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Measuring Solar Radiation based on Photovoltaic Cells
#31

(29-05-2015, 18:40)AllyCat Wrote:  Do you know or have access to a compensation table for silicon diode Photo-current versus temperature?

Not really. But I guess you may find food for thought in PVsyst's documentation where models used and calculations done are explained: http://files.pvsyst.com/help/index.html

"muIsc (often named Alpha) is specified on the main page of the parameters, and used as such in the model. For crystalline modules, its value lies usually between 0.04 and 0.08 %/°C

muISC = temperature coefficient of the photocurrent (or short-circuit current) / temperature coefficient on Isc [mA/K] or [%/K]"

etc...

Judging by the above, it would seem we could get an error anywhere from ~1,6% to ~3.2%, depending on the cell used, under 65º Celsius (0,04-0,08*40º difference from reference temp.).

For instance, if I do a simulation of the panel I am currently using to power the WeatherDuino under 1000Wm2 it should have a short circuit current of 5,34A at 25º TOper and of 5,42A at 65º (+1,5%, muIsc=0.039%/ºC).
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#32

Three ideas to ideally improve the measuring of solar radiation for your consideration and criticism:

Limitations to be addressed:
a. Capacitance issues limit the extension of wires to be used and the placement of measuring cells while potentially inducing a slight error due to voltage drops;
b. Arduino reference voltage is not always perfectly accurate;
c. Temperature of the solar cell ought to be taken in consideration in calculations an values read corrected accordingly.

Solution:
a. Use an additional Nano placed right next to the solar cell to do the readings and send them back as data (not voltage) to the TX unit (already there a couple of unused pins that might serve for this purpose?);
b. Use an external reference voltage feed to the appropriate pin by using a dedicated diode;
c. Stick a thermistor to the solar cell (does not seem easy to do to a diode) and program a compensation value to correct readings according to temperature.

This might imply having an additional board with its own power supply and regulator (?).
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#33

The WeatherDuino Pro2 Solar/UV sensor module has not yet been released, and you already have suggestions and solutions to improve it. Fantastic!

Have you thought about the implications of your suggestions?
Are they really applicable to the existing project?

If a sensor that displays the readings that can see below, and it is working stable since the last two years is not good enough for you, you should go for a real pyranometer!

[Image: solar.png]

Note: The low readings at the beginning of the day are caused by natural geographic elements existing at my location, that of course I can't avoid, unless I will remove a great hill east of my house.

Sometimes seems that you start with preconceived ideas and refuse to change them, even when facing important information.

(29-05-2015, 18:40)AllyCat Wrote:  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.

Do you will continue insisting on the issue of temperature compensation?

Almost everyone would like to use long USB cables, but that is not possible. Have you also complained about it to USB protocol developers?
I'm always open to ideas and sugestions that could improve this project, but those ideas and sugestions should be framed in the existing concept, and warrant compatibility with the existing hardware.

To be really honest, sometimes seems that us Portugueses, are the most critics of our own projects, always ready to enhance the less good points, instead of realce the good ones. Unfortunately this is a constant reality!

Despite of this pré-release, negative feedback about a thing that you has never seen or used, the WeatherDuino Pro2 Solar/UV sensor module will be released very soon!

One of these modules is already working in AU, and from what I know its owner is very happy with it. The solar radiation readings from this unit, nicelly follow the readings from other nearby weather stations.



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#34

That's perhaps a tidy bit oversensitive.

-> This was not intended as "negative" feedback in any sort of fashion. Just supplying a few topics to steer the waters and animate discussion.

-> If indeed it makes sense (perhaps it does not, that is why I asked for criticism) and or it fits the existing frame is something I leave for your better knowledge [hence the added “(?)”]

-> I am honestly happy with what is to be offered, in fact I was not the one raising the issue of temperature and it has been shown that other systems do not take it in consideration as most of the time it has a very minor effect.

-> It can be easily argued that what is suggested makes little sense as it would introduce a lot of complexity to harvest just minor benefits (who knows, perhaps a 1-2% accuracy increase?)

Take it just as a "good pupil trying to show some commitment and progress" Smile
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#35

Quote:That's perhaps a tidy bit oversensitive.

Sometimes I become very sensitive... especially with certain behaviours.

Quote:-> This was not intended as "negative" feedback in any sort of fashion. Just supplying a few topics to steer the waters and animate discussion.

I will try to forget this method to animate a discussion.

Quote:-> I am honestly happy with what is to be offered, in fact I was not the one raising the issue of temperature and it has been shown that other systems do not take it in consideration as most of the time it has a very minor effect.

If you think so, why you enumerate this possible issue, as one of the "Limitations to be addressed"

Quote:-> It can be easily argued that what is suggested makes little sense as it would introduce a lot of complexity to harvest just minor benefits (who knows, perhaps a 1-2% accuracy increase?)

From a free Google translation of the original article published in Portuguese language:

For purists, it is clear from the outset that I will not write about building a real pyranometer, but about simple equipment Constructed using economic materials and easy to purchase. Do not have a laboratory quality, but it has enough to get the measurements that can be observed in Meteocercal, and that over time have proven very comparable to those obtained by reputable brands, quality stations located nearby.

Lets be realistic, there is any importance for an amateur weather station, that the solar radiation readings are some one digit percent away from a precision reading from a trustable pyranometer?
Regarding solar radiation readings, as far as I know, if you put side by side two weather stations from a reputable brand, likely both will not have the same readings.

I know that I'm using on this, the most simplest OP amplifier circuit possible, a single stage non inverting amplifier, and that a lot better can be done, but this way, for start, almost everybody could understand how it works (it's not difficult using the second free op on the chip plus some extra parts, and build a much more output capacitance tolerant circuit).
Somebody published (many thanks for that) the schematic of a reputable brand solar radiation sensor, the first stage amplifier uses what seems to me an integrator configuration. hvalentim, could you please just calculate the used gain for me? Do you find the formula, or wish I put it here?

I'm not an engineer, I'm just a simple person that try to do simple things that work, and that almost everybody can understand how they work and build them themselves. This is the WeatherDuino concept since the beginning. Only after learning the basic, we can aspire more.



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#36

(03-06-2015, 17:33)Werk_AG Wrote:  If you think so, why you enumerate this possible issue, as one of the "Limitations to be addressed"

Because, although their prioritizing may be questionable, that's what they are. How come is such a "problem" looking for ways to improve?

Anyway, here's a very interesting article:
Martínez, M.A.; Andújar, J.M.; Enrique, J.M. A New and Inexpensive Pyranometer for the Visible Spectral Range. Sensors 2009, 9, 4615-4634.

Unlike trying to craft some sort of arithmetic compensation for temperature change they did it the other away around by building a system where temperature is kept stable at 40º.

Also interesting and potentially useful is the explained option for the use of a teflon diffuser to mitigate directional issues or "cosine error".

Among others, offers some cues as to why values tend to diverge early in the morning as well.

Now, unlike someone may think, as far as I see it, this does not serve to prove how "bad" the idea is but in fact how smart and much more cost effective is the option for a silicon based sensor compared to a thermopile solution.

If fact, before I am accused of fixation with temperature again - and before you jump into your own - I draw your attention to the "conclusions":
"As shown (...) an unregulated temperature version (...) is even cheaper and perfectly suitable for not excessively demanding applications at a very low cost."
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#37

Good day and hello
I'm wondering if there's any ready to make kit with instructions for the pyranometer, need around 9-12 sets for my project, or if there's any parts list?
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#38

We are in the final stages of preparing something like a "ready made" module to add Solar Radiation / UV readings to the WeatherDuino pro 2 system. Don't know if it will be suitable for your needs.

The tricky part for most will be physical construction of the unit: It will require some imagination and a knack for hand work.



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#39

When the ready made module is ready, please could you inform me. As I have been waiting for this so I can add it to my weatherduino set-up.

Regards
tyntop
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#40

I'm planning to release it on the first or second week of July. Then I will do an announcement on the forum.



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