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Measuring Solar Radiation based on Photovoltaic Cells
#21

Not properly a schematic, but is something like this:

[Image: UV_SolRad_Sensor_v300.png]

Forget the greyed part of this circuit. R1 value will deppend of characteristics of solar cell used.



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#22

ok, thanks ! R1 is the shunt resistor, as you say it'll need to be chosen based on the cell in use, then R3 and R4 are your adjustable resistors to get the correct bias/offset, in your case you planned to get to 1.1v (and you replaced them with fixed resistors when you had selected everything)
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#23

That's right! After calcs, I keep R3 as a variable resistor just to allow some further small gain adjustment.
The software changes the Arduino ADC Vref to 1.1V, before making the readings, this way we get a better resolution and accuracy than let ADC Vref at 5V, and just raise the OP gain to reach 5V.



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#24

Hi, I am interested in this part of the project and I am just wandering do you have any of the PCB's available,

please PM me if they are still available

regards
tyntop.
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#25

(02-02-2015, 23:52)tyntop Wrote:  Hi, I am interested in this part of the project and I am just wandering do you have any of the PCB's available,

please PM me if they are still available

regards
tyntop.

Not yet. Maybe in Spring / Summer 2015 something like an almost ready kit will come out.



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#26

Hi,

The PV panel shown in post #6 looks as if it is a "real" crystalline silicon panel, but beware that many cheap "solar garden lights" use amorphous silicon panels which have several potential "issues":

Firsly, amorphous panels detect mainly visible radiation (light), whilst crystalline panels detect about as much also in the (near) Infra-Red band (as is required, I believe, for a correct watts/m2 value). Amaorphous panels may give a value nearer to a "Lux" measurement.

Secondly, amorphous panels are said to "age" (lose sensitivity) particularly in their first few hundred hours (which may be more of an issue for a product manufacturer than a Home Made system).

Thirdlly, I have measured a few amorphous panels which have had a relatively high reverese leakage current which might affect the accuracy/sensitivity of the measurement.

Finally, for all "PV" type panels, although they are basically a "flat plate collector", there is the potential issue that a high proportion of light at very low angles (to the horizon) will be reflected away by the glass surface and not reach the panel.

Personally, I am using a "normal" silicon photodiode such as the BPW34, which does at least have its sensitivity ("absolute" and "directional") specified in its data sheet. I'm not sure if this is very significant for "watts/square meter" measurements, but it might be significant if one is attempting to determine if "the sun is shining" when it crosses the horizon (at dawn or dusk).

As for the Op-Amp configuration, I prefer using it in a "virtual earth" current-mode configuration (similar to that shown in Appendix A of this data sheet.

Cheers, Alan.
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#27

(01-04-2015, 17:06)AllyCat Wrote:  Hi,

The PV panel shown in post #6 looks as if it is a "real" crystalline silicon panel, but beware that many cheap "solar garden lights" use amorphous silicon panels which have several potential "issues":

Firsly, amorphous panels detect mainly visible radiation (light), whilst crystalline panels detect about as much also in the (near) Infra-Red band (as is required, I believe, for a correct watts/m2 value). Amaorphous panels may give a value nearer to a "Lux" measurement.

Secondly, amorphous panels are said to "age" (lose sensitivity) particularly in their first few hundred hours (which may be more of an issue for a product manufacturer than a Home Made system).

Thirdlly, I have measured a few amorphous panels which have had a relatively high reverese leakage current which might affect the accuracy/sensitivity of the measurement.

Hi Alan

You are absolutely right. During my tests I discovered it by the hard way... testing a lot of different kinds of cells. Some of them, as you said, are not suitable for this job, specially the "amorphous" ones.
Its output current is very dependent on the angle of incidence of light, producing bad results at low angles.



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#28

I am trying to make a solar radiation data logger my self based on Arduino.
I have read all this interesting topic, its a great job you all have done.
I have a question.
Of course one should use a poly-crystaline PV cell as measuring device and not a thin film based one (amorphous or CIS or other type), however poly PV cells are strongly influenced by temperature:
Power output is about -0.45% per Celsius degree above 25 oC and voltage output is about -0.35% per oC above 25 oC. And off course the temperature is the temperature of the cell, not the ambient temperature. Normally the temperature could be 65 oC during a sunny day.
How this is can be compensated?
The error that is inserted is relatively high to my opinion.
Thanks
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#29

Well, you could always glue a suitable temperature sensor to the solar cell and program the compensation...
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#30

Hi,

Do you know or have access to a compensation table for silicon diode Photo-current versus temperature? It is well known that the output Power of PV panels falls as they get hot, but is that not because the output Voltage falls (by about 2 mV per degree C for each diode junction). However, for a watts/m2 measurement, the sensors are normally operated in "short-circuit" (zero voltage) mode, where the output power must by definition (P = V * I) be zero.

In the absence of any better information, I think we have to use one of the Davis Solar sensors as a "Reference" Design and from post #12 in this thread, it appears that the Davis sensor does not compensate for the diode/panel temperature, nor report it (for the station to perform the calculation).

I'm still "researching" the most economical method for measuring Solar watts/m2, but am currently planning to use a BPW34 photodiode which seems well-specified, and with a good (Co-)Sine angular response.

Cheers, Alan.
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