Hacker News Reader

Comment by kenned3 | Hacker News Reader

I've actually considered this many times, but the math never works out. I ride 50KM 3 times a week, Strava's "estimated power rating" is all over the place, given i ride the same trail every time i would think it should not be like this.

Lets be reasonable and say 160W (somewhere between all the ratings strava gave) avg for my 2 hour trip.

I think I am a pretty strong rider, at the peak of the season I can beat electric bikes on flat ground as my top speed is higher then theirs (I dont have a speed limiter, they do)

I can tell you this is NOT easy and once you realize how much physical effort it is you appreciate being able to plug things into the wall.

it is the sustained power output that will kill you.. You can probably dump 10w of power for a long time, but putting out serious power for extended periods of time isnt easy.

View on HN

The math never works out, by a long stretch. Electricity is just too cheap and humans too weak.

“One kWh” sounds like a tiny amount, but it is 3,600,000 joules. If you want to store that as potential energy, you must (taking g=10m/s²) lift a mass of 360,000 kilograms by a meter.

Another way to see how large a kWh is: a human body, ballpark, needs 3kWh per day (125W an hour)

⇒ If you want to produce 3kWh a day, you’d have to eat twice what you normally eat.

And that, even ignoring heating and airconditioning, doesn’t power an average household in the western world.

To put things even more into perspective: The average American uses about 10kW continuously on average.

(Primary power consumption in the US is about 10^20 Joules per year, that's around 9.7kW/person).

This x1000

I actually built a bicycle generator (an AC motor with a belt around the rear wheel) and would sell organic smoothies at music festivals. The fun part was the buyer had to pedal the bike to generate the electricty to power the blender to make the smoothie.

Even with only a 150w blender, fit adults had to work HARD to get the smoothie.. well.. smooth.

Kids basically couldn't do it, or not well enough.

And that was about 20-30 seconds of pedalling.

I wonder how much more efficient it would be to mechanically hook the pedals up to the spinny part of the blender?

A lot more efficient. This sounds like a novelty, but in reality, bicycles are for endurance more than sprint. Using a battery as a buffer would enable a customer to pedal until the battery is charged enough to blend a smoothy. That could be a slow 20 minutes cycle out a 5 minutes sprint.

shark tank 2 years ago https://www.cnbc.com/2019/01/20/shark-tank-entrepreneur-pitc...

A lot. In fact most "pedal smoothie machines" you see work that way.

But I wanted to build it the way I did to be educational as well. When kids struggle after 15 seconds I say "You'd have to pedal a LOT harder continuously to power your xbox. That really gets them thinking.

There was a video of an Olympic cyclist powering a toaster with a bike. He wan't able to keep it up enough to toast one piece properly.

I was super in to cycling a while ago and I was only able to average 200w over an hour but thats not quite accurate because I was going super hard and then taking rests every 20mins which stops the clock.

This is the video: https://www.youtube.com/watch?v=S4O5voOCqAQ Spoiler: the toast gets toasted

That’s a bad demo, as toast takes high power for a very short time.

Just about any human in decent health could probably cycle long enough (with effort) to charge a battery to provide the requisite energy (over the course of, say, 30 minutes) and then output that energy at high power to toast over 3-4 minutes.

I don't think thats much of an improvement. If it takes you 30 minutes to power a single appliance for 3 minutes its not a very useful idea.

As someone that bikes with a power meter, looking at my power curve for some rides: I've held 300W avg for 20 minutes, 200W avg for 3 hours and 160W avg for 9 hours. These are from races, a steady state effort could probably be a bit higher.

I think that may be some usable numbers for what an avid amateur cyclist can achieve (ftp ~4Ws/kg). And then one will have to take into account that the body efficiency of cycling is about 25%, so one will have to eat 4x that amount in calories...

Indeed. To put some numbers to this, here's an account of a 90 km ride I did several years ago:

I was roped into a team entry for a Half Ironman. Despite being more of a runner, I did the 90 km bike leg which was effectively a solo "time trial" (no drafting allowed) on a standard road bike with clip on aerobars. After averaging 35.0 km/h on the mostly flat (~600m elevation gain) course, I was exhausted. The SRM Powermeter I borrowed for the race said I had averaged a "measly" 215 W!

https://www.cyclinganalytics.com/blog/2018/06/how-does-your-...

I haven't done the maths but I have a feeling the jellys you need to eat to sustain that power output marginally cost more than the electricity you would consume otherwise.

uhm. is there any device that converts that low power (say 60W) into something higher? or would it be more dangerous?

Watt(energy) = Volts(voltage) * Ampere(current)

e.g. USB phone charger rated for 5V/2A = 10W, laptop adapter for 19V/65A = 1.2kW, electric heater rated 110V/15A = 1650W

Voltage and current are like height and width of a 2D box named wattage or energy, and you can stretch or shrink the height thus changing width, but not the total area

Now way a laptop uses 1200W. More like 6.5A -> 120W

yea, mobile devices are limited by the ammount of thermal energy they can dissipate. most laptop cooling systems top out at around 30W, anything more is peak load or charging.

my i7-6600u idles at around 3W

Sure, a battery does that. You could charge a battery slowly with 60 W for an hour and then discharge it quickly, powering a 1000 W device... but it would run out in a couple minutes.

Reply on news.ycombinator.com