy before maintenance. If the spring storage couldn't be mounted on the bogie some flexible drive arrangement would be required to connect the axles to the springs.

Instead, many underground systems combine regenerative braking with placing the stations at the high points on the line so that they get free deceleration into a station and free acceleration out of the station.

Regenerative electric braking is more efficient, the control gear requires only slight modification, it is tried and tested and avoids major mechanical mechanisms.

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    $\begingroup$ Also a point that thermodynamically, electricity and kinetic energy are both forms of work, and converting between electricity and motion is similar to converting one speed/direction of motion to another. Electric motors and transformers are comparable to gearboxes in efficiency and power density (unlike batteries which are more comparable to engines), so it's ideal if you can transfer the electricity to another train or a big power grid (where every spinning turbine in a power plant is a big flywheel that you don't have to pay for or carry), rather than storing it yourself (which is hard) $\endgroup$
    – sqek
    May 31, 2022 at 18:15
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    $\begingroup$ As an aside, 5.6MJ is the equivalent of about 7 Krispy Kreme donuts... $\endgroup$
    – Jon Custer
    May 31, 2022 at 21:25
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    $\begingroup$ This assumes the regenerative braking power gets fed onto the power grid. In the Wellington NZ metro (which is DC) I read somewhere that regenerative braking power can be used to power another train which is trying to accelerate at the same time, but otherwise it gets wasted in a resistor bank because rectifiers do not work in both directions. $\endgroup$
    – user253751
    Jun 1, 2022 at 11:12
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    $\begingroup$ Steel springs have energy density of about 0.1 Wh/kg, so storage of 5.6 MJ would take a 15 500 kg spring (over 50% mass increase for the whole train :) Carbon nanotube springs can manage 10 Wh/kg, so a mere 155 kg spring would be enough - almost plausible, if we had a way to produce 155 kg of carbon nanotubes. $\endgroup$
    – jpa
    Jun 1, 2022 at 18:20
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    $\begingroup$ @jpa, thanks for that. I like to have some of those rule-of-thumb numbers in my head for order-of-magnitude calculations. I had never heard of the energy density of steel springs before. $\endgroup$
    – Transistor
    Jun 1, 2022 at 18:57
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On the London Underground, many stations use gravity. The line runs uphill into the station, and then departs downhill. This gives a large amount of energy storage, for free, with no additional moving parts requiring maintenance. It's somewhat inflexible, the amount of rise needs to be tied into the train's operating speed.

It obviously can't be applied on surface railways, or for road vehicles, but underground you have that extra degree of freedom.

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Note also that springs are subject to eventual (and very sudden) failure as they develop cracks with age and a very sudden 5.6MJ energy release inside a railroad car would be exceptionally dangerous!

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Mechanical energy recovery has been tried on rapid-transit systems, in the form of flywheel storage.

This is known as KERS in the motorsport world, but usage on a railway requires much higher reliability, so the cost, and the need for a containment system that can handle the rapid release of energy if the flywheel fails, means that the system hasn't been widely adopted.

Feeding current back into the supply (regeneration) is simpler & cheaper, though it recovers less energy when the line is unreceptive, i.e. at off-peak times, when there are few electrical loads in the vicinity of the regenerating vehicle.

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    $\begingroup$ I believe KERS is used to recharge batteries in a hybrid power system. A mechanical system would be unable to react quickly enough to the delicate control of braking required in motorsport. $\endgroup$ Jun 1, 2022 at 9:43
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    $\begingroup$ @Paul_Pedant the Audi R18 used both, mechanical and electrical energy recuperation. It was a very successful car and could compete with pure electrical recuperation systems. For F1 where the term KERS was coined, you are correct, it's an electrical systems. A mechanical was offered, but the efficiency was not as good as the electrical system. $\endgroup$
    – Arsenal
    Jun 1, 2022 at 12:35
  • $\begingroup$ KERS as alternative to metal spring is explained in this response sustainability.stackexchange.com/questions/10444/… $\endgroup$
    – Noil
    Jun 6, 2022 at 13:53
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Existing systems for trucks use hydro-pneumatic accumulators as the springs. As Eaton Corp said in 2010: "The system is best suited for vehicles that operate in stop and go duty cycles such as refuse trucks—the initial commercialization target."

For more information, search for 'hydraulic regeneration'. (But note that hydraulic regeneration is also used for things other than just energy saving).

Trains don't stop and start as often as garbage trucks, and have other restrictions on the permissible acceleration, deceleration and jerk. More recently, with new battery technology, interest in electrical regeneration has extended to trucks and buses.

For some applications, an alternative is to just use the recovered energy to provide compressed air or hydraulic power, without using the power for 'launch assist'.

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