Use of phase change materials - anyone?

MarkC

This link got me interested in "PCM" as a psuedo-energy storage method. 
https://www.vikingcold.com/

Probably quite "off the wall" for this forum, however with the breadth of knowledge about energy, I got to wondering if anyone has actually tried using something like sodium sulfate decahydrate (spiked with sodium chloride) to retain/shift cooling with conventional air conditioning?  The goal would be to "freeze" the PCM during cheap electricity (night/early morning) and "thaw" it during the peak electricity pricing during the day.  I pay for actual ERCOT set energy costs - that went to $13/kWh (plus) recently for fairly extended times.  No typo   

Just curious - seems there are obstacles of refreezing, containment, heat transfer, economics, etc, but maybe someone has overcome??

BB.

Here are some links to study:

https://pdfs.semanticscholar.org/0f08/258f19b88e6421fb309a824e314fd2f3d990.pdf
https://www.cedengineering.com/userfiles/Thermal%20Energy%20Storage.pdf

Since you can get air sourced mini-split heat pumps that get a COP of 4 down to something like 13F... These systems seem to have become the goto devices (inverter powered--VFD controlled compressors) for AC and Heating.

You start looking at other systems like ground source heat pumps--A fair amount of power and complexity is used to run the external buried heat exchangers (and expenses to install/maintain)--The simple and efficient mini-split systems have been amazing for many folks (inverter-VFD controls give almost no starting surges, and can be throttled back to very low power (Watts) usage.

And getting rid of heating ducts and their losses/extra insulation requirements vs mini-split point source cooling--Another advantage.

-Bill

Marc Kurth

I did thermal storage load shifting systems for a living, for years. Some small, some large, but all commercial. In the 80's we did a chilled water storage system that took the cooling load of an 18 story building "off peak" in Mesa, AZ. Try as I might, I could not justify it for residential.

Eutectic salts were a popular medium but the phase change temperature was too high (46F) and cost was prohibitive at such a scale.

The utility company, Salt River Project, did not believe that it could be done, so they offered us nighttime power at $0.015/KWH in exchange for a $150/KW demand fee if we ran the monstrous chiller plant between Noon and 6:00 PM.

At the time, the building was Western Savings, at the corner of Alma School and Southern. During the first six years, ALL cooling was provided by the stored chilled water (33F) produced at night.

With our low local power rates, any sort of thermal storage is not economically feasible. Conservation and efficiency is the key. My current 1970's era home is built into the side of a hill, with the back half of the lower floor being underground. All ductwork is contained within in the conditioned area, with floor registers feeding the upper floor. This combination lets me hold 73F inside on a 105F day with only 3 tons for a 2500 sq ft home.

MarkC

Marc;
I've been able to mitigate the stated $13/kWh energy costs by using my home as "storage" - ranging from pre-cooling to 72 and "float" to 80 degrees on days with extremely high energy costs.  It has worked OK, but If there was a simple retro-fit to enhance the capacity to hold these pre-cooling temperatures longer, that is what attracted me to the "viking cold" concepts of simple "ice cubes" with a small amount of circulation.  The sodium sulfate decahydrate spiked with sodium chloride "freezes" at 66 degrees (F), so it seems like a natural.  I cannot think of such a simple analogy to the "viking cold" that would work to retrofit A/C homes in Texas, but that does not mean someone else has not - lots of pragmatic geniuses out there!

Estragon

The basic idea has been used for a long time.  Locally, large blocks of ice were cut out of the lake in winter to be stored in insulated and/or partially buried cold houses for summer cold storage.  

BB.

I don't think you can use 66F directly for cooling. At best, you would use that as a thermal sink for your A/C heat pump.

I am not an A/C guy... But if you want 70F and 60% maximum relative humidity and use the solution for direct cooling (not heat pump energy source/sink), then from a dew point calculation:

https://www.calculator.net/dew-point-calculator.html?airtemperature=70&airtemperatureunit=fahrenheit&humidity=60&dewpoint=&dewpointunit=fahrenheit&x=73&y=12

You end up with 55.5F heat exchanger maximum temperature to even begin to control moisture/humidity. With a 67F heat exchanger temperature for a 70F room you are looking at 90% relative humidity. That would be pretty clammy, and give you major mold issues (you need to get to 60% relative humidity or lower at least once every three days to stop the mold/spore process).

-Bill

Marc Kurth

Oh yeah! The concept is ancient and simple. It all depends upon what a person considers to be acceptable performance for the investment.

I did two swimming pool systems using cupro nickel heat exchangers feeding heat pumps to cool the homes while heating the pool. Both worked quite well. I was not convinced that they were cost justified, but the customers loved their bragging rights - and it was their money.

icarus

I used to cut ice every winter...stored in an icehouse in big stacks, insulated with dry sawdust.  The ice house was ~30x30’ stacked 8’ high with 24” cubes.  We still had ice into November when we didn’t need it any more.  A couple of blocks a day moved into the “cooler”  portion of the building kept milk and vegetables cool for 100 in a bush camp.  In many ways I’m sorry I don’t do it any more, but my back isn’t!  

Tony

MarkC

BB. said:

I don't think you can use 66F directly for cooling. At best, you would use that as a thermal sink for your A/C heat pump.

I am not an A/C guy... But if you want 70F and 60% maximum relative humidity and use the solution for direct cooling (not heat pump energy source/sink), then from a dew point calculation:

https://www.calculator.net/dew-point-calculator.html?airtemperature=70&airtemperatureunit=fahrenheit&humidity=60&dewpoint=&dewpointunit=fahrenheit&x=73&y=12

You end up with 55.5F heat exchanger maximum temperature to even begin to control moisture/humidity. With a 67F heat exchanger temperature for a 70F room you are looking at 90% relative humidity. That would be pretty clammy, and give you major mold issues (you need to get to 60% relative humidity or lower at least once every three days to stop the mold/spore process).

-Bill

Yes, the concept would be to reduce the rise in temperature as a thermal stabilizer and maybe allow more shift in energy to night time cool down  - The A/C would still do the dehumidification.  Maybe build a home with wall "tubs" of this stuff.  OK - going off the deep end here with mental gymnastics

*****Thanks all*****

jonr

You have a much better chance of making a cost effective system using a water tank.  Hydronic heat pumps (like Chiltrix) help.