Sunday, July 30, 2017

pushing electrons

The photovoltaic phenomenon is described as photons knocking electrons out of their valence orbital and creating a net charge in a semiconductor. There are holes, and dopants, and depletion zones, but I don't pretend to understand the nitty gritty physics of it. Sure is a handy property of semiconductors.

Lots of physics research, material science, high tech fabrication techniques, and government encouragement went in to the solar panels we installed this summer.

The key here, is that once the PV cells and panels are built, they keep generating electricity from sunlight for a really long time. Big payment up front, then "free" power for years.

I know that the federal tax credit for installing residential renewable energy is phasing out over the next few years, so decided to make our move now.

We are now producing our sun driven electricity, to the tune of 4.9 kW nameplate capacity. So far, our peak output was 4.4 kW, but don't know what has to align to get the full capacity. For the year, this is prime solar weather, as the winter and spring are more cloudy.

We decided on ground mount, as we have plenty of unobstructed southern facing land, and I'd just as soon not mess with the roof unless that was a much better option. I also had initially planned to design and build the rack myself. ( maybe another blog entry on how that went down).


Here I am, driving a Bobcat skid steer for the first time, with the 24" (600mm ) dia. auger for drilling the holes for the concrete piers. I am still a terrible operator, but did not kill the guys who came to help. It helped that they were both nimble and quick on their feet.

Here are the rack columns in the concrete forms, braced and ready for the  pour. The holes are six feet deep. This rack is not going anywhere.


The design for the rack required huge piers, totaling 2.5 cubic yards ( 1.9 cubic meters) of concrete between the three of them. I looked at renting a mixer and doing it by hand, but the cost was not that much higher to order the truck, and WAY less labor.

Had to dig the trench from the array to the inverter in the barn by hand. No fun in the heavy clay mixed with large rocks. Code requires top of conduit to be 18" (450mm) below ground.


Here is the finished array, soaking up the sunshine. The panels themselves are super easy to install ( as long as the rack was put up level, straight, and plumb!), and the cabling is all premade with simple plug ends. This particular design is a single string, 350VDC output with optimizers at each panel.

Pulling the home runs, terminating, tying in to the power panel were done by an electrician. I might have been able to figure it out, but he was way quicker than I would have been, and one of the grants we are going for requires a master electrician to sign off on it.

If we decide to go off grid, we will need to spend for the batteries, as well as the charge controller and a different inverter. We are a ways off from that, as we would need to reduce our load, and really do some self reflection on how much lifestyle change we would be comfortable with.



2 comments:

  1. Hi Steve,

    Well done, the setup looks great! Yeah, I hear you about concrete pours and that is the way to go for that much cement.

    You'll have to watch the output so as to get a feel for the output during the changing seasons. Does you inverter have a remote monitor or data recorder?

    Very high and thin cloud will produce more than that rated peak output. The light bounces around the high thin clouds and acts in ways that you would not expect.

    On the other hand your panels are angled in such a way as to produce a good compromise between summer and winter sunlight (with a more favourable winter output). As an explanation summer optimised panels would be almost flat as the summer sun is high over head and the panels have to face the sun dead on to produce the maximum output. Winter panels would be very steep as the panels would need to be face on to the winter sun which is much lower in the sky. Hope that makes sense and it explains why you are not seeing the rated maximum output.

    The general rule of thumb is to angle your panels at whatever latitude you are located at to ensure a good compromise. Yours look about right to me and should favour winter production. Your shed roof would favour summer solar power production, so you picked well.

    I have to laugh every time someone says: You should get a tracker. Like they are really simple machines... Every single example of a solar tracker that I have ever seen installed in a private installation of solar has been broken. They're a good idea, but don’t seem to stand up to the rigours of day to day existence.

    Total respect!

    Chris

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  2. Chris- yes, the inverter talks to our wireless router, so I can access data through the internet on my computer, and the system designer can access it remotely, so very easy to monitor.

    Trackers- nope. I've seen several that were aimed all wrong, some obvious breakdown that the owner had not yet figured out how to fix.

    I left the array angle selection to the designer. As you say, the balance between seasonal sun angle, as well as typical cloud cover patterns makes for a compromise that I leave to the experts to choose.

    Thanks for stopping by.

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