How to use solar cells?

[ashour]

Hi,

I'm planning to create a very simple electronic circuit running on solar energy for a presentation in my university to show how energy materials can be used in engineering applications.


i've read the sticky topic above about solar cells and it was so helpfull but i couldn't understand how to use it.

I'm not trying to create a generator or anything i just want to take power directly from the cell to some LEDs.

My question is:
How to use it actually? will i just point it to the sun and take the power out of two terminals or i need some more stuff to be added to it in order to take power out of it?

if i could buy one i'd buy the one at this link
**broken link removed**
caz its the cheapest one , and i don't have a big budget for this project.

someone have any knowledge about it?

Thanks

 

[Nigel Goodwin]

You also need a current limiting resistor feeding the LED's, bear in mind that the actual output will be greatly less than that specified, which is probably at noon in the sahara desert in the middle of summer, and only for a few minutes then.

No matter where you are, you will need to be outside in VERY bright sunlight to get reasonable amounts of energy out of them.

 

[ashour]

Well about the sun , we are planning to create it as a video , so we'll do it in a sunny day , and i live in Egypt so we have alot of sunlight - which is actually the east of the sahara desert .

so i put the solar cell , take the positive terminal and connect it to a 220ohm resistor then to the leds and then the leds back to the cell thats it?

 

[ashour]

Is it better to put a capacitor with the resistance to make an RC circuit which would have a smoother current so that the leds wouldn't be shocked

 

[RCinFLA]

You won't need a current limiting resistor for that solar array. At 6v, 100 mW that's only a maximum of 17 mA at full sun.

A solar cell is an illumination based current source that is clamped by forward biased inherent diode. At full sun you will get 17 mA when loaded down below point where inherent diodes start to conduct. This means you will get close to 5.6 volts when loaded across array with a 330 ohm resistor, 3.75 vdc when loaded with 220 ohm resistor, and about 1.7 vdc when loaded with 100 ohms.

At half sun you will get half the 17 mA's. An array stating 6v has about 12 cells connected in series.

The array will put out 17 mA's at full sun across the loading range from 0v to close to 6 volts where the generated current starts to conduct the cells' inherent diode. No load means all the generated current is shunted down the inherent diode's of the cells so you are measuring the cumulative voltage drop of 12 forward biased diodes. Since diodes have a negative temperature coefficient, the diodes conduction point gets lower at higher temperature.

There is some equivalent series resistance and shunt resistance of the cells that will vary the appearent output slightly from an idea current source. The poorer the quality of cell the more the series resistance and lower the shunt resistance may be.

Looking at picture, I count 13 tabs across the center so there is 13 cells in that array. Maximum power point loading at 35 deg C will be about 6.5 vdc with about 430 ohm loading on array.

You can put two LED's in series if their total forward voltage drop is less voltage then PV array's inherent diode stack (white & blue LED's have higher forward voltage drop and may not be able to put two in series). If your LED have less then 2.0 vdc forward voltage drop you might be able to put three LED's in series.

**broken link removed**

 

[ashour]

WOW! ,thanks ! thats great

So you mean by "inherent" diode stack that it would come with the solar cell and i just connect it to the LEDs?
So how do i pinpoint the Positive and Negative terminals of the Solar Cell Array?

We have the avg temp of about 37 these days so at full sunlight at noon it may get about to 45 deg C! - so hot but its great for out little project.

Another question would be is there a special semiconductor material for which solar cells are made of or its just silicon based semi conductor?

Thanks

 

[Nigel Goodwin]

Well about the sun , we are planning to create it as a video , so we'll do it in a sunny day , and i live in Egypt so we have alot of sunlight - which is actually the east of the sahara desert .

You should be OK then!

As also mentioned, with such a small panel you won't get enough current to overdrive an LED anyway.

From a demonstration point of view, I'd connect a multimeter in series with the LED to show the current provided to the LED.

 

[RCinFLA]

WOW! ,thanks ! thats great

So you mean by "inherent" diode stack that it would come with the solar cell and i just connect it to the LEDs?
So how do i pinpoint the Positive and Negative terminals of the Solar Cell Array?

We have the avg temp of about 37 these days so at full sunlight at noon it may get about to 45 deg C! - so hot but its great for out little project.

Another question would be is there a special semiconductor material for which solar cells are made of or its just silicon based semi conductor?

Thanks

Solar cell is a diode (PN junction). Just use a voltmeter to find out which terminal is positive.

There are three basic types of silicon solar cells. Monocrystaline which is made similar to integrated circuits from cut wafers. They are highest quality. Polycrystaline is second, but pretty close to performance of monocrystaline. Amorphic cells are third and lowest quality (toys). Amorphic cells have the bluish 'chipped wood board' appearance.

Mono and Polycrystaline cells produce about 35 mA's per square centimeter of cell area in full sunlight and run about 15% efficient to the 1000 watts per square meter of sea level, full sun power. Amorphic cells produce about half that current per unit area. Current is nearly directly proportional to illumination.

Silicon diodes have about -2 mV/deg C temp coefficient so voltage of diode conduction drops about 0.33 % per deg C rise in cell temperature. Temperature has a very small effect on the illumination current generated. At low illumination levels any shunt resistance of cell (leakage) will consume the small amount of current generated.

There are other types of solar cells. Gallium arsenide based cells are used for high efficiency space based cells (about 25% efficient) and are very very expensive. At other extreme, there are low cost thin film deposited on milar plastic film that are about 5-7% efficient.

 

[ashour]

Thanks , for your help , i've bought the stuff and it works !! , never thought it would be that easy , but i didn't get the description of the amorphic cells so would someone look at its picture and tell me if its amorphic or not
i didn't buy the one i mentioned before because it was out of stock i bought another one which was a bit more expensive but in same low cost range so i think they must be amorphic.

**broken link removed**

Thanks for everything.

 

[RCinFLA]

The first one looks amorphic. The second one's picture is not too clear and hard to tell.

You can measure the area of one of the 13 cells and calculate the mA's/sq. cm. to get an idea of type.