Regenerative braking on Locomotives with AC transmission

[OmCheeto]

The electric light rail system in Portland Oregon uses regenerative braking. About 70% of the energy is transferred from a braking car to other cars on the system. According to http://www.fta.dot.gov/documents/TIGGER_OR-88-0001_TriMet_Portland.pdf I just read, they've added 1 kwh capacitors to 20 of the 127 vehicles, hopefully bringing the regeneration closer to 100%.
They claim it can accelerate the vehicles to 25 mph. Seems kind of fast for just 1 kwh.

43,700 kg empty [ref Siemens S70 streetcar]
25 mph = 11.2 m/s
ke = 1/2 m v^2 = 1 kwh = 3,600,000 joules
v = sqrt ( 2 * 3,600,000 joules / 43,700 kg) = 12.8 m/s

I guess it does work.

I wonder how big a capacitor you'd need to do the same with Jim's 15,000 ton train from post # 26.

15,000 tons = 13,600,000 kg
25 mph = 11.2 m/s
ke = 1/2 * 13,600,000 kg * (11.2 m/s)^2 = 853,000,000 joules = 237 kwh.

At $4200/kwh, that would cost almost $1,000,000 per train.

Seems a bit much.
Never mind.

 

[rollingstein]

At $4200/kwh, that would cost almost $1,000,000 per train.

Seems a bit much.

A new diesel electric loco (say 5000 hP) with all the modern bells and whistles runs somewhere around 2-3 Million $$ isn't it?

 

[OmCheeto]

A new diesel electric loco (say 5000 hP) with all the modern bells and whistles runs somewhere around 2-3 Million $$ isn't it?

I know absolutely nothing about trains. But I like regenerative braking. So I'm usually up to doing the math to see if it's worthwhile.
I ran some more numbers, and I don't like the idea on trains at all.

37.95 kwh/gal diesel = 6.24 gallons of diesel
@$3.00/gallon
means accelerating a train to 25 mph will cost $18.73 using diesel fuel
Dividing that into $1,000,000 yields 53,130 start/stop cycles
Which if you do it once a day, it will take 145 years to recoup your investment.

I don't really know how often trains start and stop though.

Portland's light rail trains start and stop every few minutes, don't weigh much, and a majority of the energy is transferred through the overhead electrical lines, so I can see the logic there.

 

[OmCheeto]

... I don't like the idea on trains at all.
...

Capacitor based that is.
There may be less expensive options.
I would imagine a counter-rotating set of 5 ton fly wheels might be a bit cheaper.
I usually don't give up, until all options have been examined properly.

 

[rollingstein]

Capacitor based that is.
There may be less expensive options.
I would imagine a counter-rotating set of 5 ton fly wheels might be a bit cheape

I've never heard of commercial use of capacitor / flywheel based systems on mainline rail. So your analysis showing it doesn't make sense is probably right.

The two ways to make regen pay are: (a) feed it back to transmission line (obviously can't do for diesels) &

(b) feed it to hotel load (only for PAX trains)

 

[anorlunda]

I will have to eat my words. Rollingstein was correct about the 500 KVA.

I dug deeper and found this article on "head end power" http://en.m.wikipedia.org/wiki/Head-end_power

The article confirms the 20-150 kW per car number. It says that 380 V 3-phase power distribution is most common.

Perhaps someone can help me understand how they accomplpish self make-break connections at the car couplings capable of switching 500 KVA. It needs to conduct the power, to extinguish the arc without permanent damage in case of unexpected decopuling, and to be safe for yard workers.

 

[jim hardy]

hmmmm considerable info on this link.
looks like they US has a standard, 480 VAC using four 4/0 cables per phase.

http://www.apta.com/resources/standards/Documents/APTA-PR-E-RP-016-99.pdf

 

[anorlunda]

hmmmm considerable info on this link.
looks like they US has a standard, 480 VAC using four 4/0 cables per phase.

http://www.apta.com/resources/standards/Documents/APTA-PR-E-RP-016-99.pdf

Thank you Jim. That document had a lot of relevant information. Including these facts that exceeded my imagination.

• The standard includes systems up to 1 MW total load.
• 1600 amps max load current (133 amps per conductor, or about 30% of the max rating of 4/O cable)
• The four 4/0 cables per phase connect between cars using four connectors like this one.
  
  
  
  
• The circuit breakers in each car must be able to interrupt 14,000 amps symmetrical.
• They use controls with time delays to stagger application of loads when first connected to power.
  The delays are random on each car to provide diversity.
• Distributed power rather than head-end power is being researched.
• At least on British Rail, they use automatic connections like below.

A tightlock coupling on a British Rail Class 321.
Note the large rectangular multi-connector below the coupling,
which automatically connects head-end-power and control lines.

Still a mystery to me is how they avoid an arcing three phase fault if those connectors get pulled off under load. Those plug connectors are meant for switching, not interrupting. That contingency is not mentioned in the referenced document.

It seems that head end power is a pretty rich engineering field.

 

[jim hardy]

Still a mystery to me is how they avoid an arcing three phase fault if those connectors get pulled off under load.

It'd be logical for a mechanical interlock on the coupler to grant permission for an adjacent circuit breaker to be closed.
Permission would be granted only when coupler is fully closed and latched, so as to open breaker before the cars accidentally separate and pull out the plugs
but that's only a guess.

 

[rollingstein]

It seems that head end power is a pretty rich engineering field.

+1 Who would have thought! Glad we dug deeper into this. I learned a lot of new stuff for sure!

 

[rollingstein]

Stated another way, 500 KVA is roughly the size of the load of the MGM Grand Hotel in Vegas, or the hotel load of the biggest cruise ships with 6000 passengers. But we are talking about a train with cars that can be easily decoupled.

As an aside, don't know about MGM Grand but regarding cruise ships I think your estimate is quite a way off. 500 kVA sounds way too low for a 6000 passenger cruise ship.

That'd be approx. 80 Watt per passenger. Sounds too low even based on just what those passengers would consume at home.

PS. @anorlunda: Hope you don't think I'm picking on you or being nitpicky! I find these ballpark estimates & back of the envelope calculations a lot of fun! No offense intended!

 

[anorlunda]

No problem Rollingstein. I don't mind being wrong sometimes.

All those numbers are far from my life on a sailboat. My wife and I consume 0.6 kWh/day, for an average of 12.5 watts per person. 80% of that goes to refrigeration.

 

[rollingstein]

All those numbers are far from my life on a sailboat. My wife and I consume 0.6 kWh/day, for an average of 12.5 watts per person. 80% of that goes to refrigeration.

My guess is that the largest cruise ships consume approx. 20 MW as hotel load.

 

[anorlunda]

My guess is that the largest cruise ships consume approx. 20 MW as hotel load.

About MS Oasis Of The Seas, wiki says

Installed power: 3 × 13,860 kW (18,590 hp) Wärtsilä12V46D
3 × 18,480 kW (24,780 hp) Wärtsilä 16V46D

Propulsion: 3 × 20 MW (27,000 hp) ABBAzipod,
all azimuthing
4 × 5.5 MW (7,400 hp) Wärtsilä CT3500
bow thrusters[10][11]

 

[rollingstein]

nstalled power: 3 × 13,860 kW (18,590 hp) Wärtsilä12V46D
3 × 18,480 kW (24,780 hp) Wärtsilä 16V46D

Propulsion: 3 × 20 MW (27,000 hp) ABBAzipod,
all azimuthing
4 × 5.5 MW (7,400 hp) Wärtsilä CT3500
bow thrusters[10][11]

Thanks @anorlunda

One crude estimate of hotel load might be to subtract propulsion power from total generation. In this case

Genration - Propulsion = 3 x 13.8 + 3 x 18.5 - ( 3 x 20 + 4 x 5.5 )
= 97 - 82
= 15 MW

So 15 MW might be one estimate of peak design hotel load.

 

[rollingstein]

Don't know about MGM Grand's power rating but MGM's Mandalay Bay has a connected load of approximately 30 MW.

I quote:

"MGM Resorts International has partnered with NRG Energy on the planned installation of a large rooftop solar photovoltaic array at the Mandalay Bay Resort Convention Center in Las Vegas. The 20,000 panel, 6.2 MW installation will be MGM Resorts’ first commercial solar project in the US, and it will be among the largest in the world, MGM said. At peak production, the rooftop array is expected to produce nearly 20 percent of the Mandalay Bay’s power demand."

http://www.energymanagertoday.com/m...ns-6-2-mw-roof-top-solar-installation-093352/

 

[mheslep]

I've never heard of commercial use of capacitor / flywheel based systems on mainline rail. So your analysis showing it doesn't make sense is probably right.

The two ways to make regen pay are: (a) feed it back to transmission line (obviously can't do for diesels) &

(b) feed it to hotel load (only for PAX trains)

Or eventually feed regen power to batteries in some future diesel-electric. There have been some attempts apparently; I suspect the battery tech is not quite there yet. Given the problems transmitting power along the train (as discussed above) only energy from the locomotive would be economic to recover, and then the loco-batteries would have to be particularly inexpensive to justify.

 

[rollingstein]

Or eventually feed regen power to batteries in some future diesel-electric.

My impression was batteries were not a practical solution yet for those sort of capacities.

But I just dug up some numbers. I see $ 11 / kWh mentioned as the cost of storing in a NiCd battery (1000 cycle life assumption). Assume a braking load of 3000 kW as per Jim Hardy's calculation previously in the thread. That's a steep slope & say we design for storage capacity for 60 minutes of this downhill run.

That's 3000 kWh which would cost $33,000 worth of NiCd batteries. That doesn't sound too bad. What gives? Am I using a bad estimate of storage cell cost?

This other graph let's me estimate the weight of such an assembly. 50 Whrs/kg sounds typical. That would weigh 60 tons. Not a lightweight but is that a game killer. A loco weighs upwards of 200 tons.

If you bill power at even a cheap 3 cents / kWhr that's a $250 saving on every such 1 hr downhill run. If you factor in start stops & lesser hills etc. say, you are saving $500 / day. That'd mean a 2-3 month payback. Not bad.

I assume I'm getting some numbers wrong?http://batteryuniversity.com/learn/article/cost_of_power

http://www.mpoweruk.com/images/energy_density.gif

 

[mheslep]

The problem in this case is the unit cost of power with batteries, not so much energy. Your source has Li-ion batteries currently at $1000/kW, or $3 million for your 3000 kW loco braking application, and continued fast charging typically shortens the battery life. Capacitors on the other hand are excellent for quick charge/discharge and sustain millions of cycles. Capacitors are inferior to batteries for energy density (per unit mass, unit cost). So, eventually regen braking might pay for locomotives but not yet.

 

[Doug Huffman]

What does Battery University have as LiPo cycle life? Last I looked, and BU is my go to, it was still =<500, twice your 1000.

 

[rollingstein]

What does Battery University have as LiPo cycle life? Last I looked, and BU is my go to, it was still =<500, twice your 1000.

I used the 1000 cycles life for NiCd not Li. Battery University reports a lower cost per kWh for NiCd than Li. $11 / kWh vs $24 / kWh

Should I not be using NiCd? With Li cost would double approximately but that still means a 6 month payback. Not too shabby for such projects?