Solar Panels: Solar Power Components

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Solar Panels

This blog post is about Solar Panels. This is the first in a series as we take a look at some of the main components in a solar electric system: the solar panels, batteries, solar charge controllers (SCC) and inverters. Technology is moving forward at a very fast rate. Most of the generalities are true today, but there are likely to be exceptions.

If you missed the earlier blog posts, I recommend that you read them to get a good foundation:

All other solar power articles may be found HERE.

Review of Series and Parallel Circuits

Solar Panels

Wiring solar panels in series: Wiring the negative of one wire to the positive of the other, results in the current staying the same. The voltage increases.

Wiring solar panels in parallel: Wiring the two positives together and the two negatives together, results in the voltage staying the same. The current increases.

It s important to note that regardless of the way it is wired, the power, or watts, remains the same. Since Watts equals Volts x Amps, it doesn’t matter if it is series or parallel.

Solar Panels (PV Modules)

Solar panels (or PV modules) generate DC electricity when exposed to sunlight via the Photovoltaic Effect. This was first observed by a French physicist Edmond Becquerel in 1839.

A simple explanation is that the photons from sunlight are absorbed by a semiconductor material, generally silicon. The negatively charged electrons are knocked loose from their atoms, and flow from the negative side to the positive side to recombine with available holes there. This creates a direct current flow. This flow of electrons can then be used to:

directly power a DC device, like a pump or a fan
can be used to charge a battery bank
can be inverted to AC power to use in your home

Each solar cell generates about 0.5V. That’s not much for practical use. So multiple cells are wired together in series to create a higher voltage, creating a solar module, commonly referred to as a solar panel.

A typical 12V solar panel has 36 cells in series. The larger a solar cell is, the higher the current. So the cells of a 200W panel are generally bigger than a 100W panel. Multiple solar modules wired together then creates a solar array.

You can see the difference in the look of a 12V module compared to a 24V module.

Each module has a label on the back, stating their specs. Here s an example of a Kyocera 140W 12V module.

It lists the rated outputs for the panel, as well as any certification it has. The ratings are actual outputs under standard test conditions, so the numbers you measure in the real world may be slightly different. Let s go over each of the specs for a solar module:

Open Circuit Voltage (Voc) is the voltage you will measure when nothing but a voltmeter is connected to the solar panel. This is the highest voltage the module will output at 77 degrees Fahrenheit with the sunlight intensity at 1000 watts per square meter, which are just a few of the details of standard test conditions, or STC. The voltage will be higher when it is colder out, and lower when it is hotter.
Short Circuit Current (Isc) is the amps output with no load on the panel. It is the highest current possible at STC. There are times when the output could be higher, for instance when the sun is coming out from behind a cloud, you can see the silver lining , where the edge of the cloud is magnifying the sunlight, causing the intensity to be brighter than STC.

This brings us to the two specs which are when the module is connected to a load, so more real world conditions. But still at the temperature and brightness listed under STC.

Maximum Power Voltage (Vmp) is the actual voltage the module will output when connected.
Maximum Power Current (Imp) is the amps output while under load. PV modules were originally designed to charge battery systems, so it is typical to see panels listed for what voltage battery bank it is able to charge.

Nominal voltage is a shorthand grouping term, originally based on battery voltages (for example, 12V, 24V, 48V). To charge a 48V battery bank, you simply wire four 12V modules or 2 24V modules in series to add up to 48V.

In general, you can determine what nominal voltage the module is by the number of cells on the panel. A 12V nominal panel usually has 36 cells, and its Open Circuit voltage is about 22 volts, and its Maximum Power voltage is around 17 volts. However, as grid-tied solar systems that don’t use batteries have become more popular, you start to see different size nominal panels that don t logically line up with battery bank sizes. The most popular size modules used in grid-tied systems today are 60 cell, 20V modules.

The wattage of available solar panels has been increasing, and many manufacturers are achieving that by increasing the number of cells, increasing the voltage of the solar panels. As you recall, watts equals volts times amps, so increasing the volts while maintaining the same amps increases the watts. As such, there are 80 cells and higher available these days.