More on PV.

The beginning of this year I started installing a twelve volt system to power the low voltage tier lights and the door bell and provide charging power for a Zappy electric scooter and charging rechargeable batteries for household electronic devices. The system is made up with surplus 12 volt panels I have acquired over the years.

I've been hoping to get all this on a web site, and have finally gotten around to it. Please email me with any suggestions, questions, etc.

Little by little I'll try to put on here step-by-step calculations, designs, procedures, with pictures. Not enough time in the day, but I'll do it little by little, along with everything else.

It is hard writing all this out. On the one hand, I don't want to make a whole bunch of unnecessary reading to go through. On the other hand, I don't want to leave out anything important or lose anyone by making this too hard to understand.

For now, this will look like unrelated topics, but I'll try to clean it up a little as I go along and add the pictures in a couple of weeks.

Why did I choose the specific hardware I did over other available PV panels, inverters and charge controllers? I chose the PV and batteries because they are manufactured locally, are good quality, and the price is within reason. I went with the Trace SW inverter and charge controllers because of their features and track record of dependability. The wire, mounts, boxes, etc., I tried to obtain locally and ordered through the internet advertisements when they were not available here.

There are a bunch of steps involved in putting up a PV system. First I decided the best place to mount the panels was on the roof. It is an old roof, but is all solid wood and supported by 2x4's on 22 inch centers. I figured the weight of the panels would be negligible in comparison to the shingles and make up a very small percentage of the total weight on the roof. With the mounts, the weight of the panels comes to less than 2 pounds per square foot, but I plan to add extra support in the attic as I add a reflective barrier. One can never really say what the snow load on the roof will be some day, so I figure it is better to err on the conservative side. Calculating the actual strength and load on the roof would be hard because it was made so long ago and the dimensions of the lumber are larger than the current lumber. If the roof was strong enough to support what was already on it and I add enough support for the additional weight, there should be little possibility of problems.

The panels are mounted on aluminum frames manufactured by Atlantic Solar in Baltimore in accordance with Solarex's design. I felt these were required to stiffen the flexible large panels and ensure they were solidly attached to the roof. These are actually ground mounts, so they also give me the capability of tilting the upper rows of panels in the winter months.

Next I had to decide where to put the batteries. The system was going to be grid intertied and the grid power from the electric utility in this area rarely goes down. I decided, therefore, to skimp on batteries because I could always pull power off the grid if there was not enough saved up in the batteries. The utility AC Load Center was located in the basement, so I figured this would be the logical place to put the inverter and batteries. This had the additional benefit of keeping the batteries at room temperature, which makes battery maintenance easier. An additional plus is that the batteries represent a substantial "thermal mass" to help stabilize the temperature in the basement and hold heat in the winter.

For those who are not familiar with inverters, a grid interactive inverter will synchronize its wave form with the electric company's power so solar powered appliances can use electricity from either the sun or the power company. When it is raining (no sun), they can still operate off the grid electricity through the inverter.

The AC service from the utility has #4 aluminum wire from the meter to the AC Load Center. Rather than replace this, I decided to reduce the load on the utility interface with solar. I put the 4KW (>30amp) SW4048 inverter on a 15 amp circuit breaker on the grid AC load center, and set the maximum current draw of the inverter to 15 amps in the inverter firmware setup. Any current the inverter-driven loads draw in excess of 15 amps comes directly off the solar panels or out of the batteries instead of from the Grid.

To avoid excessively charging the batteries off the grid rather than solar, I set the charger in the inverter to provide a maximum of 1 amp to the batteries. This is equivalent to a trickle charge for this bank (<120 watts). This prevents running the batteries down in the event of several shady days in a row in the winter, but ensures the batteries will be "floated" on AC charge at night, whenever possible. The batteries have not been deep cycled once, yet.

Some may not be familiar with the care and feeding of lead acid deep cycle batteries. Let it suffice to say here that the length of life of these batteries depends to a great extent on the number of charge and discharge cycles and the depth to which the batteries are drained in each "cycle". My batteries should be good for 300- 1200 cycles, so they should have a lot of life left in them if they haven't been deep cycled even once. On the other hand, they haven't really been tested and one or more of them may fail prematurely due to manufacturing defects or damage in shipping which I could only detect by cycling them deeply a few times. There is also a "break-in" in the first few cycles, so one might say these batteries have not even been broken in, yet.

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