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Comment by pgroverman | Hacker News Reader

What is preventing this technology from becoming mainstream?

I doesn't matter how much power it generates, what matters is how economically it does so. Add in wear and tear costs in the form of maintenance salaries and new parts, and pretty much anything like this becomes worse than useless very quickly, and that's ignoring the initial costs to manufacture\install and the extra effort people expend to open it! And, eh, you know, I'll let everybody else tear apart their insane power production claims, but, "Each time you pass through the door you generate enough power to make a cup of coffee." Yeah, and I've got a bridge to sell you. ;)

well you are right.. but wear-and-tear is there whether you are harnessing the power or not. it is a sunk cost... it is essentially a matter trapping a bit of that energy...

ok wear-and-tear of the motor is a different issue - perhaps an opportunity for innovation to increase efficiency and decrease W and T?

Physics:

dmd said on December 4th, 2008 at 3:47 pm

Ok, check my math here:

The owners estimate that the door will generate approximately 4600 KWH per year.

Let’s say they’re open 14 hours a day (say, 7am-9pm) for 360 days a year. That’s about 5000 hours.

Now, divide 4600 kilowatt hours per year by 5000 hours - that gives 920 watts.

920 watts - or 1.2 horsepower - from opening to closing, all year. I know Dutch people are (a) generally healthy and (b) like cafes, but somehow I doubt those figures.

I did a quick check on powered automatic revolving doors. The one in the example brochure is probably bigger than the Dutch one but it requires 230V @ 10Amps or 2300 Watts. So, 920 watts to turn a smaller revolving door doesn't seem too outrageous.

http://www.dorma-usa.com/prod/entrance_systems/crane_revolvi...(code)/product_overview/(parent)/29532

Also, it appears that they have hooked a power gauge to the door so patrons can measure the amount of power they generate. So, their power estimates do have some basis.

http://www.expatica.com/nl/articles/news/Revolving-cafe-door...

People are not using it all the time and starting the electric motor to move the revolving door uses more power than it takes to move the door.

Edit: They could have their units mixed up or just measuring the peak output which is not the same as average output.

. Each time you pass through the door you generate enough power to make a cup of coffee.

Ben said on December 4th, 2008 at 4:46 pm

I call bullshit on this - there is no way one person generates the energy to heat a cup of coffee.

Energy needed = mass * temp raise * specific heat capacity = ~0.25kg * ~70C * 4.187 kJ/kgK = ~73kj

If it takes you a generous 10s to walk though the door you would need to be producing 73kj/10secs = 7.3kWatts!!!

According to http://www.ncbi.nlm.nih.gov/pubmed/10090637 the maximum a normal person can produce is ~500W so they are way off.

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