solar cell question

Skyko

hi guys,

I have a little device that needs a solar cell which is somewhat efficient at very low light levels but also is very efficient at full sun. Size and weight are also major considerations. Amorphous cells will provide enough power/voltage at the lower light levels where crystalline fail, and monocrystalline are about good enough at full sun for the power requirement.

I looked briefly into the triple junction cells, but the only source who *might* sell to a non-government non-satellite agency is spectrolab. They have some triple junction 27% efficient cells which are about 61mW each, and the price isn't too outrageous ($250/100, which is about 6 watts worth of cells for $250) but I think the minimum order is $10,000

The other option might be the cells used in the Sanyo HIT panels. Evidently these use both a monocrystalline layer and a amorphous Si layer to achieve precisely the same results that I desire. The panels are about $6 a watt or less, but I can't find a single source online where you can buy the individual cells. Does anyone know how I could get my hands on 3 or 4 of the individual HIT 23% efficient cells?

BB.

Re: solar cell question

Just out of curiosity--What do you consider to be "very low light levels"?

some numbers:

• sunlight on an average day ranges from 32 000 to 100 000 lux
• TV studios are lit at about 1 000 lux [i.e. 1000 lumens per square metre]
• a bright office has about 400 lux of illumination
• At sunset and sunrise, ambient outdoor light is also about 400 lux (if the sky is clear).
• moonlight represents about 1 lux
• starlight measures a mere 0.00005 lux

"Playing" with the numbers (some back of the envelope calculations):

So, a solar panel that is rated at 100 watts in full sun, would generate (roughly) at sunrise/set / average bright office around:

• 100 watts * 400 lux / 100,000 lux = 0.4 watts

Or 0.4% of the cell's/panel's rated 1 Sun (earth's surface) rating assuming a cell scales 100% over such a wide range of lighting.

At some point, the internal cell resistance is going to bleed off power and reduce the power to zero before it hits zero light level--and there may be some other physics about recombination at the crystalline level that I have no clue about.

Here is a small amorphous cell that is designed for a 200 lux office light (such as a calculator?). Its output is around 17uAmps (microAmp) at 3 volts or 51 uWatts (or 0.000051 watts).

Item #SC-01 Indoor Solar Cell

The SC-01 Solar cell is made for use under fluorescent lamps. At approximately 200 Lux illumination, at 25 degrees Centigrade, the typical voltage output is 3.0 volts. Open circuit voltage is 4.0 volts. Short circuit current (200 luxs) is 17.0 uA. (note: output current is greater in sunlight)
Operating Temperature -5 to 60 degrees Centigrade.
Size of the Solar cell is 55 mm x 20 mm x 1.1 mm (2.1" x .78" x .04").

Out of curiosity--What is the conversion efficiency. Assume 1,000 watts / sq meter = 100,000 Lux and the office is 200 lux, and the cell is 55 mm x 20 mm x 1.1 mm...

• 1,000 w/sqmter * 55mm*20mm*1/1,000,000 sqmm per sqm = 1.1 watts full sunlight on cell
• 1.1 watts * 200 lux / 100,000 = 0.0022 watts at 200 lux
• 0.000051 watts from cell / 0.0022 watts of office light = 0.023 = 2.3% efficient in low office light

The math seems reasonable--Amorphous solar panels are around 8% efficient in full sun, and crystalline around 12-18% efficient...

A 10% (pick something 1/2 between) efficient 55mm*20mm cell in full sunlight would develop:

• 1,000 w/sqmter * 55mm*20mm * 1/1,000,000 sqmm per sqm * 0.10 eff = 0.11 watts in full sun

-Bill

Skyko

Re: solar cell question

very low light levels would be raining/snowing, cell pointed away from the sun, possible shaded also.

Even in these conditions I still need a voltage generated with a small current (4V at 20uA would be enough). Some of the crystalline panels I tested would not do this, but all of the amorphous would.

At full sun, I need the small panel (0.5 watts or less) to be as efficient as possible.

BB.

Re: solar cell question

For very low currents--Many devices simply use a Lithium Battery for uAmp range current--they can last for years/decade or more. Lithium primary cells also have a very wide operating temperature range (especially important for sub freezing operation).

-Bill

Skyko

Re: solar cell question

BB. wrote: »

For very low currents--Many devices simply use a Lithium Battery for uAmp range current--they can last for years/decade or more. Lithium primary cells also have a very wide operating temperature range (especially important for sub freezing operation).

-Bill

Yes, that is an option. I was also thinking of using a small supercapacitor, but they only have a rated lifetime of 2000 hours. If I can't get a multijunction cell like the Spectrolab ones or the Sanyo HIT hybrids, I guess a lithium coin type battery would be a good alternative.

Thanks

phred01

Re: solar cell question

BB. wrote: »

For very low currents--Many devices simply use a Lithium Battery for uAmp range current--they can last for years/decade or more. Lithium primary cells also have a very wide operating temperature range (especially important for sub freezing operation).

-Bill

There are rechargeable Lithium Batteries. Amorphous panel would be the most cost effective way to go

BB.

Re: solar cell question

Rechargeable Lithium Batteries--That is a real interesting subject... There are many differing chemistries and constructions out there--and some of the more "high tech/high capacity" type can get very "upset" if over discharged or over charged...

And, primary cell lithium batteries usually have a very low self discharge rate (decade)... Rechargeable batteries, typically, have a much higher self discharge rate (months to year).

If occasional battery replacement is OK (years between changes?)--I probably would skip the whole rechargeable lithium thing.

-Bill

Skyko

Re: solar cell question

Actually, I am using quite a lot of Lipo (lithium polymer) batteries in my electric assist bicycle projects. My recumbent tadpole trike has 3 KWH of Lipo which fits in a space the size of a shoebox and is about 22 pounds. Equivalent lead acid would be well over 100 pounds. The new Lipo also has a cycle life greater than 1000 cycles to 80% DoD. Good stuff.

BilljustBill

Re: solar cell question

BB. wrote: »

Rechargeable Lithium Batteries--That is a real interesting subject... There are many differing chemistries and constructions out there--and some of the more "high tech/high capacity" type can get very "upset" if over discharged or over charged...

And, primary cell lithium batteries usually have a very low self discharge rate (decade)... Rechargeable batteries, typically, have a much higher self discharge rate (months to year).

If occasional battery replacement is OK (years between changes?)--I probably would skip the whole rechargeable lithium thing.

-Bill

I have a friend that moved near Muncie, In., and flys R/C electric model aircraft. When these type batteries came to market, it provided a major boost to electric flying both outdoors as well as indoord. When flying outside, one of his model planes went down over 100 yards away, by the time he got to it, the locked prop/motor and model had caught fire and then started a small surface fire....

Along with this long lasting power source came charging problems, too. In the beginning, several modelers not only burned up their aircraft due to not properly or safely fusing when landings locked the prop and motor, but also burned down their workshops. During the charging cycle, battery pack fires started. Most using these LIPO packs began using those hollow gray concrete building blocks and charging them with one block on the bottom laying flat and putting a hollow block on top of it. With the battery pack surrounded on five sides with a noncombustible material it became the norm when charging.

drees

Re: solar cell question

Yeah - risk of fire with some lithium based batteries when over charged, over discharged or otherwise abused is real.

There are plenty of lithium based batteries which do not have these issues, but they typically do not have the energy density of the less robust batteries and are more expensive.

AntronX

Re: solar cell question

LiFePO4 are the ones that do not burn. They cost about the same per Wh or even little cheaper than LiPo. You can get 3.2Ah 3.2V cell for $6 if you buy 20 or more. That's what I paid a year ago.