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Author Topic: How do solar panels work? Various questions..output voltage, etc.  (Read 60364 times)

Offline joshk577

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Hello. I'm looking for information on solar panels. Any help would be very much appreciated. Here are some of my questions:
1) how does a solar panel convert sunlight into electricity?
2) what is the common output voltage on a solar panel? (btw, I live in the united states).
3) what is the typical amount of power (kilowatt and kVA) produced per square foot of solar panel? (or metric equivalent)
4) is the electricity produced AC or DC?
5) what is a solar panel made of?
6) is there a means for regulating output voltage and frequency?
7) what is the typical frequency of the output power, if output is AC?

Thank you.

Josh


 

Offline CliffordK

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Whew...
Lots of questions.

Lets start with a few basics.

kw per square foot is not a good way to think about it, and therein lies the problem with solar panels.

A typical 6"x6" cell puts about ½ Volt, and about 7 Amps DC Power.
Watts = Volts x Amps
So one gets about 0.5 Volts * 7 Amps = about a maximum of 3.5 Watts from the cell.

If you put 4 of the cells together, you get about 14 watts per square foot, or about 0.014 kw per square foot (with 1000 watts per kilowatt).  This is more or less the maximum power you might expect to get with noon-time sun during the summer.  You will get a lot less with morning or evening sun.

Solar panels are made up of multiple solar cells.  If the cells are wired in parallel, one increases the amps.  If the cells are wired in series, one increases the volts.

The Open Circuit Voltage and Short Circuit Amps that the cell puts out are more or less theoretical maximums.  You find that the greater the power draw, the lower the volts that the cell puts out.  Thus, there is a tradeoff between Amps and Volts.  Most panels now have a "Peak Power" rating.

A typical 12V Car battery actually is about 12.6V when fully charged.  But, it is generally charged at about 14V.

So...

Older solar panels were often rated to put out 12V, but actually had a peak output of about 18-20V. 

More modern (consumer grade) panels usually have power ratings of about 30-50V.  Usually one uses a charge controller to isolate the system or batteries, and the charge controller can automatically adjust to the "Peak Power" output of the panel.

I have some panels that are about 3' wide, 4.5' tall, with an output of about 200W, maximum voltage of 68V, and Maximum Power Point (MPPT) of about 56V.

AC vs DC.
If you are just running lights and such from a solar panel, then it is easy enough to set up a DC system, and use DC light bulbs.  No inverter to turn on and off, and no inverter loss. 
However, as most of the appliances in the USA require 110V/220V AC current, one can also attach an AC/DC inverter to provide the correct voltage.  And, of course, people typically choose 60 HZ for USA and 50 HZ for Europe.

You can run the system entirely "off grid" meaning that one has to generate 100% of one's own power needs, and setup one's own batteries for backup power.  Or, one can run the system "on grid", essentially using the power company's system as a super-battery.  One sends the surplus power to the grid, and can take extra power from the grid as needed.  No batteries are required, although some systems use them for backup power.

Traditionally the inverters have been centralized, but some companies such as Enphase have an option of connecting mini-inverters directly to each panel, in effect converting the DC panels to AC panels.

Here is a good picture of the construction of a solar cell/panel.  The top most layers listed are actually part of the panel, and outside of the cells.

http://www.specmat.com/Overview%20of%20Solar%20Cells.htm

Most cells are silicon cells with a layer of pure silicon covered by a layer of silicon "doped" with Arsenic or Phosphorous, then with electrodes attached.

There are also variations such as cells sprayed onto ordinary glass, or flexible cells.  Boeing & Emcore make what they term as a "triple junction" cell that absorbs a wider bandwidth of energy, and thus has higher efficiency ratings than the typical household cells.  These high efficiency triple junction cells are used in space applications where space & weight is a premium, as well as in commercial concentrated light solar panels.

Heat vs Electricity.
A typical solar panel is only about 10 to 20% efficient at converting sunlight to electricity.  An appliance such as a hot water heater will also have efficiency losses.  For hot water, it is much cheaper, and more efficient to heat the water directly (or with an antifreeze heat exchange medium) than to generate electricity, then using the electricity to heat the water.  Likewise, solar panels can be devised to heat air directly, again with efficiency gains over electric solar panels and electric heating.  However...  at least in Oregon, peak power needs are also during the winter when the sun is most scarce. 
« Last Edit: 09/06/2011 00:11:00 by CliffordK »
 

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