IoT / microcontroller based power switching

[Jeremy Harris]

8 hours ago, TerryE said:

@joth,

 

 

This struck a cord for me and something that I leant through my slab modelling.  I described in this in my blog post Plumbing Design – Part II, and which incidentally has been supported by 2 years of actual data on slab performance.  If we want to limit the output temperature of the inline heating element to say 75°C, then from "cold" the PCM cells will be in solid phase and may 30+°C colder than the circulation input.  The phase transition alone from melting the salt will absorb the full 2.8kW output of the element.  The cells will reach thermal equilibrium at the melt point of around 58°C (is that correct @Nickfromwales)  with the absorbed heat melting the salt and changing the % liquid.  At some point the % of solid salt and the melt rate will be such that for the flow rate of the pump, the cells will be incapable of 2.8 kW without the return water temperature rising to the point for the flow rate where the output from the element will rise above the 75°C maximum target temperature for circulation.

 

At this point we have a number of heating strategies:

1. Stop heating and accept that the cells are only charged to say 80-90% of thermal capacity.
2. Modulate the heat output of the element down and in practice this means some form of PWM modulation.  OK, the modulation epochs used to control something like a stepper motor aren't needed, but we do need to keep the temperature ripple on the circulating water within some form of tramline, say <5°C.  I need to do the sums but the on-off cycle is probably in the 10s of seconds rather than sub-second or super-minute.

Case 2 makes using a relay a no-no, IMO, and pretty much mandates the use of a zero-crossing SSD.  The pump relay is only switching a few times a  day and is carrying maybe 2-300 mA so a relay would work OK here.

 

@JSHarris would you agree with this?

 

 

 

Yes, that's what the Sunamp controllers seem to do.  The UniQ is a bit "all or nothing", in that it just turns the heater off when the temperature of the PCM, as measured at three (possibly four on the newer models) points reaches the target temperature(s).  The Sunamp PV modulates the heat flow into the PCM using the variable speed pump in the charge loop, together with the heating element relay, so if the PV diversion unit is only sending a small amount of power to the heating element the pump slows down to increase the temperature of the water leaving the heating chamber.

[TerryE]

5 hours ago, JSHarris said:

The Sunamp PV modulates the heat flow into the PCM using the variable speed pump in the charge loop

 

My PV manual shows the pump being driven by 240V through a relay which would tend to indicate that it is on / off rather than PWM.  I am going to have to trace the circuitry and get my mobile camera in at the right angles to get the part numbers of the components.  Maybe it's time to take the control board out of the unit.

[Jeremy Harris]

Just now, TerryE said:

 

My PV manual shows the pump being driven by 240V through a relay which would tend to indicate that it is on / off rather than PWM.  I am going to have to trace the circuitry and get my mobile camera in at the right angles to get the part numbers of the components.  Maybe it's time to take the control board out of the unit.

 

I think the pump may have a separate speed control wire, and that the relay is only used to turn the pump on and off.  When standing next to the Sunamp PV when it was charging, it was noticeable that the pump speed kept changing, as the power output from the heating element changed (this was with excess PV charging).

 

I think the pump they use has either a 0 - 10 VDC control signal or maybe a 4 - 30 mA one, a check of the pump model number should show which it uses.