
Sisyphus’s train set - Tomte
http://www.economist.com/news/science-and-technology/21701466-novel-idea-storing-electricity-sisyphuss-train-set
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dbcurtis
Efficiency numbers, please? Is there such a thing as a motor or generator that
is more than 65% efficient? 0.65 * 0.65 is 0.42. Then add mechanical losses,
and I^2*R losses in the power lines to, from, and along the train lines.
Commutation losses for the pantographs (assuming a typical electric loco). Is
off peak electricity really 4x cheaper than peak? Otherwise the losses kill
the benefits. The article is short on math, which is sad, because the only
meaningful question here is one that simple arithmetic answers.

~~~
labawi
Motors and generators tend to be way more 65% efficient. Random internet
search mentions 70-98%. If they were not, pumped hydro would not be useful
either.

Linked article is short on details, but the link wiredfool gave has more
details including a video with numbers (2nd video 02:00). They claim >93%
(Combined M&E) one-way efficiency. I don't know what one-way is, but 0.93^2 =
86%, which would make the loss noticeable, but I would still consider it good
enough for storing solar/wind.

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dmurray
The problem is that lifting things up stores and consumes so little energy.
You can lift yourself to the summit of Mount Everest for the calories in a
single large meal, if you just consider the vertical climb.

~~~
owenversteeg
Yep. Although I'm always surprised at how much energy humans consume. A 1200
kcal meal eaten quickly, for example an apple with 180g peanut butter (which I
often consume in a few minutes or so) is the equivalent of "charging" my body
at the rate of almost 21 kilowatts!

For comparison, that's the power consumption equivalent of running 40
household washing machines, a ceiling fan, a router, printer, bright LED
lighting, a fridge+freezer, a large color TV, ten smartphones, a laptop, an
electric heating blanket, two table fans, two smaller TVs, an electric shaver,
a scanner, twenty more smartphones, three more table fans to keep them cool,
two fountains or other water features, a small vacuum cleaner and a partridge
[0] in a pear tree.

[0] Potts, G. R. 1986. The partridge: pesticides, predation, and conservation.
(Daily food consumption of a partridge ranges from 71 kcal/day at 15C to 155
kcal/day at -15°C, which works out to be roughly 3.5-7.5 watts, or roughly the
power consumed by a few smartphones.)

~~~
amelius
True. But that's a bit like unplugging a harddrive from one computer and
plugging it into your computer, and claiming that you just had a data transfer
rate of petabytes per second, and then claiming that that's the equivalent of
the bandwidth of all US citizens and corporations combined.

~~~
owenversteeg
Hmm, not really. First of all, unplugging my laptop's 256gb hard drive and
plugging it into another laptop, in ten seconds, would be equal to 25.6
gb/sec, or 0.0000256 petabytes per second. Even if you're doing the swap with
desktop computers that are right next to each other, the largest hard drive
[0] you can buy, and really fast switching, you would get a transfer rate of
0.003 petabytes/second. The US currently consumes an absolutely enormous
amount of data, more than 23 exabytes/month [1]. (I looked up these numbers
mostly because I was curious, and I gotta say 23 exabytes a month surprised
me.)

Secondly, the power that the human body can store in just a few minutes is
pretty cool. You can think of a person who eats 2300 calories a day as a 2700
watt-hour battery. Such a battery would be pretty powerful: it holds the
equivalent of roughly 300 smartphone charges. You can "charge" the human body
in just a few minutes of eating calorie-dense food like nuts or peanut butter
or fatty food. But to charge a 2700 watt-hour battery, comparable to the human
body, you'd need to draw the maximum power you could (from a wall in a house
with standard wiring) - for about two hours.

Sure, I agree that the human body isn't really consuming the power until it
gets fully digested. But the reality is that if I'm in a rush, I can wolf down
an apple+190g peanut butter and be done 4 minutes later with 1200 calories in
my body. Which is pretty useful in real life, at least for me. If my body
instead "charged" from a wall outlet (which can usually deliver a lot of
power, 1440W or so continuously) it would take an hour to provide me with 1200
calories. Sure, people often do eat slowly. But there have been many times in
my life when there's not enough time for whatever reason, and taking an hour
to eat a meal would be unacceptable. And it's times like that when I really
appreciate that I can eat something fast, and "charge" my body at the rate of
tens of kilowatts.

[0]
[http://www.pcworld.com/article/3096292/storage/seagates-10tb...](http://www.pcworld.com/article/3096292/storage/seagates-10tb-
barracuda-pro-is-the-worlds-largest-consumer-hard-drive.html)

[1] [http://www.cisco.com/c/m/en_us/solutions/service-
provider/vn...](http://www.cisco.com/c/m/en_us/solutions/service-provider/vni-
forecast-highlights.html#)

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adrianN
I wonder how they get clean power out of the trains. At least in normal
operation the power lines of the train network are _terribly_ noisy. I guess
there'll be some signifikant losses in cleaning that up.

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wiredfool
There was a vox article 6 months ago that didn't generate much discussion that
has a little more context, and a video rendering of the larger scale storage
system: [http://www.vox.com/2016/4/28/11524958/energy-storage-
rail](http://www.vox.com/2016/4/28/11524958/energy-storage-rail)

------
Someone
Another proposed solution for buffering wind and solar energy is
[https://www.ecn.nl/news/item/energy-train-de-revolutie-
voor-...](https://www.ecn.nl/news/item/energy-train-de-revolutie-voor-
energieopslag/): a train going round in a 5km diameter circular vacuum tunnel
doing up to 2000km/h.

That hasn't been built yet (it seems more or less at the 'back of an envelope'
stage to me) and would be more expensive to build, but it promises to store
quite a bit more energy (20 GWh vs 12.5 MWh), so it might be cheaper per kWh.

I also think it can be built using existing technology and will be safe enough
if the tunnel is built underground (derailments would be catastrophic)

One problem is that this only works at scale. Halve the diameter of the ring,
and you likely have to halve the top speed of the train, decreasing capacity
by 75%.

~~~
dmurray
Is this like a very heavy flywheel?

~~~
Someone
In a sense: yes. Heavy, large, and all its mass is near its rim. For a
flywheel, it's not _that_ fast, though; only about 125 revolutions/hour.

They got rid of the forces acting on the center of a flywheel that tear apart
large fast flywheels by getting rid of the axle and using a rail on the
outside of the wheel to counteract the centrifugal forces.

I would guess they want to use maglev as a rail system, as a conventional
track may break down too soon at 2000 km/h. On the other hand, it is 'only' 15
km of rails in a controlled environment, and they can use enormous wheels, so
it may be possible to lay conventional track a lot smoother than outside.

