Friday 30 March 2012

Solar PV - the basics

As I mentioned in the original post, as well as solar water heating we also have (as of last week) solar electricity, or solar PV as it's more commonly known.

I thought I'd start out with a brief description of how a solar PV system goes together. First, a diagram!

A generic domestic solar PV system
Ok, So...

The two main components of the system are the PV cells themselves, and the inverter.

PV is short for "photo-voltaic' from the Greek photos, light, and Alessandro Volta, who invented the first battery - basically PV cells turn light into DC electricity, and the more light, the higher the voltage they generate.

Now, DC is not much use for running household equipment, as that requires 240V AC. Enter the inverter, whose job, pretty simply, is to turn DC into AC. This is where things get a little less simple, since inverters have a minimum voltage at which they will 'kick in', so if it's not bright enough to generate that minimum voltage? No electricity. Once it does kick in, the higher the voltage off the PV cells, the more power (and thus the more units of electricity) the inverter kicks out.

In our configuration[*], the inverter feeds into a spare breaker on our fuse box, via a meter that measures the total units of electricity generated (very important, as this is how you get your Feed-In Tariff, of which more in a later article). If there's more being generated than the house is using, then (by the magic of electricity) the excess is fed back to the grid, otherwise the grid supplies the balance.

And that's pretty much it. Other than to note that, if you were dreaming of being able to go 'off-grid' when the power goes out? Most systems don't, and more awkwardly, they actually shut down when the power goes off. Annoying and somewhat paradoxical - the reason behind it is that in order not to cause disruption to the grid, it's essential that the inverter syncs up to the frequency of the grid supply, and it's a requirement that if the grid goes off, it shuts down until it comes back.

Next in this series of posts, I'll look at how to calculate what kind of power you can get from a PV system.

[*] In some systems, the inverter is connected up in the meter cabinet instead of to the fuse box: this makes little or no difference to how things work.

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