Increasing fridge efficiency with PCMs

[Exeric]

I guess I should add that there might be one benefit of adding a PCM without solar. If your fridge has short periods between cycling on and off it will lengthen those cycles, both in ON and OFF modes. That would be good and would save a little bit of money by lengthening them. But it wouldn't save much unless the fridge is already doing a bad job. In most cases I think it would make more sense to just buy a better one that has more insulation and is more efficient.

[Mikesolar]

A m3 of water can store about 48kwh of heat (10C to 50C). A proper PCM system can allow that m3 of water to store 100-125kwh of heat from the same volume assuming that you design the paraffin to change state at the right temp, perhaps 45C. That way, you could use a 400L (100usg) instead of 800-1000L tank which is way more expensive and hard to get through a door. Time to get some stuff for the open tanks.

[ecomodded]

When i added the 10 gallons of water jugs to my fridge it ran and ran for hours on end.
But then all was quiet, the water was up to temperature and no longer required the long charge cycle. My fridge thread has the gains noted, and they were gains, it does work, to the tune of 10% increased efficiency.

[Exeric]

If it works for you then more power to you! I can't explain your results from a physics standpoint.

[ecomodded]

You should study up on the subject Exeric.
It not only works but it makes perfect sense why it works.
Thermal storage likes Mass ! not empty space.

[AC_Hacker]

Originally Posted by ecomodded

You should study up on the subject Exeric.
It not only works but it makes perfect sense why it works.
Thermal storage likes Mass ! not empty space.

Thermal storage does not 'like' anything.

It is inviting flawed thinking to assign a human emotion to a physical process.

You are better off studying the behavior of vapor compression machines to search for an answer.

When a vapor compression machine begins it's cooling cycle, there is a period of time at the beginning of the cycle, when the vapor & liquid components are coming into an equilibrium. This includes the heat that is given off by the compressor, and the temperature changes to the HXs as it runs. The period before equilibrium is reached produces less efficient work that the period after equilibrium is reached. This period of inefficiency is an unavoidable part of each cycle. When you load the refrigerator up you are increasing it's thermal mass, and especially if you use a material that goes through a change of state at a temperature that falls between the Start Temp and the Stop Temp (I don't think an appropriate PCM material has been chosen yet. Olive oil might be a better choice.). In so doing, you are reducing the number of run cycles that occur over a period of time. and you gain efficiency because you have less equalizing cycles per day.

An even better line of thinking might be to recognize that when your refrigerator is loaded up with thermal mass containers and possibly PCM containers, that the space that is left in the refrigerator is much reduced, but is still sufficient for your needs.

This would indicate that a smaller refrigerator, with a smaller surface area to lose heat, and a smaller compressor required to cool the interior could be used and less power would be required. What a concept!

I am using a refrigerator that is 18.5" wide by 18" deep by 32" high and it is quite cheap to run. It takes up less valuable space in the kitchen, also.

You have to be careful in selecting a very small refrigerator, because it is a rare one that receives the same attention to operating efficiency as full size refrigerators... but they do exist, and they are cheaper to operate.

I am very pleased with mine ($20 at a thrift store), but I am trying to find one with no freezer. Sometimes they are referred to as "all refrigerators".

Best,

-AC

[Exeric]

I would have to agree with AC on this. Thermal storage does not "like" anything. Cooling of mass requires the expenditure of energy to move the heat outside of the interior of the fridge. All theory that I've ever studied implies that the total energy for input and output does not change. If the volume of air in the fridge is reduced by the volume of mass put to use as thermal storage then there will be less volume of air inside to cool. This does not make the fridge more efficient but just reduces the volume of storage.

If you want to use a large refrigerator then far better idea that was mentioned earlier was to get enough thermal mass to sustain the temperature for a period when there is no solar energy to harvest. With this method you could use photo voltaics to run it during the day and let it coast through the night with the use of the thermal mass. I haven't investigated the possibility of getting that much thermal mass so I don't know if its feasible. If not, then just get a smaller fridge.

[Exeric]

Originally Posted by Exeric

If you want to use a large refrigerator then far better idea that was mentioned earlier was to get enough thermal mass to sustain the temperature for a period when there is no solar energy to harvest. With this method you could use photo voltaics to run it during the day and let it coast through the night with the use of the thermal mass. I haven't investigated the possibility of getting that much thermal mass so I don't know if its feasible. If not, then just get a smaller fridge.

Actually, the more I think about this whole discussion of using thermal mass the more it reminds me of storage of electricity in capacitors or inductors. If you have a steady DC supply of electricity capacitors or inductors are useless. They are only meant to fill in where there are temporal gaps in the flow of electricity. This is exactly the same function thermal mass serves in a refrigerator. If you don't have gaps in electricity flow there really is no purpose or function that a thermal mass can serve in a refrigerator.

In fact, even using thermal mass in a refrigerator along with a grid tied solar voltaic system wouldn't make any economic sense. In that situation the grid would act as the capacitor storage much more efficiently that the thermal mass in the refrigerator would. In most cases, at least around here, utilities credit you for any excess electricity you produce over what you use. That means even if your fridge runs at night when there is no electricity being produced by solar any excess electricity you produced during the day will be paying for running the fridge at night. This is a much better economic model than putting in thermal "capacitors" in the fridge that would drastically reduce the volume that is usable in it.

[ecomodded]

I am curious if my 10 gallons of water jugs is non functioning as the previous posters surmise.
So for the sake of science i am going to baseline my fridge yet again, then remove the array of water jugs and measure again.
This will be accurate, it will be measured using my kill-a-watt meter and i will open the fridge door a equal amount of times on each of the days to maintain continuity.
I had planned to do this long ago, but was happy or pleased with the water jugs so neglected or elected not to pull them out.

I feel the frozen water jugs in the freezer section aids the refrigerator as well,it appeals to my common sense.

[diyEthic]

Originally Posted by Mikesolar

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.

Free solar energy or no, water is far more efficient as a thermal mass material. Water - Thermal Properties