Exercise bike that can store 2KWh of electricity : technology

[–] [email protected] 75 points 6 months ago* (last edited 6 months ago) (4 children)

TLDR: It's a great concept but it's about 100× more exhausting than you imagine.

I absolutely love stuff like this, and I also love cycling.

However, there is a big caveat here: I've been cycling for years and know my own power output:

Output -- Time Window -- Heart Rate

1400 W* | 60 seconds | 208 bpm
385 W | 20 minutes | 162 bpm
148 W | 6 hours | 110 bpm

*(yes, I know. My thighs are larger than some people's torsos and it scares me too)

That means that on average, around 13 and ½ hours of pedalling to charge this thing. (2 KWh is also worth $0.68¢ where I live at standard residential rates).

Humans are not great at converting their physical and thermal energy into kinetic or electrical (20-24% for most bodies).

Pedal power is amazing for things like charging phones or powering small devices and computers though, or for something completely meta: Charging up a eBike or electric scooter (120W), to then use without pedalling later.

Which then begs the question -- if our "human/person power output" is like 150 watts constant .. and the sun provides 1.4 kW/m² of energy -- why not just lay down a 150W photovoltaic solar panel ($89) in the sun and sip on some unsweetened iced tea instead?

[–] [email protected] 16 points 6 months ago (3 children)

While what we can generate is negligible. If, for example you had these in all gyms, that's generating something. Not a lot, but more than nothing. Also all houses should have batteries and being able to remove the cost of a phone charge from what you suck from the grid would be nice.

[–] [email protected] 17 points 6 months ago (1 children)

Wasn't that the second episode of Black Mirror?

[–] [email protected] 4 points 6 months ago

True, and the ads are everywhere too.

[–] [email protected] 11 points 6 months ago (2 children)

It's still unlikely to pencil out to do this given the opportunity cost of actually going through the effort of building, buying and connecting these things, to be honest.

I'm fascinated by the idea but it's important to remain realistic.

Here's a good article on the concept of bicycle generators: https://solar.lowtechmagazine.com/2022/03/how-to-build-a-practical-household-bike-generator/

Personally, I think the best application of this concept is probably direct use of the mechanical energy, without converting the energy to electricity at all. See the bicimaquinas-concept: http://www.mayapedal.org/index.en

One of the wonderful things about bicycles is how extraordinarily efficient they are - very little energy is required in relation to how much transportation work you get out of it. This works against us in the case of power generation, though - little power going in means little power going out.

[–] [email protected] 2 points 6 months ago

I love the bicimaquinas.

[–] [email protected] 2 points 6 months ago

There's not enough focus on direct energy usage or storage in general. If you want thermal energy, collect and store thermal energy. If you want mechanical energy, use that directly (and I guess compressed air and hydraulic head count as mechanical energy storage).

What I think would be cool for an exercise bike is to just have a power takeoff of some sort. Lots of bikes use a flywheel already, but even if they didn't, but you could hook up a PTO to a flywheel or a charger so that in a pinch, you could charge your phone or whatever. Probably wouldn't want to use it if you had a better option, but nice to have in an emergency. Like those wind up flashlights.

[–] [email protected] 9 points 6 months ago (2 children)

It's a really good business idea that I've had more than once -- a self sustaining "green gym".

Members would leave their electronic devices in special hardened lockers, plug them in, and then go to work out, and the power from central battery bank would then charge everything that's in the lockers.

I also had an idea for credit system where the more power you generated the cheaper your monthly bill/subscription would be for the gym (only a few bucks here and there), or some kind of perks, like a free t-shirt once a year if you generated over 1 MWh (1000 kWh) or whatever.

The power generated by such a place would probably be negligible but it would give people the same emotional high as other pro-social tasks.

[–] [email protected] 11 points 6 months ago* (last edited 6 months ago)

I saw a report on a gym with the idea that the exercise equipment would power the gym. It wasn't a good business model. They brought the reporter in and only turned on some of the lights - they had to be super stingy with electricity to get anywhere near net positive energy generation.

[–] [email protected] 2 points 6 months ago

I've also had this same idea more than once.

I think the power output of the earlier commentor is probably for a "basic" fit person. But what about those huge bodybuilding hulks? How much is the power output when they're pulling maximums for a day?

In the best scenario, there'd actually br extra electricity to sell back to the grid, and the gym might be free for some of the "power-users".

However I think as the earlier dude points out, stable power output isn't that high and probably way more reliable to just put down solar panels, and a whole gym worth of generative weight lifting equipment would probably cost quite a lot.

[–] [email protected] 9 points 6 months ago (1 children)

1400W for a minute is insane. You are either a pro track cyclist, or you should quit your job and become a pro track cyclist!

[–] [email protected] 8 points 6 months ago

Haha, I wish -- they call it the hour record for a reason!

I've got explosive short-term speed, but it doesn't last, and I get dropped within 20 minutes, as my power output dips below the optimized pros who can do 400W constantly for entire Tour stages.

I appreciate the comment though!

[–] [email protected] 4 points 6 months ago

If you look at the "about us" section on their site, I don't see anyone with human performance expertise (though who knows if any of them are hobby cyclists with a huge depth of knowledge). The seat and pedal design is definitely geared more towards aesthetics than performance, too. I probably wouldn't want to do more than a hundred watts on there.

I definitely appreciate that the portability and aesthetic quality is a major design consideration for them, though. I see this more as a battery pack that you can pedal, than a bike that stores power.

[–] [email protected] 2 points 6 months ago (1 children)

Pedal power is **amazing** for things like charging phones or powering small devices and computers though, or for something completely meta: Charging up a eBike or electric scooter (120W), to then use without pedalling later.

Skip that step and get a chainless digital drive bike. Same concept but you charge it as you ride.

[–] [email protected] 4 points 6 months ago

With abysmal conversion efficiency

[–] [email protected] 20 points 6 months ago* (last edited 6 months ago) (1 children)

3,442 Calories (14,388 kJ) is required to generate 2 kWh at a (generous) overall storage-work-storage efficiency of 50%.

For comparison, that is roughly:

one pound of body fat
a marathon
a full day of cycling

[–] [email protected] 6 points 6 months ago (1 children)

Rule of thumb efficiency of humans is about 25%, which is about the kJ to Calorie conversion, too

so 14kJ of energy output burns ~14,000 Calories.

[–] [email protected] 3 points 6 months ago

True, I only included kJ equivalent because some countries use kJ on nutritional panels instead of Calories. Actual conversion rate of human energy stores to usable power is, at a guess, more like

Calories * 0.25 * 0.6 * 0.8 - dt

where t is time, d is the self-discharge rate of the battery, 0.6 is the efficiency of the generator, and 0.8 is the efficiency of the battery.

So, user better eat their wheaties.

[–] [email protected] 12 points 6 months ago* (last edited 6 months ago) (1 children)

Love things like this, but at 3.500€, that's a massive ask. Hopefully the concept catches on.

[–] [email protected] 11 points 6 months ago (2 children)

2kwH is worth about $0.18 here. I'd have to charge it ~19,500 times to offset the cost in electricity.

[–] [email protected] 8 points 6 months ago

So only about 30 years of constant pedaling.

[–] [email protected] 6 points 6 months ago (1 children)

It's not about saving money. It's about using the energy that would otherwise not be used, and incentivizing exercise. If I could keep my phone charged from exercising, that would be cool. But yeah the price point is quite high.

[–] [email protected] 2 points 6 months ago

Most people could probably never even break even in the cost of energy that it takes to produce one of these things.

[–] [email protected] 6 points 6 months ago (1 children)

A fun toy. But a normal exercise bike already converts the energy to heat. This heat then warms up the apartment.

Of course, with electricity, a heat pump for heating or cooling could be used, which is more efficient. But it's not like the energy remains unused with a normal exercise bike, as long as it is cold outdoors.

[–] [email protected] 3 points 6 months ago

But sometimes it's hot outside 😓

[–] [email protected] 4 points 6 months ago

They need to put these in all of the gyms in the cardio equipment. Just think of the power they could store.

[–] [email protected] 4 points 6 months ago (1 children)

I always wondered why are they not generators as well. I mean, free electricity right there, however not much there is.

[–] [email protected] 3 points 6 months ago

Especially for gym owners

[–] [email protected] 3 points 6 months ago (1 children)

I've been wanting this for years. But maybe not at that price.

[–] [email protected] 6 points 6 months ago* (last edited 6 months ago) (2 children)

As it seems to double as a balcony solar inverter if you add some PV pannels the price seems to be not so bad. A 2KWh LiPO4 battery, charge-controller and inverter alone will cost you at least half of the asking price.

[–] [email protected] 6 points 6 months ago

Maybe making a bike for half the price that can just plug into an existing inverter / battery would be more versatile then.

[–] [email protected] 2 points 6 months ago

That definitely changed things a bit there.

[–] [email protected] 2 points 6 months ago (1 children)

I'm pretty sure this was the premise of a Rick and Morty and a Black Mirror.

[–] [email protected] 1 points 6 months ago

Peace among worlds 🖕

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