refrigeration

Suggestions on keeping the cook happy
mahnamahna
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Location: Gosford NSW

refrigeration

Post by mahnamahna » Fri Sep 17, 2010 6:55 pm

Jim,

You are quite right. I guess once an idea gets into your head its a hard concept to shake. But as you say Murphy will always put what you want at the bottom and I am sure that will get tired very quickly. I am also hoping to have a large solar array and a wind gen so hopefully our lifestyle of reading the papers online in the morning and tv or a dvd in the evening and a front load fridge can be accomodated. If not we will have to get an excersize bike connected to a dynamo and we can keep the batteries topped up that way!!

Thanks for your advise. Paul

Smooth Cruiser
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refrigeration

Post by Smooth Cruiser » Sat Sep 18, 2010 7:42 pm

Because I am an engineer and because it is a quiet Saturday morning I decided to actually quantify the heat losses on opening a fridge door and what it means in running the fridge compressor. Sad I know but it may add some "science" to the discussion . . . .



Most of the 12V fridges use a Danfoss 35 or Danfoss 50 compressor - these are rated at 48W and 78W respectively. Lets use the BD50 compressor for this example.



Let's assume a 150L upright fridge, that is completely empty of food or drink and just containing cold air. If you open the door and dump the cold air out completely and allow it to fill with room temp air again then the compressor will have to work to cool this air volume.



The energy required (Q) = mc (delta T)



m = mass of air = 150L x 1.2 g/L = 180g

c = specific heat constant = 1.006 J / g / K for air

delta T = room temperature of 32 degrees back to fridge temperature of 2 degrees, thereofre change = 30 degrees K (or C)



So Q = 180 x 1.006 x 30 = 5400 J



Compressors have an efficiency of around 50%, for every joule of cooling energy they consume around 2J of power.



So the compressor needs to use 10,800 J of electrical energy.



The compressor is 78W (W=J/s) so this means that the compressor runs for 10800/78 = 138 seconds to cool the air. This is the figure for cooling the contained air, inefficiences like leakge etc exist all the time and put a base load on the compressor - the additional load to cool the air space is the same as running the compressor for just over 2 minutes each time the door is opened.



This was with an empty fridge - if your fridge is half full of delicious icy cold beer then the air volume is halved and the the energy required is halved.



So assume in the average fridge that is just less than half full that the compressor has to run for 90 seconds each time the door is opened. This is fairly conservative given the assumptions used (all air is lost completely etc).



I have tried to envisage how many times I would open my fridge every day and reckon a minimum of 15 and a more typical average of around 40 (it adds up - 2 times for each meal at least, a few times for drinks and snacks during the day and a few more for drinks later).



So the compressor is likely running for around 40 x 90 = 3600 secs = 1 hour just to cool the air volume.



So I reckon an upright fridge runs for an hour a day more than a chest type fridge and therefore uses around 6Ah more each day. This could be provided with a single 75W solar panel in good sunlight for an hour.



Use this information as you see fit (but take it with a grain of salt . . . or with an icy cold beer).



I still like chest fridges because you can fit a lot more in them (although it does require good packing and logistics) and because they seem to use available boat space more efficiently. I still think you could buy a camping fridge with a Danfoss compressor and plates and retrofit them into a custom built chest fridge/freezer box fairly cheaply.

mahnamahna
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Joined: Wed Aug 02, 2006 4:48 pm
Location: Gosford NSW

refrigeration

Post by mahnamahna » Sat Sep 18, 2010 8:53 pm

Ok Smooth now that you have started it, now you have to finish it. :D :D



You have estimated how long an upright compressor will run based on its average usage but to finish the efficiency equation we need to subtract how much a chest would work given the usual usage as per the following scenario.



Assume now a chest fridge packed with 2 baskets and the item you want is in the bottom of the bottom basket. Assume how long it would take to remove the top basket, remove the bottom basket, unpack the bottom basket to get the item you want, repack the bottom basket, replace the bottom basket, replace the top basket. During this time, the exposed surface area of every item is changing temperature (rising) and is therefore stroring heat which will transfer back into the fridge when you pack it all back in. Multiplied by the number of times this would happen in a day.



How much temp is being transferred into the fridge or more appropriately how much running time to remove it.



BTW, the one thing I had not considered was effective use of space. I agree that with an upright you lose a lot of space to the airspace above the things stacked on the shelves, whereae with a chest gravity helps you stack things to more effectively use the space. The flipside of that is the convenience of retaining the useablity that we take for granted in a home style upright.

Jim
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refrigeration

Post by Jim » Sat Sep 18, 2010 9:26 pm

Very interesting Smooth, but you really need to get another boat and channel all that spare time and energy into it instead of calculating stuff.

Jim.

Whimsical
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Location: Fremantle W.A.

refrigeration

Post by Whimsical » Sat Sep 18, 2010 10:01 pm

Smooth

How do u factor in the COP for refridgeration.

I have no idea what the coeficient is for these systems but modebrn aircons are around 3 to 3.5



You realy do seem to have too much spare time on your hands so i second Jims idea. :lol:



Mike

Smooth Cruiser
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refrigeration

Post by Smooth Cruiser » Sun Sep 19, 2010 12:50 am

mahnamahna wrote:Assume now a chest fridge packed with 2 baskets and the item you want is in the bottom of the bottom basket. Assume how long it would take to remove the top basket, remove the bottom basket, unpack the bottom basket to get the item you want, repack the bottom basket, replace the bottom basket, replace the top basket. During this time, the exposed surface area of every item is changing temperature (rising) and is therefore stroring heat which will transfer back into the fridge when you pack it all back in. Multiplied by the number of times this would happen in a day.


I lived on my boat for 7 years and didn't find this a problem - you have to be fairly organised but I could open my fridge, find what I wanted and get it out without too much disturbance at all.

Smooth Cruiser
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refrigeration

Post by Smooth Cruiser » Sun Sep 19, 2010 12:54 am

Whimsical wrote:Smooth
How do u factor in the COP for refridgeration.
I have no idea what the coeficient is for these systems but modebrn aircons are around 3 to 3.5

You realy do seem to have too much spare time on your hands so i second Jims idea. :lol:

Mike


The COP for a small fridge compresser is probably around 2. The COP will rise the closer the temperature differential is so for a fridge with a delta T of 30 degrees or so the COP is a lot lower than for an aircon with delta T of around 10 degrees. The biggest losses are in the electrical motor though whcih basically offsets all the COP gains for the heat pump side. That's my theory anyway. And I agree - i need another hobby other than calculating fridge thermodynamics - i am at work though so at least it isn't a sailing day I am wasting!

Whimsical
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Joined: Sat Apr 28, 2007 8:01 pm
Location: Fremantle W.A.

refrigeration

Post by Whimsical » Sun Sep 19, 2010 5:01 am

Did u factor in the COP in your example or do we have to halve the 10,400 joules and be back to 1/2 an extra hour. I could live with that little bit extra but don't like the idea of a full hour just to cool the air again. I will have to have some serious rules on fridge openings. But then i am planning 1kw of solar so maybe i won't have to care.



Mike

tuskie
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refrigeration

Post by tuskie » Sun Sep 19, 2010 7:31 am

Very interesting discussion. Thanks gentlemen. It seems to me that there are two approaches to the refrigeration dilemma, as with all power useage problems:

(a) Make the fridge really efficient by top opening, thick quality insulation and remote eutectic cooling unit (= expensive fridge + fewer solar panels), or

(b) Lower energy efficient standard front opening, thinner insulated off the shelf unit (=cheaper fridge + more solar panels).



It seems that cost imposition of the efficient fridge isn't covered by the savings of less solar panels, so option (b) seems the way to go. This option is also the least work and the best looking, if there is room for the extra panel.

Smooth Cruiser
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refrigeration

Post by Smooth Cruiser » Sun Sep 19, 2010 4:35 pm

I loosely factored in COP, it is difficult because in a small fridge without a fan you don't get much mixing of air so often the air near the plates is very close to the plate temperature while the air in other parts of the fridge could be quite a lot warmer (more so in a chest type fridge and more so if the fridge is fully packed. In my chest fridge I could just about freeze water at one end next to the plates and have my temperature sensor showing 8 degrees at the other end of the fridge - actually a pretty good arrangement if you stacked the fridge accordingly and had drinks at one end grading through to vegies at the other).



My calcs intentionally err on the conservative side, if the energy efficiency is higher, if the fridge is smaller (150L is a fair size), if some of the air in the fridge is warmer than 2 degrees, if not all the air is lost each time, if the air hasn't cooled right down to 2 degrees the next time the door is opened, if the fridge is reasonably full, if the COP is higher, if . . . , if . . . . then the extra compressor running time would be reduced. I think it would be fair to say that worst case the compressor might run for an hour more, and this would probably not pose too much of a problem for anyone with a decent solar / wind / alternator set up. The choice then might come down more to other factors of packing efficiency / boat space / price / reliability.



I am still a fan of the chest type. 8)

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