Domestic Thermal Storage 2: Phase Change Material

Friends, this is the second of three articles in which I am comparing three different types of thermal storage.

In the last article I looked at the humble domestic hot water (DHW) cylinder, and in the next article I will look at large thermal stores. Here we will look at the use of a phase-change material (PCM) to store heat.

In practice a PCM thermal store looks like a regular ‘white goods’ metal box and is typically placed wherever the domestic hot water (DHW) cylinder would have been in a dwelling.

Click on the image for a larger version. Publicity images from the Sunamp web site demonstrating the small physical size of their PCM thermal stores.

But a PCM thermal store has a big advantage over a DHW cylinder: it is typically one third to one half the size for the same amount of thermal storage. Dimensions are typically 1 metre high, 60 cm deep and 40 cm wide.

Click on the image for a larger version. On the left-hand side is a commercial PCM thermal store. On the right-hand side is a schematic explanation of how it works. In this versions of the device, the PCM material is charged using an electrical immersion heater. In other versions it can be charged using a heat pump. In operation, cold water flowed into the device is rapidly heated and discharged.

Additionally a PCM store is cubical, and so makes use of the corners of spaces that DHW cylinders – being cylindrical – can’t use.

Functionally it works like a DHW cylinder. When a tap is opened, cold water flows into the device and is heated as it flows through pipes embedded in the hot PCM material – and hot water flows out.

However, the PCM thermal store has a trick up its sleeve. If the PCM stored heat in a substance at high temperature, then the temperature of the substance would have to be high initially – with high losses – and the storage medium would cool as heat was withdrawn.

PCM thermal stores get around this by using a material which melts – typically at around 55 °C to 60 °C.

  • Charging the PCM involves heating it up to its melting temperature, then supplying the so-called ‘latent heat’ required to change it from one ‘phase’ (a solid) to another ‘phase’ (a liquid). It is then heated further as a liquid.
  • When cold water flows through pipes embedded in the PCM, the PCM cools and freezes around the pipes. When it freezes it stays at its freezing temperature releasing its so-called ‘latent heat’ until the entire charge of of PCM has solidified.

In practice this means that one gets the benefits of a DHW cylinder in a smaller space. PCM thermal stores are particularly well-suited to smaller single-person dwellings.

PCM’s: Home experimentation

A common PCM with which you can experiment at home is candle wax.

While my wife was out at work, I put two candles into a glass container and melted them (one at a time) by putting the container in a jug of boiling water.

Click on the image for a larger version. Top-Left: Melting a candle in a jug of hot water. Right: A partially melted candle.Bottom-left: Measuring the temperature as the molten wax cooled.

When both candles were melted, I put a thermocouple into the wax, wrapped insulation around the glass vessel and then measured the temperature as the molten wax froze – i.e. changed ‘phase’ from liquid to solid (to use the technical terms). The data are shown below:

Click on the image for a larger version. The graph shows the temperature of a thermocouple embedded in 55 g of wax as it froze. Note that there is a sharp change in cooling rate when the wax starts to freeze due to the release of so-called ‘latent heat’. This allows the wax to stay above 50 °C for almost 3 hours, while if it had continued cooling at the initial rate, it would have fallen below 50 °C in under 1 hour.

What one sees is that as the molten wax cools, it looks like it will fall below 50 °C after about 50 minutes. However, once the wax starts to freeze (at about 57 °C), the cooling rate is reduced to roughly one tenth of its previous rate, and the liquid/solid mixture stays above 50 °C for around 160 minutes.

Using a very rudimentary analysis based on googled data:

  • Heat Capacity of wax ~2.5 J/g/°C – assumed the same in liquid or solid state;
  • Latent Heat of wax ~176 J/g;

…one can roughly estimate how much heat is released at temperatures above 50 °C.

Click on the image for a larger version. Analysis of cooling curve in the previous graph allows an estimate of the amount of heat released at temperatures above 50 °C. The latent heat of 55 g of wax amounts to just under 10,000 joules.

Although I had followed the golden rule of experimental physics, I still failed to anticipate just how long it would take the wax to solidify – the experiment took 4 hours and I was almost late preparing my wife’s dinner!

This extended experiment indicates just how much ‘latent’ heat a material can store compared with ‘sensible’ (i.e. sense-able: which can be detected with a thermometer) heat storage.

Based on the latent heat alone, 100 kg of wax – which would occupy a cube with a side of 50 cm – could store 5 kWh of thermal energy – the equivalent of a small DHW cylinder.

Commercial PCM Devices

I don’t know, but I am pretty sure that commercial PCM devices do not use wax as a storage medium.

Update: A Twitter Source tells me that the Sunamp uses “Sodium acetate tryhydrate (plus a few secret additives).”

Sunamp’s list of patents includes a variety of chemicals which can be used, but the particular chemical used and the way it is prepared is likely a trade secret. Nonetheless, I suspect their basic properties are not so different from wax.

They will have a phase change temperature ideally around 55 °C. If the phase change temperature is much higher than this, then the store will operate at too high a temperature and lose more energy. If the phase change temperature is much lower than this, then water will not be sufficiently hot when discharged.

Early models of the PCM stores were designed to be ‘charged’ electrically with a heater immersed in the PCM material. This could be powered either from the grid – ideally using off-peak electricity – or from solar PV panels. However recent versions can also be charged using a heat pump.

Summary

PCM thermal stores  represent a clever way to incorporate thermal storage in dwellings where space is at a premium. They are particularly useful in flats and households with just one or two people.

However, like all thermal storage devices, they are not perfect.

One disadvantage is that unlike a DHW cylinder, the storage medium has to ship with the device – it can’t be shipped empty. This makes the devices heavy: A PCM store equivalent to a 200 litre cylinder weighs ~ 172 kg. Of course a DHW water cylinder holding 200 litres of water would weigh more – but it can be filled and emptied in place!

Heating losses are similar to DHW cylinders – with roughly 10% of the stored energy being lost each day – and like DHW cylinders, it can be tricky to know how ‘full’ the store is because it can be difficult to work out what fraction of the PCM material is liquid or solid.

But all-in-all, the PCM thermal stores devices seem to have found a niche where they can make themselves genuinely useful.

4 Responses to “Domestic Thermal Storage 2: Phase Change Material”

  1. Stuart Gibbons Says:

    I am exploring energy storage using non lithium batteries ; Supercaps
    I would be interested in your views.
    The concept can be used for residential and commercial overnight storage, plus on a much large scale, the grid.
    I appreciate this might not be referencing your write up, but clearly you are an expert in your field and I would appreciate your opinion?

    • protonsforbreakfast Says:

      Stuart

      Good Evening. I don’t have any data to hand at the moment, but my recollection is that super capacitors can store charge with low loss very rapidly, but they don’t have the energy storage density that lithium batteries do.

      I have just taken a moment to check with wikipedia and this confirms what I thought.

      https://en.wikipedia.org/wiki/Supercapacitor#Comparison_with_other_storage_technologies

      They store between 10 and 100 times less charge than a lithium ion battery.

      Is there a reason that you think super capacitors may be the way to go?

      Best wishes

      Michael

  2. Stephen L Says:

    There’s no real trade secret as to what’s been added to the Sodium Acetate Trihydrate inside the SunAmp thermal batteries. There will be an amount of Carboxy Methyl Cellulose (wall paper paste) to prevent phase separation of the SAT. The other ingredient they use is something that stops the SAT from entering its supercooled state. I can’t recall what it is off the top of my head, but there are plenty of research papers that contain that information. Rather than allowing the SAT to supercool, once the temperature gets to around 45°C, the added ingredient initiates nucleation of the SAT, thus releasing the ~270kJ/kg Heat of Fusion. There’s lots of research looking into non-mechanical methods of initiation of nucleation that would allow SAT to remain supercooled from summer to winter, ultimately allowing seasonal storage of the abundance of solar PV in the summer months.

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