Phase Change Materials

[Paul L]

     For those who don't know, phase change materials (pcm) are usually salts or waxes that melt at certain temperatures - the ones I'm interested melt at a little higher than room temperature.  As these salts or waxes go from one state to another (solid to liquid) they absorb enormous amounts of heat.  Once the temperature of the now liquid salts/wax drop (like when the sun goes down) the liquid begins to solidify and release the heat it had stored during the phase change.  Apparently they can store more than five times the energy when compared to conventional thermal mass reservoirs (trombe walls, concrete floors) relative to the volume.
   PCMS were first implemented by the solar energy pioneer Maria Telkes in the late 50's for domestic heating.  She used two tons of Glauber salts (sodium sulfide decahydrate) and heated a Minnesota house for two years using only direct solar.  The Glauber salts however, proved to be an unsuitable PCM, and the search began for other options.  The most commonly used salt for PCMS today is a mix of Calcium Chloride and other salts.  There are also several suitable waxes, but waxes are flammable, which makes me wary of using them when high temps are being used.           
     As someone who builds R2000 houses (the Canadian precursor of the LEED building standard), I know that the best way to make a house energy efficient is to have a small footprint, and insulate the hell out of it.  The problem is that conventional thermal storage and small houses don't mix, since traditional thermal masses take up quite a bit of space - space which is in high demand in a small house.  PCM's solve this problem since they occupy less space, but still pack in the energy.
     There are drawbacks however - compared to conventional thermal mass storage systems (rocks), they are costly.  And they do have a life span - some commercially produced are guarantees for only 10 years after which their efficiency lessens.  They also need to be stored in proper containers, since they are corrosive.  Someone who had spoken to Mr.Lane (see below) suggested using 1L pop bottles would be appropriate.   Apparently in Europe, where their energy costs are more significant, PCMS are more common.
     BASF makes PCM impregnated gypsum board, which I think is a great idea, but I'd like to make my own pcm.  You can see a patent filed by George Lane, who worked for Dow Chemicals at http://www.freepatentsonline.com/4613444.pdf.  I'm trying out some of his "recipes", but have no experience in chemistry or the like.  Things are moving along though, and I have several bottles sitting on my windowsill containing several variations of Mr. Lanes work.  They all change phase when I dunk them in hot water, and then revert to solid once I take them out, so I'm doing something right!  Now I just need some sunshine!  The biggest problem I'm running into is getting my hands on a suitable gel like substance to suspend the salt crystals.  If they aren't suspended in the liquid, the eventually filter out, which renders the mixture useless.  If anyone out there has tried something like this out, it'd be great to hear from you.               
     

[Gabriel]

I don't know much about phase change materials, but it is something I am interested in learning more about for the same reasons you mentioned. You might try writing some documentation about how you did it and posting it on something like www.instructables.com (or my site even if you preferred). I don't see anybody else there with any information about pcms, but I'd bet that you would get a decent response in the comments if you asked.

I had never considered making the stuff from scratch before. One of my long term goals is to design and build a 100% solar powered house, so something like this could come in handy.


P.S. The patent seems to have disappeared from the link. I found some other similar ones, but not the one you mentioned.

[Paul L]

    shoot, i added a period on at the end of the link!  this one should work -  http://www.freepatentsonline.com/4613444.pdf
for some reason, it's hard to find high purity calcium chloride here in canada, which is making things difficult/expensive for me.  it's funny since we usually have more snow up here than in the states...  i've gotten my hands on a bag of Dowflake Xtra and am trying out example #10 right now, since the concentration of salts are fairly close to the mixture found in Dowflake Xtra.  i ordered my strontium from United Nuclear http://unitednuclear.com/index.php?main_page=product_info&cPath=16_17_69&products_id=808 , though it looks like they are currently out of stock.  comparatively, it's expensive to calcium chloride, but it goes a long way since it comprises roughly 1% or the total final weight of the pcm.  as far as my method, i'm basically just following what is written in the examples.  I haven't taken specific temperature measurements other than picking it up in my hand and saying, "yup, this one is warmer" but the pcm definitely holds the temp longer than plains water.  I'm using corn starch right now to thicken the mixture and suspend the crystals, but I've read that is not a good way to go...at this point I'm just experimenting.  I've also tried using Bentonite, since it is similar to Fuller's Clay, which was used by Telkes.  Bentonite is basically the clay you would find in clumping cat litter, so kitty litter is what I used(non scented).  Needless to say, it didn't work too well.  On the bright side, I also found a video online explaining that Bentonite is also the type of clay they use in fancy salons for facials.  My fiancee was not impressed when she came home to find me with dried cat litter smeared on my face...(if you're wondering, it works, feels great, and takes years off your appearance! I believe most PCMs use Sodium Polyacrylate as a thickening agent.  I really should take temperature readings and plot them to see how effective they are, and if they are degrading with cycling, but I don't have any thermometers around
   
As an aside, my extra arduino r3 isn't working - have you ever run into this as an error when uploading -

avrdude: stk500_getsync(): not in sync: resp=0x00
avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x51

i've gone over the forums people have suggested fixes, but noting seems to work for me.  don't look it up, just wondering if it's ever happened to you.  i think i may have fried the ATMega328 somehow....   the heliostat, on the upside, is preforming fabulously!

[Gabriel]

"My fiancee was not impressed when she came home to find me with dried cat litter smeared on my face..." You're a very odd man aren't you.

I can't say that I remember having seen that error before. I'd try it out on another computer just to rule out a software glitch. You can also buy just the Atmega chip with the Arduino boot loader on it and try replacing it for cheaper than buying a whole new board. I keep a spare one around so it won't slow me down much in case I do fry one by accident at some point. Amazingly I've managed to avoid it despite all of the weird things I've done to my Arduinos. 

I recently bought a data logging shield from Adafruit for long term data collection. The Arduino does quite a nice job of it. All you would really need though is a $2.00 temperature sensor to get started.

I'll have to give that paper a read through when I get the chance. I was never a huge fan of Chemistry lectures, but working in the lab was always fun.

[Paul L]

     I never take myself too seriously, which means I do strange stuff like that all the time.   

     So this is where I am right now - the PCMs that I made with the DowFlake Xtra didn't work.  I think there was too many impurities in the mixture, so I had to source a purer source of Calcium Chloride.  I emailed Tetra, and they informed me that they currently aren't producing any now because hydrochloric acid is so pricy.  After contacting Pool supplies and Janitorial stores to no avail, I was surprised to find some at a local hardware/everything store.  Bulk, 94-97% pure, $1.50 a pound, which is expensive, but alot cheaper than Dri-Z-Air, which is what I was using in the beginning.  I contacted their supplier, and could shave 50 cents off a pound if I bought from them in bulk. 

     My current PCM is working really well; I've been cycling it from hot to cold, and it seems to be holding up.  Once I heat it up and stick it in the fridge, it maintains it's temp around 23C for roughly 3 hours (fridge temp is roughly 5.5C), whereas the same quantity of water heated to the same temp looses more than 10 degrees in 20 minutes.  The sample I'm using is 300grams and is kept in a plastic drinking bottle.  I'm now working on trying to find a suitable container - I'd like to increase the surface area of the bottle to help with getting the heat out.  I tried to use my air compressor and a heat gun to expand a bottle in a mold, but that didn't work, so now I'm going to see if I can find a cheap Foodsaver vacuum sealer on Craigslist and see if it could be used to hold the PCM.  I read that the plastic must be at least 3.5mil, and I think foodsaver bags just make the mark.  I've also ordered some Sodium Polyacrylate to use in future PCMs to replace the cornstarch gel I'm using now, so we'll see how that goes.     

[Gabriel]

I really want to try this sometime. Have you figured out how much heat you can hold with it? I skimmed through the pdf, but didn't see anything on it. It maybe that I missed it though.

[Paul L]

I don't think it says anything in the PDF about heat storage capacity, but by judging on what's written here, http://www.builditsolar.com/Experimental/PCM/DIYPhaseChangeMaterial.htm it should be around 225 BTU a liter.  That means nothing to me, all I know is that it's warm when everything else around it is cold.    I encourage you to try and make some - it's pretty neat to find it sitting in the morning on the counted 6 degrees hotter than anything else around it.  And with your data logger, you could get some better numbers than what I can using an indoor/outdoor thermometer. 

Also, I came across this page - http://www.nateful.com/diysolar/diysolar.html that I thought you'd find interesting if you hadn't seen it already.  Looks like something I might consider doing later down the line now that I'm getting comfortable with the whole arduino thing....   

[Paul L]

I got the Sodium Polyacrylate I ordered, tried it out, and it doesn't work for as a gelling agent for this PCM.  I'm thinking I'll try Hydroxyethyl Cellulose if I can get my hands on any... 

[Gabriel]

I've been reading more into this PCM stuff recently and came across this pdf.
http://www.umaine.edu/MechEng/peterson/Classes/Design/2011_12/Groups/Comfort/finalreport.pdf

It's a pretty easy read, and this seems to be the only resource I have found that gives any sort of pricing information (It's in the appendix).

They even gave the name of the company where they bought the stuff, Alfa Aesar.
The link is http://www.alfa.com/en/gh100w.pgm

If I understood correctly (I only skimmed, I haven't read through the whole thing yet), the PCM they are using is n-Hexadecane.
The link to the cheaper 95% stuff is.
http://www.alfa.com/en/gp100w.pgm?dsstk=43283#

I don't know if $385 is a good deal or not for 5 gallons, but I suppose its a good place to start researching cost feasibility.

The PDF says that this PCM has a melting temperature of 64 degrees Fahrenheit (17.7778 degrees Celsius), and it also a latent heat of 228 KJ/kg.

Let's see, 5 gallons = 18.927 liters. The density of the PCM is 0.7733 kg/liter, which gives a mass of 14.64Kg.

So the PCM would be able to hold 14.64Kg * 228 KJ/Kg = 4315.35 KJ of energy.

We'll use an example of water being raised by 1 degree Celsius for the sake of comparison.

It takes about 4.186KJ to raise the temperature of 1 liter of water by 1 degrees Celsius at the melting point of this PCM. So, it would take about 1031 liters (or 272 gallons) of water to hold an equivalent amount of heat as the PCM.

If you were willing to let the temperature fluctuate by say 5 degrees C, then you would only need 206 liters (or 54 gallons) of water to hold an equivalent amount of heat.

Aside from the fact that you would obviously save space, you would also save money on the cost of the tank since you could get by with a smaller one if you used a PCM. 

[Joe]

Hi Paul,
Calcium Chloride is also sold to farmers as a calcium supplement.  Purity won't be great but it's sold in 50 lb bags.  I read the free patent document and I am interested in any progress you make.

[Paul L]

The calcium sold to farmers isn't suitable since the purity is low...
The Sodium Polyacrylate worked as a thickening agent and I'm just keeping an eye on the mixtures - cycling them and eventually I'll check with a thermometer once I find it again (I'm in the middle of a move, so everything is packed away) to see how well it's holding the heat.  But just from looking, it seems to be working - i.e. melting at 26 and freezing below.  I have a funny feeling that this is cheaper than n-Hexadecane....

[stepper121]

Have seen a house use paraffin wax as a PCM the owner had a wheelie bin full about 240 litre he had old solarwater panels on the roof pumping water around to a heat exchanger in the bin. Then another pumping around to  heaters in the house. Worked well the first time I had seen a PCM being used. The paraffin wax has a 2.14–2.9 J g−1 K−1 (joule per gram kelvin).

do not know the cost of paraffin wax but a mate had a though why not use bees wax  a lot tastier