Thermodynamics: Better to Stuff a Fridge ?

[sck5]

What seems to be missing from the discussion is the concept of thermal mass. Some substances can hold more energy than others. E.g. that fake ice they sell to put in your cooler scores higher on this than does, say, air. Green heating systems often incorporate significant thermal mass in the form of rocks, concrete, or water tanks. Yes, it takes a long time to heat or cool them but they also stay cool or hot for a long time.

On a boat, you would want to cool down your thermal mass while plugged in to the dock or while running your engine. Then the box can go quite a while before getting warm again. Almost anything has more thermal mass per unit volume than air does. So a full box will both require more cooling (which should be done when energy is available) and will stay cool longer when energy is more expensive or unavailable.

[Dockhead]

Originally Posted by sck5

What seems to be missing from the discussion is the concept of thermal mass. Some substances can hold more energy than others. E.g. that fake ice they sell to put in your cooler scores higher on this than does, say, air. Green heating systems often incorporate significant thermal mass in the form of rocks, concrete, or water tanks. Yes, it takes a long time to heat or cool them but they also stay cool or hot for a long time.

On a boat, you would want to cool down your thermal mass while plugged in to the dock or while running your engine. Then the box can go quite a while before getting warm again. Almost anything has more thermal mass per unit volume than air does. So a full box will both require more cooling (which should be done when energy is available) and will stay cool longer when energy is more expensive or unavailable.

The phrase was mentioned a few times.

You mention a different consideration, however -- not just total energy consumption, but consumption when energy is abundant.

I think you're right in theory. How do you take advantage of it, though? Do you turn your fridge up on the dock and then, think about it, and go rummage around in the fridge to turn it back down again, when you're sailing? Then back up when you're motoring? I have never known a sailor to fiddle with his fridge like that. And if you don't do that fiddling, then there will be no advantage from thermal mass.

A different story is a eutectic holding plate. This is an extreme example of what you are talking about. Freeze it once or twice a day with your engine, and otherwise you're not putting any energy at all into your fridge. Certainly, there's something to be said for that approach.

[Event_Horizon]

Taken from Permafrost Soluations:

At the moment air cycles (on/off) typically produce a minimum of 12 cycles per hour, and in some open display cases 20 cycles per hour. By using the cube as a cycle control mechanism a reduction of refrigerated cycles can be lowered by as much as 80%. (Example: Dairy Case: air cycles 3 minutes on, 2 minutes off = 12 cycles per hour. With the cube fitted it changes its cycles to 8 minutes on and 7 minutes off = 4 cycles per hour. This example equates to a 66% reduction in starts).

As the start up of a refrigerator compressor uses 3 times more power (i.e. start up amp is 12 amps, run amps is 4 amps) this will result in a power consumption reduction of 16% to 22% (refrigeration is effectively switched on when the food demands it, and switched off when the food does not need it to operate!). This is further explained by the start period which, although very short, nevertheless produces excessive heat into the start winding, and because of the refrigeration process i.e. winding suction, this then has to be cooled by the refrigerant gases (i.e. the heat removed), affecting the efficiency of the compressor, and its energy consumption.

Taken from Fixitnow.com

In a normally functioning system, the current draw when the compressor first starts will be higher than when it’s been running a while. For the first few minutes after the compressor starts, you’ll typically see a current draw of something over 1 amp. Then, as system pressures reach steady state, you’ll see the compressor current draw drop to something less than 1 amp.

^for some fridge running R-134a.

Sorry, I don't have a whole lot of time at the moment to give you a fully satisfactory explanation. Just some fast google snippits.

[cfarrar]

Drink cold beer at anchor. Restock the fridge with warm beer just prior to starting the engine and getting under way!

Seriously, there's a fridge thermostat at Sailor Solutions that lowers your temp setting when the alternator is running, then brings it back up when the alternator's off. I doubt it would pay for itself, but it's an interesting concept.

[Dockhead]

Originally Posted by Event_Horizon

Taken from Permafrost Soluations:



Taken from Fixitnow.com



^for some fridge running R-134a.

Sorry, I don't have a whole lot of time at the moment to give you a fully satisfactory explanation. Just some fast google snippits.

Well, it's interesting. I accept the idea that every startup of the compressor introduces inefficiency into the process. Still no numbers, however, to show how significant the effect is, so still not sure that it's important.

[Dockhead]

Originally Posted by cfarrar

Drink cold beer at anchor. Restock the fridge with warm beer just prior to starting the engine and getting under way!

Seriously, there's a fridge thermostat at Sailor Solutions that lowers your temp setting when the alternator is running, then brings it back up when the alternator's off. I doubt it would pay for itself, but it's an interesting concept.

But that makes perfectly good sense. That sounds like a useful device which probably would pay for itself, for those whose refrigeration causes premature battery exhaustion.

In absolutely practical terms, it is not an issue for us. We can live for 24 hours without a charge on our 440 amp/hours of battery capacity (at 24v), with the refrigeration running away (front-loading fridge and separate top-loading freezer). I can't remember ever starting the genset just because the batteries were low; there's always some other reason. We have Isotherm water cooled compressors, two of them.

[sck5]

I suppose you could fiddle around and optimize somehow. What I do is fill the fridge with all the warm drinks and water first thing while still at the dock. Then if there is any room I throw a bag of ice in there. Then with any luck the drinks are cold and the ice remains frozen for a couple of days without drawing much power out of the batteries while I am sailing and anchoring out. I guess I could freeze bottles of water whenever I am motoring but I dont ever think of it. Not very scientific I guess but the idea is to cool everything down when either plugged in or running the engine.

[barnakiel]

Makes a lot of sense.

Every time you open the fridge the cold air will be replaced with warm air which will then have to be cooled down.

If you have say cold water there, the water stays there and there and it hardly changes the temp, so nothing really to cool down again.

I do not use my fridge as it is only 25 degs cent here, but my guess would be to keep it stuffed up.

b.

[Stillraining]

Originally Posted by Dockhead

I can't remember ever starting the genset just because the batteries were low; there's always some other reason. .

Yep.....Popcorn from the micowave...Ummm!...

[Dockhead]

Originally Posted by Stillraining

Yep.....Popcorn from the micowave...Ummm!...

LOL. Not popcorn, but usually some other item of decadence -- make toast to go with those omlets, run the nespresso machine, run power tools.

For that sort of thing we need an inverter, I admit, which would be much more efficient than cranking up the genset. Once we add some inverter loads then the batteries will need charging more frequently. Yes, I understand that.

Sorry for the thread drift.