Just for fun: treadmill air conditioner efficiency calculation

[DynV]

What approximate portion of generated heat from being active on the treadmill, both the person on it as well as the machines themselves would be compensated/cooled if the treadmill resistances would be disabled/removed and instead 1/2 end cylinders would be connected to an air conditioner compressor? I'd like both machines used in this playful consideration to be the best (or amongst the top) average priced widely available one (so not a bad machine nor a difficult to obtain good one).

 

[DaveC426913]

I think the most obvious and very short answer is that air conditioners are very inefficient converters. They take a LOT of power and mechanism to cool just a small amount.

Witness the fact that there is no such thing as a "Micro Air Conditioner". The smallest AC I've ever seen is about a cubic foot in size and weighs about 40 pounds. I guess those little beer coolers are smaller and lighter, but they're really bad at cooling things down; they're better at keeping previously cool things cool.

But one wonders why no one has ever thought to attach a human-powered fan to a treadmill...

 

[berkeman]

But one wonders why no one has ever thought to attach a human-powered fan to a treadmill...

A rowing machine that I used to use at a previous workplace's gym had a built-in fan on it for a load, pointed at the rower.

 

[jbriggs444]

I think the most obvious and very short answer is that air conditioners are very inefficient converters. They take a LOT of power and mechanism to cool just a small amount.

A decent air conditioner will run an EER of between 11 and 13. Call it 12. That is 12 BTU/hour cooling per watt of input power.

Human power output can vary. Peak mechanical power output in the 300 to 400 watt neighborhood is possible. But 100 watts is more accurate for sustained output. If we hook a 100% efficient generator to the treadmill, that means that we can get 1200 BTU / hour of cooling.

How many watts of cooling is that?

1 btu per hour =
0.29307107 watts

So from our 100 watts of input power we are getting $1200 \times 0.29307107 = 350$ watts of output cooling. That is not a "very inefficient converter".

However, the human body is not a particularly efficient machine. A ball park figure is 25%. In order to generate those 100 watts of output mechanical energy, we burn 400 watts of food energy and produce at least 300 watts of waste heat to be dumped via the 350 watts of air conditioning.

That sounds pretty darned close to break even in terms of energy.

In terms of economics, it is a non-starter. At ten cents per kwh, you are buying yourself a penny per hour. $7.20 / month if you run the thing 24/7. Then you have to worry about condensate, cleaning, maintenance, freon leakage, initial expense and the fact that the resistance is not as smooth as something like magnetic eddy currents. And you still have to plug the thing in because goodness knows the customers will want their animated LED display and fancy programs.

The exercise bike in my home cost about $350 when I bought it. Google says $500 or so today. It dumps energy with magnetic eddy currents and has a little one or two amp power supply to run the LED display and manipulate the magnets.

 

[russ_watters]

A decent air conditioner will run an EER of between 11 and 13. Call it 12. That is 12 BTU/hour cooling per watt of input power.

I was thinking the same. FYI, that's a good average of older units and window units but today the minimum allowed by federal law for residential split systems is 13 SEER (EER averaged over a season), and new high efficiency ones can up to around 20.