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ASHP sizing - value of Heat Transfer Coefficient

 
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Air Source Heat Pumps (ASHPs)
Last Post by Jancold 1 year ago
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Toodles
 Toodles
(@toodles)
Illustrious Member Contributor
Joined: 3 years ago
Posts: 2066
16/05/2024 9:11 am  

@jancold The PCM is heated up by a 3 kW element in the version I have (there are others where a high temperature heat pump can do the same thing instead), the unit itself is very well insulated and loses no more than 0.75 kWh of energy in 24 hours. When hot water is required, it passes through a length of pipework inside the PCM material and is heated to approx. 50- 55 degrees C - then to your tap / shower or whatever. (Rather like a Combi Gas Boiler might supply the hot water) I think if my memory serves that the PCM is heated up to 65 - 70 degrees C. and this is why an HT heat pump is required as an LT can’t raise the temperature to that required. We have a Sunamp supplied regulator on the output so that the water is supplied at 45 degrees C. for safety and comfort.

The energy can be stored in the PCM for days and the unit we have takes approx. 11 kWh of storage; sometimes, we don’t recharge for several days if it is dull (little solar PV) and the Agile price is too high for my Scrooge nature to allow it to run. The control wiring for the Sunamp is connected to a Myenergi Eddi and that indicates ‘Max Temp Reached’ when fully heated and then the control system monitors the temperature of the unit. The system may be a little ‘granular’ though as the LED status indicator shows ‘Full’ then a sort of ‘More than Half full’ , then ‘Half Full’ and then ‘Less than Half’ and finally ‘Empty’ there is also a ‘Mains Connected’ LED that indicates charging or at a standby status. This granularity worries some people a little I gather but I am happy with this as we rarely need more than 25% of the stored energy in a day - and the unit is only a quarter the volume of a conventional DHW tank and does not feature all that pipework that tanks seem to bristle with!

The system is also very unlikely to suffer from Legionella problems as water is not stored in any quantities at dangerous temperatures. As you might have detected, I rather like the system! Having very little space for a conventional tank went a very long way towards our decision to have a box in the airing cupboard and still have some room for the linen! Regards, Toodles.


Toodles, he heats his home with cold draughts and cooks his food with magnets.


   
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bontwoody
 bontwoody
(@bontwoody)
Noble Member Contributor
Joined: 4 years ago
Posts: 850
16/05/2024 10:55 am  

@jancold To add to what @toodles has said, you can imagine the phase change material as something like candle wax, which will melt before the maximum temperature of the store is reached. A property called the latent heat of fusion means that a lot more energy is needed to raise the temperature between solid and liquid and so this is stored in the liquid material.

I would imagine that the material closest to the coil would solidify initially as it heats the water but might melt again as heat is transferred from the outer volume.

Thermino's are a good idea and small but also a bit pricy and as toodles says need to be heated higher than a cylinder would.


House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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Toodles
 Toodles
(@toodles)
Illustrious Member Contributor
Joined: 3 years ago
Posts: 2066
16/05/2024 11:05 am  

@bontwoody I might add that the control wiring indicates to Eddi that the temperature required by the PCM closest to the element has reached temperature - at which point, the system stops calling for power. The Eddi is set to (I think) 20 minutes of rest and then it will try again to see if the ‘local to the element’ material has dissipated the energy into the mass of the unit and cooled down sufficient to request more energy to heat the element further. I have observed this happening and noticed the Eddi apply more power after this ‘dissipation’ cycle. Regards, Toodles.


Toodles, he heats his home with cold draughts and cooks his food with magnets.


   
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Jancold
 Jancold
(@jancold)
Reputable Member Member
Joined: 3 years ago
Posts: 129
16/05/2024 9:19 pm  

@toodles Thanks for the explanation I get it now as I had not realised the temp needed to charge it.


Daikin 6Kw ASHP.
MG4 EV
1926 Semi with loft and wall cavity insulation, all radiator.


   
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