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Posted (edited)

I never thought I'd spend so much time considering wax as a constituent building material. It's all to do with the latent heat of fusion being so high in comparison with heat capacity and that waxes can be chosen to melt at useful temperatures.

 

There are two places it has occurred to me to use it:

  1. In a ASHP buffer tank - during off peak electricity hours you could heat a buffer tank to just above the melting point of the wax within it. For example, if you had a total of 100kg of wax in capsules inside a buffer tank, melting that wax would take about 2MJ of heat or 5.6kWhr. A wax that melts at the temperature you want to run your UFH water at and so as the wax solidifies it will give a constant temperature. You wouldn't have to heat the floor itself whilst you are running the ASHP or run the ASHP when you are heating the floors. Has anyone done this? Shouldn't be particularly expensive.
  2. Underneath rafters in a loft space - lofts can get too hot. If you had wax under the rafters below the insulation it would stop the plasterboard getting hot. As an idea, if the under tile void is 20C higher than the wax's melting point and you had U=0.2 of insulation, 4W/m2 would pass through the insulation into the wax. If you had a 1.1mm layer of wax it would weigh 1kg/m2 and absorb 200kJ in the process of melting. That would take 14 hours so never actually melt unless the average inside temperature went above the wax's melting point, averaged across about a day. You would choose a wax at a comfortable upper limit to the room temperature. I have seen that Knauf produce a plasterboard (ComfortBoard) with beads of wax in on this principle. I think it is very expensive though. Has anyone used that or done anything similar?

 

Useful information of paraffin wax as a phase change material (PCM): https://www.intechopen.com/books/paraffin-an-overview/paraffin-as-phase-change-material

 

Edited by MortarThePoint

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Posted (edited)

As a comparison, just a panel of plasterboard behind massless U=0.2 insulation would heat up at a rate of dT/dt = 4W / (10kg * 1090J/kgK) = 3.6e-4 C/s gives ~1hour per degree C.

Edited by MortarThePoint

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23 minutes ago, MortarThePoint said:

In a ASHP buffer tank

Could call it a Sunamp.

 

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32 minutes ago, MortarThePoint said:

Underneath rafters in a loft space

Just remembered, a guy I know, researched some plasterboard replacements that used phase change waxes to store energy.

He fell for the marketing bluff (he was an ex marketing bloke), but when we all looked at the technical limitations, it was not so impressive as you are basically stuck with the one temperature range of about 3K.  You could make boards with variable ranges, in layers to increase that to say 10K, which may be useful.

Or you could go back in time and put a water tank in your loft and let the ambient air increase the temperature, a tank with a larger surface area to volume ratio would heat up easier.

Here is what has happened to mine the last few months.

image.png.16707644400a9ce02906855963e9d6ca.png

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Posted (edited)

Interesting to see the tank temperature.

 

The wax plasterboard won't do anything for you up until it starts to melt and then it should clamp the plasterboard, and therefore likely the room temperature too, until the wax has all melted. After that the temperature would continue to rise, but it's a lot of heat by then.

Edited by MortarThePoint

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Posted (edited)
1 hour ago, A_L said:

@MortarThePoint In case you don;t have it - https://www.pcmproducts.net/Encapsulated_PCMs.htm in particular look for 'BoardICE'

 

Thanks, I'll look in to that. I have also seen this which is for greenhouses so may be cheaper, but I notice the question in the top right corner of the website which may suggest expensive crops. Also greenhouses probably want to be warmer than 22C

Edited by MortarThePoint

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7 hours ago, SteamyTea said:

Could call it a Sunamp.

 

I hadn't heard of that before and have found the website: https://sunamp.com/

Not surprised it's been thought of before and I have found a thread in here as well. 

 

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I'm not sure the economics of a Sunamp compare favourably to batteries unfortunately. Comparing two approaches:

  1. 12kWh Sunamp charged up by running the ASHP off peak. Expected cost ~£167 - 250 per kWh.
  2. 12kwh Lithium batteries charged up off peak, inverter to then run ASHP. Expected cost around £150 per kWh [£100/kWh for LiFePO4 batteries and £600 for charger and inverter guestimate]

I'd expect both to have a similar efficiency (about 80-85% round trip?). Economy 7 tariff prices are around 21p/kWh day and 12p/kWh night. Storing "12kWh" saves:

 

(£0.21/kWh * 85% * 12kWh) - (£0.12 * 12kWh) = £2.14 - £1.44 = 70p/day

 

That's £256/yr or £2560 across 10 year service life. Not fantastic if the outlay is £1800 up front. Not sure if off peak electricity is more polluting than peak (suns not out, but demand for wind may be lower). If it's the same, the it's less environmentally friendly to store and use due to the losses.

 

If you charge from your own solar, or can have a cheaper homebrew PCM heat store then that's more likely to pay off.

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Oh dear, I think I have thought of two more issues unfortunately. I hope I'm wrong on the first as that's an absolute killer for commercially available PCM for everything other than immersion heater based applications in which case they are still debatable (e.g. you'd be better of with the batteries or having a small ASHP or showering in the morning which is most likely anyway).

 

Someone please spot the mistake as this is a bloodbath. @SteamyTea or @Jeremy Harris can you throw it a lifeline?

 

1. Capacity measured in Heat or Electricity Equivalent?

 

If the 12kWh Sunamp capacity is for heat, which I suspect it is (above assumed it was electricity equivalent) then the numbers are much less favourable:

  1. 12kWh(heat) = 4kWh(electricity) [1] Sunamp charged up by running the ASHP off peak. Expected cost ~£500 - 750 per kWh electricity.
  2. 4kwh Lithium batteries charged up off peak, inverter to then run ASHP. Expected cost around £200 per kWh [£100/kWh for LiFePO4 batteries and £400 for charger and inverter guestimate]

(£0.21/kWh * 85% * 4kWh) - (£0.12 * 4kWh) = £0.71 - £0.48 = 23p/day

 

I also falsely assumed you'd have the heating on all yeah (D'oh). Guess 200 days per year so £46/year or £460 across 10 years.

[1] assumes a constant COP of 300%.

 

2. COP worse at night

 

The Sunamp side of things gets worse due to reduced night time COP of the ASHP as well unfortunately. At night the outside temperature will be lower and so the COP likewise. Using the graph below, the COP at 3C is around 225% and the COP at 8C is around 300% (based on 50C flow temperature which has the smallest kink).

 

(£0.21/kWh * 85% * (12kWh/(300%/225%))) - (£0.12 * 12kWh) = £1.61 - £1.44 = 17p/day  [Assuming the capacity is 'electricity equivalent']

(£0.21/kWh * 85% * (12kWh/300%)) - (£0.12 * (12kWh/225%)) = £0.71 - £0.64 = 7p/day [Assuming 1 above]

(£0.21/kWh * 85% * (12kWh/300%)) - (£0.12 * (12kWh/275%)) = £0.71 - £0.52 = 19p/day [Assuming 1 above and avoiding the kink]

 

Guess 200 days of heating per year so £34/year or £340 across 10 years best case, £14/year or £140 across 10 years worst case. Perhaps 50% of the heating days manage to avoid the kink (overnight temperature above 3C would mean £26/year or £260 across 10 years.

 

First google result for "ASHP COP graph"

image.png.019d81c4be0b5f427aa5ba5756e96b43.png

https://originaltwist.com/tag/cop-for-ashp/

Edited by MortarThePoint

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5 hours ago, MortarThePoint said:

at 3C is around 225% and the COP at 8C is around 300%

You have to use the Kelvin scale to get the true difference.

To be honest, we should all use the Kelvin scale, for everything.

 

Phase change thermal batteries are not cheap.

I think in the order of things it is:

PV (if used when generating)

Water Storage

Battery Storage

PCM Storage

 

This is more to do with the scale of production and the usefulness (entropy)

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1 minute ago, SteamyTea said:

You have to use the Kelvin scale to get the true difference.

 

I'm just reading of the graph rather than calculating the Joule-Kelvin effect.

 

I suspect commercially available PCM looks to be a 100year payback candidate, i.e. never as it wouldn't last that long. If so, the environmental cost of manufacture would also never be recouped. It might work for a solar thermal hot water system, but again other technology would probably beat it there too (PV+batteries or just batteries).

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2 minutes ago, MortarThePoint said:

or just batteries

The trouble with batteries is that you have to oversize them.  So if you want 1 kWh delivered, you probably have to buy in 3 kWh.  This does depend on the load they are expected to deliver, the coulombs, as well as the chemistry and operating conditions.

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Just now, SteamyTea said:

The trouble with batteries is that you have to oversize them.  So if you want 1 kWh delivered, you probably have to buy in 3 kWh.  This does depend on the load they are expected to deliver, the coulombs, as well as the chemistry and operating conditions.

 

Yes, of course I haven't factored that in. Good point.

 

With LiFePO4 I think you get near best case life from keeping the state of charge between 20% and 80% (don't charge to 100%). That increases the capacity required to 167%, so 4kWh of usable battery capacity would require 6.7kWh of nominal battery capacity. I think your 3x thoughts are more to do with AGM type batteries. The charger and inverter wouldn't get any more expensive and I feel I was generous enough with £400 for those. That makes for a 4kWh usable battery costing £670 + £400 = £1070 or £270/kWh. That's still near halve the cost of a likely PCM store. PCM might start to have a look in if there is no ASHP available.

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1 minute ago, MortarThePoint said:

I think your 3x thoughts are more to do with AGM type batteries

Probably, it was a number I remember for years ago.

May be worth calling it 2 times now.

1 minute ago, MortarThePoint said:

PCM might start to have a look in if there is no ASHP available

I am lead to believe that was why the Sunamp was developed. It is a Scotch product and they have lots of low carbon energy, and domestic heat loads are the biggest in a house (up there, usually).  So as a direct replacement for an E7 system, there are not so highly priced and offer some space saving and user benefits.

Still way to expensive for a box of candles mind.

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It feels like there are a whole lot of environmentally focused products that perhaps work or are needed when you are Passive house, but simply don't stack up for a house with an ASHP or GSHP. The likely 10 year payback in the PH case is still not great and there are other solutions that, like you've pointed out, have better bang for the buck.

 

I'd still like some cheap wax in the walls though.

Edited by MortarThePoint

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4 minutes ago, MortarThePoint said:

I'd still like some cheap wax in the walls though.

I wonder about the fire risk with a wall full of lard.

 

I think, assuming that insulation is fitted correctly, airtightness is the important one, even without MVHR.

It is also the sort of thing that anyone can do, it is not a skilled job, just a bit of training and understanding why it is important.

 

Off to work now.

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