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Grid-scale Electricity Storage Hits the Rails

Posted June 17, 2016 12:00 AM by Engineering360 eNewsletter

Pumped hydro is an excellent energy storage method but requires very specific geography. However, a train-based system under design can simply use gravity and a hill to store and dispatch electricity in a similar way for grid stabilization. When a surplus of electricity is available, it's used to propel a fleet of electric trains and heavy flatcars uphill about 3,000 ft. When electricity is needed, the railcars descend the hill and generate power via regenerative braking. A full trip could store and release 2 MWh. The design is claimed to be economical with a small environmental impact and a 40-year service life (see video).


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#1

Re: Grid-scale Electricity Storage Hits the Rails

06/18/2016 12:00 PM

I wonder...could this be incorporated into San Francisco's cable cars? Transportation plus capturing waste energy plus storing energy.

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#3
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Re: Grid-scale Electricity Storage Hits the Rails

06/20/2016 12:29 PM

Not sure how it would work, since the cars clamp onto a moving underground cable to travel, and don't have any onboard batteries to store the energy from regenerative braking.

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#5
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Re: Grid-scale Electricity Storage Hits the Rails

06/20/2016 5:56 PM

The cable is driven by a motor at a central location. It could do the regen braking when the trolleys go down a hill. Would probably replace the motorman (the guy who operates the brake now) with some electronics to get max efficiency.

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#7
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Re: Grid-scale Electricity Storage Hits the Rails

06/22/2016 10:20 AM

The cable runs at a fixed speed 24/7/365, that one cable is pulling ALL the cable cars on that circuit. From a documentary in the cable cars I saw long ago, the drive pulleys are HUGE: something like 20-30 feet across. with that much bass, they must behave like flywheels; probably takes 4-6 hours to spin one up from stopped to running speed, and the same amount of time to bring it to a stop for maintenance.

And the motorman is not someone who can be replaced with 'some electronics.' Sure, he only has three controls: Clamp, Brake, and Bell. but he's the one controlling the entire show. It takes YEARS of training and apprenticing to learn the route, and to gain the knowledge of where you deviate from the 'standard operating procedures' to ensure a smooth, comfortable ride for the passengers. He's also responsible for watching the road and dealing with track hazards. Remember, the cable cars share the street with cars and pedestrians. Sometimes it takes more than just sight, hearing and 'road feel' (touch) to operate the cars safely, there are moments where you have to rely on 'gut sense' to tell you if a pedestrian or driver seems like they're going to do something stupid, like try to cross too close to the front of the car, or to tell you that that glint you thought you saw on the track might be a penny put down in a dangerous spot because that section of track needs to be clear of debris for safe braking on that hill/curve.

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#2

Re: Grid-scale Electricity Storage Hits the Rails

06/19/2016 3:59 PM

To put it in perspective:

1 KWH = 2.65 million ft lb. To store 1 KWH (without losses) requires raising 1 ton (2000 lb) over 1300 feet!

https://www.unitjuggler.com/convert-energy-from-kWh-to-ftlb.html

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#4
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Re: Grid-scale Electricity Storage Hits the Rails

06/20/2016 12:37 PM

They're lifting 3,000 feet, so to keep the math simple, I'll lowball and call it 2KWH per ton moved.

They're storing 2MWH

2MWH = 2,000KWH

2,000KWH at 2KWH/ton would require a train that weighs in at 1,000 tons.

That does not seem too outrageous, a common weight limit for American Hopper Rail cars is 125 short tons of cargo, so, even if we assume that the cars and the engine weigh nothing, that's 8 cars worth of ballast.

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#6

Re: Grid-scale Electricity Storage Hits the Rails

06/22/2016 12:22 AM

A missing piece of the puzzle is the total length of the track and the speed of descent. Typical grades for heavy freight lines are usually limited to 1.5%, the limiting factor being the adhesion of the drive wheels to the track. At that gradient it will take approximately 37 miles to climb 3,000 ft! If it moves at 5 mph you could get a little more than 7 hours of storage though.

Somehow I think that finding that much right of way with the proper gradient is less probable (and more costly) than finding a convenient reservoir/lake for a pumped hydro system with a similar storage capacity, and water can be pumped at a much steeper gradient. Of course you could move the train up a much steeper incline by having a headend power house do the pulling and and regenerative braking on the trip down.

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#8
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Re: Grid-scale Electricity Storage Hits the Rails

06/22/2016 10:27 AM

Yeah, when you boil it down to the base physics, the train and reservoir systems are functionally the same: use electrical energy to store potential energy, then release the potential energy, which converts into kinetic energy, and convert the kinetic energy into electrical energy.

It's all Motor hoists weight, 'brake/clutch' holds weight until needed. Dropping weight turns generator.

But still, you've got to give these guys props for bringing a train into the mix; trains make everything cooler.

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