How do Sunamp heat batteries work?

Martyn1981

Reed_Richards said:

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. 

First para. They stay in the liquid form, unless triggered.

Second para. That's why I suspect you are wrong that they don't fully phase change or there would be no truth to their claims that they don't lose heat, like a hot water tank. Or, you're spot on, and their claims are iffy.

Third para. Sunamp do seem to claim that in a domestic situation the phase change means energy is stored. But again, I don't see how that is true. Whether they have better insulation or not, I'd assume that the time it takes for the box fill of PCM to cool from ~58C to ambient (but stored energy), would be longer than the delay before hot water is needed, so any extra heat (after phase change storage, but before release) will mostly be used too.

Just to be clear, I'm not being critical of their product, I suspect you feel the same. I just don't think any savings v's a hot water tank are significant enough to justify the high cost of their hot water alternatives. Does that make sense.


Regarding this comment you make, please tell me you aren't suggesting that I or anyone else has said that the PCM is triggered in the same way? If that's your belief, then please provide a quote.

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.

@Coastalwatch - Hiya mate. The domestic heating units are effectively the same idea, but with larger energy storage. Think storage heaters, but less leaked heat before you ask for it.

My concern though would be how the heat is added. If it's just resistive heating, then even a simple A2A alternative would be more efficient. If the store is heated via a heatpump, then that's a bit better, more complexity, but a store. Seems to me that a HP running 24/7 would be simpler, but I suppose a store might add a buffer, but eventually you'll be running off the HP (via the Sunamp) once exhausted?

But countering my own argument, if you deployed a lot of heat batteries in say a block of flats, then you might be able to time shift some of the energy demand v's a basic HP, but again, the same is possible to an extent with the HP, but using the inside of the house as the battery, just less well insulated.

I'm guessing at a lot of this.

Going way, way back to the start. I'd have thought buying a few extra hot water tank jackets is a much simpler and easy solution.

70sbudgie

I believe that SunAmp use the same PCM as the handwarmers, but a different mechanism for the phase change bit. 

Phase change does seem to be a bit "magic". I believe it is to do with physical chemistry. A chemical has a different amount of energy within its inter molecular bonds depending on whether it is in a solid phase or a liquid phase. PCM technology is an attempt to harness this inherent energy. Every chemical has a melting point, which is the temperature at which it changes from solid to liquid and vice versa. Because physical chemistry isn't perfectly exact and is about probability, it is more like a temperature range. 

If you can maintain your PCM at its phase change temperature and find another way to trigger the phase change, you can control the energy flow. The chemical will always try to return to the lowest energy phase (solid), so to keep a PCM in the higher energy state (liquid) you would have to keep inputting energy. I think this is where the losses occur. The small amounts of energy required to keep the PCM just above its "freezing point" cannot be recovered. 

This input energy doesn't have to be heat, so I doubt the Sunamp includes a resistive heater. Electricity would be an adequate source of energy. 

And there is no limit on how often a chemical can change phase, so in theory the PCM can't run out. (I'm sure the laws of physics have some input into the reality of this). 

The chemistry and physics of PCM technology make it totally different to a hot water tank where all the stored energy is heat. In a PCM, the stored energy isn't heat. I think that the phase change temp of SunAmp being 58°C is misleading. The temperature of the PCM isn't how it provides heat, the heat comes because changing from liquid to solid is exothermic. In theory, you could have a PCM with a phase change temp of 25°C and it still provides the same amount of heat. But I imagine that the control would be more difficult. 

Martyn1981

Yep, I assumed a different mechanism too. I get the impression RR thought I'd assumed that the Sunamp had manual clickers ..... so to speak.  

Doesn't have to be resistive heating, that was just in a leccy based solution I was giving, which could work with PV, night rate leccy etc. Could also be heated by a gas boiler. I'm thinking heatpump, might be their best variant?  

I think the PCM used here has a phase change of around 58C (in fact I'm sure I found some reference to it actually being called PCM58, but that may just be their 'nickname'), in order to heat the water high enough. However, I do recall a fun idea with furniture where someone was selling a large table that contained a PCM that operates at about 20C, or so, the idea being that bas the heat rises, the PCM abosrbs heat to stop the room heating too fast, and then as the room cools, the PCM again helps to stabilise the temp. In this case it won't be a PCM that 'locks' for want of a better word, just something like ice to water to ice but at around 20C. Sounded like a great idea, but I don't know how much impact it would have, and it was pretty expensive. 

Edit - We may be at crossed purposes, but I'm not sure about your 25C PCM example. That would be more complex as you say, reguiring a heatpump to provide hot water at around 50C. My understanding of PCM's, like the table example, is that you want one with a phase change temp that roughly matches the role for which it's needed. For instance, simply using ICE (OK water) as a storage material in conjunction with large A/C units, is being used to improve efficiency, as they can 'store' cold during low demand periods, for later daytime use. Effectively the water, via a phase change to ice, can help to store excess RE leccy, yet another form of battery. [I may however be contradicting myself regarding the use of PCM58 in Sunamp's space heating role, but RR may be able to help me out here, as space heating does require a decent temperature differential, hence why PCM58 would be suitable for ~20C space heating .... I assume?]


Sorry for the edits, but I'd missed this bit:

The chemical will always try to return to the lowest energy phase (solid), so to keep a PCM in the higher energy state (liquid) you would have to keep inputting energy. I think this is where the losses occur.

I think this is where the 'magic' of sodium acetate trihydrate comes in, since if heated thouroughly into its liquid state, then it will happily sit for years in this stored state until 'triggered'. Hope that doesn't seem petty.

So you wouldn't necessarily need to keep adding energy. But as RR mentioned, and I can see his point, if you use the PCM within the phase change window, as a way of storing greater energy (than water) for a given space*, then yes you'd have to keep the energy supplied to match any energy escaping, such as heat loss.

*I'm not sure if we've mentioned this benefit, and yet again I feel bad for criticising Sunamp, as it is a benefit - a Sunamp water heater can provide more heat for a given volume, than a hot water tank, thanks to the PCM.

Reed_Richards

The energy that you have to supply to cause a phase change from the colder phase to the hotter phase is called "latent heat".  It's necessary because the molecules of the hotter phase are less tightly bound to each other and releasing the tightness of the binding requires energy.  Going from the hotter phase to the colder one releases energy.  Sunamp boxes work by taking advantage of this latent heat to store more energy per unit volume than a hot water tank can.  Other than that they are like any other thermal store, AFAIK.  You can heat them with an embedded electrical element, just like an immersion heater.  And some models can be heated by passing hot water through them

Phase changes don't usually "lock" so don't usually need a "trigger".  Usually phase changes happen spontaneously as the temperature gets hot/cold enough.  But when you freeze a liquid that is crystalline in its solid state, it tends to take less energy to grow a crystal on an existing crystal surface than it takes to form the first crystal when there is nothing but liquid.  Sometimes a bit of debris or a sharp point can substitute for that first crystal but in the absence of anything like that it you may be able to "supercool" the liquid below the temperature at which it would normally solidify.  That's part of how the reusable handwarmers work.  And @Martyn1981 tells us that the handwarmers are so good that they can stay in this supercooled state for at least a year.

I have never read a claim from Sunamp that their boxes don't lose heat, just that they are better (and more easily) insulated than a hot water tank.  Do we have a reference for this claim?  Clearly it is possible to make a "heat battery" that stores latent heat at ambient temperature because that is exactly what the handwarmers do.  

But if I was designing the Sunamp box I would make sure it did not supercool and did not require a "trigger" because that adds extra complexity and no benefit for domestic use that I can see.  You don't want to store the heat for a long time and you don't want the release of that heat to be all or nothing as it is with a handwarmer.  Nor do you want to have to fully charge the unit with heat in order to reset it.    
     

Martyn1981

Good point RR about phase changes not normally 'locking'. I think that's quite important to this discussion.

I'm going way off topic now but a decade or so back there was mention of a product that could store heat seasonally. It's a type of stone that's extremely porous, and if dried out thoroughly and kept dry (that's the hard part) will only release the heat (energy) when it gets wet. I think the name of the stones in Greek literally translates into steam stones or heat stones, or something like that. The idea being you might have tonnes of this stuff in your house providing seasonal storage ..... well perhaps theoretical dream, I'm not sure if it progressed.

Also an Aluminium oxide can do this, but my memory is beyond exhausted, as I read this stuff, so, so long ago.


Edit - BTW RR, I've tried to find a reference for how long sodium acetate trihydrate can store the energy, and have read a few papers, but they tend to say 'long time with no heat loss', 'seasonal or longer', or 'biannual or longer'. 

Solarchaser

This discussion has moved on somewhat.
I'm not sure if sunamp ever claimed they don't lose heat.

I claimed they lost heat in the conversion,  RR claimed that they didn't lose any heat.
That's pretty much been what started this 6 page long conversation. 
I said I recalled about 85% conversion,  so 15% loss.
70sbudgie came upon a similar figure by looking at the different sunamp units. (Also thanks for your other half's contributions,  helped with my understanding 👍)

CW suggested that the ceo had a hairpiece and should perhaps be second guessed on any other presentation, as that was some form of deception.... I've always felt the same about the ceo watching the sunamp presentations.

Sunamp mostly work with pcm58 as Mart stated, but do offer several other "concoctions" at different temperatures. 

Sunamp market themselves on simplified pipework, which is true, smaller package for the same amount of storage, which appears to be true, and less losses overall... of which there seems very little evidence.

On any of the electric powered or backed up models they use a 2.8kw immersion.
There are heat pump models, but they only work with specific heat pumps due to the high temperature required. 

Side note, Mart the Stone you mentioned about, I believe that's at the heart of the warmstone offering, which is a mahoosive heavy block of £15k last time I looked, and the internals are like 600C iirc

I've probably misquoted everyone 🤣

Martyn1981

Solarchaser said:

This discussion has moved on somewhat.
I'm not sure if sunamp ever claimed they don't lose heat.

I claimed they lost heat in the conversion,  RR claimed that they didn't lose any heat.
That's pretty much been what started this 6 page long conversation. 
I said I recalled about 85% conversion,  so 15% loss.
70sbudgie came upon a similar figure by looking at the different sunamp units. (Also thanks for your other half's contributions,  helped with my understanding 👍)

CW suggested that the ceo had a hairpiece and should perhaps be second guessed on any other presentation, as that was some form of deception.... I've always felt the same about the ceo watching the sunamp presentations.

Sunamp mostly work with pcm58 as Mart stated, but do offer several other "concoctions" at different temperatures. 

Sunamp market themselves on simplified pipework, which is true, smaller package for the same amount of storage, which appears to be true, and less losses overall... of which there seems very little evidence.

On any of the electric powered or backed up models they use a 2.8kw immersion.
There are heat pump models, but they only work with specific heat pumps due to the high temperature required. 

Side note, Mart the Stone you mentioned about, I believe that's at the heart of the warmstone offering, which is a mahoosive heavy block of £15k last time I looked, and the internals are like 600C iirc

I've probably misquoted everyone 🤣

I never said that, how very dare you sir, how very dare you! 

Decided to go looking for my Greek steam stones, what a weird Google search that was, but finally found them ... drum roll ..... zeolites, and 10yrs ago (time flies and I'm getting old).

Zeolite Stores Thermal Energy For Unlimited Amount of Time

Zeolite is a mineral that can store up to four times more heat than water. And what’s better, unlike water which gradually cools off, zeolite retains a hundred percent of the heat for an unlimited amount of time.

Zeolite – which means ‘boiling stone’ in Greek- was named for its peculiar properties. Zeolite is extremely porous. So much so, that a gram of the stuff has a surface area of a 1000 square meters (10,764 sq ft). When water comes into contact with zeolite it is bound to its surface by means of a chemical reaction which generates heat. Reversely, when heat is applied the water is removed from the surface, generating large amounts of steam.

The transference of heat to the material does not cause its temperature to rise. Instead, the energy is stored as a potential to adsorb water. The Fraunhofer scientists used these particular properties to turn zeolite into a thermal storage system. They created a storage device and filled it with zeolite pellets. To charge the pellets, they exposed them to heat. To retrieve the energy they simply added water.

70sbudgie

I'm going to start this comment by saying that since I heard about PCM technology, it has bugged me that I can't get my head around it - it still seems like black magic. Physical chemistry was the only bit of my A level chemistry that I could understand, because I understood the physics. But it is still right on the edge of my understanding. I have previously discussed it a couple times with my oh (I know, we have strange conversation topics) but I have never managed to get the concept to click. I have made more progress with this conversation than ever before! Though I appreciate I have yet to have that lightbulb moment! 😂

I said I recalled about 85% conversion,  so 15% loss.

70sbudgie came upon a similar figure by looking at the different sunamp units. 

You did mis quote me 😉 but in doing so you have credited me with finding an answer I didn't! I couldn't find any energy data on the SunAmp data sheet. So I used the unit dimensions to calculate a volume and then used this graph:

  
   
This is from the Sunamp literature (comparing their material to something like water). 

(which shows the latent energy of the phase change as ~95Wh/l) to calculate a maximum energy output of a single, complete phase change from the smallest unit. That is when I applied your figure of 85% efficiency, to understand what it's working capacity might be. I can understand what's going on when working with kWhrs. Though it isn't possible to calculate a power output without understanding the speed of the phase change.

Someone mentioned two types of efficiency, I actually think there is a third. Which is how complete the phase change is. I.e. what % of molecules start in the higher energy state and what % finish in the lower. From what I understand (from my oh), it won't ever be 100% to 0% because of the randomness of molecular physics (if that is the correct terminology). I'm sure he has told me about the challenges of 100% crystalisations.

The ~95Whrs/l appears to correspond to less than 1° temperature change.

Phase changes don't usually "lock" so don't usually need a "trigger".  Usually phase changes happen spontaneously as the temperature gets hot/cold enough.  But when you freeze a liquid that is crystalline in its solid state, it tends to take less energy to grow a crystal on an existing crystal surface than it takes to form the first crystal when there is nothing but liquid.  Sometimes a bit of debris or a sharp point can substitute for that first crystal but in the absence of anything like that it you may be able to "supercool" the liquid below the temperature at which it would normally solidify.  That's part of how the reusable handwarmers work.  And @Martyn1981 tells us that the handwarmers are so good that they can stay in this supercooled state for at least a year.   

My oh called this introduction of crystals "seeding". 

If you hold a chemical at its phase change temperature, what causes it to then change phase? I understand that it needs some sort of catalyst. As RR said (sorry if that is over familiar, I appreciate that I am a relative newby to this board), it can be spontaneous. But then it will seed itself and you get a runaway exothermic reaction. It seems to me that the magic is in finding a PCM that has a narrow phase change temperature so that you can control the process - it doesn't spontaneously solidify, but will solidify easily if you somehow "trigger" it. 

Adding the bit about power above, made me realise that you would also want to control the speed of the phase change. And how would you stop it part way through?

I understand that the handwarmers use super saturation. (My oh explained this when helping my daughter grow pretty crystals). By heating the solvent, you can dissolve more of the salt (?). Then when the solution cools, it is super saturated and it is easy to start the exothermic, solidification phase change. Somehow the clicker in a handwarmer does this.

It also isn't true that the handwarmers last almost indefinitely. I reckon the ones in our kitchen drawer lasted about a month before spontaneously solidifying.

70sbudgie

I have totally skipped past the Zeolite stones as that may make my head explode!

Reed_Richards

If you are used to boiling water you should be able to get your head around a PCM.  If you put a pan of water on the stove and turn the light on under it then the water gets hotter and hotter until it reaches 100 C and then it doesn't get any hotter than 100 C!  Instead steam comes off.  This continues to happen with the water remaining at 100 C until it has all turned to steam and you boil the pan dry.  You have still got the light on the stove so you are supplying energy but your material, water in this instance, isn't getting any hotter so where is the heat going?  It's supplying the energy necessary to turn water into steam.  We are all so used to this that it seems perfectly natural but it's just the way that a PCM works.

You have probably never done this but you could take some crushed ice out of the freezer, put it in a pan on the stove, put a light on under the pan and put a thermometer in the ice to measure the temperature.  As it has been in the freezer it might start at -10 C, it will heat up to 0 C (you would probably need to stir it to distribute the heat from the pan). But when it gets to 0 C it won't get any hotter for a while as the ice changes to water.  Once all the ice has become water the temperature will start to increase again.  This is exactly what happens to the PCM in a Sunamp box except that it happens at 58 C instead of 0 C (and it looks as if the heating elements are well-spread inside the box so you don't need to stir it).

I wondered to myself if you could supercool water in the same way that you can the PCM inside the reusable handwarmer.  Turns out that you can so that might be a fun experiment to try at home: https://www.thoughtco.com/how-to-supercool-water-605972