27-08-2017, 23:28

last one ...

Knowing this, you should feel comfortable using multiple cells panel for better precision and less components. Also, we do not really need to calculate the short circuit which requires an expensive multimeter having low resistance in current measurement. We can just use a voltmeter and measure the voltage across a trial value as shunt resistor and adjust it to be close or right on the target : 0,1 v / cell.

Start with higher value (like 10 ohms for a 5 volts panel) and go down till you get 1 volt across the shunt (for 5 volts panel or 0,1 volt/cell your panel has) at full sun. Not possible to have full sun at this time, like for me in Québec. How will you know how much sun you have while testing ? Apogee instruments has a Clear sky calculator http://clearskycalculator.com/pyranometer.htm that gives you the theorical value of irradiance according to you location, temp, humidity, etc ... It's used by pyranometer's owner to validate their pyranometer calibration to see if it requires calibration. So that's perfect to calibrate ours. We are going to pick the shunt and do our calibration in 2 steps.

Calibration :

Lets suppose your yearly maximum sun should be 1300 w/m2. Suppose Clear Sky calculator gives you 720 W/m2 when you do your testing. Current being proportional to irradiance, at this sun level, target voltage should be : 0,1 volt X ()720 /1300) = 0,0554 volts per cell. If you have a 10 cells panel like me, target would be 10 times X 0,0554= 0,554 volt. You probably won't be have a shunt giving exactly that voltage. Don't go too much over the target voltage, a bit lower is safer, closer gives more precision but generates more heat. Solder your shunt and do another test with clear sky value.

Clear sky irradiance : ex. 1050 w/m2

Your maximum sun irradiange : 1300 W/m2

Full sun target : 1 volt

The measured voltage with you shunt in place : 0,791 volt

How many w/m2 is you pyranometer ?

791 mv -> 1050 w/m2

1 mv = 1050/791 = 1,327 W / m2

Suppose now, you want to calculate irradiance from a voltage :

791 mv X 1,327 W/m2/mv = 1050 W / m2 ( as expected it was ou calibration value.)

As basic Arduino gives 4.88mv / step. Our precision would be : 4.88 X 1,327 = 6,48 w / m2 if we take the 5 volts as reference.

To have better precision, someone could buy a 16 bits I2C ADC board with 4 inputs to be used elsewher in your Weather station or use 2 shunt resistances to split the total shunt voltage and be able to use the 1,1 v reference. An easier way is to use this 1,1 v reference, is to ensure you are below 1 volt at full scale and even higher. As in the example, you could consider using 1500 W / m2 even if your maximum is 1300. This way your shunt voltage should never be over the reference.

I hope these long posts could help some people.

J guy

Knowing this, you should feel comfortable using multiple cells panel for better precision and less components. Also, we do not really need to calculate the short circuit which requires an expensive multimeter having low resistance in current measurement. We can just use a voltmeter and measure the voltage across a trial value as shunt resistor and adjust it to be close or right on the target : 0,1 v / cell.

Start with higher value (like 10 ohms for a 5 volts panel) and go down till you get 1 volt across the shunt (for 5 volts panel or 0,1 volt/cell your panel has) at full sun. Not possible to have full sun at this time, like for me in Québec. How will you know how much sun you have while testing ? Apogee instruments has a Clear sky calculator http://clearskycalculator.com/pyranometer.htm that gives you the theorical value of irradiance according to you location, temp, humidity, etc ... It's used by pyranometer's owner to validate their pyranometer calibration to see if it requires calibration. So that's perfect to calibrate ours. We are going to pick the shunt and do our calibration in 2 steps.

Calibration :

Lets suppose your yearly maximum sun should be 1300 w/m2. Suppose Clear Sky calculator gives you 720 W/m2 when you do your testing. Current being proportional to irradiance, at this sun level, target voltage should be : 0,1 volt X ()720 /1300) = 0,0554 volts per cell. If you have a 10 cells panel like me, target would be 10 times X 0,0554= 0,554 volt. You probably won't be have a shunt giving exactly that voltage. Don't go too much over the target voltage, a bit lower is safer, closer gives more precision but generates more heat. Solder your shunt and do another test with clear sky value.

Clear sky irradiance : ex. 1050 w/m2

Your maximum sun irradiange : 1300 W/m2

Full sun target : 1 volt

The measured voltage with you shunt in place : 0,791 volt

How many w/m2 is you pyranometer ?

791 mv -> 1050 w/m2

1 mv = 1050/791 = 1,327 W / m2

Suppose now, you want to calculate irradiance from a voltage :

791 mv X 1,327 W/m2/mv = 1050 W / m2 ( as expected it was ou calibration value.)

As basic Arduino gives 4.88mv / step. Our precision would be : 4.88 X 1,327 = 6,48 w / m2 if we take the 5 volts as reference.

To have better precision, someone could buy a 16 bits I2C ADC board with 4 inputs to be used elsewher in your Weather station or use 2 shunt resistances to split the total shunt voltage and be able to use the 1,1 v reference. An easier way is to use this 1,1 v reference, is to ensure you are below 1 volt at full scale and even higher. As in the example, you could consider using 1500 W / m2 even if your maximum is 1300. This way your shunt voltage should never be over the reference.

I hope these long posts could help some people.

J guy