Building your own fridge or cooler

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Building your own fridge or cooler

Postby GeorgeTelford » Sun Jun 25, 2006 1:51 pm

To give an idea what to do to make an efficient fridge or cooler I am going to run through the notes I have taken over the years, but also explain the why's and wherefore's of each part, because knowing why can help you make informed decisions about each step.

First I will define what a fridge or coolbox is...

A coolbox is an openable heat insulated box, a refridgerator is simply a coolbox that adds some means of removing heat from the inside to the outside.

What are we trying to achieve? to keep certain items cool/cold. It is very important to understand that cold is merely the abscence of heat and that heat is energy.

Consider air at 100 degree C and water at 100 deg C they both have the same temperature but contain entirely different amounts of heat energy
Put your hand in boiling water and you will be scalded, air at that temperature will just be uncomfortable, you may be wondering what the hell this as to do with fridges and coolboxes? actually everything because air contains so little heat energy, it takes very little energy to remove it from the box, however the solids in our fridge/coolbox would take masses of energy to remove.

Lets take a few figures to heat (or cool) 1 KG (1 KG = 1 Liter) of water by 1 degree C takes 1.15 Watts

as an average generally the contents of a fridge have a specific heat capacity of 75% of water

say we are putting warm contents in at 31 C and of course we actually want them to be 4 C (reccomended temperature for fridge)

lets say we have a 60 Litre fridge 2/3 rds full thats 25 KG of food and drink

we have (25 x 75%) X 31 X 1.15 = 668 Whrs

Thats how much power we would need to get the food cool, the 1/3 rd air space takes that little energy to cool its not worth mentioning.

This is why we should always put food/drink into the fridge/cooler that is as cold or colder than required, big energy saver stage 1

Now to show the effects of insulation....

If how 60 Ltr fridge/cooler had 2" of insulation it would allow 320 Watts of heat back in per day, with 3" of insulation that would be 213 Watts per day and with 6" insulation only 107 watts per day for efficiency Insulation is of Major importance....

The maths will come in handy later ( I will be making a spreadsheet up to do all the maths required including the later Insulation and fridge/cooler size maths ) for a cooler it will show how long the contents will stay cool for a fridge it will show the energy required to keep the temperature down.

I will try to do one in Imperial (American weights and measures too, should not take to much converting)

btw although it takes energy to cool things do not forget there are free sources of energy out there, the ambient temerature is also a source of energy....

ImageImage

The Easicool Returns.


Cooling units powered by natural evaporation, clean & eco-friendly.
First built in 1962 the Easicool is an environmentally friendly coolbox,
which works on the evaporative principle,the transfer of latent heat by
absorption. No gas or electricity is required just a cup full of water
added daily to keep the liner moist. The water is then carried around the
inner cabinet & as it evaporates the interior is cooled.
It is not a refrigerator but it will continue to work long after an ice
pack has melted in a standard coolbox.
They are ideal for camping, caravanning, boating & all outdoor pursuits.

The Easicools are independent of any power source & therefore more
versatile than an ordinary coolbox.

Fitted by motor caravan manufacturers as original equipment in the 60’s.

Dimensions (cms) : H51 x W36 x D31
Capacity: 32 Litres
Features: Wine rack & shelf
Price: £65 plus £7.50 for UK p&p


You can buy one here

In a normal fridge a compressed gas is circulated, this absorbs the heat and radiates the heat out of the back.

an Absorbtion fridge (absorbtion fridges were invented and patented by Einstein) like the electrolux 3 way relies on the circulation bought on by heat constantly turning the amonia to gas which circulates absorbs the heat the which is radiated out the back, the amonia is in a constant cycle of turning to gas then reforming as a liquid.

The Easicool works on a total loss evaporative principle, the water evapourates taking the heat energy with it, in hot dry countries this method is still used to keep food cool.

You will notice a significant drop in temperature if you ever stand beside a waterfall, the water is attracting heat energy from the surrounding air.

Another easy test is damp a clean piece of cloth and then breath through it.

Depth of cooling can be up to 12 degrees or so below the ambient temperature.

To see how it works you could place a milk container in a bowl of water, damp a piece of cloth, drape it over the milk container and make sure the bottom edges are in the water in the bowl. Place it in the airing cupboard, when checked several hours later you will find.

1. the water level as dropped
2. the milk will still be cool

There is a fair bit more to come, hopefully thats wetted your apetite a bit....
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Re: Building your own fridge or cooler

Postby sjptak » Sun Jun 25, 2006 6:14 pm

GeorgeTelford wrote:Consider air at 100 degree C and water at 100 deg C they both have the same temperature but contain entirely different amounts of heat energy
Put your hand in boiling water and you will be scalded, air at that temperature will just be uncomfortable, you may be wondering what the hell this as to do with fridges and coolboxes? actually everything because air contains so little heat energy, it takes very little energy to remove it from the box, however the solids in our fridge/coolbox would take masses of energy to remove.


Ok George, now remember, I'm going to be your pupil here. Is that why you would not want to put hot leftovers directly in the fridge, but should allow them to cool first? Another question that I have is that I'm under the impression that a full fridge (or freezer) is more efficient that an empty one. True? I've always used that argument with the wife in that I told her I was helping the efficiency of the fridge by keeping it full of beer.

As I read further, there will be more questions to follow, I am sure. I have no knowlege of anything related to HVAC.

Thanks,
Stan
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Re: Building your own fridge or cooler

Postby sjptak » Sun Jun 25, 2006 6:19 pm

GeorgeTelford wrote:Lets take a few figures to heat (or cool) 1 KG (1 KG = 1 Liter) of water by 1 degree C takes 1.15 Watts

as an average generally the contents of a fridge have a specific heat capacity of 75% of water

say we are putting warm contents in at 31 C and of course we actually want them to be 4 C (reccomended temperature for fridge)

lets say we have a 60 Litre fridge 2/3 rds full thats 25 KG of food and drink

we have (25 x 75%) X 31 X 1.15 = 668 Whrs

Thats how much power we would need to get the food cool, the 1/3 rd air space takes that little energy to cool its not worth mentioning.

This is why we should always put food/drink into the fridge/cooler that is as cold or colder than required, big energy saver stage 1



Sorry. I'll read the whole thing before I make another comment. You just answered my first question. I promise I'll try not to ask any more dumb questions.

With a red face,
Stan
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Postby mikeschn » Sun Jun 25, 2006 6:28 pm

Okay... I too will have to read it again. But the question I have is, how much ice will I need to keep 20 pounds of food cold at an ambient temperature of 80* F if the ice box has 3" of insulation all around?

And if that was too easy, will the thermoelectric cooler element (removed from the Coleman and installed in a lid of this icebox) keep 20# of food at 4*C at the same ambient temperature.

Here's the info on the coleman thermoelectric...
http://www.coleman.com/coleman/images/pdf/5640.pdf

And here's the info on the icebox...
http://tnttt.com/viewto ... ght=icebox

Mike...

P.S. Will your spreadsheet tell me all those good things? ;)
The quality is remembered long after the price is forgotten, so build your teardrop with the best materials...
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Postby PaulC » Sun Jun 25, 2006 7:04 pm

The Easicool Returns.
Limited Batch of 50 to be manufactured very soon.
Units will be £85 each plus £7.50 p&p to mainland UK by courier

Cooling units powered by natural evaporation, clean & eco-friendly.
First built in 1962 the Easicool is an environmentally friendly coolbox,
which works on the evaporative principle,the transfer of latent heat by
absorption. No gas or electricity is required just a cup full of water
added daily to keep the liner moist. The water is then carried around the
inner cabinet & as it evaporates the interior is cooled.
It is not a refrigerator but it will continue to work long after an ice
pack has melted in a standard coolbox.
They are ideal for camping, caravanning, boating & all outdoor pursuits
*From the Easicool web page


The two different types of fridges & freezers
There are two types of fridge available to the mobile user and both types have advantages and disadvantages.

Absorption fridges use a cooling process which is almost completely silent, very reliable, low cost and very effective. Their one big drawback is that, when run on electricity, they consume a massive amount of power - 8 amps per hour on 12V even for a small portable unit. This high consumption is the reason why most absorption fridges can also run on gas. When operated in this way, absorption fridges are very economical to run and are perfect for use in situations where electricity is not available.

Another potential disadvantage of absorption fridges which should be noted is that, in order to work correctly, they must be kept almost level. Different models have different requirements but, in general, 5° of tilt is the maximum that should be aimed for.

Compressor fridges work in just the same way as domestic models. A small D.C. compressor pumps coolant into an evaporator plate and down goes the temperature. Modern compressor-type fridges are so economical on D.C. power that they are increasingly to be found in motor homes and other vehicles as well as large boats where they have long been popular.

Compressor fridges are unbeatable in terms of cooling efficiency, can be used without regard to angle of tilt and are so quiet that they simply cannot be noticed in normal circumstances.

However, although a fridge of this type can consume as little as 1 amp per hour under average conditions, it will still be using well over 20 amps per day. Larger models will use considerably more power and, if the ambient temperature is high or if the fridge is being kept down to an internal temperature of -15C° for example, consumption may reach 4 amps per hour or more depending on the model.

However, in the near future, compressor fridges should become available which use less than 1/4 amp per hour and then this type of fridge will become a practical possibility for most applications.

A bit of info I gleaned from the Internet.
Cheers
Paul :thumbsup:
PS By the way George, the evaporative type fridge you show was originally invented in Oz--The Coolgardie coolsafe
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Postby GeorgeTelford » Sun Jun 25, 2006 7:16 pm

Hi Mike

A good aproximation will be possible. I will assume that 4 Degree C is the temp to keep it below, there is one factor missing from your question how long for.......

The calculation is simple enough it takes so much heat energy to decrease the temperature of water 1 deg C once the coolbox size is established (not forgetting space for the ICE! we then know what the heat loss will be in watts per day, that heat loss (in watts) will be melting the ICE at a set rate, thus we will have a time till internal temperature exceeds 4 Deg C

Unfortunately its early AM here and I dont fancy making a complete hash of the calc through tiredness, I have also got to get all the American temps to Celcuis calculations sorted

The thermo electrics can only achieve a set drop in temperature relative to the ambient temperature, this is due to the suface area there is only so much loss pr sq inch that can be achieved, but as long as the ambient doesnt exceed a certain point yes it can keep the temperature down.

The downside is that at elevated temperatures it will still be drawing full amps but actually heating the inside up not what you want at all.

Yes once I have plumbed the figures in it will tell you vuirtually everything you need to know, I may even had a rough guide to the thermo electric bit.
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Postby mikeschn » Sun Jun 25, 2006 7:21 pm

Yes George, you are right....

I'd like to be able to get by with refilling the ice box just once a day.

But maybe it makes more sense to work it backwards... start with the thermoelectric, and design it so that it keeps stuff cool, and then add in the ice part...

Of course, that's what the goal solver in Excel is for! :lol:

No hurry of course... I'll be designing for the next several months... :designing:

Mike...

Oh BTW, 80*F = 26.7*C
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Postby sjptak » Sun Jun 25, 2006 7:52 pm

I might be wrong, but if I remember correctly, and my memory ain't what it used to be, but I thought I read in the destructions for my Coleman thermolectric that you were not supposed to add ice. I don't remember if it said why, but I thought it advised against it.

Stan
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Postby GeorgeTelford » Sun Jun 25, 2006 7:55 pm

Hi Paul

There are more than 5 types of "fridge", Compresor, Absorbtion, Evaporative, Thermoelectric, eutectic.

Whoever wrote this

However, in the near future, compressor fridges should become available which use less than 1/4 amp per hour and then this type of fridge will become a practical possibility for most applications.

Lets deal with this bit first (bearing in mind that they do not say the fridge size in litres)

It takes a set amount of energy to remove a set amount of heat energy, this is a physical law and cannot be sidestepped.

1/4 amp hour is 3 watt hour note the smallest fridge size in my first post even with 6 " of insulation its losing 107 watts per day, so 73 watts per day cooling will not keep even a tiny 30 Ltr fridge at constant temperature. This is not taking into account that compressors are inefficient and will require 40% more than the calculated amount to do what the maths says

Next

However, although a fridge of this type can consume as little as 1 amp per hour under average conditions, it will still be using well over 20 amps per day. Larger models will use considerably more power and, if the ambient temperature is high or if the fridge is being kept down to an internal temperature of -15C° for example, consumption may reach 4 amps per hour or more depending on the model.

again these are extremely optimistic estimates, even with extreme insulation on a small freezer -15C° is going to take more than 4 Amps

Mike

OK I will factor in one day ice.

Back to the thermo unit

Where is the electric power coming from for the thermo unit?

Even without working it out, a mains compressor fridge is going to use less ampage (than the thermo electric) and the mains will do a far better job. The Coleman thermo is set to use 4amps @ 12v for 37 Litre cooler and a 120 Litre real mains fridge uses less than 3 amps @12v

so a 3 X larger mains fridge uses 25% less power to refridgerate properly regardless of ambient temp, why would you want toi use a thermo?

If you have enough power to run the thermoelectric you have more than enough to run the mains one
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Postby PaulC » Sun Jun 25, 2006 7:56 pm

You're right Stan. Coleman do say not to add ice. The best way to improve a thermoelectric coolers efficiency is to have cool air blowing over the intake side of the vent. Using the a/c in the car, while mobile, is one way of doing this.

Cheers
Paul :thumbsup:
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Postby mikeschn » Sun Jun 25, 2006 8:02 pm

sjptak wrote:I might be wrong, but if I remember correctly, and my memory ain't what it used to be, but I thought I read in the destructions for my Coleman thermolectric that you were not supposed to add ice. I don't remember if it said why, but I thought it advised against it.

Stan


I can see why Coleman would say that. I imagine that a person would rotate it 90* and set it down like a normal cooler. Then fill it with ice. That would then melt and water would run into the thermoelectric unit which is now on the side instead of the top, shorting out the unit, and causing excessive returns to the store for which Coleman would be charged back. Not a pretty situation.

My intent is to keep the thermoelectric unit in the lid of the icebox... ;)

George,

My thought was the thermoelectric unit was small, and it was 12v, and I could build it into the lid of the ice box.

If you can show me how to do the same with something 120v that I can run off of 1 battery, I'm all ears!

Mike...
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Postby sjptak » Sun Jun 25, 2006 8:15 pm

[quote="mikeschn"]I can see why Coleman would say that. I imagine that a person would rotate it 90* and set it down like a normal cooler. Then fill it with ice. That would then melt and water would run into the thermoelectric unit which is now on the side instead of the top, shorting out the unit, and causing excessive returns to the store for which Coleman would be charged back. Not a pretty situation.[quote]

I forgot. There are dumb people out there. Buddy of mine was camped next to a couple that had a thermolectic. Set up Friday nite and plugged the cooler into the cig lighter. Got up after sleeping in late on Sat AM and couldn't figure why all there food was so hot. Bacon grease all over the bottom of the unit. Somebody didn't pay attention to the instructions.

Their car had a dead battery, too.
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Postby GeorgeTelford » Sun Jun 25, 2006 8:36 pm

Hi Mike


My thought was the thermoelectric unit was small, and it was 12v, and I could build it into the lid of the ice box.

All true, but with a major problem 92 Amps a day that needs to be one hell of a battery.

I can hear you thinking only run it part of day

If you only intend to run it part of the day, another big problem rears its ugly head, when its switched off, it becomes a thermal link to the outside temperature which means rapid warming of the interior.

The other reason they dont like ice inside is because the unit is a thermal bridge condensation will quickly form on the thermo unit, its ok to use thermal packs (those sealed blue eutectic blocks)
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Postby PaulC » Mon Jun 26, 2006 2:15 am

George, I've been looking for these 120v or mains thermoelectric jiggers that you mentioned earlier and I can only find 12v units. Where do I go to find them?
New thermoelectric devices, prepped with a temperature cutoff switch. Originally intended for 12Vdc use in picnic and automotive coolers/ heaters. 127 thermocouples per device. deltaTmax=79degC, Thot=50degC, Vmax=16.1V. 40MM x 44MM x 3.3MM. Qmax=80.6W, Imax=8.1A

Cheers
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Postby madjack » Mon Jun 26, 2006 2:46 am

Paul, I don't know specifically just which ones George is referring to but several of the thermoelectric units that I have had in the past came with 120vac adapters to run them, although they were 12vdc units, this includes the one we presently have in our tear...
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