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Post by cye on Jun 11, 2011 7:11:51 GMT -5
i found this a useful introductory guide, being someone who know nothing (yet) about good design practice for pv panels... Attachments:
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Post by cye on Jun 25, 2011 3:47:27 GMT -5
Here's a simple guide on installing a blocking diode. Schottky's have a lower voltage drop than normal diodes which is why they are popular, but they are a more expensive Attachments:
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Post by caveman on Jun 25, 2011 7:31:05 GMT -5
I took a look at those documents and I thought I would add a bit and striaghten a bit out. Looking at the attachment I have drawn a simple pv system. Each block represents either a cell within a panel or a panel in a system. Thinking of the drawing as a a three panel system with the sun shining, it would work, sort of. There are a number of problems with this setup but there are two big ones. The first is that there is no provision to care for the battery. On a bright day the panel voltage will rise far above the correct voltage to charge the battery. This is the solar thermal equivalent of boiling the system. The battery will last no time with this sort of treatment. The second problem is that at night there is nothing to stop current flowing from the battery back through the panels, heating them up and discharging the battery. Attachments:
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Post by caveman on Jun 25, 2011 7:45:48 GMT -5
There is an easy solution to one of these problems. If we put a diode in the circuit we can stop the current flowing back to the panels at night. The diode works like a one way valve in a plumbing system, it will only allow flow in one direction. Another analogy is the valve on a bike tyre- it allows air to be pumped into the tube but stops it coming out again. So a diode in the system improves things, this diode is refered to as a blocking diode. Attachments:
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Post by caveman on Jun 25, 2011 7:58:57 GMT -5
The second problem of caring for the battery is best dealt with by using a charge controller. That is a story for another day. Other issues arise when all the cells in a string or panels in a system are not supplying the same output. This can happen when part of a panel is shaded or cells have been damaged. The important thing to note is that the affected cells not only fail to contribute to the output of the system but they actually impede the flow of current in the system. This will cause cells heat up, and in the case of shaded cells they may be damaged by the heat. In the diagram, the broken cell and the shaded one will 'constrict' the flow of current to the battery. Attachments:
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Post by caveman on Jun 25, 2011 8:10:35 GMT -5
The solution to this is to fit diodes that will allow current to bypass faulty or shaded cells. These diodes are called bypass diodes. They can be the same sort of diodes as would be used as blocking diodes The difference is in what they do. In a perfect world all cells in a system would be producing the same current and the bypass diodes would never be needed, only the blocking diode would do a days work. Attachments:
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Post by caveman on Jun 25, 2011 9:36:09 GMT -5
There are two types of diodes useful for this sort of job. One is the common junction diode and the other is the Schottky type. The differences are outlined in the diagram. When choosing a diode there are a number of considerations. The first is the voltage drop across the diode. This is always reduces the voltage available to charge the battery and so the lower the better. Schottky diodes have the lowest voltage drop and so are the most popular ones for this sort of work. However, as was pointed out by Cye, they are the most expensive. The next consideration is how much current the diode will have to handle. The blocking diode will have to handle all the system current. Looking at the picture you can see that diodes are available in a number of different packages. There are a few more I didn't have to photograph. The higher the current handling the bigger the package. The biggest one shown is fit for 30amps and is in the form of a bolt so that it can be fitted to a finned piece of metal called a heat sink. This allows the diode to dissapate heat. It would get too hot otherwise and fail. The smallest diode will carry 1 amp and is too small for anything PV except perhaps a trickle charger. There is no easy way to tell what sort of diode each is without reading the number and looking up the data sheet. The last thing you will need to know about a diode is how high a voltage it can withstand when it is not conducting. This is recorded as the Peak Inverse Voltage (PIV). In the ever useful plumbing analogy this would equate to how much pressure a one way valve can withstand before it bursts. Choose a voltage which is higher than the system maximum voltage. In an arrangment of parallel and series cells it may be possible to use a lower voltage rating for cell bypass. I would have to think about that a bit more and perhaps work it out with an example. Attachments:
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Post by cye on Jul 27, 2013 1:59:17 GMT -5
here's a paper explaining why one should understand how the internal bypass diodes are configured in a panel before you decide on the panel orientation, taking into consideration even minor shadows Attachments:
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