Photovoltaic Module and Solar Panel Information

electricity from the sun by John Drake II | home

Basically there are three common types of photovolaic modules.

Single or Mono Crystal

This type of photovoltaic cell was the first kind made around 1954.

It looks pretty much the same over its surface, some manufacturers

etch grooves in each cell to increase surface area.

These are grown from a doped silicon ingot and then sliced into

individual cells.

The single crystal is the most efficient with a given amount of light and

also the most expensive.

By being the most efficient a single crystal panel will offer the most

watts per square foot.

Where mounting space is a concern, this is the best route.

Poly or Semi-Crystalline

This type of cell structure came about in the late '80s.

It looks like a bunch of broken crystals arranged hap-hazardly on the

module.

These are a little cheaper than the mono crystalline panels but also

are a little less efficient.

Thin Film or Amorphous

This type of cell came out in the late '80's.

I remember seeing some of the first ones made by Arco Solar.

Part of the Arco Solar school included a tour of their plant in

Camarillo. We were allowed to go anywhere in the plant except for

the thin-film production area.

A thin film panel is made by spraying, in a vacuum, a doped silicon

material onto a glass or metal substrate (backing).

These panels use the least amount of silicon per square foot which

affects pricing but they are also the least efficient.

These work fine in an installation where panel size is not a concern.

A quick note: even though these are not as efficient as the mono

or poly crystalline panels, they will start to produce power with less

light. Earlier in the morning and late in the afternoon, as well as on

heavily overcast days, these panels will produce power when the

other two types do not.

Another quick note: thin-film panels will degrade about ten percent in

the first year and then level off and age like single or multi crystal panels.

Manufacturers of thin film panels will often derate the label specifications

by 10 percent.

After about a year the module will be at the label specifications.

When putting together a system using thin film modules,

please over size (amp ratings) the components so the

system will handle the higher output for the first year.

There is a lot of confusion regarding the specifications of photovoltaic

modules.

Let's start with the wattage.

Wattage is the amount of power a module can produce in optimum

conditions when connected to a system.

The maximum wattage listed is where the voltage curve (0 volts up to

open circuit voltage) and the amperage curve (0 amps up to short circuit

amperage) meet in the "sweet spot."

And then we have voltage.

You will see two voltages on the pv module label.

The first is the Voc or voltage open circuit. If you set the panel in full sun

and measure the voltage, without the panel connected to a controller or

battery, that would be the open circuit voltage.

The second is the Vmp or voltage maximum power. This is the sweet

spot where amperage and current meet to produce the most power.

A quick note about voltages:

If you take a 12 volt panel with an open circuit voltage of 20 to 22 volts

and connect it directly to a 12 volt battery, the voltage will drop

down to around 13.5 to 14.5 volts.

The battery acts as a load and drags the battery voltage down.

If you connect the panel to a conventional charge controller and

then to a battery, the same thing will happen.

A 12 volt battery needs around 13.5 to 14.0 volts for normal charging.

Most people do not know this and can not understand why a panel

having an open circuit voltage a few volts above the nominal

battery voltage will not charge their battery.

How about current, or amperage.

You will see two current ratings.

The first is Imp or amperage maximum power where the current curve

meets the voltage curve for maximum power output.

The second is Isc which is the current short circuit, this is the most

current the panel can put out under normal conditions.

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