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Old 03-01-12, 02:15 PM   #1
Piwoslaw
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Default Phase change materials

This thread will be about phase change materials - parafin wax for higher temperature applications (solar, automotive), water/ice for lower temps, and anything else. I'll start with olive oil.

I found this page about using phase change thermal mass in the refrigerator to extend its off time. In this case it is olive oil, which has a freezing temperature only a few degrees higher than water, allowing it store more thermal energy at refrigerator temps than water.

OK, this got me excited, so I read up on freezing olive oil. According to this, olive oil doesn't really freeze because it doesn't have a regular crystal structure. Instead it just becomes more and more viscous. This page explains it even further. So, if olive oil doesn't technically "freeze", but only "slows down", then does it really change phase and store more energy than just changing temperature?

On the other hand, the author of the this page cites some numbers for olive oil's heat of fusion, so maybe it is a phase change? Here's a quote:
Quote:
Water has a heat of fusion of about 100, this means it takes 100 times as much energy to freeze 32 degree water as it does to raise water 1 degree from 32.
The problem with using water for a fridge is you will freeze your food if you try to use the heat of fusion of water to keep cool.
Olive oil has a heat of fusion of 48 but it starts to freeze at 40 degrees Fahrenheit, it also has one half the heat capacity of water , this means it takes 4 gallons of olive oil to hold as much heat as 1 gallon of water( at fusion temp), but it thaws at 32-40 degrees which means it will hold the refrigerator at 32-40 degrees for a very long time.
So I'm a little lost: If I have about 11 liters of water bottled up in my fridge, then will replacing it with a similar amount of olive oil extend its "coastdown time"?

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Old 03-01-12, 03:46 PM   #2
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Olive oil turns solid at a temperature a bit higher than the refrigerator is kept at. How do I know? My olive oil is solid in the cupboard downstairs.
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Old 03-01-12, 10:15 PM   #3
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As I understand it, it takes a calorie to raise the temp of a gram of water one degree but it takes three calories to raise frozen water that one degree to make it liquid, not 100 calories...
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Old 03-02-12, 06:22 AM   #4
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Here are some statistics I found online. I think the calculations are correct:

For water at its normal freezing point of 0 C, the specific heat of Fusion is 334 J g-1. This means that to convert 1 g of ice at 0 C to 1 g of* water at 0 C, 334 J of heat must be absorbed by the water. Conversely, when 1 g of water at 0 C freezes to give 1 g of ice* at 0 C, 334 J of heat will be released to the surroundings.

One joule is 0.239 calories.

0.239 calories x 334 J g-1 = 79.826 calories removed to form ice per gram or cubic centimeter of water
1 US gallon = 3,785.41178 cm3
3,785.41178 cm3 x 79.826 calories = 302174.28 calories to melt I gallon of ice
1 BTU=0.252164401 kilocalories or 252 calories

so 1 gallon releases 302174/252 or about 1200 BTU when it melts but there is no temperature change

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