Increasing fridge efficiency with PCMs
 

I have a few bottles of water in my fridge for volume reduction/thermal mass, but to take it a step further I've thought about phase change materials. A quick search found a bit of research done in this field. Here a few calculations show that PCMs can reduce energy consumption by 40%. The trick is to store as much "coolth" as possible at night when:

• Electricity is cheaper,
• Outdoor temperatures are lower, allowing better COP by increasing radiator coil efficiency.

For more number crunching check out Phase Change Materials For Domestic Refrigerators To Improve Food Quality And Prolong Compressor Off Time.

I also found another research paper with more good info, but it appears to have disappeared. The title is APPLICATION OF PHASE CHANGE MATERIALS TO DOMESTIC REFRIGERATORS by C. Marques et al., and I found it here. If it shows up again, or I find it elsewhere, then I'll post a summary.

All of this research is very new, ie from the last 2 years, so this field of improving efficiency has some potential.

I tried your link, and it worked, but the paper came in very slowly.

I also searched the same title and got this same-titled paper by different authors.

I know that sailboats have used refrigerators with holding plates filled with a eutectic solution for years, to store cold for greater efficiency.

I don't want to discourage your research & experimentation here, but to my way of thinking, the idea of using a small chest freezer with a different thermostat that forces it to work in refrigerator temperature range is the 'coolest idea' yet. The reported energy use of such a setup is aprox. 25% (or less) of normal refrigerator power consumption.

No reason that both ideas couldn't be combined...

Best,

-AC

Are you talking about doing something like I did last year with my winter fridge assist using jugs of ice?

My testing didn't show a large improvement, although the conclusion basically was that I needed more ice jug surface area to shed the cold out of the ice faster. With 2 milk jugs and 1 juice container I was seeing ~5% reduction in refrigerator electricity consumption.

No, this isn't about getting "coldth" from outside. The PCMs are supposed to do what thermal mass does, ie store the "coldth" that the compressor produced for a longer time. This means that the compressor is on for longer (unless a larger compressor is used, which one of the articles claims would increase efficiency), followed by much longer off-time.

I had a look at the pdf file,without doing a 100% read of it more of a skim really i could not find what PCM substance they were using at sea. I suspect sea water ? as a sea water hose was on the parts list.

My concern with cooling the water storage is the energy needed to maintain its mass. Once its down to fridge temperature it is still going to warm with the fridge although slower it will need a longer cycle to completely cool and require a faster duty cycle to kick in, as the water saps the fresh extra colder degrees the fridge gained with the compressor run time.
So with my overly cautious concern over the energy expended cooling the extra water mass i am going to try sealed containers full of easily if not instantly cooled air.
For my freezer i think the frozen water is a stupendous idea that i do do and may well be best for the refrigerator half as well.
After my fridge gets its baseline run time numbers finished with its new exterior insulation i just did i am going to add air tight mass and see how that effects things. I am sure at that point i will want to add water and see how that goes.

Do you have any plans to try this out?

I'm thinking about it. The evaporation coils in the fridge part aren't exposed, so that complicates things. I have noticed that the water which condenses on the rear wall sometime freezes, so the temperature there is below zero. I might make a small shelf to "insulate" the rear wall with thin packets of water, not yet sure how to work out the details. Not even sure if and when I'll decide this is worth doing. Maybe in a few years I'll change the electricity tariff to be cheaper at night, then this will be worth while.

For my refrigerator
I am planning on using some 12" ceramic floor tiles with 1/2 inch spacers for surface area expansion for a PCM.
I have 10 boxes of tile and some loose that I can scavenge for the job.
With 50 of them in the bottom of the fridge counting both sides of the tile , the surface area would be 100 sq-ft and fair bit of weight.

Here is a link to a PCM company:

Phase Change Material Products, including thermal storage balls

These materials are best used when you have a free resource like solar and you want to get as much storage capacity possible or when you have intermittent power. I would need to be convinced that putting some in a fridge makes sense.

Mike, I think you are exactly right. All phase change materials do is act as thermal mass. While adding thermal mass enables one to keep the refrigerator cool longer without running it also will require you to run the refrigerator longer to get it down to the desired temperature. What you add on one end requires subtraction on the other and does not add anything to the efficiency of the refrigerator. It's basic physics.

If you have free energy from solar then it makes sense to use pcm to maintain the low temperature when the sun goes down. If you don't have a cyclical free energy source you are just spinning your wheels putting in pcm materials.