Solar radiation meter - reasonably priced. Do they exist?

izmail

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Dec 8, 2021
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I have a simple off grid system - two 450w panels, SCC, inverter, AGM batteries and controls. It runs pool pump - so far so good. The setup is as follows:
1. Timer turns system on, it starts in battery mode.
2. If there it enough sun, pump runs all day on solar.
3. If cloudy and solar panels do not generate enough juice, system switches to grid after battery voltage drops to 24v.
4. Runs on grid power until battery is recharged, then switches back to solar and cycles repeats.
I'd like to run the pump on solar only when there is enough sun, and on grid on cloudy days without cycling the battery, saving it for blackouts.
For that I'm looking for a device ( reasonably priced) to use as a solar radiation meter. I'd like to switch system to solar on a signal from a device only when there is enough sun power to run the pump, without draining the battery. For now I'm using a tiny panel from garden solar light, loaded with 300ohm resistor. It does generate voltage across the resistor from 1.5 ( cloudy) to 2.8 ( full sun). I use this signal for control. But signal does not look stable. One day shows 2.75v, next day 2.65 at visually same sun conditions. Tried different size resistors, does not make a difference in stability ( voltage of course does change a little) Does anybody know such a device- in $20-$50 range? Getting an industrial grade pyrometer is not an option.
Should be a device with voltage, current or relay output, not a hand-held meter with a display for field measurements.
 
Last edited:

MurphyGuy

It just needs a bigger hammer
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May 20, 2020
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1,428
You could get yourself a day/night sensor like they use for outdoor security lights.. Point it down at a white surface.. When the white surface is illuminated bright enough by the sun it will trigger the sensor.

You could probably also point the sensor at the sky and cover it with layer(s) of semi-translucent material until such time as it only activates under full sun..

The problem with your sensor idea is that a cloudy day can produce 10% sunshine and 90% clouds.. or 90% clouds and 10% sunshine. Without obtaining a device capable of time travel, any sensor you get is only going to be capable of reading moment by moment.

You can have solid sunshine until noon and then dark clouds roll over for the rest of the day.

A better way of doing this is to only run the pump when the batteries are full and the solar panels are producing extra energy. Plenty of charge controllers have relay outputs when they stop charging.
 

740GLE

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Apr 30, 2021
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970
Do they make DC pool pumps? Just run it off a SCC controlled output.
 

izmail

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Dec 8, 2021
Messages
21
Running directly from SCC - good idea. No sun- no voltage. More sun- more voltage, pump starts to turn. Full sun - full speed.
Unfortunately for me it is an expensive option, solar pumps cost $1000+ (((.
As to the system I have - yes, the idea is to run the pump only on solar, using light sensor to detect sunlight. Using SCC controller relay may not work, since it detects end of charge only. So when SCC output is say 5amp, which is not enough to power the pump, the controller will not trigger relay output. If I don't find a light sensor, I'd have to use classic arrangement - shut and mV to V transmitter to measure SCC output. If it is more then 10amp ( need 10amps to run the pump) - pump is on.. If current falls below 10 amps - pump is off. Not as simple as using some kind of analog lux meter though
 

Don B. Cilly

Energetic energy padawan
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Mallorca ES
Your micro-panel is simply too microscopic.
You can get 12V ones for some $10 that will do 200mA, that should be stable enough under a small load.
Photoresistors and photodiodes are dirt cheap. Pre-wired light sensor switches, cheap enough.
 

izmail

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Dec 8, 2021
Messages
21
Thank you for good ideas. Arduino solution is also based on solar cell, same thing, Instead of Arduino I use a cheap TECO PLC with analog inputs. It also does all other controls. Very handy by the way - 24v DC, around $100 - ideal for DIY solar system with 24v battery.
Problem is to find right solar cell to provide stable input. So next step will be getting a 3w 12v panel, something like this - 3w solar panel. It indeed generates 250ma and should be stable enough. Will also run some tests with photo resistor.
 

jall

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Joined
Jul 12, 2021
Messages
17
I have a simple off grid system - two 450w panels, SCC, inverter, AGM batteries and controls. It runs pool pump - so far so good. The setup is as follows:
1. Timer turns system on, it starts in battery mode.
2. If there it enough sun, pump runs all day on solar.
3. If cloudy and solar panels do not generate enough juice, system switches to grid after battery voltage drops to 24v.
4. Runs on grid power until battery is recharged, then switches back to solar and cycles repeats.
I'd like to run the pump on solar only when there is enough sun, and on grid on cloudy days without cycling the battery, saving it for blackouts.
For that I'm looking for a device ( reasonably priced) to use as a solar radiation meter. I'd like to switch system to solar on a signal from a device only when there is enough sun power to run the pump, without draining the battery. For now I'm using a tiny panel from garden solar light, loaded with 300ohm resistor. It does generate voltage across the resistor from 1.5 ( cloudy) to 2.8 ( full sun). I use this signal for control. But signal does not look stable. One day shows 2.75v, next day 2.65 at visually same sun conditions. Tried different size resistors, does not make a difference in stability ( voltage of course does change a little) Does anybody know such a device- in $20-$50 range? Getting an industrial grade pyrometer is not an option.
Should be a device with voltage, current or relay output, not a hand-held meter with a display for field measurements.
I have been thinking in the same idea since 1 month ago. In my case, my interest is running the heat pump for "free heating" the house. Since my system is a grid tied inverter with zero injection to the grid, I must keep an eye continously in the amount of irradiation to decide if turning the heat pump on or not. I want to make this process automatic, but I need the sun irradiation signal since it is the mandatory parameter to decide if the generated power will be enough to supply the needed energy to the heating system.

"sun irradiation signal" is NOT "sun light signal", and therefore, forget any photodiode or photoresistor as the source of the information (the light value in a foggy day can be almost the same of a sunny day but the power of the PV array will be almost zero). Also the voltage of the panel array is not usable as signal because it provides almost the same voltage early in the morning, at noon, or on a rainy day. Also forget any termistor based system since the sensed temperature will change with the sourronding air temp.
What we need is the DC AMPS reading at the panels array, but it depends on the load, and since the heat pump is not working untill the irradiation value is measured (and found ok), the drawn current from the PV is almost zero.

I have been looking for a "sun irradiation measuring module" veeery long, but I didn´t found any. So, decided to proceed to the DIY way: a mini solar panel (for instance 3 watts) with the same tilt and orientation that the PV array (basically installed on the array itself) feeds to a filament light bulb of 1 W in front of an adjustable light sensor relay module that will decide when the generated power is enough to keep the heat pump working for free. With this cheap and easy setup, you can control the settings for turning ON and OFF your pump unnatended. The "OK signal" will be real and consistant with the sun irradiation received by the panel array

Hope this helps, but if anybody has a better solution, please, let me know!

best regards from Madrid,
Jose
 

izmail

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Joined
Dec 8, 2021
Messages
21
Jose! Thank you for interesting reply, very useful information. Glad I found somebody out there who has and working on the same problem. Idea with using light bulb is very "outside of the box" solution. I'd like to ask couple of questions and share what I have so far. Light power emitted by a light bulb will be proportional to the current through the bulb? If yes, then the bulb is just resistor - the more power small panel generates, the higher the current and therefore the higher voltage drop across the resistor. Power generated by a solar panel with fixed load is proportional to available sun power, right? That what they say for example here. And this is the curve that they give - power vs voltage at given load resistor.
1643257716979.png


So in my current setup I'm measuring voltage across the resistor ( see pic)

1643257753596.png
I changed a toy solar pane to a 3W panel and so far I see that voltage generated across the resistor is proportional to available main panel power.
How do I know - I'm looking at current generated by panels vs voltage across resistor. For my pump I need around 10amp. In cloudy condition, when main panels can generate only 4-5 amps, signal is 1.7 to 2v. When panels can generate at least 10 amp, sensor voltage is around 2.7. Noting is perfect, so I turn solar power to the pump when signal is 2.75v. There is also another problem and you most likely have it too. During semi-cloudy day, when clouds are in and out, panels are often on the edge of generating needed 10 amps. So light signal bounces around threshold value, causing pump to cycle on and off. This can quickly destroy the motor. But this is inevitable and I deal with it through the logic -I have delay on and delay off for the pump. It runs though short clouds on battery ( 2 min delay off ) before switching to utility. If you want to run your heat pump on solar only ( zero grid), then your heat pump will be cycling in semi-cloudy day. I'm also thinking about implementing logic to counting cycles - if pump cycles say 3 times, then system decides that sun conditions are not good enough and quit using solar. Or to look at light signal duration- if it is good for say 20 min, then consider solar conditions acceptable and switch to solar. Anyway, there are many possible tricks on the logic side. Bottom line - looks like small panel and resistor solution works. But - more observation is needed. I'll program some diagnostic logic to record light sensor signal vs battery drain to capture an instance when sensor voltage shows good sun, but system uses battery ( not enough power from panels). This will prove that my solution is no good and it is time to try something else - starting with your light bulb solution. Will let you know if you interested.
 

AE4KR

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Jun 2, 2020
Messages
62
Location
Near Salt Lake City, UT
It does generate voltage across the resistor from 1.5 ( cloudy) to 2.8 ( full sun). I use this signal for control. But signal does not look stable. One day shows 2.75v, next day 2.65 at visually same sun conditions.
Have you just created a perfectly functional meter which is simply measuring with more accuracy than what you can determine visually?

Call me crazy, but this is why I use a meter rather than taking a WAG based on looking up at the sky!
 
Last edited:

svetz

Works in theory! Practice? That's something else
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I read the w/m² from a neighbor's house with an HTTP GET for free. For example, check out the table in https://www.wunderground.com/dashboard/pws/KFLOVIED91/table/2022-01-20/2022-01-20/daily.

Usually, you can find someone nearby from the https://www.wunderground.com/wundermap but be sure to zoom in, more stations will appear.

It's not perfect, occasionally a cloud passes over them that doesn't pass over me or vice versa. Depends on how close you can get. Worse case, you can always just get a weather station that reads solar radiation.
 

justgary

Solar Addict
Joined
Jan 1, 2022
Messages
227
Consider adding a capacitor in parallel with your load resistor to get a moving time average of your sun signal. That should help smooth your signal. Something with a time constant of a few minutes might work, but you'll need to tune the system to meet your needs.

This is also an advantage of using a microprocessor since you can do all of the averaging and other data manipulation very easily. Your TECO system may allow you to do this as well. Nothing would stop you from doing a bunch of data processing (including talking to your SCC) and then output an analog control signal to your TECO.
 

RVLiFe

Solar Addict
Joined
Jun 14, 2021
Messages
194
I like the idea of using an ardueno or raspberry pi with a relay. You can pull the solar irradiance from a local weather station for free and use that information to trip the relay. It might not be as accurate as having a meter connected directly to your system but finding a station close to you would probably be good enough and I’m certain you would get the outcome that you are looking for. I’d use node-red to extrapolate the data and to trip the relay. Very easy to do and learn. I’ve used this method to open/close a damper on a A/C unit before while using outside temperatures as my control.
 

izmail

New Member
Joined
Dec 8, 2021
Messages
21
As to my initial statement about unstable voltage signal
"It does generate voltage across the resistor from 1.5 ( cloudy) to 2.8 ( full sun). I use this signal for control. But signal does not look stable. One day shows 2.75v, next day 2.65 at visually same sun conditions."
As I said in later post, I changed ( using good advice here) a toy solar panel to a better one - 3.5x3.5", 3W and now signal seems to be stable.
Also about using a capacitor - it is pretty much the same as using time delay in the control logic. I have 2 min delay for turning solar off and on to ride through short clouds. Just less hardware in the panel - and more flexible. And just for a research, I'll try to use SCC output current as a control signal as follows -
1.Turn solar on
2. Check SCC output current
3. If 10amp or more - run on solar
4. If not - switch back to grid, check later.
5. All switching with time delay - 2 to 3 min.
Finding cheap DC current sensor is little bit of a challenge, good old 75mV shunt will require mV/V transmitter. There are inexpensive battery monitors on Amazon, may get one and try to pull amplified mV signal out of it.
 

jall

New Member
Joined
Jul 12, 2021
Messages
17
Jose! Thank you for interesting reply, very useful information. Glad I found somebody out there who has and working on the same problem. Idea with using light bulb is very "outside of the box" solution. I'd like to ask couple of questions and share what I have so far. Light power emitted by a light bulb will be proportional to the current through the bulb? If yes, then the bulb is just resistor - the more power small panel generates, the higher the current and therefore the higher voltage drop across the resistor. Power generated by a solar panel with fixed load is proportional to available sun power, right? That what they say for example here. And this is the curve that they give - power vs voltage at given load resistor.
View attachment 81378


So in my current setup I'm measuring voltage across the resistor ( see pic)

View attachment 81379
I changed a toy solar pane to a 3W panel and so far I see that voltage generated across the resistor is proportional to available main panel power.
How do I know - I'm looking at current generated by panels vs voltage across resistor. For my pump I need around 10amp. In cloudy condition, when main panels can generate only 4-5 amps, signal is 1.7 to 2v. When panels can generate at least 10 amp, sensor voltage is around 2.7. Noting is perfect, so I turn solar power to the pump when signal is 2.75v. There is also another problem and you most likely have it too. During semi-cloudy day, when clouds are in and out, panels are often on the edge of generating needed 10 amps. So light signal bounces around threshold value, causing pump to cycle on and off. This can quickly destroy the motor. But this is inevitable and I deal with it through the logic -I have delay on and delay off for the pump. It runs though short clouds on battery ( 2 min delay off ) before switching to utility. If you want to run your heat pump on solar only ( zero grid), then your heat pump will be cycling in semi-cloudy day. I'm also thinking about implementing logic to counting cycles - if pump cycles say 3 times, then system decides that sun conditions are not good enough and quit using solar. Or to look at light signal duration- if it is good for say 20 min, then consider solar conditions acceptable and switch to solar. Anyway, there are many possible tricks on the logic side. Bottom line - looks like small panel and resistor solution works. But - more observation is needed. I'll program some diagnostic logic to record light sensor signal vs battery drain to capture an instance when sensor voltage shows good sun, but system uses battery ( not enough power from panels). This will prove that my solution is no good and it is time to try something else - starting with your light bulb solution. Will let you know if you interested.

Izmail,
Interesting topic here.

I understand that the "light bulb method" sounds today as arcaic and obsolete (in fact, finding the bulb itself can be a dificult task if you don´t have in your garage a surplus case full of old stuff ;-), but a more sophisticated and complicated system is not necesarily a better solution.
Although an Edison bulb is not an ideal resistor (the filament´s resistance is much lower during the startup than when hot), you get in its output an automatic integration of the available electrical power generated by the PV (light output from the bulb = V x A, no need to make any sophisticated calculations. Also, the light sensor relay gives us a direct solution to the hysteresis issue (you can easily trick it with an RC at the light sensor side in order to modify the time delay if needed).
In my case, I don´t need to turn off and offwhen clouds are passing. I can accept delays of 30 min or even more (my inte3ntion is not to have "zero grid", but to use the house´s mass as thermal accumulator during the day in order to minimize the need of heating when ariving home at night.
To be honest, as I don´t need a super-accurate measuring system approved by the NASA, I prefer a very simply setup that I can build and test on a single sunday morning without investing months in hardware and software developping (that developping, although sounds funny and exciting, doesn´t seem to be a cheap solution in terms of time and resources).
I have clear that the target of the device is just to switch on and off a machine when the available power is enough without having to watch to the sky for a clear sun.
But anyway, I am really interested in your way and hope to hear about your progress (and headaches ;) )

Best regards,
Jose
 
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