On 1 Jul, 23:34, Tom Barry wrote:
John B wrote:
Ish. On AC, absolutely right; on DC, you need banks of resistors as
well because putting it back to the grid if there isn't a conveniently
placed train to take it is Too Bloody Hard. However, given the traffic
density on LUL, most of the time there'll be someone accelerating
while you're breaking so it should work out OK...

[hangs head in shame at 'braking' typo]

Yes, that's what I was getting at. In mitigation, I hadn't read Cap'n
Deltic's latest screed in MR at that point, which suggests that
receptivity in DC networks is higher than previously thought, so you
might not need as much resistance capacity and associated cooling around
the place.

My last bout of crazy-travelling-about-the-place ended just before the
latest edition came out, so I've only seen the preview email. Bring on
the next train voyage...

What's the effect of the suggested upping of the voltage to 750v DC on
some lines? Being a concrete'n'steel type engineer (manqué) I don't
understand electricity as well as I should.

transmission losses [hence, here, heat gains] = current^2 / resistance

Since resistance is ~constant and power delivered to the train =
current * voltage, increasing the voltage from 630V to 750V reduces
the current required to provide the same power by 16%, which reduces
transmission losses by 29%.

This ignores the effect of almost everything real-world (especially
the fact that you're dealing with two rails at +420 and -210 rather
than a single rail at +630), but you get the idea.

--
John Band
john at johnband dot org
www.johnband.org

In message , at
00:07:52 on Wed, 2 Jul 2008, Tom Anderson
remarked:
on DC, you need banks of resistors as well because putting it back to
the grid if there isn't a conveniently placed train to take it is Too
Bloody Hard.

But there are places to send it other than the grid, surely? Supercaps?
Pumped storage? Flywheels? A giant laser firing into space?

How about a plant that's generating hydrogen by electrolysis. That
wouldn't need a steady flow of current, and could simply absorb whatever
was available from one second to the next. Then use the hydrogen to
power those buses they have. Or did that experiment end now?
--
Roland Perry

On 2 Jul, 10:20, Roland Perry wrote:
wouldn't need a steady flow of current, and could simply absorb whatever
was available from one second to the next. Then use the hydrogen to
power those buses they have. Or did that experiment end now?

For it to be a proper experiment they need some published results.
Haven't seen any yet. More likely it was just green bandwagonning by
Ken.
I haven't seen the buses themselves for a while now but they weren't a
common site in the first place so they could still be bumbling about
somewhere.

B2003

On Wed, 2 Jul 2008, Tom Barry wrote:

Tom Anderson wrote:
On Tue, 1 Jul 2008, Tom Barry wrote:
...

This seems like an extraordinary proposition to me. Could you refer me to
any documents giving more details?

Yup, someone took notes at a TfL talk in March, which are online he

http://www.freewebs.com/going_underg...ogspot.com.doc

Excellent document, thanks!

Relevant section:

Ah, now the thing that i found extraordinary was the idea that heat was
building up in the deep tubes over the course of decades - that the reason
things are hotter today is that heat generated in the 70s or whatever has
yet to escape. I just can't imagine the time constant for diffusion of
heat from the tubes to the surface being that long. I have no basis for
this in geophysical knowledge or engineering experience, but i feel it in
my gut.

And what our corresponded writes on this subject is:

"However, in the closed environment of the deep lines, the movement of
rolling stock, working equipment and people using the lines emitted heat
over time. This dissipated into the walls and then into the subsoil,
raising the ambient temperature and drying out the clay and reducing its
ability to conduct heat away. Thus they noticed after 20-30 years of
operating the early deep level lines that the ambient temperature had
slowly but steadily increased over that time..."

That is, it's not a matter of heat buildup, but of the heat from the
tunnels changing the nature of the surrounding ground. That i can believe
- although i would never have thought of it myself!

He also mentions what i suspect is the main reason for the increased
temperatu more services. We have a train every two minutes on many
lines, which i would guess is more than a hundred years ago, and i would
hazard a guess that they accelerate harder and go faster, and so use more
power per train, too (perhaps except the latest stock). Again, this is
purely my gut talking!

"Most heat is generated from the equipment, brakes etc underneath the
train. Plans are being made to draw this heat out from under the trains
when they stop at platforms and then vent this heat out of the stations.
Obviously this would only work in those stations that have adequate
ventilation to street level..."

Interesting stuff, particularly since the TfL tube cooling press release
quoted the Mayor thus:

?It always perplexed me that boffins could produce mobile phones the size of
a credit card yet passengers would emerge dripping with sweat from Tube
trains that lacked air conditioning.'

Oh christ, don't get me started. I'd never presume to imagine that our
glorious leader's grasp of physics would leave him up to speed on this
issue, but you might hope that he'd have someone explain it to him.

Someone like this:

http://www.boriswatch.co.uk/2008/06/...s-a-new-voice/

The 'boffins' do at least appear to be on the case, here (already ruling
out air-conditioned Tube trains straight off, for obvious reasons),
although whether or not any of the current crop of ideas reaches
operational status is open to question. This effect was known about
when the Jubilee was extended, so it's hardly new stuff.

What's probably true is that it's one of those engineering problems
where there's no magic bullet big ticket way of fixing it (other than
total replacement), but a few judicious improvements could add up to a
considerable improvement.

No magic bullet, no. I do have high hopes for the impact of regenerative
braking, though - my own physics is pretty wafty, but if the 30% reduction
in power consumption (which ISTR is claimed - and why isn't it more? boo!)
translates to a 30% reduction in heat output, that should mean a 30%
reduction in the temperature differential between the tubes and the
atmosphere. That should be a good few degrees.

On top of that, i think our best bet might be heat pipes. I'm really not
an expert on these, but the point is that they're thing which conduct heat
incredibly well, far better than a metal rod. Run some of those along the
tunnels, and install surface-linked air conditioning at substations and
stations to pump heat out of them, and you have a way of cooling the
tunnels. The two questions are whether these can move enough heat to be
useful, and how much they'd cost, and i have no idea about answers to
either.

Anyway, some more info here (along with a repeat of the 'ground warming
up' theory):

http://en.wikipedia.org/wiki/London_Underground_cooling

And some splendidly technical knowledge from Profs Maidment and Missenden,
who, refreshingly, really do know their physics:

http://www.cibse.org/pdfs/Cooling.pdf

tom

--
Links are content.

On Wed, 2 Jul 2008, Tom Anderson wrote:

On Wed, 2 Jul 2008, Tom Barry wrote:

Tom Anderson wrote:
On Tue, 1 Jul 2008, Tom Barry wrote:
...

This seems like an extraordinary proposition to me. Could you refer me to
any documents giving more details?

Yup, someone took notes at a TfL talk in March, which are online he

http://www.freewebs.com/going_underg...ogspot.com.doc

Excellent document, thanks!

Relevant section:

Ah, now the thing that i found extraordinary was the idea that heat was
building up in the deep tubes over the course of decades - that the reason
things are hotter today is that heat generated in the 70s or whatever has yet
to escape. I just can't imagine the time constant for diffusion of heat from
the tubes to the surface being that long. I have no basis for this in
geophysical knowledge or engineering experience, but i feel it in my gut.

Hang on, hang on, it now seems like that *really is* what's happening:

http://www.ianvisits.co.uk/blog/2008...n-underground/

Astrordinary!

And also, we're already regenerating almost as much as we can.

And also, in most bat**** but brilliant underground engineering idea i've
heard recently news: building drilling rigs in the Northern line deep
level shelters (currently used as document archives etc) in order to tap
cold, deep aquifers.

Further bat**** cooling ideas, this time of dubious brilliance:

http://www.undergroundcooling.net/

tom

--
Links are content.

Tom Anderson wrote:

Hang on, hang on, it now seems like that *really is* what's happening:

My gut said 'hang on, isn't that what Roger Ford was talking about with
the Southern electricity supply upgrade' - the average gap between
heatings isn't sufficient to dissipate the heat produced. The net
heating doesn't have to be large, as long as it's positive, and Father
Time will do the rest.

Tom

Tom Anderson wrote:

http://www.freewebs.com/going_underg...ogspot.com.doc


Excellent document, thanks!


NP

Someone like this:

http://www.boriswatch.co.uk/2008/06/...s-a-new-voice/


*blush*

Actually, we do seem to be getting some influential readers on there
now, but I didn't think we'd reached the heady heights of u.t.l
attention. Wow.

One I haven't written up on there yet is last week's question time,
where Boris attempted to explain the difference between conduit and
overhead tram electrification in London. Trying to explain the
difference, he first made a back-and-forth motion and talked about a
'funny little metal groove', then waved his hand over his head and
referred to an 'overhead gizmo'. The point he was trying to make (about
it being better if the city government and the boroughs work together
rather than go their own ways) was rather lost in all this flailing around.

More pedantically, I think he erred in referring to Croydon as the first
electric tramway in London?

It's about 1:04:30 into the webcast, which I think is still available.
http://www.london.gov.uk/webcast/jun08/mqt_180608.asx

Tom