How do Sunamp heat batteries work?

Martyn1981

Solarchaser said:

Martyn1981 said:

Reed_Richards said:

Solarchaser said:

... In fact here is how hand warmers work
https://www.sciencefocus.com/science/how-do-re-usable-hand-warmers-work/

The key word in the reference is reusable.  These are hand warmers that you charge up "in a pan of boiling water" (or similar).  They come out boiling hot, cool quite rapidly to the phase change temperature then release a lot of heat at that temperature but eventually cool down completely.  So they start very hot but cool slowly whilst releasing heat.          
  .     

But that's what I described before you replied:

Hiya. If you think about one of those handwarmers after you've reset it, it will be hot, but will cool down (lose heat/energy), so as you say, after the pcm has changed phase, any additional energy applied will raise its temp (just like the water getting hotter), but also, just like the water, it will then slowly lose some of the heat to the environment.

My bold in my quote to match your bold in yours.

I think we've been talking about the same thing but confused each other, and would have helped immensely if I'd been more clear that I was talking about the re-useable type. I didn't mention that as I thought it was well known, a poor assumption looking back, and one I shouldn't have made.

So going way back, I (and I think SC) haven't been claiming that any losses from the Sunamp are as 'bad' as a hot water tank, but in my waffle v's the economics (back at the start) I felt the economics of the Sunamp with their 'claimed' zero losses, didn't work. And as SC has pointed out, there will almost certainly be some small losses from the nSunamp too.

TBC, as we've all said, any losses in the system, during the heating months isn't really a loss, though there may be different values depending on leccy v's gas heating etc etc.

Hi Martyn I think we are thinking similarly, however reading your post has made me realise that it's me that's caused this confusion right at the start.

I'll try and briefly explain what I understood the sunamp pcm to do and hopefully explain why I think what I think.

My understanding of the sunamp is that the pcm is in liquid state at temperatures below let's say 60C for arguments sake.
And so any heat below that is lost, now RR is saying it isn't, I'll explain why I think it is, in a sec.
It's reported through forums that the pcm wants to be about 65C for phase change and then changes back when... let's say asked to, and the result is 55C water out.

From this I ASSUMED that the pcm in its solid form sits at room temperature (like the hand warmers) in theory for ever, giving great long term heat loss figures as its inert while storing its heat.

I ASSUMED that when hot water is required there is something in the sunamp that commands the material to change phase, (again based on the hand warmers) releasing its heat into the water through heat exchangers. The fact it is changed at 65C and gives back 55C means there is loss in the conversion.

Why do I assume/reason all heat is lost below the phase change, well that's because in the various forums people report the water goes from hot to suddenly cold, not a gradual cooling as you get with hot water tanks which essentially release the same amount of energy as they cool... or at least can we agree its linear, vs the sunamps which *appear* to give all and then nothing suggesting to me that when it's changed all back to liquid at 55-60c then there is no more heat to give and so instead of going from 55 to 50 to 45 etc, it seems they go all goof all good, empty.

RR says I'm wrong and they do absorb heat the whole time.
I'm OK with being wrong, it happens all the time 🤣 ideally though, id like to be proven wrong

Totally tongue in cheek, but if you like being proved wrong then don't these products sit in a liquid state when charged ....... 

From this I ASSUMED that the pcm in its solid form sits at room temperature (like the hand warmers) in theory for ever, giving great long term heat loss figures as its inert while storing its heat.

I'm so naughty.

Solarchaser

Hahaha darn it, yep you got me 👏🤷‍♂️

Reed_Richards

Solarchaser said:

I stop my hot water tanks wicking away heat by a combination of one way valves and loops, all very available to your average Joe public.

Will I have more heatloss than sunamps, probably, but I also have alot more usability, and a far clearer view on the "temperature" of my tanks. An often complained about negative of sunamp is you have no idea if its half full or empty as it has no way of telling you, there is no temperature guage or capacity guage on phase change boxes.

My point is that one way valves and loops work well for heat leakage via the water in the pipes but do little or nothing for heat leakage via the copper pipes themselves.  Or do you use lengths of plastic pipe, @Solarchaser?  That would give you much better insulation than copper and would only need to be in the vicinoty of the tank but I have never seen it done.

You probably get some volume change in the material inside a phase change box.  So you'll need a little air (or some other gas) inside to accommodate this.  So some sort of sensitive differential pressure gauge might function as a capacity gauge.  As you rightly state, a temperature gauge would be useless.  

Reed_Richards

Okay, it's the re-usable handwarmers thing that is causing all the confusion; they're a very bad analogy for how a Sunamp box works.

With the reusable handwarmer you boil the hand warmer to turn crystals into a liquid.  You have to make sure that all the crystals are converted.  The liquid cools but remains in a meta-stable state. It wants to revert back to crystals but hasn't got a starting point to initiate the process.  You click a bit of metal and I read that this cause small shards to break off.  These initiate the crystallisation process and the you get the latent heat of crystallisation to warm your hands.  The meta-stable state is the really clever feature (that I did not know about) and it's what differentiates a hand warmer from what goes on inside a Sunamp box.  I don't know what happens to the metal shards when you boil the hand warmer again, they must be lost somehow or it wouldn't work a second time. 

With a Sunamp box I imagine you never heat the thing enough to completely liquify it (if that is the type of phase change going on inside).  You don't get the high temperature phase in a meta-stable state (and you wouldn't want to).  There is no clicker or equivalent trigger, there might even be bits of grit or something pointy to make sure the material recrystallises when it reaches the phase transition temperature.  It's really much less clever than the hand warmer, the only clever part is getting a material with the right phase change temperature.   

      

Solarchaser

Reed_Richards said:

Solarchaser said:

I stop my hot water tanks wicking away heat by a combination of one way valves and loops, all very available to your average Joe public.

Will I have more heatloss than sunamps, probably, but I also have alot more usability, and a far clearer view on the "temperature" of my tanks. An often complained about negative of sunamp is you have no idea if its half full or empty as it has no way of telling you, there is no temperature guage or capacity guage on phase change boxes.

My point is that one way valves and loops work well for heat leakage via the water in the pipes but do little or nothing for heat leakage via the copper pipes themselves.  Or do you use lengths of plastic pipe, @Solarchaser?  That would give you much better insulation than copper and would only need to be in the vicinoty of the tank but I have never seen it done.

You probably get some volume change in the material inside a phase change box.  So you'll need a little air (or some other gas) inside to accommodate this.  So some sort of sensitive differential pressure gauge might function as a capacity gauge.  As you rightly state, a temperature gauge would be useless.  

There is no guage on the sunamps, that was the point.
For dhw I use copper into the tanks via one way valve to stop thermosyphon back up the cold line, coming out of the tank I have a one way valve after around 300mm of pipe and then plastic pipe after that.

Martyn1981

Reed_Richards said:

Okay, it's the re-usable handwarmers thing that is causing all the confusion; they're a very bad analogy for how a Sunamp box works.

       

I'm not with you? It's the very same analogy Sunamp use, when talking about their product, and the salesmen use the hand warmer (let's say heatpack) in their demo's.

Plus as I said, way, way back (edit - well 10am  ), it's the same product, give or take, sodium acetate trihydrate in the heatpacks, and the Sunamp products. So how is it a very bad analogy?

Sunamp - What is a phase change material and why is it important

These highly patented PCM formulations form the backbone of our Plentigrade platform. Our PCMs have an enviable high energy density, avoid costly or scarce components and there is no question over their sustainability as the salts are either mined, are by-products of the existing chemical industry or are simple commodity chemicals with diverse supply chains.

Its green and safety credentials are impressive too, for example, Plentigrade P58 is a food-grade product. It’s certainly not advised that anyone eats it – but sodium acetate trihydrate – its core ingredient – is actually a flavouring used in salt and vinegar crisps. There’s definitely no combustion risk and the PCM can also be safely recycled back into the ground if necessary.

[My bold]




LATENT HEAT STORAGE

Sodium acetate is mainly known for its use in heating pads, hand warmers, and hot ice. Sodium acetate trihydrate crystals melt at 58 °C. When they are heated beyond their melting point and subsequently allowed to cool, the aqueous solution becomes supersaturated. This solution is capable of cooling to room temperature without forming crystals. By agitating the solution it crystallizes back into solid sodium acetate trihydrate. The bond-forming process of crystallization is exothermic. The process can be repeated indefinitely.

[My bold]

Martyn1981

LOL, time flies.

I'd forgotten how long we've been chatting about Sunamp. I remembered posting a variant idea of their's on a G&E thread a 'while back', so Googled 'Sunamp + barges', and it was much longer ago than I'd realised. Not sure any progress has been made with this idea since:

Sunamp to transport waste heat by barge in project with UK city council

The company has been in discussions with the local authority for months and is currently in the process of applying for funding from Innovate UK. Credit: Jen Jones/Moxy Inc
Sunamp is preparing a landmark project with the UK's Bristol City Council that will see the company take excess heat from waste treatment facilities by barge to be used in the city’s district heating scheme.

Trewin explained that the company would then transport up to 32 containers carrying 64MWh of stored heat by barge to the city’s district heat scheme, which is currently under construction to move away from gas.

Reed_Richards

Martyn1981 said:

I'm not with you? It's the analogy Sunamp use, when talking about their product, and the salesmen use the hand warmer (let's say heatpack) in their demo's.

I have always believed that the quiescent state of the Sunamp unit in-use is that the material inside is at the phase change temperature, 58 C.  You can extract a lot of heat at that temperature when the unit is charged and add a lot of energy to recharge it without causing its temperature to change.  It seems obvious to me that that is what you want to achieve for anything that is used frequently.  The fact that the liquid state can become supersaturated/meta-stable is an inconvenience which I hoped you could avoid, perhaps by adding some sharp objects.   

Martyn1981 said:

Sodium acetate is mainly known for its use in heating pads, hand warmers, and hot ice. Sodium acetate trihydrate crystals melt at 58 °C. When they are heated beyond their melting point and subsequently allowed to cool, the aqueous solution becomes supersaturated. This solution is capable of cooling to room temperature without forming crystals. By agitating the solution it crystallizes back into solid sodium acetate trihydrate. The bond-forming process of crystallization is exothermic. The process can be repeated indefinitely.

But if you used the Sunamp unit very infrequently (say less than once a day) then the heat wasted in letting it cool to room temperature and then somehow triggering the phase change, warm itself back up and then start heating the water might be less than the heat wasted from keeping the unit at a constant 58 C.  The paragraph above suggest that there might be an internal stirrer.  You might need a stirrer to stop the pipes being coated with solid phase material.  You would have to link it to the pump so it comes on when the pump comes on.  Then what happens when it breaks?  If you can get away without a stirrer it would be much better.  If there is a stirrer inside that is something I never knew before.  
    

Martyn1981

Right, I'm with you about the temp. Using the properties of a phase change, you try to keep the energy in, and the energy out within the phase change window, have I got it now? A bit like operating a battery between 10% and 90% but not (in this example) go full solid, or full liquid?

What I wasn't getting was how comparing to an 'identical' product chemically, you thought was a bad analogy, but you mean (hope I've got this right now), not the product comparison, but the fact that those heatpads go above and below the phase change ...... am I getting warmer (heating pun intended)?

As I said before, I think we were missing what the other was saying, and perhaps talking about different parts of this.


So, going back to the start, and this is only my thoughts, I don't pretend to understand all of the chemistry and physics, but yes I can see the point you are making, that if the product is kept within the phase change, then it may not lose any energy. That's energy input v's energy output. Yep I see that. But whilst in the phase change window it'll be at around 58C, so well above ambient, and therefore potential losses?

One question though, and admittedly this is based on reheating those pads. If you don't get them 100% liquid, then they will go back to solid as they cool. So if the PCM doesn't reach 100%, or perhaps 101% (with some waste heat), what would stop it cooling down, and losing all heat, as it returns to a solid? And going back to the point I made when I first mentioned the pads, once they've fully phase changed, and the energy is locked in, then unless hot water is wanted immediately, that material will still want to cool from ~58C to ambient, with potential losses to the environment.

I appreciate that a well insulated box would mean that it takes a long time to cool down, but that would defeat the whole purpose though wouldn't it, that the heat is stored permanently without loss, once full phase change has happened?

[Made some edits, additional ponderings.]

Reed_Richards

The way my hot water cylinder works is that it gets heated up to 50 C by passing water that's a bit hotter than 50 C through it.  I can set the hysteresis and in my case it's set to 5 C so when the tank temperature drops below 45 C it gets charged up to 50 C again.

I presume the Sunamp unit has a thermostat (or thermostats) that causes it to call for heat as soon as temperature drops a bit below the magic 58 C and turns off the charging process once the the temperature rises a bit above 58 C.  The difference to my hot water cylinder is that if the material inside is in its supersaturated liquid state and it drops below 58 C then it takes very little energy to heat it back up.  If it's in its solid state it takes in a lot of energy.  But it has to be compatible with standard heating controllers and fortunately these don't worry if a process that took little energy one day takes a lot more energy on another day.

Whichever state it is in, it will lose some heat over time, as any well-insulated box containing something at 58 C will.

If you go away and turn off your hot water heating then the Sunamp might cool down to room temperature and possibly retain it's stored energy (because the material it uses can be supersaturated).  That's good because when you return and turn on your hot water, the controller will see that the unit is calling for heat, take it back up to the bit-more-than 58 C but that won't require much energy.  If that's truly how it works (and I'm just guessing) then Sunamp would be particularly well-suited to a holiday cottage that you only visit at weekends (if a combi boiler is not an option).