
The train that powers its station - SlashmanX
http://www.bbc.com/autos/story/20151002-the-train-that-powers-its-station
======
mhandley
The tube already does regenerative braking. This trial is about better
regenerative braking. Currently, the trains return power to the track when
braking, but if the track is already at capacity the remainder is dumped via
resistors on-board the train. If I understand correctly, this new substation
can sense when this is about to happen, take that extra current and feed it
through an inverter to supply it back to the power grid.

Some more details are here: [http://eandt.theiet.org/news/2015/sep/tube-brake-
energy.cfm](http://eandt.theiet.org/news/2015/sep/tube-brake-energy.cfm)

~~~
lucaspiller
I don't fully understand the physics behind it, but how exactly does can the
track be 'at capacity'? Isn't the track just fed from the power grid, so if
the demand on energy on the track is lower, won't it just take less energy
from the grid?

~~~
Animats
The London Underground uses 630 VDC traction power. Power distribution is
11KV/22KV 50Hz, converted to DC at substations every few stations along the
right of way.[1] 630V is too low a voltage for long distance transmission
without big losses, so distribution is at a higher voltage. AC to DC
conversion is with rectifiers, which are one-way; they can't make AC from DC.
With inverters, that's possible. Inverters are basically big switching power
supplies. This is just an inverter installation at a substation. It's probably
bi-directional, converting AC to DC or DC to AC as required.

The whole transit system isn't one big DC circuit; it's in sections of a mile
or so. Regenerative braking can only power trains in the same section, unless
traction power can be up-converted back into the AC system. If more trains in
a section are braking than accelerating, the excess power has to be dumped
somewhere, usually into big iron resistance grids that waste it as heat. Using
inverters eliminates that energy dump.

[1]
[http://www.trainweb.org/tubeprune/tractioncurr.htm](http://www.trainweb.org/tubeprune/tractioncurr.htm)

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STRML
I found this confusing:

    
    
      For each day of the five-week test, the system captured a full megawatt hour — enough to power more than 100 homes for a year.
    

This doesn't add up. I'd like to believe what they mean to say is "enough to
power more than 100 homes", but the average UK home uses about 4.6 megawatt-
hours per year, or 12.6 kWh per day. This is, more accurately "enough to power
less than 80 homes _each day_ ".

Perhaps that's per-train, and I don't doubt that this is a significant energy
(and heat) savings. But this is a surprising inaccuracy from the BBC.

~~~
jessaustin
The whole energy-vs-power confusion seems problematic. This is a grid-
connected system; there are no batteries. Thus it seems average power would be
a more helpful measure than energy, when talking about train-brakes and
households both. If one insists on discussing energy, why not quote it in
joules or BTUs or whatever? One rarely sees average power used in these
contexts however, so there must be something about it that the power companies
don't like.

~~~
fnordfnordfnord
>why not quote it in joules or BTUs or whatever?

Retail electricity is sold by the kWh, consequently that is the unit that most
readers will be familiar with. Of the readers who prefer to use strictly Si
units in their daily lives, those people, aside from frequently converting
to/from various units, are aware or can quicky find that 1kWh is 3.6MJ. For
anyone who wants to use BTUs, well, it's a free country, I suppose.

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nraynaud
Strange, I though all trains had that.

There is this little anecdote about regeneration in trains: there is a mine
ore train next to the Swedish/Norwegian border that goes downhill to the
harbor full of ore and uphill empty, even with the efficiency factor, it's
still sending quite some power to the general electric grid.

~~~
lolwat
Nearly all railway vehicles use brake energy. Older trams wasted the energy
with heating resistors. Newer ones feed back energy back to the grid if
possible. Otherwise banks of supercaps are charged to allow going short
distances without power (like old town where contact wire is not desirable).

[http://www.bombardier.com/en/transportation/products-
service...](http://www.bombardier.com/en/transportation/products-
services/propulsion-controls/products/energy-management.html)

~~~
fnordfnordfnord
>Nearly all railway vehicles use brake energy.

Do you mean nearly all new ones, or nearly all in service today?

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sandworm101
> Spread across the entire Tube system, that technology would result in
> savings of £6m, about $9m.

Ok, but my understanding is that doing anything across the entire tube system'
would no doubt cost trillions. Repainting all the guard rails would probably
cost more than 6mil. So the proposition won't save a dime. It might be
environmentally friendly and might be a good idea for new stations/trains, but
the money would be better spent on other projects.

In short: Does spending a million on this regenerative breaking produce more
electricity than installing a million's worth of solar panels on the roof?
Which saves the most carbon per pound spent?

~~~
Retric
Presumably you would only add this to the train cars not the track. Also
electric motors can generally also act as generators so the additional costs
on new cars would probably be minimal.

PS: Cars only weigh 85,000 lb or so which means you only need to store
something like 10kwh depending on top speed.

~~~
sandworm101
I'm only going from the OP. The savings they describe seem minimal (6mil)
whereas they are dealing with a notoriously expensive tube system. Carbon
savings are great, but one should always ask if that same money might result
in greater carbon reductions if spent elsewhere.

~~~
Symbiote
London Underground would, I expect, /invest/ $6M in reducing heat in the
tunnels. I can't find costs, but there have been other projects to do this.

In the summer, the temperature can rise to uncomfortable levels (over 30°C).

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biot
Is it impractical to have the braking spin up a flywheel which then gets used
to start the train moving when it departs? With the energies involved, I can
only imagine that would end up being a monster flywheel with size, safety, and
reliability concerns. And the energy from the flywheel would also be wasted
overcoming its own inertia.

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jimktrains2
> In the first experiment of its kind, the London Underground has installed an
> inverter system that converts braking energy into power.

[http://www.septa.org/sustain/blog/2011/07-06.html](http://www.septa.org/sustain/blog/2011/07-06.html)
SEPTA has been doing something similar (but using the electricity more
generally than for just stations) since 2011. I'm sure SEPTA hasn't been
leading the way in this type of hardware either.

~~~
dalke
Not only that, but the London Underground already uses regenerative braking.
Here's the press release from 2013 from when Alstrom got the contract to put
in the system that is now being tested, [http://www.alstom.com/press-
centre/2013/5/alstom-to-supply-g...](http://www.alstom.com/press-
centre/2013/5/alstom-to-supply-ground-braking-energy-recovery-system-for-
london-underground/) :

> Alstom was awarded a contract by UK Power Networks Services to supply its
> innovative Harmonic and Energy Saving Optimiser (HESOP) energy recovery
> system for the Victoria Line of the London Underground. ...

> London Underground already makes good use of regenerative braking but, by
> adding HESOP to the power supply arrangements, the residual energy that is
> currently wasted in braking resistors can be made use of – this will help
> prevent tunnel temperatures rising.

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kcorbitt
The central line can get quite toasty in the summer, especially at peak travel
times. I'd be interested in an estimate of how much of that heat is a direct
result of the current friction braking system. Will the stations get
noticeably cooler if induction braking is adopted? "Enough [electricity] to
power more than 100 homes for a year" coming out of a single station over just
five days sounds like a ton of energy, all of which is currently being
dissipated as heat.

~~~
SlashmanX
The article does allude to that.

> Additionally, regenerative braking doesn’t produce the heat that
> conventional friction braking does, so the tunnels themselves stay cooler,
> requiring less energy expenditure on climate control (and keeping Tube
> riders happier in the process).

~~~
CydeWeys
That statement in the article didn't make any sense to me. By my understanding
it's a violation of the second law of thermodynamics. You can't take a process
that creates waste heat and convert that into energy with less entropy,
electricity in this case, and have less total resultant waste heat. Any
physicists care to chime in?

~~~
joncrocks
Sure you can. As long as overall entropy increases, you're AOK.

Imagine a thought experiment not too dissimilar. You have a bike, that you
pedal up to speed. You then let yourself slow to a stop naturally. All the
energy you put in goes to heat.

Then you do the same again, pedal the bike up to speed, but this time you
click a dynamo (attached to a battery) into place and let yourself stop
naturally. This time some of the energy is converted to heat, and some into
energy in the battery.

It the same principle of hydro electric power. You can let water just move
down some tunnels from high to low gravitational potential energy (GPE ->
water velocity), or you can put some turbines in the way.

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sandworm101
Idea: Why are the resistors on the train? Why not make them stationary?
Regulator-resistors attached to the rails could be used in fixed locations to
heat water. Perhaps they could dump energy into capacitors to feed back onto
the rail when needed. The trains would dump energy into the rail, raising the
voltage and triggering the stationary regulators. They might not need to be
anywhere near the busy stations.

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tonyedgecombe
Perhaps they should just put the stations on the top of an incline, then as
the train enters the station the momentum would be converted to potential
energy and back again as it leaves the station.

~~~
Symbiote
This is already done — originally (in the 19th century) so that seeping water
didn't pool in the stations.

