27-08-2017, 23:09

Hello,

For those of you less familiar with electricity and maths, I bring your attention to some specific sections of Chuck Wright document and give my understanding. Then, it will be easier to have a commun understanding on what's good and what's not.

1- Basic Electricity : " A Current Source is an element that delivers a constant current, regardless of the voltage across it."

The voltage across the PV comes from the shunt resistance we add. As we expect constant current, voltage will be higher if we use a large shunt resistance and lower with smaller resistance.

2 - Appendix A, pv cells : "Over part of its operating region, PV cells act like a current source."

That's where we want to operate : when PV is acting like a current source which gives constant current. Constant current gives reading proportional to sun irradiance, required for a pyranometer.

3- "Note that when voltage is very low, the cell acts like a current source. That is, it delivers a rather constant current, independent of the voltage."

To get proportional reading, we have to have the output voltage low. How low ?

Appendice A, Pitfalls : "The method described above limits PV output voltage to about 0.4 volts, which keeps the measurement reasonably linear with radiation variation."

As an example, for the blue line (1 sun), current is really constant from 0 to 0.285v. So I would not go up to 0,4 volts as indicated in the document to limit heat. I would stay close to 0,1 - 0,15 volts / cell. So linearity does not require to be exactly on 0,1 v per cell at full sun. No need to pay for 1 % resistance. All we need is to keep the shunt resistance to have a voltage lower than 0,285 volts (at full sun) and we are sure current will be proportional to irradiance, But target around 0,1 volt per cell to limit heat. Later I will give you an easy way to avoid the pitfalls in measuring short current.

Next, I'll look at one cell panel vs solar panel with multiple cells.

J Guy

I just hope image can be seen

For those of you less familiar with electricity and maths, I bring your attention to some specific sections of Chuck Wright document and give my understanding. Then, it will be easier to have a commun understanding on what's good and what's not.

1- Basic Electricity : " A Current Source is an element that delivers a constant current, regardless of the voltage across it."

The voltage across the PV comes from the shunt resistance we add. As we expect constant current, voltage will be higher if we use a large shunt resistance and lower with smaller resistance.

2 - Appendix A, pv cells : "Over part of its operating region, PV cells act like a current source."

That's where we want to operate : when PV is acting like a current source which gives constant current. Constant current gives reading proportional to sun irradiance, required for a pyranometer.

3- "Note that when voltage is very low, the cell acts like a current source. That is, it delivers a rather constant current, independent of the voltage."

To get proportional reading, we have to have the output voltage low. How low ?

Appendice A, Pitfalls : "The method described above limits PV output voltage to about 0.4 volts, which keeps the measurement reasonably linear with radiation variation."

As an example, for the blue line (1 sun), current is really constant from 0 to 0.285v. So I would not go up to 0,4 volts as indicated in the document to limit heat. I would stay close to 0,1 - 0,15 volts / cell. So linearity does not require to be exactly on 0,1 v per cell at full sun. No need to pay for 1 % resistance. All we need is to keep the shunt resistance to have a voltage lower than 0,285 volts (at full sun) and we are sure current will be proportional to irradiance, But target around 0,1 volt per cell to limit heat. Later I will give you an easy way to avoid the pitfalls in measuring short current.

Next, I'll look at one cell panel vs solar panel with multiple cells.

J Guy

I just hope image can be seen