The Power of Curiosity

On a brisk November day in 2017, I entered the south-facing stairwell of Union Square’s subway station in NYC. It was peak rush hour with commuters bustling home from work.

Rifling through my pockets to locate my MetroCard, I found myself staring at the row of turnstiles. Swipe, beep, shling - The 6 turnstiles barely had time to stop their rotations before the next passenger was gliding through. Swipe, beep, shling. A glazed metamorphosis of spinning steel was occurring in front of my eyes. The rhythmic 3-pronged turnstiles were looking like miniature windmill turbines.

Is it possible that those turnstiles could generate(or recover) electricity as they rotated, much like a wind turbine? Why wasn’t the subway system capturing this energy generation? My mind hummed with growing curiosity the rest of the way home.

The next day I explored my subway turbine concept further with some quick back-of-napkin math. According to NYC transit data on the previous year, 6 million riders took the subway each weekday. So combining entering and exiting, that is 12 million rotations. Assuming the majority of those riders took the subway back from work as well, that would make 24 million rotations. Hmm could we get to 48 million rotations by making them double-sided in some way?

Out of my expertise jurisdiction, I emailed two accomplished professionals in the energy space for their input.

David T: Mhh, these sorts of Human - energy recovery systems are interesting. For instance, some gyms have included them. When you pull weight you do work, and energy = work * distance.

Essentially, humans are like batteries that store energy (a bit like the matrix), and we could try to get this energy back. Calculating the electricity out of such a system should be very easy to do, I'll ask my electrical engineers to crunch a few numbers. I am inclined to think that there will not be much power out of it. Mainly because Power = Rotational Speed * Torque and either one or the other has to be high. For instance, very large wind turbines rotate very slowly but have very high torque, whilst gas turbines will rotate very quickly but with little torque.

Will let you know what the numbers are :) - but you are thinking in the right direction, energy recovery is an important and growing business.

John W:

That’s actually a very creative idea. I am trying to figure out how to transfer the energy being generated to a useful form of energy. It is such a low-grade intermittent form of energy I think it probably needs to be stored (battery) so it can be built up to a higher grade and then used. I’ll have to ask a few engineers to see what they come up with though.

David T:

I agree with John that it's a creative idea. However, I feel you will not extract much energy from it. I'll give you an example: If we take an extreme athlete like Lance Amstrong, he can produce cycling, with his strongest muscles about 300Whr. From Wikipedia:?An adult of good fitness is likely to average between 50 and 150 watts for an hour of vigorous exercise.?Realistically the energy we use to rotate the subway doors is not vigorous exercise, so I would say 25 watts per hour.

In a day based on your assumptions, you would produce 25 watts x 48 million (rotations) which = 1,200 million watts. That's a pretty decent amount of energy, however, it will be produced only during the time the rotations are made (i.e. you do not rotate the doors continuously for 1 hr). Power is a unit of energy over a period of time (i.e. watts or as you see in your bill, kW hr). We can assume that it takes us about two seconds per rotation. So in other words, you are able to produce 1200 million watts over two seconds.

You want to be able to produce that power continuously to give it any $$$ value. As an example- if you spread this power over one hour, you would produce 1,200 million watts x 2 seconds / 1 hour = 1,200 million watts x 2 seconds / 60s x 60s =600kWh.

Essentially, you would produce about 600kWh of electricity. The average price you pay in the USA for electricity is 12p/kWh so you would get about 600x12p = $72. This is just fast number crunching and I may have missed something :)

Present

Watching the recent acceleration of innovations and progress in the energy industry reminded me of this 2017 story. While I didn’t pursue my concept further, feeling a little intimidated at the time by its scope, I am excited and appreciative that others have taken on the challenge. I am particularly impressed by the product work being done at companies like Arcadia, Sense, Patch.io, and Span.io.

It's so easy to let interesting visions, ideas, or questions be fleeting. In a world of convenience, taking the moment to explore an idea, even modestly, can feel difficult. But it isn’t. One quick email can start a chain of thought-provoking, insightful discussions.

The next time an interesting thought comes to mind, be curious. Pull the thread with a few follow-up questions, you never know where it will lead you.

-Alex Stroud