How do they power the fridge in the Going Merry? When the crew (Sanji and Ussop) saw the Arc Maxim powered by Enel's Lightning they were surprised by how the machine worked autonomously without a power source. But then how does the crew power the fridge?
Although nothing related to electricity/fridges is mentioned in text in the Wikia article on Going Merry, you can find the following image there (with a rough Japanese counterpart here):


  


Presumably, this Water-Drawing Machine is used to power the refrigerator. As pointed out in the comments, all you need is a rotary motion source to use electricity at all times.

Although, the crew stores most of their food in the Storage Room, as indicated by this image:


  


You may also note that there are 3 orange trees, taken from Bellamere's plantation, which are used as a source of emergency food: