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noamross
23-02-2014, 02:45 AM
Hi all,

I'm not an expert on this topic but I have an idea for a hobby project that I hoped I could get some feedback on here.

I'm interested in trying to make a pedal-powered ice-cream-and-frozen-drink maker that runs a compressor directly. I'd like to know the feasibility of this. Some back-of the envelope calculations:

- A typical biker can put out about 200 watts (~1/4 hp) continuously
- I'd be aiming to cool about 1 quart of liquid from room temperature to frozen. Given the specific heat and heat of fusion of water, this amounts to change of energy of 120 Watt-hours.
- If the coefficient of performance of the system is 2, then one should be able to cool the liquid in about 20 minutes (120 W-h/ (2* 200 W)).

So this seems theoretically possible, but one would need a compressor and refrigerant system that fits the bill. It would have to be a compressor that could be efficient when run by an external belt/crank giving about 1/4 HP. One can use gearing to increase RPM, but it would be best if the RPM required were < 2000 RPM. And the refrigerant should evaporate at a below-freezing temperature.

Main questions:
- I'm sure I'm missing some major considerations - what are they?
- What compressor would fit this bill? I thought that maybe something designed for A/C of compact car might do. As far as I can tell, all compressors designed for small fridges (or countertop ice cream machines) are hermetically sealed combined motor/compressor units.
- What refrigerant would make sense? If this could be done with propane, it would make the DIY aspect easier.

Thanks for your help!

mikeref
23-02-2014, 07:20 AM
"Legging it" has taken on a new meaning in this post.:p
If i had to provide stable cooling in your situation, i would use a 300 Amp/Hour battery to power a Danfoss 12/24 DC compressor and top up the battery by leg power.... driving an old 35 amp vehicle alternator.
That way you can take a break to catch your breath. :D

Brian_UK
23-02-2014, 07:28 PM
Don't forget that you also need to power a condenser as well.

noamross
23-02-2014, 08:10 PM
Could the condenser be passive? That is, just a set of coils that the refrigerant passes through with a lot of surface area? I suppose a small mechanical fan blowing air over the coils would boost the efficiency a lot without a lot of additional effort for the biker.