💬 The Harvester: ultimate power supply for the Raybeacon DK
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@NeverDie My thought was that amorphous silicon (a-Si) cells have better spectral response to artificial light than crystalline cells (c-Si). However, after investigating this a little bit I've found that this doesn't seem to be true. Instead, it's shown everywhere that c-Si cells have better response to every wavelength:
Moreover, when the light source has wide spectrum (like the sun or an incandescent bulb), c-Si panels take the advantage and produce significantly more energy from the same source, and this all explains why a-Si cells are almost two times less effective than c-Si (roughly 8% vs 20%). Please note, because of narrow spectrum a LED lamp will be obviously inefficient for a PV panel.
But at the same time, there are reports of a-Si cells being 4x more effective in low light than crystalline. Indeed, both crystalline and poly-crystalline cells may degrade a lot:
The seem happens due to low parallel resistance of c-Si type cells. Shunt resistance of amorphous cells is naturally higher which results to less degradation of Vmpp and hence higher efficiency in low light conditions. Some paper show the shunt resistance rather low, when other mentions it relatively high, but at extremely low power conditions even 20 kOhm may be too much.
In short, a-Si cells are tend to produce fairly better results in very low light environments. But they can't leverage from wide spectrum sources, yet are subject to the Staebler-Wronski effect when exposed to direct sun (which can be reversed to some extent by heating the panel). In case if the light source is bright enough (around 1000 lx and above) a c-Si pannel should be preferred.
Finally, there are some other kind of solar cells, in particular those made from III-V semiconductors compound and promising even better low light performance.
@Mishka Have you found a good candidate for an amorphous cell to try? I see a lot of cells/panels advertised as amorphous, but without a datasheet showing performance under low light conditions, selecting one seems a bit like throwing darts at a map.
I've seen some flexible amorphous panels that claim to stack materials with different light sensitivities to get a better spectral response:
But are they any good, or is it just puffery?I've seen articles claiming that CIGS have efficiencies of 20% to other articles saying that CIGS are barely better than amorphous. Some also make claims that CIGS perform well under "low light," but without the detailed datasheet, there's just not much to hang one's hat on when it comes to selecting one to try....
And then there's powerfilm, which I had linked to earlier above, which claims to be optimized for 200 lux and below. At least they were selected by TI for TI's BLE demo kit, so presumably they were a good choice, at least at the time the choice was made....
Is amorphous better than these other choices at low light, and if so, which amorphous solution has the best efficiency under low light?
NREL seems to be an objective independent source for testing, but for high brightness conditions (according to wikipedia, the standard test conditions for solar cells are "the AM1.5 spectrum as the reference. This air mass (AM) corresponds to a fixed position of the sun in the sky of 48° and a fixed power of 833 W/m2. "):
https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20200218.pdf
https://www.nrel.gov/pv/cell-efficiency.html
At least on paper, the multi-junction cell efficiency looks really quite amazing. There are some for sale on ebay in the $20-$35 dollar range, depending on quantity. So, if you absolutely had to have one to meet your size requirements, there they are. No datasheets though, so again, just a cat in a bag. One claims 35% efficiency. No indication at all as to low light efficiency. -
@NeverDie Right, the good thing about thin-film solar cells that they can be relatively easily stacked up to gain better efficiency. Don't know about CIGS, but some III-V compounds like GaAs are known to be very effective in low light environment (please see the last paper in my previous post). Such, some manufacturers are making tripple-junction GaAs cells with power effectiveness up to 15 μW/cm² at 200 lx - just compare it to Amorton which have it at about 6 to 8 μW/cm² under the same conditions. Sounds like a huge difference, especially taking in account the Panasonic offers rather high quality cells. Unfortunately, the cost is as high as the satellites carrying these cells.
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Last night I hooked up the keychain solar cell to my simple solar circuit, and at 5 lux it could still charge a 100uF capacitor to 2.7v and blink a red led without any boosting. It looks like it's probably amorphous. So, pretty good performance considering its low cost, but perhaps not as small as what you're looking for.
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Last night I hooked up the keychain solar cell to my simple solar circuit, and at 5 lux it could still charge a 100uF capacitor to 2.7v and blink a red led without any boosting. It looks like it's probably amorphous. So, pretty good performance considering its low cost, but perhaps not as small as what you're looking for.
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@NeverDie Well, 5 lux is ridiculously low. It's about the same illuminance you may have at 45 cm from a candle. Are you sure your lux meter is working? :-)
Interesting to measure Voc and Isc at that light. What's dimension of the cell?
@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
Are you sure your lux meter is working?
I'm not at all sure that it's accurate, but that's what the lux meter said. It's one of these: https://www.amazon.com/Dr-Meter-LX1330B-Digital-Illuminance-Light/dp/B005A0ETXY/ref=sr_1_3?keywords=lux+meter&qid=1582903100&sr=8-3
I've misplaced the manual, but someone posted this on amazon as to its specs:
The specifications in the instruction manual reflect the following:
Light-measuring level from .1Lux to 200,000Lux
Accuracy +- (3%rdg+10dgt) <=20,000Lux/2,000FC
+- (5%rdg+10dgt) >= 20,000Lux/2,000FC
Repeatability +-2%
Photo detector lead length ~150cm
Spectral Sensitivity- curve shows mostly betweeen 500nm and 650nmPerhaps I should get something better, or else maybe find some way to calibrate it. What is it that you're using?
I assume that for "Accuracy +- (3%rdg+10dgt)" it means plus or minus 3% of the reading, which is fine. Not sure what the 10dgt means though. If that means it could be plus or minus 10 lux, then I guess it's useless for measuring 5 lux.
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@NeverDie Well, 5 lux is ridiculously low. It's about the same illuminance you may have at 45 cm from a candle. Are you sure your lux meter is working? :-)
Interesting to measure Voc and Isc at that light. What's dimension of the cell?
@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
What's dimension of the cell?
37mm x 22mm
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@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
Are you sure your lux meter is working?
I'm not at all sure that it's accurate, but that's what the lux meter said. It's one of these: https://www.amazon.com/Dr-Meter-LX1330B-Digital-Illuminance-Light/dp/B005A0ETXY/ref=sr_1_3?keywords=lux+meter&qid=1582903100&sr=8-3
I've misplaced the manual, but someone posted this on amazon as to its specs:
The specifications in the instruction manual reflect the following:
Light-measuring level from .1Lux to 200,000Lux
Accuracy +- (3%rdg+10dgt) <=20,000Lux/2,000FC
+- (5%rdg+10dgt) >= 20,000Lux/2,000FC
Repeatability +-2%
Photo detector lead length ~150cm
Spectral Sensitivity- curve shows mostly betweeen 500nm and 650nmPerhaps I should get something better, or else maybe find some way to calibrate it. What is it that you're using?
I assume that for "Accuracy +- (3%rdg+10dgt)" it means plus or minus 3% of the reading, which is fine. Not sure what the 10dgt means though. If that means it could be plus or minus 10 lux, then I guess it's useless for measuring 5 lux.
@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
I'm not at all sure that it's accurate, but that's what the lux meter said.
Wow, relatively to my built into the smartphone Sensortek STK3x1x ambient light sensor this one looks very serious.
The cell has surprisingly high voltage (2.7V) at so low light. My amorphous cell has Voc = 1.8V at 50 lux (2 m from a fluorescent lamp). Maybe yours has many more cells in series. I'm going to order some Amorton panels of suitable size (less than 25x25), it will be interesting to compare them with my other amorphous cell.
I'm also wondering would it be good o bad to connect two cells of different types - one amorphous and one crystalline.
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@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
Are you sure your lux meter is working?
I'm not at all sure that it's accurate, but that's what the lux meter said. It's one of these: https://www.amazon.com/Dr-Meter-LX1330B-Digital-Illuminance-Light/dp/B005A0ETXY/ref=sr_1_3?keywords=lux+meter&qid=1582903100&sr=8-3
I've misplaced the manual, but someone posted this on amazon as to its specs:
The specifications in the instruction manual reflect the following:
Light-measuring level from .1Lux to 200,000Lux
Accuracy +- (3%rdg+10dgt) <=20,000Lux/2,000FC
+- (5%rdg+10dgt) >= 20,000Lux/2,000FC
Repeatability +-2%
Photo detector lead length ~150cm
Spectral Sensitivity- curve shows mostly betweeen 500nm and 650nmPerhaps I should get something better, or else maybe find some way to calibrate it. What is it that you're using?
I assume that for "Accuracy +- (3%rdg+10dgt)" it means plus or minus 3% of the reading, which is fine. Not sure what the 10dgt means though. If that means it could be plus or minus 10 lux, then I guess it's useless for measuring 5 lux.
@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
If that means it could be plus or minus 10 lux, then I guess it's useless for measuring 5 lux.
Well, maybe not completely useless. If the specs are valid, then it's surely less than 20 lux, assuming I'm giving the right interpretation to "10dgts".
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@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
I'm not at all sure that it's accurate, but that's what the lux meter said.
Wow, relatively to my built into the smartphone Sensortek STK3x1x ambient light sensor this one looks very serious.
The cell has surprisingly high voltage (2.7V) at so low light. My amorphous cell has Voc = 1.8V at 50 lux (2 m from a fluorescent lamp). Maybe yours has many more cells in series. I'm going to order some Amorton panels of suitable size (less than 25x25), it will be interesting to compare them with my other amorphous cell.
I'm also wondering would it be good o bad to connect two cells of different types - one amorphous and one crystalline.
@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
I'm going to order some Amorton panels of suitable size (less than 25x25), it will be interesting to compare them with my other amorphous cell.
I'm thinking of ordering the AM-1816CA, which AFAIK is the largest one rated for indoor and low lux. https://www.mouser.com/datasheet/2/315/panasonic_AM-1816CA-1196985.pdf
My only reason for ordering the largest would be to see what the limit is on how dim things can get in that series and still have something that can function. Maybe ordering smaller panels would make more sense, though, as they could always be added together in parallel or series. Yeah, that would make more sense I think.In addition, if you let me know what models you order, I may order one of the same too just so we can have something in common to compare results.
At very dim levels I notice that my Fluke 87v multimeter actually draws too much current off the solar cell to get an accurate open circuit voltage measurement. So, I'll have to rig up some kind of voltage following op amp buffer as an aid to doing these measurements.
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@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
I'm not at all sure that it's accurate, but that's what the lux meter said.
Wow, relatively to my built into the smartphone Sensortek STK3x1x ambient light sensor this one looks very serious.
The cell has surprisingly high voltage (2.7V) at so low light. My amorphous cell has Voc = 1.8V at 50 lux (2 m from a fluorescent lamp). Maybe yours has many more cells in series. I'm going to order some Amorton panels of suitable size (less than 25x25), it will be interesting to compare them with my other amorphous cell.
I'm also wondering would it be good o bad to connect two cells of different types - one amorphous and one crystalline.
@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
I'm also wondering would it be good o bad to connect two cells of different types - one amorphous and one crystalline.
Only one way to know for sure, but I would guess that the crystalline one would drain off the current produced by the amorphous one (based partly on your theory as to why amorphous is better in low light). Worth a shot though: maybe as a compromise solution you can have the best of both worlds.
Thinking out loud here, I have read about some research solar harvesters where they use a separate "pilot" solar cell to power the control electronics past the cold boot threshold. These days, with nanoamp current drains from control components, it would mostly need to produce adequate voltage and not much current, so a simple approach would be optimize the pilot configuration for exactly that--perhaps putting a few tiny cells in series. Perhaps any extra current could then spill over into the main accumulating capacitor. That would be yet another way to use more than one type of panel.
The ideal solution would be if there were some way to re-configure multiple cells in series or parallel depending on the lighting conditions. It could default to series to push past the cold start and then switch to parallel (or some appropriate combination of series and parallel) for the energy harvesting. I haven't seen much on that topic, but I'd be keen to know if there are ways to do reconfiguring that consume very little power in overhead.
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@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
I'm not at all sure that it's accurate, but that's what the lux meter said.
Wow, relatively to my built into the smartphone Sensortek STK3x1x ambient light sensor this one looks very serious.
The cell has surprisingly high voltage (2.7V) at so low light. My amorphous cell has Voc = 1.8V at 50 lux (2 m from a fluorescent lamp). Maybe yours has many more cells in series. I'm going to order some Amorton panels of suitable size (less than 25x25), it will be interesting to compare them with my other amorphous cell.
I'm also wondering would it be good o bad to connect two cells of different types - one amorphous and one crystalline.
@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
less than 25x25
That probably limits you to a couple of AM-1456 (25mm x 10mm) or a single AM-1606 (15mm x 15mm) as your only choices.
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@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
I'm also wondering would it be good o bad to connect two cells of different types - one amorphous and one crystalline.
Only one way to know for sure, but I would guess that the crystalline one would drain off the current produced by the amorphous one (based partly on your theory as to why amorphous is better in low light). Worth a shot though: maybe as a compromise solution you can have the best of both worlds.
Thinking out loud here, I have read about some research solar harvesters where they use a separate "pilot" solar cell to power the control electronics past the cold boot threshold. These days, with nanoamp current drains from control components, it would mostly need to produce adequate voltage and not much current, so a simple approach would be optimize the pilot configuration for exactly that--perhaps putting a few tiny cells in series. Perhaps any extra current could then spill over into the main accumulating capacitor. That would be yet another way to use more than one type of panel.
The ideal solution would be if there were some way to re-configure multiple cells in series or parallel depending on the lighting conditions. It could default to series to push past the cold start and then switch to parallel (or some appropriate combination of series and parallel) for the energy harvesting. I haven't seen much on that topic, but I'd be keen to know if there are ways to do reconfiguring that consume very little power in overhead.
@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
Only one way to know for sure, but I would guess that the crystalline one would drain off the current produced by the amorphous one (based partly on your theory as to why amorphous is better in low light). Worth a shot though: maybe as a compromise solution you can have the best of both worlds.
I mean connect them in series with bypass diodes so the amorphous cell can be used to bootstrap the harvester, and then crystalline cell will be workhorse during the day. Unfortunately, can't check it right now - left all my cells in the office.
Thinking out loud here, I have read about some research solar harvesters where they use a separate "pilot" solar cell to power the control electronics past the cold boot threshold.
That's a smart idea. Perhaps connect a dedicated tiny charge pump and an amorphous panel parallel to the buck-boost harvester storage capacitors?
The ideal solution would be if there were some way to re-configure multiple cells in series or parallel depending on the lighting conditions. It could default to series to push past the cold start and then switch to parallel.
A mechanical device? :-)
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@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
less than 25x25
That probably limits you to a couple of AM-1456 (25mm x 10mm) or a single AM-1606 (15mm x 15mm) as your only choices.
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@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
I'm going to order some Amorton panels of suitable size (less than 25x25), it will be interesting to compare them with my other amorphous cell.
I'm thinking of ordering the AM-1816CA, which AFAIK is the largest one rated for indoor and low lux. https://www.mouser.com/datasheet/2/315/panasonic_AM-1816CA-1196985.pdf
My only reason for ordering the largest would be to see what the limit is on how dim things can get in that series and still have something that can function. Maybe ordering smaller panels would make more sense, though, as they could always be added together in parallel or series. Yeah, that would make more sense I think.In addition, if you let me know what models you order, I may order one of the same too just so we can have something in common to compare results.
At very dim levels I notice that my Fluke 87v multimeter actually draws too much current off the solar cell to get an accurate open circuit voltage measurement. So, I'll have to rig up some kind of voltage following op amp buffer as an aid to doing these measurements.
@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
At very dim levels I notice that my Fluke 87v multimeter actually draws too much current off the solar cell to get an accurate open circuit voltage measurement. So, I'll have to rig up some kind of voltage following op amp buffer as an aid to doing these measurements.
Heh, we seem dived below 1 µA / µW level here :fish: :-)
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@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
At very dim levels I notice that my Fluke 87v multimeter actually draws too much current off the solar cell to get an accurate open circuit voltage measurement. So, I'll have to rig up some kind of voltage following op amp buffer as an aid to doing these measurements.
Heh, we seem dived below 1 µA / µW level here :fish: :-)
@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
Heh, we seem dived below 1 µA / µW level here
Looking at the datasheets for the op amps I have on hand, I'm guessing that the LTC2063 will allow an accurate measurement: https://www.analog.com/media/en/technical-documentation/data-sheets/LTC2063-2064.pdf I'll try it after my uv glue arrives.
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@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
Only one way to know for sure, but I would guess that the crystalline one would drain off the current produced by the amorphous one (based partly on your theory as to why amorphous is better in low light). Worth a shot though: maybe as a compromise solution you can have the best of both worlds.
I mean connect them in series with bypass diodes so the amorphous cell can be used to bootstrap the harvester, and then crystalline cell will be workhorse during the day. Unfortunately, can't check it right now - left all my cells in the office.
Thinking out loud here, I have read about some research solar harvesters where they use a separate "pilot" solar cell to power the control electronics past the cold boot threshold.
That's a smart idea. Perhaps connect a dedicated tiny charge pump and an amorphous panel parallel to the buck-boost harvester storage capacitors?
The ideal solution would be if there were some way to re-configure multiple cells in series or parallel depending on the lighting conditions. It could default to series to push past the cold start and then switch to parallel.
A mechanical device? :-)
@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
Perhaps connect a dedicated tiny charge pump and an amorphous panel parallel to the buck-boost harvester storage capacitors?
Maybe, but which one? I would have suggested this one, but it's no longer available: https://media.digikey.com/pdf/Data Sheets/Seiko Instruments PDFs/S-882Z.pdf
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@NeverDie said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
I'm not at all sure that it's accurate, but that's what the lux meter said.
Wow, relatively to my built into the smartphone Sensortek STK3x1x ambient light sensor this one looks very serious.
The cell has surprisingly high voltage (2.7V) at so low light. My amorphous cell has Voc = 1.8V at 50 lux (2 m from a fluorescent lamp). Maybe yours has many more cells in series. I'm going to order some Amorton panels of suitable size (less than 25x25), it will be interesting to compare them with my other amorphous cell.
I'm also wondering would it be good o bad to connect two cells of different types - one amorphous and one crystalline.
@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
Wow, relatively to my built into the smartphone Sensortek STK3x1x ambient light sensor this one looks very serious.
Looks as though you can get a fairly inexpensive digital light sensor from adafruit that will tell you the lux level: https://www.adafruit.com/product/4162
https://www.amazon.com/Adafruit-4162-VEML7700-Lux-Sensor/dp/B07S9TD2W1/ref=sr_1_1?keywords=Adafruit+VEML7700&qid=1583129068&sr=8-1It doesn't have the little translucent plastic dome on it though that one typically finds on lux meters. Not sure how important that is or isn't. Seems like such domes would shade the light and skew low light level readings, so maybe you'd be better off without it.
I may get one myself as a check on my lux meter.
There's also this one, a little cheaper: https://www.amazon.com/Adafruit-TSL2591-Dynamic-Digital-ADA1980/dp/B00XW2OFWW/ref=sr_1_1?keywords=Adafruit+lux+sensor&qid=1583129190&sr=8-1
I checked the adafruit library, and it prints sensor readings in lux.Not sure which one is more accurate.
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I built the op-amp circuit, and now the open circuit readings on a solar cell are much higher than when I was taking the readings with a regular multimeter. As long as I can keep the control logic current at just a couple hundred nanoamps or so, I think I'll probably have enough voltage under even very dim lighting that I doubt cold start will be an issue.
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@NeverDie Wow! Are you sure the lux meter is working properly? 15 lux is about as low as 25 cm from a candle fire which is awfully low.
So, since the ADP5091 is also on the list now, I think it becomes necessary to put all the mentioned power harvesters side by side so we can compare at least basic parameters. Please take a look to the Google Spreadsheet listing some of the tiny harvesters. Please note, The Analog Devices has about ten harvesters which may be described as "tiny", but so far the sheet is covering only ADP509x series. I'm going to add the LTC series later.
From brief analysis, it looks like the Startup Input Voltage and the Startup Input Power are placing the major constraint on PV panel. Of course, the panel should be also able to supply voltage required to cold-boot the harvester. Such, under the low-light conditions (50 lx to 100 lx in a dim room) power capabilities of both of my panels are simply not sufficient to bootstrap the SPV1050: the IXYS is too weak and works starting from 150 lx, and the SCNE is good for boost, but still require 150 lx to reach 2.6V at STORE (the data is for the charts I've posted earlier):
Therefore any panel which can reach required voltage and provide enough power should make the harvesting IC work. Please also note, that after cold-boot almost any harvester can work on a lower power (usually 1.5 - 3 times lower than it was required to start).
Also, in the table there is a class of super-tiny harvesters, namely the Cypress S6AE102(3)A and Ricoh R1800K. They can charge a store by harvesting source with less than 1µW capability. At the same time, the EM8500 looks like the most sophisticated embedded solution with lots of features. The rest of harvesters are quite nice ICs for a modular system.
@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
What method or methods are you using to characterize your solar cells? I'm guessing in this instance that for the different illumination levels you are recording the open circuit voltage and the short circuit voltage? So as to at least try to compare apples to apples, I want to collect data on my cells in the same way that you are.
Another, complementary approach is described here: "The key characteristic of a solar cell is its ability to convert light into electricity. This is known as the power conversion efficiency (PCE) and is the ratio of incident light power to output electrical power. To determine the PCE, and other useful metrics, current-voltage (IV) measurements are performed. A series of voltages are applied to the solar cell while it is under illumination. The output current is measured at each voltage step, resulting in the characteristic 'IV curve' seen in many research papers. " https://www.ossila.com/pages/solar-cells-theory I suppose with this approach a series of curves could be produced, each for a different illumination level. Since doing that would be a lot of work, I'd like to somehow automate the testing process, but first I need to either know or decide what the process is that I want to automate.
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@Mishka said in 💬 The Harvester: ultimate power supply for the Raybeacon DK:
What method or methods are you using to characterize your solar cells? I'm guessing in this instance that for the different illumination levels you are recording the open circuit voltage and the short circuit voltage? So as to at least try to compare apples to apples, I want to collect data on my cells in the same way that you are.
Another, complementary approach is described here: "The key characteristic of a solar cell is its ability to convert light into electricity. This is known as the power conversion efficiency (PCE) and is the ratio of incident light power to output electrical power. To determine the PCE, and other useful metrics, current-voltage (IV) measurements are performed. A series of voltages are applied to the solar cell while it is under illumination. The output current is measured at each voltage step, resulting in the characteristic 'IV curve' seen in many research papers. " https://www.ossila.com/pages/solar-cells-theory I suppose with this approach a series of curves could be produced, each for a different illumination level. Since doing that would be a lot of work, I'd like to somehow automate the testing process, but first I need to either know or decide what the process is that I want to automate.
What method or methods are you using to characterize your solar cells? I'm guessing in this instance that for the different illumination levels you are recording the open circuit voltage and the short circuit voltage?
@NeverDie exactly. That was a quick and dirty measurement using a multimeter. The P (µW) value was calculated as V * I * 0.8 (MPP assumed 80%, I must multiply to 0.8^2 instead). My intent was to describe the panels in dependency of different illuminance (which must be also denoted by E instead).
Finding MPP on IV curve is the right method to characterize a cell. But that would require fixing illuminance at some point (and is more complicated), when I was more interested in different light conditions. Most cells are rated at 200 lux indoors, and one sun (more than 100k lux) outdoors. Perhaps 50 lux indoors (a typical light at home) and 1000 lux (cloudy day) is more practical for low-light purposes so I could trace IV curves for the cells I ordered at that illuminance levels.
Looks as though you can get a fairly inexpensive digital light sensor from adafruit that will tell you the lux level
It seems my phone uses Sensortek STK3310 or similar. At low light might be as accurate as those two, but is limited at higher levels indeed. Would be nice to replace it with more reliable solution, will try to find out a luxmeter in a local fablab or get one of those you've suggested, thanks!
