The system is simple in design. Solar panels provide power that we use. Any extra power is stored in the batteries. When there is no sun, the batteries provide power. In between each part we added circuit breakers for protection and to allow us to isolate them.
If the solar panels output just as much power as is used, nothing goes to the batteries.
Determining System Size and Location
We did not determine the system size by going through a list of all the things we had to power. We did not have the funds to set the size at will. Instead, we are building the system from 2 panels (what we could afford) up.
We are making sure that all the components work together well. Right now, the solar controllers can handle an allowable current of 48 Amps (A). Since each panel outputs around 7 to 7.5 A, we can say the safe total number of panels we can hook up without purchasing more controllers is 6.
If we add more panels, we must also add more batteries in parallel to handle the increase in power output.
Right now we have two Crown deep cycle batteries. The electrical capacity (how much electricity they can store) of the batteries depends on how fast it is being used. They are rated for 140 Amp-Hours (AH) for 20 hours (Hr) or 110 AH for 5 Hr. This is how ratings are given by the battery manufacturers. Batteries have a greater capacity the slower they are charged or discharged and a lower capacity the faster they are charged or discharged. The chemical reaction that happens in the batteries is more efficient when it is slower.
What this means is that each battery can give out 12 A per hour for 20 hours (140 AH / 20 H = 12 A) or up to 22 A per hour for 5 hours (110 AH / 5 H = 22 A).
We want to maximize the lifetime of our batteries, therefore we will not be exceeding the lower rating by much. If no power from the panels is used during the day, the batteries will charge at a maximum of 14 A (2 panels @ 7 A each).
The circuit will run at 24V, with the two panels in parallel and two 12V batteries in series (providing a 24V battery bank). Two identical Tristar controllers are used, each set to a different mode. The first (top in the picture) one provides power to the battery terminals from the solar panels and charges the batteries. The second one (bottom in the picture) takes power from the battery terminals and sends it to the left breaker box for use.
The location of the panels also affect their power output. It is best not to have any shadow at all on them, even in a corner.
In the norther hemisphere, where we are located, we must orient the panels to the south, towards the sun. The angle the panels should be at depends on what the goals are. If the panels need to output near their maximum power in the winter, they should be placed at a high angle (sun is lower on the horizon). If requiring maximum power in the summer they should be placed at a low angle (to catch sun that is more overhead). For good overall performance during the whole year, a middle angle should be used. The website for the Solar Handbook which we have used almost exclusively as a guide, has a great solar angle calculator online.
For example, in Chicago, during June, the angle is give as 72 degrees. In December is it given at 24 degrees. A 90 degree angle would mean the sun is directly over head. For maximum power production during the summer, the solar panels would be but at 72 degrees. For maximum power production during the winter, the solar panels can be set at 24 degrees. For good performance all year rounds, they can be put at (72 + 24) / 2 = 48 degrees.
Our panels can be manually tilted for maximum efficiency all year.
That will do it for this part. Part II will be more details about the wiring.
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