COP calculation with primary/secondary current

[dandjo]

Hi Altherma Fans.

So far I have calculated the power of the compressor and the COP using "INV Primary Current" and the voltage of my grid. However, this results in large fluctuations and apparently time-shifted data. If I divide the current of the pulsed 3-phase output signal "INV Secondary Current" by 3 and multiply it by the voltage "Voltage (N-phase)", I get much more plausible values for the power of the compressor and ultimately for the COP. Additionally I consider the power factor at lower currents. I packed this data into a function based on some measurement data from my smart meter. Here is the result of the compressor power calculation (openHAB ECMAScript).

var current = parseFloat(itemRegistry.getItem('espaltherma_compressor_secondary_current').state);
var voltage = parseFloat(itemRegistry.getItem('espaltherma_compressor_voltage_n_phase').state);
var emergency = itemRegistry.getItem('espaltherma_emergency_active').state;

current = current / 3;  // 3-phase sum

var power = 0;

if (current > 0 && voltage > 0 && emergency === OFF) {
  var power_factor = 0.752385 * Math.pow(current, 0.11962) - 0.00141164;
  power_factor = power_factor > 1 ? 1 : power_factor;
  
  power = current * voltage * power_factor;
  power = Math.round(power * 100) / 100;
}

events.sendCommand('espaltherma_compressor_secondary_power', power);

The heating power is calculated as follows (openHAB ECMAScript).

var power = 0;

var operation_mode = itemRegistry.getItem('espaltherma_operation_mode').state.toString();
var defrost_operation = itemRegistry.getItem('espaltherma_defrost_operaton').state;

if (operation_mode === 'Heating' && defrost_operation === OFF) {
  var flow = parseFloat(itemRegistry.getItem('espaltherma_flow_sensor').state);
  var lw_temp = parseFloat(itemRegistry.getItem('espaltherma_leaving_water_temp_before_buh').state);
  var iw_temp = parseFloat(itemRegistry.getItem('espaltherma_inlet_water_temp').state);

  var diff_temp = lw_temp - iw_temp;
  
  var water_density = 0.993;  // for approx 35°C

  var convert_flow = 60 * water_density; // converts l/min to kg/h for water
  var thermal_capacity = 1.1626; // of water in Wh/(kg•K)

  power = flow * convert_flow * thermal_capacity * diff_temp;
  power = Math.round(power * 100) / 100;
}

events.sendCommand('espaltherma_heating_power', power);

These are my graphs of the measured data (Grafana from InfluxDB).

What do you think? Would appreciate a discussion about it.

[sahni2021]

I also noticed large fluctuations calculating the COP using the primary current, but never thought of any for the secondary current. Now when I use the calculation below (inside Home Assistant) I agree with @dandjo in that it delivers more plausible values.

{{ ((state_attr('sensor.althermasensors','Flow sensor (l/min)')| float * 0.06 * 1.16 * (state_attr('sensor.althermasensors','Outlet Water Heat Exch. Temp. (R1T)') | float - state_attr('sensor.althermasensors','Inlet water temp.(R4T)')|float) ) / (state_attr('sensor.althermasensors','INV secondary current (A)') | float / 3 * state_attr('sensor.althermasensors','Voltage (V)')|float / 1000)) |round(2) }}

[dandjo]

What I have seen so far: primary current always precedes the heat generated, i.e. the compressor starts up, produces heat and shortly afterwards this results in higher temperatures. That's not the case with Secondary Current, which makes it implausible again. Also, dividing the current by 3 may not always result in correct values, especially at higher values.

[sahni2021]

I think to solve the problem with the current ambiguity the only solution might be to use an external device (e.g. Shelly 3EM) to measure the currents in real-time. Something I rather avoid due the high costs for the Shelly device and the cable clutter in the circuit breaker box ;-)

[Kepro]

I think the issue is with Voltage (N-phase) (V) it's not stable from 75 it goes up to 320 and my unit is on 1 phase so there should be around 230-240V always...

edit: it looks like with higher temperatures also Voltage is higher causing COP to drop... as I started with COP 12.1 and now it's 2.16 (voltage 340)

edit2: 57 celsiuz -> voltage 430

[dandjo]

Hi @Kepro be aware that N-Phase is the current after the SSR as I understood, not the current of your grid. It's the current after the transformation for the compressor motor itself. It's a PWM signal.

[Kepro]

hi @dandjo thanks for the quick reply, I just found a couple of formulas for COP, so I used it and many people complain about the same from my understanding... so is this then correct? https://github.com/raomin/ESPAltherma#calculating-cop

[dandjo]

@Kepro compared to my SmartMeter sometimes it's correct, sometimes not. I think the problem is the asynchronous reading respectively the offset the data from the registers. In average it's pretty close.

[Kepro]

@dandjo one more question, it's possible to use Voltage (N-phase) (V) or INV secondary current (A) or INV primary current (A) to calculate current load? I have 1 phase external unit 6kW

also today I'm heating and COP (outside 18C, 33 water temp) is above 10... that seems too high if I'm using formula from readme

[dandjo]

Voltage (N-phase) (V) is the voltage after the "transformer" for the compressor. INV secondary current (A) is the current after the "transformer" for each phase (there are always 3!). We assume that all 3 phases have the same voltage (I'm not sure if this is the case).

The other option is to mutiply INV primary current (A) with the voltage of your grid, since this is the current before the "transformer".

I'm not quite sure which method is better or more accurate in the end.

[pwwukpw]

Sorry new here ..

I'm getting

INV primary current (A): 0
INV secondary current (A): 0
Voltage (N-phase) (V): 203

Though COP appears to be calculated

Would be great to get accurate COP

[pwwukpw]

Managed to grab a capture with Amps and Volts showing .. V way above Grid voltage and both primary and secondary current. Installer did put a grid meter just before the heat pump but I think it's dumb. Any consensus as to which formula to use for accurate COP. With so many variables I'm keen to optimise operation for best lowest consumption.

[raomin]

I'll move this to the discussion section, and I would welcome any suggestion to improve the COP calculation on the readme :)