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    How Solar Panels work

    A solar array looks like a fairly simple piece of equipment; it consists of a few solar panels tied in series and parallel; however an entire solar array is a complex piece of electrical equipment. The diagram below shows how we are going to approach the discussion of a solar array.

     

    Solar Cell Solar module solar panel and solar array work together

    Many Solar cells are combined to create a solar module (most people know them as solar panels). Many solar panels are combined into a solar array, and it is the solar array that we use to power your house, or charge batteries, etc.

     

    So lets get started... fair warning this is going to be quite technical (and some may say boring).

     

    1. The Solar Cell

    The solar cell is fundamentally a semiconductor made up of two materials known as P type and N type semiconductor material. Semiconductor material is most commonly highly purified silicon. The deliberate addition of impuraties such as boran and phosphorus are used to create the different P and N types.

     

    The internal structure of a silicon solar cell made of up P type and N type semiconductor material

    Whenever P type and N type semiconductor material are placed together an internal voltage is created at the junction. When light photons hit the N type region one of two things happen; Heat is generated or the light photon energises an electron which allows it to travel up towards the top plate, where it is collected by the conductive plate and move to the load (in this case a light bulb). At the same time the holes created by the elctrons moving up towards the top plate are pushed down towards the bottom conductive plate; and hence an electric current is created.

    If you didnt understand any that (because you dont have a degree in rocket science) then dont stress... the only  important thing is a solar cell produces an electric current proportional to the surface area exposed to light and the intensity of that light.

     

    There are three types of solar cells that you are likely to come across.

    Monocrystalline Solar cell

    monocrystal solar cell

    These are made by creating a single grain of silicone and cutting thin wafers from it.

    It sounds like an expense exercise, and it is. Monocrystal solar cells are the most expensive silicone based solar cells, however they are the most efficient.

    The cells have a single matte appearance as shown to the left.

    Polycrystalline solar cell

    polycrystalline solar cell

    Polycrystalline solar cells are also made from silicone, however the silicone is cast into square ingots, the result is that multiple silicone grains are formed. The image beside shows the characteristic poly cell look which is something akin to broken glass.

    These cells are a lot cheaper to produce, however they are not quite as efficient as monocrystal cells because of the effect of silicone grain boundaries.

    Amorphous solar cells

    amorphous thin film solar cells

    Amorphous solar cells are produced by depositing the semiconductor material directly onto a substrate, that substrate can be any number of things, both flexible and rigid.

    Amorphous solar panels are typically made as one big solar panel rather than a number of solar cells being brought together.

    Amorphous solar cells are the cheapest of the solar cells, however they are the least efficient, which means you need more of them to produce the same amount of power.

     

    2. The solar module

    Solar module - Solar PanelA solar module (or solar panel) is simply a collection of solar cells (discussed earlier) tied together in series and parallel (similar to the way you might tie batteries together). The diagram shows a pictorial view of how a solar panel is made up of individual solar cells. A commercial solar module will have many cells tied together, for example the CNPV 190W modules have 72 solar cells tied together.

     

    The internal workings of a solar module made up of many solar cells

    In order to produce usable power individual solar cells need to be tied together to create higher voltages and higher currents. Thats about as complicated as a solar module / panel gets.

    The only other thing worth mentioning in most modern solar panels are the bypass diodes. lets say for example a bird is flying past and feels the need to leave a gift behind on your solar panel; that gift completely covers one of the cells in your module. In our diagram above covering any one of the cells in the panel would cut the available power output by one third if there were no bypass diodes; the other thing it would due is create a heat spot on the panel; these hot spots can be quite dangerous on large panels because of the risk of fire.

    Bypass diodes are designed to do exactly as their name suggests; they allow the electrical current to bypass any cells that are not functioning. In  a perfect would each solar cell would have a bypass diode associated with it, however for financial reasons (any maybe some technical ones too), most solar modules will have a bypass diode protecting an area of the panel, that area will be made up of many cells... not perfect but better than nothing.

    What I am about to talk about now is actually characteristics of the solar cells not of the solar module, however it is easier to understand when discussed in the context of a solar module.

    2.1 The infamous IV solar curve

    First of all some basic electrical theory.....

    Power (measured in Watts) = Voltage (V) x Current (I)

    What this means is; for a solar panel to be producing maximum power we need to maximise voltage and maximise current. With that said lets look at the IV curve of a solar module.

    IV curve of a solar panel

    The IV curve shows the relationship between voltage (V) and current (I) of a solar panel. You can see from the IV curve that as current is slowly drawn from the solar module, voltage reduces in small amounts; however their comes a point when voltage drops away rapidly. This point is known as the Maximum Power point, the point where Voltage and Current are maximised.

    The design of the array and all electrical equipment that take power from the solar panel will attempt to operate at the maximum power point.

    2.2 Open Circuit Voltage (Voc)

    The open circuit voltage refers to the voltage of a solar module when nothing is connected to it.

    The Open circuit voltage of a solar panel is an important piece of information; it is important because any piece of electrical equipment you connect to a solar panel, or a solar array will have an absolute maximum voltage, if your solar panel open circuit voltage exceeds that value you will damage and probably destroy the equipment.

     

    A note about Voc and temperature:

    Voc specified on your solar panel rating plate will be based on the open circuit voltage at 25 degrees Celcius. Solar panel voltage is proportional to temperature. The colder it is the higher the voltage, the hotter it is the lower the voltage.

    This has a couple of important implications:

    1. A solar panel will produce less power on a hot sunny day than it will on a cold sunny day.

    2. When checking to ensure that your equipment can handle the solar panel open circuit voltage (Voc) make sure you check it based on the coldest possible temperatures in the region it will be working.

    You will need to check your individual solar panel, but basically if you assume a 4-5% increase in open circuit voltage for every 10 degrees below 25 degrees C.

     

    2.3 Short Circuit Current (Isc)

    The short circuit current is the absolute maximum current that can be produced from the solar panel if the leads are shorted together, this is the current that you need to rate all of your cabling and fusing on.... An important note about is that it assumes 1000W/m2 of solar irradiance. If you happen to live on top of a mountain like I do, then there is every chance on a clear day you will get more than 1000W/m2 . It is a good idea to assume the short circuit current can be as much as 1.2 x Isc (infact the australian standard requires you to make that assumption).

     

    3. The Solar Array

    The solar array is a combination of solar modules put together in series and parallel. There is only two things I will talk about here:

    1. Solar panels are combined together to create large voltages and large currents... this means they can be very dangerous. For example the 10kW grid connect systems we build have voltages around 600V and produce currents in excess of 20A; this is more than enough to kill a person.
    2. Solar panels must be matched together in the array; the array is only as good as the weakest solar panel in the array.

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