Sodium Acetate: Fun in the Kitchen with Phase Change Experiments

Friends, you may recall that in a recent article I wrote about Phase Change Materials (PCMs) used for thermal storage. I illustrated that article with a measurement of the temperature versus time as some molten candle wax solidified. I then tried to work out how much so-called ‘latent’ heat was released as the wax solidified.

A Twitter source then told me that the actual material used in commercial thermal storage units was sodium acetate trihydrate, and within 18 hours, a kilogram of the substance was delivered to my door.

NOTE: In this article I have used the term sodium acetate to mean sodium acetate trihydrate and in some locations it is abbreviated to SAT.

NOTE: Sodium acetate is pretty safe from a toxicity perspective: it’s an allowed food ingredient E262, but one needs to be careful not to scald oneself – or others – when handling the hot liquid.

So I began a series of experiments in which I made a great variety of very different, but similarly basic, errors. There really is nothing like a practical experiment for making one feel incompetent and stupid! Part of the problem was that I was trying to do other things at the same time as reading the temperature of the two samples (wax and sodium acetate).

To overcome these difficulties,  I eventually bought a thermocouple data-logger which can read up to 4 thermocouples simultaneously and save the data on an SD card. This allowed me (a) get on with life and (b) to do something clever: to measure the cooling curve of a sample of water at the same time. I’ll explain why this was important later.

Eventually – after a series of new basic mistakes such as setting the logging interval to 30 minutes rather than30 seconds – I began to get some interesting data. And sodium acetate really is an extraordinary substance.

Of course my experiments are not complete and I would really like to repeat the whole series of experiments based on the golden rule, but I really need to the clean up the kitchen.

Experiment#1

As shown below, I heated three samples of equal volumes of wax, sodium acetate and water to roughly 90 °C for around 10 minutes – sufficient to melt all the SAT.

I then transferred the samples – while logging their temperature – into a cardboard stand where I guessed that the cooling environment of each sample would be similar.

The results of the first experiment are shown below.

Click on image for a larger version. The temperature of the three samples of water, wax and sodium acetate as a function of time.

The first thing to notice is how odd the curves are for the wax and the sodium acetate. They both have discontinuities in their rate of cooling.

And strikingly, although they start at similar temperatures, they both stay hotter than the water for longer – this is what makes them candidate thermal storage materials. But precisely how much more heat have they released?

To work this out we need to start with the cooling curve for the water which (happily) behaves normally i.e. smoothly. We would expect…

  • …the cooling rate (°C/s) to be proportional to…
  • …the difference between the temperature at any particular time, and the temperature of the environment (roughly 27 °C during Experiment #1).

Using the magic of spreadsheets we can check if this is the case, and as the graph below demonstrates, it is indeed approximately so.

Click on image for a larger version. The cooling rate of the water  as function of the difference between water temperature and the temperature of the environment.

Because the heat capacity of water is reasonably constant over this temperature range, we can now convert this cooling rate into an estimate of how much heat was leaving the water sample at each temperature. To do this we note that for each °C that each gram of water cools, 4.2 J of heat must leave the sample. So if 1 gram of water cools at a rate of 1 °C/s, then the rate of heat loss must be 4.2 J/s or 4.2 W.

Click on image for a larger version. Estimate for the rate of loss of heat (in watts) of the water as function of the difference between water temperature and the temperature of the environment.

This last graph tells us that when the temperature difference from the environments is (say) 10 °C, then the water is losing 0.104 x 10 = 1.04 watts of heat. Based on the closeness of the fit to the data, I would estimate there is about a 10% uncertainty in this figure.

Finally, if we add the amount of heat lost during the time interval between each data point, we can estimate the cumulative total amount of heat lost.

It is this cumulative total that indicates the capacity of a substance to store heat.

Importantly, because all the samples are held similarly, at any particular temperature, I think the heat loss from each of the other samples must be the similar to that for water when it was at the same temperature – even though the cooling rates are quite different.

Using this insight, I converted the cooling curve (temperature versus time) for these materials – into curves showing cumulative heat loss curves versus time.

Click on image for a larger version. Estimates for the cumulative heat lost from the water, wax and SAT (sodium acetate) samples as a function of time. Also shown as dotted lines are the limiting extrapolations from (a) the first part of the cooling curve of the SAT and (b) the final part of the cooling curve. The difference between these two extrapolations is an estimate for the latent heat of the SAT.

We can apply a couple of sanity checks here. The first is that the heat lost from the water comes to about 10.7 kilojoules. Since the 60 g of water cooled from 70 °C to 28 °C then based on a heat capacity of water of 4,200 J/°C/kg we would expect a heat loss of (0.06 x 4200 x 42 =)10.6 kJ. This rough numerical agreement just indicates that the spreadsheet analysis has not resulted in any gross errors.

Looking at the difference between the extrapolation of the first part of the SAT curve, and the extrapolation of the final curve, we see a difference of approximately 23.8 kJ. This heat evolved from 88 g of SAT in the tube and so corresponds to 23.8/0.088 = 270 kJ/kg. We can check that against an academic paper, which suggests values in the range 264 to 289 kJ/kg. So that too seems to check out.

With everything sort of working, I tried the experiment a couple more times

Further Experiments: coping with super-cooling

The most striking feature of these experiments is that when the sodium acetate freezes, it releases its ‘latent heat’ and warms up to its equilibrium freezing temperature of roughly 58 °C.

From the first experiment – and the experiments I had done previously – it became clear that the sodium acetate tended to supercool substantially. This is the process whereby a substance remains a liquid even when it is cooled below its equilibrium freezing temperature.

[The physics of supercooling is fascinating but I don’t really have time to discuss it here. In facile terms, it is like when a cartoon character runs over the edge of a cliff but doesn’t fall until it realises that there is nothing holding it up!]

In this context, the supercooling is just an irritation! So I tried different techniques in each of the three difference experiments

  • In Experiment #1, I stirred the sample to initiate the freezing.
  • In Experiment #2, I placed spoons in each sample in the hope that some additional cooling would initiate the freezing. It didn’t.
  • In Experiment #3, I left the sample for as long as was practical in the hope it would spontaneously freeze. It didn’t.
  • In Experiment #4, I left the sample for longer than was practical. But it still didn’t freeze spontaneously.

So I didn’t manage to control the supercooling – in each case I initiated the freeze by poking, shaking or stirring. I’ll comment on this failure at the end of the article.

The data and analysis from experiments 2, 3 and 4 is shown below.

Click on image for a larger version. The upper three graphs show 3 cooling curves for wax and SAT. The water sample is not shown to simplify the graphs. The Lower 3 graphs show estimates for the cumulative heat lost from the wax and SAT samples as a function of time. Also shown as dotted lines are the limiting extrapolations from (a) the first part of the cooling curve of the SAT and (b) the final part of the cooling curve. The difference between these two extrapolations is an estimate for the latent heat of the SAT.

Conclusions

The most important conclusion from the analysis above is that a given volume of SAT releases much more thermal energy on cooling than the equivalent volume of either water or wax. This what makes it useful for thermal storage.

If we consider heat released above 40 °C, then the SAT releases around 3 times as much heat as a similar volume of water. This means an equivalent thermal store built using SAT can be up to 3 times smaller than the equivalent thermal store using a hot water cylinder.

The experiments gave four estimates for the heat related as latent heat which are summarised in the table below. Pleasingly all are in reasonable agreement with the suggested likely range of results from 264 to 289 kJ/kg.

Click on image for a larger version. Three estimates of the latent heat of Sodium Acetate Trihydrate (SAT)

Practical Devices

Scaling to a larger sample, 100 kg of sodium acetate would occupy a volume of 68 litres and fit in a cube with a side of just 40 cm or so, and release around 27MJ (7.5 kWh) of latent heat. This is roughly the equivalent of the heat stored in a 200 litre domestic hot water cylinder.

Sodium acetate is the thermal storage medium in a range of devices that can serve the same purpose as a domestic hot water cylinder but which occupy (in practice) rather less than half the volume. Clever!

Heat is stored by melting the sodium acetate in an insulated box, and released by running cold water through pipes immersed in the sodium acetate: the water is heated and emerges piping hot through your taps! As the sodium acetate freezes, the temperature remains stable – and the water delivered similarly remains piping hot.

But what about the supercooling? How do the devices prevent from the sodium acetate from supercooling? I’m afraid I don’t know. This paper discusses some practical considerations for thermal storage devices made using SAT, and it lists a number of additives that apparently rectify shortcomings in SAT behaviour. One of the additives is – curiously – wallpaper paste. I did try experiments with this but I didn’t observe any regular change in behaviour.

In any case, have fun with your sodium acetate experiments. It is available from here.

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6 Responses to “Sodium Acetate: Fun in the Kitchen with Phase Change Experiments”

  1. Matthew Hannah Says:

    Great fun Michael thanks for posting. Presumably there are compounds out there that are even better, although maybe more expensive/toxic? Is there any way for screening for the best/optimal materials or is it just a question of wading through tables of ….? What? …heat capacity or something else? Thanks again, btw at the end of the first line of the conclusion you should have wax rather than SAT (I think)

    • protonsforbreakfast Says:

      Matthew,

      Thank you for your kind words. Yes there are a lot of candidate materials. This paper

      https://www.sciencedirect.com/science/article/pii/S2352152X22011380

      describes some of them.I think the attributes one would search for are

      (a) melting/freezing temperature – this needs to be in just the right range for your application
      (b) latent heat, sometimes called latent heat of fusion – an old fashioned word for melting – or enthalpy of fusion.

      And thank you for the typo – now corrected.

      All the best

      Michael

  2. David Cawkwell Says:

    Thanks for posting. I still think water wins every time for heat storage where you have the space for it due to cost.

  3. Ross Mason Says:

    The local adventure store sells handwarmers. These have SAT in them. Boil them up in a pot of water until liquid. Stash in you pack or pocket. Each sachet has a piece of metal that acts like a cricket clicker. A quick bend of the metal sends a crack through the liquid and voila! Anti – freezing the fingers..

    • protonsforbreakfast Says:

      Somehow I forgot to mention those! This was a really hard article to write and I lost the plot in one or two places.

      All the best , Michael

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