How do Sunamp heat batteries work?

Martyn1981

So ..... if it heats to full phase change and is left, it will lose some heat to the environment, as per SC's original claim? Until it reaches ambient temp, at which point losses stop, and the remaining energy is 'permanently' stored.

And if it operates as you suggest (I don't think that's correct) and is kept from full phase change, it will sit at ~58C, losing some heat to the environment, as per SC's original claim? And if left, will continue to lose heat to the environment, just like a normal hot water tank?

But either way, doesn't that mean SC was at least partially correct, there has to be some losses anytime the heat inside the unit is above ambient? But I don't think he was suggesting huge losses, just pushing back on any suggestion that there are no losses.

[Again, for the purposes of the discussion, by losses I mean heat that 'escapes', even if that heat is beneficial to the area it seeks refuge in.]

Reed_Richards

I never suggested there are no losses, neither do Sunamp AFAIK (although they do over-sell in my opinion).  What I did state is that there can be no internal losses (heat lost inside the unit that does not get out again but has no useful value).

I don't know how long a reusable handwarmer will last after you have boiled it up in that pan of water and let it cool.  Hours, days, weeks, months?  @QrizB ?  That would give some idea of how "permanent" the heat storage could be.    

70sbudgie

So my husband is a chemical engineer and all this is his specialisation. I have asked questions, but don't claim to understand all of his responses, so apologies if I get my wires crossed in trying to relay what I have learned. 

All chemicals change phases, usually from solid, to liquid to gas. There are some chemicals that can go straight from solid to gas, which is called sublimation (carbon dioxide is an example). (He said that graphite to diamond is an example of a solid to solid phase change, but he doesn't know of any liquid to liquid phase changes. Then provided an explanation I didn't understand.)

Changing phase is when the structure between molecules changes. Going from solid to liquid and liquid to gas requires lots of energy that doesn't effect the material's observable temperature. This is why we put ice in drinks - as the water changes phase from solid to liquid it takes more energy from the surrounding drink and cools the drink more than if you just put cold water in it. 

And vice versa - going from liquid to solid is exothermic as it releases all the energy keeping it in the liquid phase. 

We also had a conversation about how the reusable hand warmers work. I got lost very quickly, so this may not stand up to verification. He said the shards coming off the clicker is nonsense. He doesn't know the answer, but went on to hypothesise: nucleation (when crystals form) without a catalyst or seeding is random. Perhaps the clicker is creating pockets of higher pressure that increases the likelihood of nucleation. Anyone that has used the reusable handwarmers will know that they don't always work on the first click.

So, my guess for how the Sunamp works is that they have found a chemical whose phase change temperature is about room temperature. You charge it by inputting energy and putting it into the liquid state (I'm only saying liquid because I imagine it is easier / less voluminous to store than gas.) When you call for heat, something triggers nucleation and enough heat is emitted to heat water to 58°C by heat transfer. This would explain why the heat suddenly runs out - all the PCM has changed to the lower energy phase. To recharge it , you need to put enough energy in to return all the PCM to the higher energy phase. I imagine that it is really difficult to measure how much of the PCM is in which state, and that is why there is no indication. 

70sbudgie

could the losses be due to having to input extra energy to ensure all the PCM is in the higher energy state? Therefore heating it to a higher temperature than can be recovered by the exothermic phase change?

Reed_Richards

70sbudgie said:

We also had a conversation about how the reusable hand warmers work. I got lost very quickly, so this may not stand up to verification. He said the shards coming off the clicker is nonsense. He doesn't know the answer, but went on to hypothesise: nucleation (when crystals form) without a catalyst or seeding is random. Perhaps the clicker is creating pockets of higher pressure that increases the likelihood of nucleation. Anyone that has used the reusable handwarmers will know that they don't always work on the first click.

Yes the "shards" thing was in the BBC reference but it doesn't make sense because you would have to get rid of them to re-use the handwarmer.

70sbudgie said:

could the losses be due to having to input extra energy to ensure all the PCM is in the higher energy state? Therefore heating it to a higher temperature than can be recovered by the exothermic phase change?

No, I don't think so because you don't need all the PCM to be in the higher energy state.  It won't be when the thing is actually heating your water.

  
   
This is from the Sunamp literature (comparing their material to something like water).  There's no hint of the metastability that hand warmers rely on (but perhaps they don't want to mention that).   

70sbudgie

That graph is interesting. It clearly shows (imho, which is struggling to keep up with this conversation!) that the phase change temperature is just above 55°C (I think someone said 58°C further up the conversation). Forcing the PCM through the phase change, results in about 95Wh/L (if I am reading the graph correctly) either being emitted or absorbed (depending on direction of change). Presumably these units come in different sizes and that's where you get the different ratings from.

Another idea of where the efficiency comes from - perhaps it is the amount of PCM that reaches the higher energy state (without over charging as I previously guessed)? Looking at the Thermino hp range, the footprint remains the same at 575mm x 365mm and the height of the unit increases for greater capacity. If the whole of the unit is full of PCM (which I know it won't be, but I need to pick something for the calcs) the 150 would have 130l of PCM and a rating of 12kWhrs and the 300 would have a rating of ~21kWhrs (I have no doubt that you will point it out if I've lost track of my units). This increased energy provision at the point of demand explains why the recommended flow rate increases with model size. 

So if testing consistently shows an output of 10kWhrs (for the 150), it makes the unit 85% efficient. From what my oh says, phase change technology is difficult to be precise with as it basically comes down to probability. He seems to work off graphs with averages and variances. This may also be why it is so difficult to find an answer on efficiency from Sunamp, because it is actually a range, not a precise figure. 

I don't think the Sunamp uses the meta stability thing. I don't think it needs to, because it is electrically connected, it will have access to other ways to trigger nucleation. PC technology "only" needs a catalyst to trigger nucleation and a method of reheating the PCM. Handwarmers only have the clicker to trigger nucleation and you return them to the high energy state by boiling them. But perhaps the challenge is in harnessing the probabilities?

Reed_Richards

70sbudgie said:

So if testing consistently shows an output of 10kWhrs (for the 150), it makes the unit 85% efficient. 

There are two types of "efficiency" I can think of in these cases.  There is the efficiency of heat transfer between the hot substance inside and the water you pass through it.  And there is the efficiency with which it retains its heat, which depends on how well insulated it is, and the pipes that connect to it are.  I could not follow which, if either, of those you are referring to.  Or is it the usable material divided by the volume, a "size efficiency"?

70sbudgie said:

I don't think the Sunamp uses the meta stability thing. 

No, neither do I.  But the demonstration with the reusable hand warmer has made such a strong impression on other forumists who have looked at this technology that they appear convinced that the Sunamp does use it.
  

Martyn1981

Reed_Richards said:

I never suggested there are no losses, neither do Sunamp AFAIK (although they do over-sell in my opinion).  What I did state is that there can be no internal losses (heat lost inside the unit that does not get out again but has no useful value).

I don't know how long a reusable handwarmer will last after you have boiled it up in that pan of water and let it cool.  Hours, days, weeks, months?  @QrizB ?  That would give some idea of how "permanent" the heat storage could be.    

The handwarmers last for years.

If you don't let the PCM completely change phase, but keep it sitting in the window, as you suggested, then it will need heating to stay at 58C, otherwise it will cool back down. That's exactly the same, isn't it, as a hot water tank?

So to get the long term storage without losses benefit of this material wouldn't you have to let it change phase? that's why I don't follow your suggestion, as I can't see any benefit over a hot water tank if you mantain a temp of ~58C, without fully changing phase.

Coastalwatch

Excuse my intervention but I've been following the thread and thank all posters for their contributions. Afraid I am still at a loss as to understand how precisely the energy is stored and released other than through, in my eyes at least, the magic of changing phase. But just as there is no such thing as a free lunch I'd be pleaseantly surprised if no losses occured during these processes. Or at least losses that could be measured.

Being a lifelong sceptic, it took months before I could accept that heat pumps were capable of an energy efficiency greater than one, I'm still to be convinced of Sunamps claim that their heat store is 100% efficient aided by it's high performing insulating jacket.

As mentioned previously if small heat "losses" are maintained within the property then perhaps this supports their 100% efficiency claim.

Our thermal DHW store of 210 litres has a heat loss of circa 81 watts or +/- 2kWh's/day with a percentage of it maintained as heat in the living area. The remainder lost in the loft area where some of the hot water pipes run, but even there it could be suggested that it may assist with maintaining some warmth and so prevent freezing pipes otherwise, but I really don't know.

From memory I believe current Sunamp units are capable of delivering sufficient heat to cover central heating and that they were planning to develop their unit further to cover DHW too.

I've searched the Sunamp website to find there are now four versions from Thermino e upto Thermino 300 but can find no data readily available relating to stated capacities of each.

I'd be interested to learn what these might be together with their anticipated costs?

I'm afraid that until datasheets are readilly available I shall remain sceptical, not aided by the representative in the video appearing to wear a hairpiece making me wonder if there isn't something more than just a receding hairline kept away from public view.

Reed_Richards

Martyn1981 said:

The handwarmers last for years.

In the liquid state or do you have to re-liquify them from one winter to the next?

Martyn1981 said:

If you don't let the PCM completely change phase, but keep it sitting in the window, as you suggested, then it will need heating to stay at 58C, otherwise it will cool back down. That's exactly the same, isn't it, as a hot water tank?

Yes, it's exactly the same as a tank of hot water.  But a lot more compact and lighter than a tank of water with the equivalent heat storage capacity.  And the claim is that the insulation is better.

Martyn1981 said:

So to get the long term storage without losses benefit of this material wouldn't you have to let it change phase? that's why I don't follow your suggestion, as I can't see any benefit over a hot water tank if you mantain a temp of ~58C, without fully changing phase.

In a domestic situation you don't want long term storage.  Whether you could heat it up, switch the unit off and let it cool without it changing phase is moot.  I don't think Sunamp claim this is possible with a domestic unit (despite the handwarmer demo).  Their waste heat transportation units might be able to do this.