Ditching gas, going electric immersion only, a wee project

70sbudgie

I've just tried asking my husband another question and now I have less understanding than I thought I did before and makes me think that what I wrote above is rubbish. Sorry!

Solarchaser

I take most of what you said on board,  but definitely disagree on the pump.
If a pump is used, that energy is used for rotation, not heat,  so it would be lost energy in your heat.

I remain unconvinced that heat that happens to be lazing about in the box would make its way inside the heat exchanger for the phase change material,  and if it doesn't, then its waste heat.

You seem to see conversion efficiency as energy, and in that respect, sure, since energy can neither be created or destroyed its totally efficient. 

The key though is heat in vs heat out.
If you put in 10 kwh of electricity for heat and get 8.5kwh of hot water out, that's a conversion efficiency of 85%.
You can certainly say the energy isn't lost or whatnot,  but thats not really relevant,  what's relevant is the usable heat you get out.

There were multiple reasons I didn't get a sunamp:-
Lack of verifiable data was a big one.
Cost, another
Weight, another (my tanks are only super heavy with the water in them.
Inability to tell if its "full" or "half full", another
Reports of being unable to take power and supply hot water at the same time, think wanting hot water on a sunny day.
And final nail in its coffin was 2.8kw heater, no larger, so for my 3 hours of off peak I could only add a theoretical 8.4kwh

This wasn't and off the cuff decision, as I said previously I was very very close to buying several sunamps, but chose not to, and will continue to choose not to.

Reed_Richards

Your still not getting it.  You need the pump to create motion but you need to keep pumping because you don't get perpetual motion, internal friction is slowing the flow of material and it would stop (or be reduced to what you get with convection) if you turned the pump off.  There is also friction inside the pump motor.  So the pump creates motion, the motion creates friction and the friction creates heat. 

If you have a sensitive enough thermometer, leave a small bowl of water to reach room temperature, then turn on an electric whisk and whisk the water.  Keep whisking and you should be able to measure the water warm up to a bit above room temperature.  You are using electricity to create motion but the motion creates heat.

Now imagine you have a box, any box, you put 10 kWh of electricity in and get 8.5 kWh of heat out so you have left 1.5 kWh behind.  In the short term we don't know what happened to that 1.5 kWh but if you do it time and again then anything that it could be doing other than heating (e.g. some physical or chemical change) will stop because you will have used up all the available material. 

Perhaps there's a car battery inside the box and your 1.5 kWh goes towards charging it.  That's fine until it becomes fully charged but after that the extra 1.5 kWh has nowhere to go so it has to come back as heat and you put 10 kWh in and get10 kWh out.  Perhaps there's a toy electric train inside the box and that 1.5 kWh goes to keeping it running.  But you need to supply energy to keep it running because of friction and that friction give rise to heat so if the box is well insulated the inside just gets hotter and hotter until the track melts or the engine overheats and after that you get 10kWh out for every 10 kWh in.  Or the inside of the box gets so hot that the standing losses get to 1.5 kWh So in the long term anything other than 0% "conversion loss" is not sustainable.  But it's possible your box, your thermal store, could get incredibly hot and then it's a huge standing loss that is your problem.

I don't like Sunmap.  I think they oversell their product and skimp on telling you the technical details.  But nothing, no matter how high or low tech can sustain an internal "conversion loss"!

 

Solarchaser

Erm, quite simply, no.

If you supply energy for heat, but some of it instead is used to move material around, then that is what the energy is used for, not heat.

Sure, you will get a byproduct of some heat, but most of the energy is used in the rotation of the pump, pushing its piston up and down, overcoming the inertia of the medium, moving the pump diaphragm up and down and keeping the fluid moving.
Of course you don't get perpetual motion, who suggested you do?
So each watt the motor uses to circulate fluid is used for that purpose, not for heat.

Using your own theory, take your bowl of water and use a 100w motor to pump the water round and round for an hour.
Now instead take a 100w element and put it in the water and power it for an hour.

Will the water temperature raise by using the pump, sure, of course it will, however if you think the water will be just as heated by the pump as by the element, then honestly I'm not sure what to say.

Either way, this is the same discussion from a month ago where you eventually admitted you had no idea what was in a sunamp while simultaneously arguing that you knew exactly how it worked and now it seems exactly how efficient it is.

No one cares how hot the box is.
What you care about is energy in = heated water out.
If you supply 10kwh of heat but can only draw out 8.5kwh of hot water, then that is a conversion efficiency of 85%.
It doesn't matter that the heat is in the box, it doesn't matter if the heat is flowing out somewhere else in the box, none of that matters, and thats where you seem to not get it.

You believe phase change is 100% efficient,  grand.
I've no idea, and thats irrelevant to this conversation. 
What matters is how much of the energy that you put in, comes out as hot water.

If the 15% is standing loss, fine, still irrelevant. 
If its 15% because 15% of the heat leaks in a day and its only expected to be cycled once a day, fine, still irrelevant. 
The only thing that's relevant is the hot water you actually get out.

You are focused on theoretical conversations about energy use, I'm talking about practical supply of hot water.

Reed_Richards

Sorry @Solarchaser, I'm focussed on theory because internal "conversion efficiency" is something that exists only in your head and not in reality.  It cannot be real because it defies the law of conservation of energy.  But standing losses are very real and should indeed be the full focus of your attention.  And you can have an external loss of energy if something outside your thermal store gets hot when you are charging the thermal store.

I know conservation of energy is difficult to grasp from our real world experience.  You charge up your electric car, drive it 75 miles to visit you mother and then 75 miles back home, park it in the garage and you need to charge it up again!  Where has the electricity gone?  You had to overcome some inertia to get the car moving but once it reached a steady speed there was no more inertia to be overcome yet still it ran down the battery, even though you had a clear run on the motorway, barely needed to brake and anyway braking gave you regenerative recharging.  The electricity you had has gone and it seems like energy provided from it has simply gone, vanished, been used up.  Conservation of energy says no, that's not true.  Your motor and your tires got a bit warmer and that heat was transmitted to the air during the journey.  The car had to push against air resistance.  For a while this created turbulent air but after a while longer this large scale turbulence was transmuted into greater motion of the individual air molecules and the air got very slightly warmer.  So even though you had a "green" electric car, the electricity you used for your journey ultimately became heat and made its own little contribution to global warming.  For the same reason the energy supplied to a pump ultimately becomes heat.

Sunamp; 

• Do I know what's inside the box? No. 
• Do I know how a phase change material stores heat?  Yes. 
• Do I know what conversion efficiency it has?  Yes, 100% because there is no such thing. 
• Do I know what standing loss it has (at some particular temperature).  No, I haven't tried to look it up.  And I don't believe reported standing loss figures for any thermal store or hot water cylinder because I think the tests are done without any pipe connections and I think these significantly degrade real-world performance.  However comparing different reported values of standing loss could give you a guide with which you can compare one thermal store to another.        

Solarchaser

Sorry, but most of that, while i appreciate that you were definitely trying for condescending, was twaddle.

Since there is no such thing in your mind as conversion efficiency, a 2 second Google is going to blow your mind:-
https://www.sciencedirect.com/topics/engineering/energy-conversion-efficiency#:~:text=The definition of the energy,into quantity and quality terms.

https://www.allthescience.org/what-is-energy-conversion-efficiency.htm

Tesla reckon their recharge efficiency is around 60%, that's because not everything you expend goes where you want it, and you are in somewhere along the right ish lines with your car analogy, although proving you know very little about cars, nor tyres, nor rolling resistance, but hey who cares right.
But roughly 80% gets to the road, and roughly 80% of that 80% comes back on recharge.

You have come on to show me what the big man knows, but have now made your argument so unbelievably focused to try and prove a theory that was never what we were talking about, that its pretty much a strawman.
I now see you adding internal, it's whole system, and always has been.

The conversion efficiency is how much energy you put in, vs how much hot water you get out, anything else, is just misdirection and nonsense. 
It was never about anything else, energy in vs hot water out!

I really really don't understand why you have come onto this thread to rerun this same silliness of a month ago.

Reed_Richards

I'm sorry you have taken offence, I was genuinely trying to be helpful.  Maybe one day you will realise this. 

In my opinion, the only way you can better the system you have at the moment would be to find something with a standing loss so much less than what you have now that it would pay back the extra cost over a reasonable amount of time (and be compatible with all your needs).  Looking at the price of alternative thermal store technologies like the Sunamp or the Caldera "Warmstone Heat Battery" I very much doubt that such a thing exists.      

Solarchaser

Offence is a strong word, Ive not been offended, I was a mixture of confused and amused if truth be told.

I am however quite ... surprised that you think your comments were to be helpful though, perhaps read the very first sentence in your previous post again, and see if you would see that as helpful rather than derogatory?

As for my existing system, I've already stated in a fair amount of detail exactly what I'm planning to make it less susceptible to standing loss, and how I'll be changing the configuration of the tanks to improve heat exchange to the main tank, and adding a 3rd tank to equip me with a larger capacity than I've required for the coldest days so far to ensure that in the two coldest months, I have more than enough capacity to deal with the harshest winter.

This is my project thread after all, not a discussion about sunamps or warmstone or tepeo, as I covered in the first couple of posts in this thread, that wasn't the route I was going to take.

Though of course I accept my original plan has not been 100% successful and the original route has changed, as it has required me to add extra heat through the day on several of the coldest days, hence the planned improvement in insulation and expansion of literage.

Reed_Richards

I'm profoundly glad that you have not been offended.  I think it is brave of you to attempt something so innovative and highly public-spirited to report how it has worked, both the successes and the set-backs.

I was trying to think of how one might improve the insulation around a water tank.  You don't require this insulation to be breathable, as you do in many domestic contexts.  I thought about a vacuum as an effective insulator, there's a nice little diagram here https://ph-instruments.eu/vacuum-insulation/  As a result, I discovered that you can buy Vacuum Insulated Panels such as these: https://www.kevothermal.co.uk/ which was something I had not known about hitherto.  Perhaps you have already considered these (or indeed are using them; I don't recall what insulation you said you used).  I didn't find a price so I have no idea if such a thing would be economic or not.    

Solarchaser

I hadn't even considered vacuum insulation to be honest.
I wasn't aware it cane in large sheets.
I'm not sure I said what insulation I was to be using, or have been using.

I plan to create a box of PIR insulation, both for the tanks to sit on, and be surrounded by, 2 layers of 50mm pir creating the box, and any space inside filled by knauf loft insulation. 
At the moment I just have knauf loft insulation laid somewhat haphazardly on top and around the tanks, but it definitely definitely can be improved upon.

I'll have a look into vacuum now, thanks.
I like the graph essentially saying its way better than pir etc, my instant though would be cost may be prohibitive, but its worth a bit of research to find out.
Thanks 👍