I live in Northern Ohio. I have a friend who just built a detached garage, unheated, behind his house. He wants to put a refridgerator in it to store beer for when he’s working on his car. Or just goofing around, away from the wife. 
Query: Since it goes down to zero sometime around here in the Winter, what does he need to do about protecting the fridge? How cold can it get before damage is done to that fridge? And why?
Can he use a small space heater near the fridge on cold nights?
Why not just unplug the fridge when the weather gets colder than proper beer temperature?
Sounds like he’s more worried about damage to the fridge than to the beer inside it.
Yes. Is there danger of the freon or whatever freezing? Is something likely to be hurt by very low temperatures?
The biggest issue would be thickening of the lubrication under low temps, which can lead to increased motor and compressor wear, as well as the potential for seizing resulting in burnout. It won’t, however, freeze solid at any reasonable temperature. Neither will the refrigerant.
My dad has a fridge out in his garage. After keeping it out there a few months, things started to go wrong with it. I’m not exactly sure what. He went to an applicance store and they came out and “winterized” it. I think they added more insulation and did something with the motor. But I’m fairly sure, you can buy a kit or have someone come out and do it. It doesn’t cost much.
Then there’s the question of keeping the beer above the temperature at which beer freezes. Unless the refrigerator has really great insulation, you’d have to heat the ambient air with something like a space heater.
My suggestion is to bring the beer into the house during the coldest months, and unpluging the fridge in the garage. How much time is he planning on spending in the garage when it’s that cold?
How warm does the garage get during a sunny day?
I have an unheated front porch with a beer fridge. Because it faces south and has lots of windows it gets pretty warm on a sunny day. Of course it gets very cold at night and will stay cold if there a few consecutive cloudy days, but overall I never had a problem with the beer or the fridge.
Just about everyone up here in the frozen north have fridges in their unheated garages and unheated camps, and I’ve never heard of any having any problems. We have had both. Our last house had an unattached, unheated garage, and we had a fridge there. No problems. We also have a camp which is heated only by a woodstove, and sits empty most of the winter. The last fridge we had there only lasted about 30 years before it was replaced. The current one is going on 5 years, and is fine.
It’s much colder here than central Ohio, so I doubt you’ll have any problems.
Most folks don’t leave beer in a fridge long enough for it to freeze, 
So, is there any truth to the old explaination about why Eskimos really bought refrigerators? It wasn’t that they were being suckered, so the explaination goes, but that the fridges they bought kept their food from freezing. If it’s true and it worked in colder places…
Most refridgerators don’t have any way to heat the inside. The insulation will prevent (or delay) things from freezing when the temperature drops and if the area the refridgerator is in gets above freezing every few days the insulation may be enough.
There are refridgerators specifically made for unheated areas that can heat the inside to maintain the temperature above freezing – Garage Storage System Solutions & Garage Organization – Gladiator It looks cool also but it does seem a bit expensive.
My furnace broke down while I was away on vacation last winter and after I recovered from slipping and falling on a patch of ice in my kitchen, I noticed that my fridge had stopped running. Once I got the house back up to a normal temp it ran fine. So no permanent damage, but it did shut down. It sounds like it was just because the house was cool enough, but the light didn’t come on; it was like it was completely shut down.