Economical Refrigeration

[livingstone]

I don't know what the idea behind the water generating its own heat is. Maybe as someone mentioned, the movement of the boat... hadn't considered that.

As far as the water jugs versus air goes, the deal is that air, though a better insulator (less conductive than water), has less mass. Less mass means it takes more energy to heat it up, and acts as insulation with more umph.

That's my take on it anyway. Physics questions intrigue me. Anyone else know if I'm on the right track?

[livingstone]

This is really interesting. Okay, I thought I posted a reply, but I don't know where it went, but I'm revising my thinking. Aside from the fact that there's air leak with the door open, or there's heat loss through the insulation, I think there's really two things at work here that are in question:

First, there really is friction in the water that causes heat from the sloshing. It's negligible, but yes, it occurs. Anytime anything liquid or gas touches a solid, the movement of one against the other is basically "rubbing." It eventually stops moving through that kinetic motion turning to heat. Imagine the amount of energy necessary to stop (or start, if that's easier) a 2 liter of water sloshing: that's how much heat is produced. Exactly.

So as far as the water or air debate goes, air is better, because it is less stuff, (less dense, less mass, fewer molecules further apart) sloshing. So less stuff in there occupying the same space, that keeps air from being displaced when it's opened, is going to be the same efficiency, but less subject to the sloshing/heat generator.

Second: A large part of a motor's energy usage comes from the initial startup, which takes a comparitively huge load than just running. So the more often it cycles, the more energy wasted. Therefore, the more mass in the fridge, the more efficient.

So third: one works against the other, though both are true. So basically, pick a hand, any hand, everyone's a winner, step right up...

[Starbuck]

After being frustrated tangling with Hellosailor wielding my blunt sword of physics knowledge, I've sat on the sidelines, watching two ideas become relatively well-established in the subsequent discussion:

1. Water molecules hitting each other in jugs in your refer. might generate a tiny, tiny bit of heat, but at most it's negligible, and it's really spitting hairs bringing that idea into a discussion about boat refrigeration: it really adds up to zero in this context. (Hellosailor, your argument is technically correct, but applicationally insignificant, here.)
   
   
2. Because of water's relative reluctance to change temperature relative to air, filling empty space with water jugs will indeed save one energy over time, because it will cause fewer compressor start-ups with longer run times each cycle, and systemically that is more efficient. Hellosailor, the idea that "energy in/energy out" of the compartment must match is true, but "thinking outside the box" in this case (again, adopting a macro, instead of a micro view) allows one to appreciate the performance of the overall system.

Those were my only two points. Not that I knew how to express it; I just had a hunch. Through no skill of my own, I'm coming out of this one just a bit justified, it would seem.
</crowing mode>

[dkall]

That is the fan over the condensor. The fan is good but the compartment must have fresh air available or you're just moving hot air over the coils and you're almost back to square one. Make sure you have vents in the compartment you have the compressor and you'll be mush happier.

Fair winds

[Esconditas]

OK..OK... May I have another Idea to add to the mix. Rather than loading up the box with water jugs, Why not use Vodka... think about it... It would help retain the "cold" and when the your net energy use goes up, just take a few drinks and it won't matter any more. And the vodka never gets solid, Just a little thick, goes down a little smoother.

Insulation, Great Stuff to fill the voids outside the box, fans (in and out), don't install the compressor where it's hot, and keep the box as full as you can... Thats my story, and I'm sticking to it... Oh, get the frigoboat speed controller, and install it on your Cold Machine, forget about all of the other frigoboat equipment....

[hellosailor]

Jeff, the business of fewer compressor starting surges is a perfect example of the how subtle "good design" can be, and why a mismatched box or compressor size (too big OR too small) is a bad thing. Or course using two small systems instead of one big one (so you can just run one when there's just a small load) has the drawback of costing money too.<G>

Water isn't reluctant to change temperature, it just has the ability to store more energy, since it is a denser medium under the conditions usually found on sailboats.<G>

I think I can wrangle an EPA grant to buy a Sundeer 72, equipped with six ice boxes of various sizes in orer to have a realistic test bench for more study on this. Probably a USDA grant as well, if I modify the test parameters to explore the more efficient way to regulate Milk. <VBG>

[Sonosailor]

This is an old thread, but had me thinking. Filling space in a refrigerator with water, such that it stabilizes the refrigerator and displaces air that might spill out, sounds like good science, and may or may not be depending on the specific physics you analyse. Me? I would not put an otherwise low value conductor in the refrigerator, especially such that it contacts an outer surface, and conducts cold to it. I would store a flat piece of styrofoam in there instead. It would insulate the particular surface and still take up that air we might be worried about spilling.

We do chill our rum, such that our ice cubes in our insulated glasses have an easier time; but don't fool ourselves in thinking it does any good to the energy budget.