How do Sunamp heat batteries work?

Martyn1981

Reed_Richards said:

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.    
     

Somethings been bugging me about this discussion. Are you sure that the Sunamp devices don't lock the energy in? That really seems to be the whole point to me, and I'm not sure why you keep suggesting that's how they work. Have you found that somewhere because it seems to run contrary to how I understood they worked?

So it was your use of 'all or nothing' with the release of heat, and your 'fully recharge' that suprised me. Are you assuming that the unit operates as one large singular unit, because that's not my assumption at all? Of course I may be entirely wrong, but I'd assumed it would work in a segmented fashion, heating some, and locking, as it heats the next until fully charged, and stored (locked), and when hot water is needed it works in the opposite fashion, it doesn't activate all of the PCM, just what's needed for the power demand, with more activated if the power demand lasts and more energy is needed. Now I'm even starting to doubt my name!

I can't think of a good analogy, and I know you don't like Sunamp's heatpad analogy, so perhaps a hot water tank with resistive heating. But instead of a single long 3kW element from top to bottom, it might have 3 3kW short elements (top, middle and bottom), operating one at a time - heat the top to desired temp, stop heating the top, and start heating the middle, and so on.

I've got half a mind to ask Sunamp to comment on this thread, but that's probably a bit rude after all the 'negativity' we've posted, and my conclusion that a hot water cylinder is cheaper. That would be a tricky e-mail - "Hi, been slagging off your product, and wonder if you could tell us how it works?" ...... maybe not.

70sbudgie

Martyn1981 said:

Reed_Richards said:

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.  

So it was your use of 'all or nothing' with the release of heat, and your 'fully recharge' that suprised me. Are you assuming that the unit operates as one large singular unit, because that's not my assumption at all? Of course I may be entirely wrong, but I'd assumed it would work in a segmented fashion, heating some, and locking, as it heats the next until fully charged, and stored (locked), and when hot water is needed it works in the opposite fashion, it doesn't activate all of the PCM, just what's needed for the power demand, with more activated if the power demand lasts and more energy is needed. Now I'm even starting to doubt my name!

I can understand not fully charging the unit. What I don't know is how you stop the solidification process part way through. The only solutions I can think of are to apply more energy to start recharging (which isn't too wild an idea) or to build it in cells of PCM, perhaps 1L each? That would make manufacture of different sized units easier, same components, just different multiples of them.

What is the SunAmp website like for asking questions? Perhaps a more tactfully phrased question asking to understand the PCM technology.

Martyn1981

70sbudgie said:

Martyn1981 said:

Reed_Richards said:

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.  

So it was your use of 'all or nothing' with the release of heat, and your 'fully recharge' that suprised me. Are you assuming that the unit operates as one large singular unit, because that's not my assumption at all? Of course I may be entirely wrong, but I'd assumed it would work in a segmented fashion, heating some, and locking, as it heats the next until fully charged, and stored (locked), and when hot water is needed it works in the opposite fashion, it doesn't activate all of the PCM, just what's needed for the power demand, with more activated if the power demand lasts and more energy is needed. Now I'm even starting to doubt my name!

I can understand not fully charging the unit. What I don't know is how you stop the solidification process part way through. The only solutions I can think of are to apply more energy to start recharging (which isn't too wild an idea) or to build it in cells of PCM, perhaps 1L each? That would make manufacture of different sized units easier, same components, just different multiples of them.

What is the SunAmp website like for asking questions? Perhaps a more tactfully phrased question asking to understand the PCM technology.

That last sentence had me laughing out loud, I do enjoy a good sense of humour.

Yes, my assumption, and it is only that, which I'm now doubting, is that the PCM would be segmented 'in cells', so it's charged and discharged in parts. Sort of the opposite of a battery bank where load is shared, more like having several battery banks, and operating them one at a time. But again, that's purely my theory, I simply hadn't considered the all or nothing approach before ..... a fun learning experience.

Reed_Richards

Martyn1981 said:

Somethings been bugging me about this discussion. Are you sure that the Sunamp devices don't lock the energy in? 

So let's think how they would do this.  The reusable handwarmer you make very hot (boiling water temperature) and then you let it cool to room temperature.  So you waste all the heat that is required to raise it from room temperature to 100 C in its liquid state but because it's inside a soft plastic bag with no nucleation centres for crystals to form you keep the latent heat of crystallisation for later use when you want your hands warmed.

If the same PCM is inside the Sunamp box you first have to make it hot enough to get rid of every last crystal or the locking mechanism would not work.  So you would have to take the temperature well above the phase change temperature for a while.  The Sunamp controller would control the internal temperature, say perhaps to reach 65 C then wait, say, 15 minutes maintaining at least 65 C, before it signals the heat source to go off.  So you might actually have to spend longer heating it than you would with a normal thermal source.

Now at 65 C (or more), it's all liquid and you want to extract some heat.  Down to 58 C you can get the heat out as per a water thermal store but at 58 C you want to start getting at the latent heat (you can't let it get cooler because then the water it is heating would not be hot enough).  But it's all liquid so you need to trigger the crystallisation somehow.  Let's assume you can do that and it starts to crystallize.  But now the thermostat stops calling for heat (CH) or the hot tap is turned off (DHW).  You have a mixture of liquid and crystals which you cannot lock.  Provided you are at 58 C this mixture is stable.  But to liquify and lock it again you have to heat it back up to well above 58 C again.

So so long as the Sunamp box is being used frequently your desire to lock it means that it spends most of it's time between 58 C and a higher temperature.  That means it will lose more heat through the insulation than if you just maintain it at a constant 58 C and don't try to lock it.

On the other hand if you go away for a few days having liquified the PCM it may have cooled to (say) 30 C by the time you get back.  No matter, you can use it immediately by triggering the phase change and it will rapidly heat itself and the water flowing though it back up to 58 C.

Martyn1981 said:

 Are you sure that the Sunamp devices don't lock the energy in? 

No , I'm not sure at all.  But locking would require the unit to be at higher average internal temperature than not locking for any normal domestic application where there are frequent calls for heat.  So you would lose more heat through the insulation and it would be less efficient.  On the other hand, if the the usage pattern was that it is off for much longer than it is on and the off times were for many hours or days then (and only then) would locking make economic sense.    





                 

Martyn1981

I get that, but again all of your arguments seem to be working on an all or nothing basis. If you heat up some of it to, I think it's 75-80C to reach 'lock in' (I'm sure that's not the correct term), then wouldn't some of that heat be available for the other sections within? Plus, your suggestion of operating in the phase change window, just means direct heating at 58C(ish) so no real energy in benefit over heating to 75C, as that heat is still within the box and can be used to heat the water.

Yes you would lose more heat if the whole box was a constant 75C than 58C. But if it's not the whole box, at the same time, and the lock allows for heat loss to eventually end (so not a constant 75C), then will it definitely be less efficient?

Edit - In fact, if it cooled down to room temperature, then losses would be less than 58C, but I totally get where you are coming from, and that temp loss (but only partial energy loss) is unlikely. Hope you are enjoying the headscrathing as much as I, but now I'm wondering how much energy is lost as the product cools from 75C to ambient, since it locks energy in. Now I'm totally baffled.

Do you see where I'm coming from?

[I settled on 75C just for the purpose of the discussion, I'm not sure what the exact temp is.]


And another edit - Sorry RR, I realise I should have said this at the start. I'm trying to find a way to make this product more energy efficient (not just smaller), as I'm somewhat baffled as to its purpose. So if it seems like I'm disagreeing about how it works, that's not my motive, I'm assuming there must be something more to the idea, otherwise ..... why?

Reed_Richards

Are there really sectors inside the box?  It would make the whole thing more complicated because you would have to control each sector independently but report to the outside world as if it's just a single entity.  You also need good thermal insulation between the sectors so there is no thermal crosstalk.  Each sector would need to be capable of being heated independently, which require multiple internal valves (heating by water) or switches (heating by electricity).  These must never fail.  You also need good insulation between the sectors.  You are adding a lot of extra complexity and so extra expense and extra possibilities for something to go wrong.  So its a lot of extra trouble.

And what have you actually gained?  You have gained the ability to lock-in the energy in some or all of the unit, something you would only make use of when you go on holiday.

That handwarmer analogy has a lot to answer for.          

70sbudgie

But once you've "locked in" the PCM to its higher energy state, you don't need to maintain it at 75°C. You only need to maintain it slightly above 58°C, so that it doesn't spontaneously change phase. (Like the handwarmers where you only have to boil it to reset it to liquid. Then you can leave it at room temperature until you trigger the phase change) If it is the same material as the handwarmers and operates under the same super saturated way, surely it would be stable at room temperature? Though, probably not for as long.

And if you have a modular (cells) design, I think you would apply the over temp to each cell in series to minimise waste / losses, from heat sharing between the cells. 

This may be what Martyn has just said. 

Martyn1981

70sbudgie said:

But once you've "locked in" the PCM to its higher energy state, you don't need to maintain it at 75°C. You only need to maintain it slightly above 58°C, so that it doesn't spontaneously change phase. (Like the handwarmers where you only have to boil it to reset it to liquid. Then you can leave it at room temperature until you trigger the phase change) If it is the same material as the handwarmers and operates under the same super saturated way, surely it would be stable at room temperature? Though, probably not for as long.

And if you have a modular (cells) design, I think you would apply the over temp to each cell in series to minimise waste / losses, from heat sharing between the cells. 

This may be what Martyn has just said. 

Yes, exactly, but to be super clear, I'm not at all sure that this is how it works, just an idea I have as to how to make it more viable.

@RR, I'm not sure you would need great insulation between sectors, just enough to allow one segment to reach the required temp (75C?) without all the heat moving on. Then any heat that does move is fine, that helps the next segment, and of course all the heat inside is still energy for water heating.

But, if I'm right and the higher temp is 75C or 80C, then I totally appreciate that the lost energy back down to 58C would, for a well insulated box, take a long time, so I totally understand your doubts, and see your point.

Reed_Richards

70sbudgie said:

If it is the same material as the handwarmers and operates under the same super saturated way, surely it would be stable at room temperature? 

Very possibly.  But it's in a well-insulated box so hopefully it would take several days of not being used at all for it to cool to room temperature.  That's only going to happen when the house is unoccupied (and everything is off) for several days.  So although it could be done, it will very rarely be of any practical use.    

Reed_Richards

Martyn1981 said:..... why?

My house has an airing cupboard that had fallen into disuse.  When I wanted to put a hot water cylinder back they had to take out a section of wall to make the airing cupboard a bit bigger in order to get the hot water cylinder and the associated pipework inside.  Something like that is not feasible in a lot of properties so a unit that performs the function of a hot water cylinder but is more compact has real utility.  I don't think there is anything more to it than that.

For example, @Solarchaser is heating their property using cheap rate electricity to charge some large tanks of water.  They are fortunate to have sufficient space to accommodate these water tanks and the necessary insulation.  With less space they might have been forced to use PCM thermal stores.