Only using fridge to make ice...?

[DaveInDenver]

Sure, if you are only factoring in using some sort of cool pack and could use a set temp of 27F that would be different, but the OP seemed to want to make ice as well. I can't remember what temp I had to set my fridge down to for it to actually freeze ice cubes/bottles, but it was something like under 15F, maybe closer to 5-10F. Either way, I don't think it would be an efficient solution.

@rruff is the OP.

 

[IAm_Not_Lost]

@rruff is the OP.

Hahaha. Of course he is. Weird this forum doesn't highlight the OP when they make a response. At least that I could see. Been awhile since I've been on here.

 

[shade]

Weird this forum doesn't highlight the OP when they make a response.

It is. I'm not sure why that isn't possible, or the feature hasn't been turned on.

 

[billiebob]

changing water to ice consumes copious amounts of energy as the water changes state
you use much less energy just keeping food cool

 

[rruff]

Sure, if you are only factoring in using some sort of cool pack and could use a set temp of 27F that would be different, but the OP seemed to want to make ice as well.

I *am* the OP. I don't want to make ice, just freeze water in packs. And the main motivation is not overall minimization of energy consumption (this will probably be a small hit), but creating a buffer when the sun is good and I have an excess of solar, that greatly reduces the needed battery capacity (~1,000W-hrs or so) to deal with cloudy/rainy periods.

I wonder how cold the freezer would need to be, to freeze the packs at the unit's capacity. Looks like ~80W is the cooling capacity at -5C. With some geometry and heat transfer coefficients I should be able to make a decent guess on whether that's enough.

I'm hoping you try this. As Deming said, "In God we trust, all others must bring data.

I prefer to learn from others mistakes (or successes) whenever possible If I go this route it will be awhile I think so don't hold your breath!

 

[rruff]

changing water to ice consumes copious amounts of energy as the water changes state
you use much less energy just keeping food cool

The amount of energy consumed in freezing is exactly the same amount you get back when the ice melts. No loss.

 

[shade]

And the main motivation is not overall minimization of energy consumption (this will probably be a small hit), but creating a buffer when the sun is good and I have an excess of solar, that greatly reduces the needed battery capacity (~1,000W-hrs or so) to deal with cloudy/rainy periods.

Ah, that makes more sense. Instead of storing cooling capacity in a battery, you want to do it with ice packs when you have a surplus of solar power available. I'd use a bigger battery and/or utilize a secondary charge source, but I can better see what you're considering now.

If you give it a try, I hope you'll let us know how it works out.

 

[rruff]

If I do it I will. The other major contender is no refrigeration at all. I did that for 13 years fulltime and prefered the simplicity over anything I might have been missing. I have more space and $$$ now, but... on a recent 2 week trip my wife didn't even seem to care. We took a crappy cooler, but after the initial ice melted we didn't bother to refill it. She might have a different opinion after years, though!

 

[dreadlocks]

We do that simply by just adding water bottles to the fridge as we empty it when the solar inputs are good, I dont freeze em because I dont want everything in the fridge frozen but they are a great mass buffer and once they are chilled they keep power consumption minimal.. then we can drink the water.. I hate bottled water but this is one of my exceptions, its too convenient and having well chilled water is really nice, might drink a few bottles each outing but most of em are kept in trailer as backup water and fridge mass.. We'll also toss our water bladders in out of our hiking packs, that feels great on a hot day.. Ive got a giant fridge (80qt Engel) and if its just a short weekend I cant even fill the fridge up so its got a bunch of pre-frozen water bottles, and the fridge dont really use any power all weekend.. if I go out for 2 weeks half of the fridge is pre-frozen meats and it still uses pretty minimal power if I really need too.

 

[DaveInDenver]

I prefer to learn from others mistakes (or successes) whenever possible If I go this route it will be awhile I think so don't hold your breath!

Exactly, why I hope *you* try it...

 

[DaveInDenver]

We do that simply by just adding water bottles to the fridge as we empty it when the solar inputs are good, I dont freeze em because I dont want everything in the fridge frozen but they are a great mass buffer and once they are chilled they keep power consumption minimal.. then we can drink the water.. I hate bottled water but this is one of my exceptions, its too convenient and having well chilled water is really nice, might drink a few bottles each outing but most of em are kept in trailer as backup water and fridge mass.. We'll also toss our water bladders in out of our hiking packs, that feels great on a hot day.. Ive got a giant fridge (80qt Engel) and if its just a short weekend I cant even fill the fridge up so its got a bunch of pre-frozen water bottles, and the fridge dont really use any power all weekend.. if I go out for 2 weeks half of the fridge is pre-frozen meats and it still uses pretty minimal power if I really need too.

I have several HDPE Nalgene bottles that get used to fill space, provide thermal mass. The rectangular ones pack easier than typical outdoor recreational round ones. You see them sometimes at outdoor stores but the larger ones are usually considered lab ware. They come in lots of sizes up to 64 ounces and they even make 5, 10 and 20L monster carboys if you want to fill lots of space.

 

[dreadlocks]

thats a damn good idea, Ive got a handful of those same small nalgenes full of various soaps (dish/hand/body/laundry) because I got sick of em exploding open w/altitude changes, those never made a mess in the last decade.. never occurred to me to look for bigger ones, they do pack very nice w/those square shapes.

seems better than ice packs IMO, at least all that weight/mass can be used as backup drinking water.. most ice packs have glycol in em to lower freezing temps so u wouldent wanna drink em heh.

 

[john61ct]

Yes tuning the eutectic solution so that you get the polar tubes **just** past the phase change temp (freezing point) is the key to effective energy storage.

But the extra space required is not small in a mobile context

the hysteresis range of the target contents much wider, so less conducive to the consistency required for food safety

and total energy required **much** higher than just using a good refrigerator.

If truly the majority of the energy used is indeed "free surplus" then fine and good

but just expanding the lithium House bank size is usually a better solution.

 

[dreadlocks]

agreed, I dont really care about energy storage now w/Lithium density.. most of my fridge were habits picked up when I was trying to scrape by with minimal battery/solar.. with 30lbs of LFP Ive now got an energy surplus and the idea of drinking anything warm seems ludicrous heh.. now just having near frozen drinking water is easy to provide and still have enough extra energy I said f'it and got a dedicated ice maker for use w/harder drinks.

Just having a battery bank that soaks up every amp of solar you can output changes the paradygm, instead of using fridge as an energy buffer to capture unused solar when my FLA bank is absorbing I'm looking at, what big appliances can I run with all these unused amps once the battery is charged? Tool Chargers, Ice Makers, Electric cooking, Electronic Toys are nearly free to operate in decent solar conditions.. fridge looks feeble now days with an incredibly efficient LFP in the heart of things.. however it was anything but cheap.

 

[rruff]

We do that simply by just adding water bottles to the fridge as we empty it when the solar inputs are good

That will help a little. Lets say you have 20lb (9 kg) of 35F (2C) water in your fridge when the electricity stops, and your heat loss rate is 250W-hr/day (10.4W). The thermal capacity of water is
4.18 J/g-C = 4.18 W-sec/g-C 10.4/4.18 (W-g-C/sec) /9000g = .000276 C/sec = 1.0 C/hr = 1.80 F/hr. After only 5.5 hrs the temperature in your fridge will have gone from 35F to 45F.

20lb of ice on the other hand will take 81 hrs to melt (computed in post #23) and at a constant temperature.