Human-powered refridgerator

[1eyedjack]

Wasn't sure whether to post this in the physics forums or eng. forums. Sort of applied physics, perhaps.

 

I was wondering if any of you kind readers could do some sums to work out how much effort it would be to run a human-powered refridgerator? I was thinking along the lines of a portable one that you could take camping where there is no power supply provided. I did some searching and could not see any on the market, so I sort of assume that it is not feasible, but would like to see it proven in black and white. After all, it wasn't that long ago that wind-up torches and radios were not available.

 

I suppose that the short answer is that it the amount of effort would depend on how good a refridgerator you want. Ie, by how much do you want the internal temperature depressed? You could probably produce one that would reduce the temperature by about half a degree without working up too much of a sweat, but that would not result in a marketable product with profit potential.

 

As regards how much ongoing regular physical repetition is required to maintain a desired internal temperature range, well that would depend on the efficiency of the insulation (assuming you don't go repeatedly opening and closing the fridge door), and doubtless a host of other variables (outside temperature perhaps being a critical one, size of refridgerator and specific heat capacity of contents (say water for this purpose) being another).

 

We could perhaps simplify the problem by assuming a perfect insulator, so that once heat is extracted it stays extracted.

 

Then I guess what I am looking for is a formula that shows how much effort in pedalling (or hand cranking) of a resistant wheel is required to reduce the internal temperature by so many degrees (and subject to whatever other key variables you consider significant).

[swansont]

The coefficient of performance will tell you how much heat you can move compared to the work you are doing.

 

http://www.aie.org.au/melb/material/resource/cop.htm

 

If you use a thermoelectric cooler, the COP will be around 0.5

[1eyedjack]

Ah thanks.

 

I wonder if greater efficiency could be achieved by using human effort to compress air into a bottle which would then gradually dissipate to drive a pump in a more conventional heat engine that uses (eg) ammonia in a reverse rankine loop.

[swansont]

Probably, but that's a bit less "portable" in my book.

[CaptainPanic]

I found it tricky to find the power (peak power and average power) for a refrigerator. But here are a few results: http://hypertextbook.com/facts/2001/SambitMishra.shtml

 

I do know that humans can do about 150 W average power, and about 400 W when doing for example sports.

 

I think that a fridge will easily consume 50W average... but all depends on size and outside temperature of course... To produce 50W constantly is not hard-core sports, but it does mean that you are busy with it a lot.

 

I propose a small wind turbine or a solar panel.

[insane_alien]

To produce 50W constantly is not hard-core sports, but it does mean that you are busy with it a lot.

 

but you don't need to produce it constantly. just average it. you can use batteries to store energy for when you need to be doing something else.

[CaptainPanic]

That is true... fridges don't have a constant electricity demand either. I overlooked that for a second.

 

But 400 W power (while doing sports) is a total. That includes heat... it's not quite mechanical output (to drive a compressor)... So, I conclude that you still need a significant and annoying amount of time to have a cold beer.

 

Then again, there are people who bike and have a beer, and actually enjoy that...

[Externet]

The rate of beer consumption to balance the amount of exercise to keep the beer cold points the camper to better carry dry ice in a polyestirene insulator box, it will be lighter than carrying the human powered generating contraption plus the refrigerator.

 

Many years ago I saw an invention of a chemical jacket for cans or jars that was activated by pulling a string or something, instantly cooling its core.

 

The modern habit of 'wanting' a drink to be cold is a psychologic 'desire' that contributes little of nothing to body hydration compared to not cooled drinks.

 

If you insist, can carry an absorption refrigerator and feed its heat with sun and lenses, -or fire-. A strong mule may barely be enough.

[Mr Skeptic]

A human-powered refrigerator seems like a bad idea. Better use an ice box, or dry ice. For longer trips, use a heat-powered refrigerator. The heat-powered refrigerator uses heat from a fire to heat water that contains a dissolved gas such as ammonia. Hot water can hold less ammonia than cold water, so the result is hot water and compressed ammonia. Then both water and ammonia are allowed to cool. Then the ammonia is allowed to expand, which provides the cooling. Finally, the cool ammonia is dissolved in the cool water, and repeat.

 

Thus, you can have a refrigerator powered by your camp fire.

[CaptainPanic]

Pity that no fridges exist for cars that can be connected to the car itself (other than to the 12 V battery). Combustion engines have lots of heat that you could use.