Regeneration - station capacitors

[Spotty]

Yes I know I'm covering old ground but here it comes again. I ride a Southern Rail London Commuter train. What does it do all day? It brakes into a station and accelerates out. All day a continuous waste of energy. Some times I toy with the idea of a massive buffer spring braking the train with the slider removed from the Third rail, then at the time of departure the compressed spring being attached to the rear and released accelerating the train back to near its origional velocity. Nonsense I know but I've wondered if there is a way to bring this into reality. A Fourth rail only running the length of the deacceleration/acceleration path connected to a massive capacitor. The capacitor can be kept at a nominal voltage by a feed from the Third rail. As the train approaches the station it switches from Third to Fourth and configurering its traction motor as a generator feeds current into the capacitor and breaks by back EMF the train. The capacitor is raised above nominal volatage. When starting up the motor is reconfigurered and uses the stored enegy to return to its original speed, due to loses the capacitor will need a slight bleed from the Third rail.

 

[Nym]

So, you mean like how almost every single DC System rectifier feeder station is currently set up, just more complex...?

Before someone gets back to me on this, even those that don't support grid feedback from regenerative braking have smoothing filters that are able to store an amount of charge / current, the newer versions use IGBTs to work in both rectifier and inverter modes.

For now I'll ignore the implications of leakage current continuously in the capacitors for this setup,
modifications to all rolling stock (where they already use the 3rd rail supply like this)
AWS and TWPS Induction Loops / Magnets down the centre of the track in place that would be rather in the way.
Additional expensive maintenance of equipment
Over-rating of traction motors to run above 750V
Big capacitors are expensive, and already provided in the rectifier / feeder station, so why duplicate?

This would have possibly being helpful in the realm of basic DC motors with resistor ladders to start, but in the world of AC traction motors over-rating the voltages does very little as synchronous machines are used to provide traction. Actually, maybe not, since the resistor ladders are used to reduce the voltage across the traction motors for when starting, eg. in stations...

 

[Yew]

The simplest way would be to build stations on hills, when it's slowing down it goes uphill, reducing speed, then when it's accelerating the trai is going downhill. It's what they do on motorway slip roads.


Unfortunately it's hard to retrofit this to stations without substantial reconstruction.

 

[John55]

Yes I know I'm covering old ground but here it comes again. I ride a Southern Rail London Commuter train. What does it do all day? It brakes into a station and accelerates out. All day a continuous waste of energy.

South Western, Southern and South Eastern all use regenerative braking on a proportion of their fleets. As electricity has become relatively more expensive this has been commissioned on the more modern trains. This does need the power supply to be able to cope with it of course. In the fullness of time the vast majority of electric trains will regenerate.

So the railway is already doing pretty much what you are suggesting. Eventually traction batteries will make a comeback (or more likely another power storage system and perhaps on the trains).

If anyone knows what happened to the trials on LUL of flywheel storage of energy? Did it get beyond the trials stage? One hears of odd installations abroad of similar devices but no comprehensive networks.

PS it isn't a continuous waste of energy as it gets the passengers from A to B but it could be more efficient which I guess is what you mean.

 

[90019]

The simplest way would be to build stations on hills, when it's slowing down it goes uphill, reducing speed, then when it's accelerating the trai is going downhill. It's what they do on motorway slip roads.

IIRC, the Glasgow subway uses that sort of concept.

 

[Nym]

IIRC, the Glasgow subway uses that sort of concept.

As will Crossrail, as does the Jubilee Extension..

If any flywheel systems are being commissioned, they're sure as heck not at the new inlet box, (that does support regenerative braking), any local installations could be in place but I don't think that any are active (willing to be corrected) as the flyback into the AC Distro Grid for LU is more efficient than involving more synchronous devices when the grid can be used for this...

I personally don't think traction batteries will make a comeback, it would add tonnes to the vehicles for a system that can easily be accommodated into the trackside, where one doesn't have to stop and start the mass, the only way I can see it coming back is in some form of backup supply, even then, a small diesel generator or gas turbine would be more space and mass efficient.

 

[Railcar B]

The simplest way would be to build stations on hills, when it's slowing down it goes uphill, reducing speed, then when it's accelerating the trai is going downhill. It's what they do on motorway slip roads.

IIRC That was designed into the original section of the Central London Railway (now the Central Line of London Underground).

 

[D365]

Victoria line too.

 

[Robsignals]

Maximum allowable line voltage is only about 50 Volts above normal which severely limits the power Regen can put into the system. With modern power equipment I wonder if substations could be intelligently linked to detect increased line volts and reduce their output. If not then trains could be fitted with real-time data links reporting available Regen power when braking to local substations which reduce output to match. To get the most from Regen a metro ATC system could delay departures by up to say 30 secs to match their acceleration with a nearby braking train.

 

[Nym]

The regenerative systems used today do feed back in at 750V, with most DC feeder stations any attempt to 'over run' the voltage by attempting to induce high voltages in the line from the train would result in damage to equipment, but small changes are absorbed into the filter circuits within the feeder.

But with the way the power electronics make use of the regenerative braking, it is fed back onto the track at 750V or 660V depending on where it is, and regulates the current to a certain level, so that nothing is pushed beyond it's rating.

Feeder stations 'reducing their output' would simply put more strain on the final stage filters causing more damage (in most implementations of power supplies)

 

[John55]

The regenerative systems used today do feed back in at 750V, with most DC feeder stations any attempt to 'over run' the voltage by attempting to induce high voltages in the line from the train would result in damage to equipment, but small changes are absorbed into the filter circuits within the feeder.

But with the way the power electronics make use of the regenerative braking, it is fed back onto the track at 750V or 660V depending on where it is, and regulates the current to a certain level, so that nothing is pushed beyond it's rating.

Feeder stations 'reducing their output' would simply put more strain on the final stage filters causing more damage (in most implementations of power supplies)

If trains regenerate at 750V into a line at 850V there won't be much braking effort. I believe trains regenerate at up to 900V and to drive the current to a useful load this has to be higher than the line and the higher the better.

 

[Spotty]

As I said I'm covering old ground I sincerely hope that works as you say although I tend to hear quite a lot of grinding of brakes! However the concept I was trying to bring up is that of storage. The system in current use does not store energy. What I was trying to make sure was that the energy lost in braking is most definetely used by the train in departing. I suspect in the present no storage system a lot of energy is wasted by voltage regulation and the lack of a suiable train being available to 'soak up' the regenerated energy!

 

[Crossover]

It's what they do on motorway slip roads.

I'm not sure that is always the case - on the way to work, where I get on is uphill and two of the junctions I can come off at are downhill. Coming home, two of the junctions I can get on at are uphill and the one I come off at is downhill...

 

[Robsignals]

As I said I'm covering old ground I sincerely hope that works as you say although I tend to hear quite a lot of grinding of brakes! However the concept I was trying to bring up is that of storage. The system in current use does not store energy. What I was trying to make sure was that the energy lost in braking is most definetely used by the train in departing. I suspect in the present no storage system a lot of energy is wasted by voltage regulation and the lack of a suiable train being available to 'soak up' the regenerated energy!

Capacitors can't store enough energy at reasonable size/cost, batteries might be better if 'instant charging' could be developed. The other 'great hope' from the past is flywheels, mounted on train (in very strong caging in case of disintegration) or line-side. Doesn't the Parry People Mover on the Stourbridge branch use a flywheel as sole power source spun-up at each turn round?

 

[JGR]

Batteries suffer from degradation and thermal issues, and are generally eye-wateringly expensive.
Flywheels are also expensive, though on the upside do have a better energy density.
"Instant charging" is not really feasible for either of them, that is more the realm of capacitors, but at any rate you'll still be limited by the current or power limits of the supply and any intermediary equipment.

Storing regenerative equipment lineside is generally easier as you can make it as big as necessary. A single large flywheel with one set of associated equipment is much easier to manage, more efficient and cheaper than lots of small ones on trains.

 

[upnorth71]

However the concept I was trying to bring up is that of storage. The system in current use does not store energy. What I was trying to make sure was that the energy lost in braking is most definetely used by the train in departing.

Spotty, this may be of interest. Successfully tested in Japan, in the Tokyo area:

Tokyo, January 18, 2011 – Kawasaki Heavy Industries, Ltd. announced today that it has successfully completed a verification test of the railway wayside energy storage system it developed, called the Battery Power System (BPS), on the Den-en-toshi Line operated by Tokyu Corporation. The test, conducted in cooperation with Tokyu Corporation, Tokyu Techno Systems Co., Ltd., and Fuji Electric Systems Co., Ltd., was designed to see how effectively the BPS can reduce the need for regenerative braking cancellation, save energy, reduce CO2 emissions, and stabilize traction power line voltage. This is the first time ever that a railway wayside energy storage device connected directly to a standard 1500 VDC traction power line has demonstrated the ability to reap these benefits.

http://www.khi.co.jp/english/news/detail/20110118_1.html

 

[apk55]

Most trains are now designed to be to be permanently regenerative but with the addition of an electronically controlled braking to limit the line voltage to a safe level. If the line voltage is low then the resistor is is not switched in, but if it rises above a set level then the resistor is switched in to limit the voltage. This also enables a train to cope with section gaps.

I can see a future for supercapacitors in railway. While at present they do not energy to mass storage capacity of chemical batteries, with the best available today having an energy to mass ratio of about half that of a lead acid battery. However they can be charged and discharged very quickly, much faster than any chemical battery and they can be charged and discharged many thousand of times without deterioration. A supercapacitor that can absorb a pulse of energy from a breaking train need only have a storage capacity of perhaps 1 to 2 KW/H per car to achieve this. Then when accelerating the energy can be used to reduce the peak power demand from the supply network improving efficiency. This is particularly relevant on suburban duties where a train may only be tacking power for less than half the time and maximum power for a few seconds.
Putting capacitors on the train does involve some weight penalty but then eliminates transmission losses to or from substation. They could also enable heavier trains or higher acceleration to be achieved without upgrading the power supply.
In Japan with 1500V supply system is less lossy so so it may be practical to mount storage units by the track side.
It would also be practical to fit super capacitor storage units to Diesel units, particularly on suburban duties where fuel savings could be achieved by recycling braking energy. I have read some reports recently where they were investigating using flywheel storage on DMU's and super capacitors would be an alternative. These trains could become similar to a hybrid car with engines switched on and off depending on the duty. I could envisage a two engine DEMU with only one engine used on suburban duties the second only used when continuous fast running is required.