Kinetic energy as motive power?

[Lampshade]

Right, it’s no secret that a train moving at speed has a hell of a lot of kinetic energy so wouldn’t it be possible to somehow ‘recycle’ this energy as motive power for the train? If for example there were generators on the axles or some other sort of turbine that is driven by the energy of the train moving along which could then charge capacitors/batteries or drive the traction motors directly, then trains would be greener as they won’t be using as much power/diesel to move, and obviously the faster the train moves, the greener it becomes as more energy is recycled.

Is this anywhere near reality or just very naive thinking?

 

[90019]

Reality.
In the form of regenerative braking.

However there is a lot of heat and friction as well, so you can only conserve so much of the energy used, increasing the speed of the train you also hugely increase the frictional forces, which is one of the reasons it gets harder to gain more speed the higher you get (it's a lot harder to get a 10mph increase at, say 140mph than it is at 40mph, because the increase is exponential IIRC - or close to anyway), so you can get more back, but the percentage increase in what you get back is much less than at lower speeds, if that makes sense.

I suppose you have to take into account the extra weight of the equipment needed for it as well, and whether the energy savings you'll make will actually be worth it; ie. it's not very useful if the equipment makes the train more thirsty and almost completely negates the point of putting it there in the first place.


Thinking about it though, with EMUs and DEMUs, I would've thought you could reverse the operation of the traction motors themselves; I'm not completely sure on the mechanics of them, so this may not be the case - But I'm presuming the drive just comes from a basic electric motor which can be reversed and used as a generator to charge capacitors, batteries or whatever.

 

[jopsuk]

Unless you were thinking of (as 90019 says) regen breaking, which is still in its infancy for use on electric routes, then I think you might be straying in perpetual motion territory.

I know Desiros are supposed to be able to put power "back into" the system as they slow down.

If you;re talking about attaching a dynamo to the wheels/axles to charge some batteries as the the goes along normally, then you need to learn more about physics!

 

[asylumxl]

Thinking about it though, with EMUs and DEMUs, I would've thought you could reverse the operation of the traction motors themselves; I'm not completely sure on the mechanics of them, so this may not be the case - But I'm presuming the drive just comes from a basic electric motor which can be reversed and used as a generator to charge capacitors, batteries or whatever.

Certain DEMUs (eg 22x family) use rheostatic braking. Now Im hoping ive not totally misunderstood it for years, but this is the reversing of the traction motors to generate electricity. This electricity is then fed into resistor banks ontop of the train, which through the huge resistance converts the electricity into heat. It's essentially the same as regenerative but without feeding back into the system.

 

[Kingfisher200262]

Right, it’s no secret that a train moving at speed has a hell of a lot of kinetic energy so wouldn’t it be possible to somehow ‘recycle’ this energy as motive power for the train? If for example there were generators on the axles or some other sort of turbine that is driven by the energy of the train moving along which could then charge capacitors/batteries or drive the traction motors directly, then trains would be greener as they won’t be using as much power/diesel to move, and obviously the faster the train moves, the greener it becomes as more energy is recycled.

Is this anywhere near reality or just very naive thinking?

There are several systems that are in the early stages of development for capturing braking energy on DMUs. One uses an accumulator to compress air during braking, which can then be re-used either to provide "silent starting" (i.e. drawing out of a station with the engine/s off) up to 40 km / hr so that noise and pollution in station confines is reduced or additional power to improve acceleration. The expectation is that this kind of system can catch / provide about 80 kW for a short period of time.

The second system that is being developed uses super capacitors to capture electrical anergy for re-use, and this is the kind of system that Forula 1 cars use and call KERS (Kinetic Energy Recovery System)

In the recent procurement process led by DfT for 202 DMU vehicles under the HLOS, it was intended that a small number of the units would be fitted with these systems, as comparators with each other and with units that were not fitted with energy recovery. Unfortunately, as most will be aware, this procurement was canned due to the "reduced need for DMUs in the coming years".

The potential benefit of these systems is reduced fuel consumption and, in the case of the hydro-dynamic system, noise and pollution in stations.

The main disadvantage of the systems (both main types) is that they are heavy (up to 3 tonnes on a three car unit) so for units that are not stopping frequently and / or operating on heavily graded routes the benefit may be negligible or even negative!

 

[TrainBrain185]

Certain DEMUs (eg 22x family) use rheostatic braking. Now Im hoping ive not totally misunderstood it for years, but this is the reversing of the traction motors to generate electricity. This electricity is then fed into resistor banks ontop of the train, which through the huge resistance converts the electricity into heat. It's essentially the same as regenerative but without feeding back into the system.

Thats essentially correct. AC electric vehicles using rheo brakes indeed feeds unused electricity back into the grid. Rheo brakes are used at higher line speed braking and when the train slows down the rheo brake loses effectivenenss and requires use of air application to slow to a halt.

 

[Wyvern]

Thats essentially correct. AC electric vehicles using rheo brakes indeed feeds unused electricity back into the grid.

Those are regenerative brakes surely.

They are based on the idea that when an electric motor is not driving but driven it acts like a dynamo. This is a DC motor. Instead of the generated current being dissipated in resistor banks, it is fed back into the power supply. The problem is in controlling it. This is compounded with AC systems as the phase has to be synchronised.

Simplistically put.

 

[royaloak]

Regenerative braking is definitely in its infancy, I mean it has only been around since the 1950s (Manchester- Wath via Woodhead) with the class 76 electrics.
Haven't learnt much in 50 years have we except how to complicate things.
--- old post above --- --- new post below ---
Regenerative braking is fed back into the supply system, but rheostatic braking it is dissipated by rheostats as waste heat and is NOT fed back

 

[TrainBrain185]

Regenerative braking is definitely in its infancy, I mean it has only been around since the 1950s (Manchester- Wath via Woodhead) with the class 76 electrics.
Haven't learnt much in 50 years have we except how to complicate things.
--- old post above --- --- new post below ---
Regenerative braking is fed back into the supply system, but rheostatic braking it is dissipated by rheostats as waste heat and is NOT fed back

Yes, thats correct. My error confusing Regenerative with Rheostatic.

 

[MrC]

I know Desiros are supposed to be able to put power "back into" the system as they slow down.

Testing has been performed at Weymouth recently for DC regen on classes 458 and 444/450. 444/450 already have rheostatic dynamic brakes - not sure about 458.

There is one major problem with DC regen braking which is that the regenerated power is currently (ho ho ) only available to other trains in the same substation area - and substations are quite close together in 750v DC areas. As a result DC regen braking is only of real advantage where there are trains available to accept the regenerated electricity, eg in suburban areas. If only one train is regenerating in a substation area then it will revert to rheostatic.

 

[pinkpanther]

Regenerative braking is definitely in its infancy, I mean it has only been around since the 1950s (Manchester- Wath via Woodhead) with the class 76 electrics.
Haven't learnt much in 50 years have we except how to complicate things

Exactly.