North-Facing Solar Panels
AntonioO
| Posted in General Questions on
Today I saw a house with solar panels on a north pointed roof slope. Does that ever make sense? About 20% of panels were on the front of the home, which faces south. The rest were on the rear north facing roof slope. The south facing roof could accomodate few panels because of decorative gables.
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Antonio,
Looking a this, it doesn't seem like that bad an idea: https://www.solarpowerworldonline.com/2016/06/much-less-efficient-north-facing-solar-modules/
Unless there's absolutely no other option its makes almost no sense at all from my persepective. My south facing rack of panels will yield around 50 amps dc in direct sunlight, while prior to the sun rising above the treetops, or after it's blocked by the trees in late afternoon - which would likely be similar to their facing north and never receiving direct sunlight - they yield in the neighborhood of 3 amps. If the roof slope is relatively flat they may get direct sunlight during late spring to early summer. Of course, most installations are grid-tied situations, so maximum sun exposure often is less priority. I see myriad installations that are covered with snow through the winter, shaded by trees much of the summer, and otherwise poorly sited because the necessity of the electricty from the panels is a far second to the grid power.
Generally shading is a much bigger issue than tilt. If they are unshaded for most of the day then it's OK.
The farther North you are, the worse North facing panels will perform. It's all about angles. The best production situation for the solar panels is when the sun is directly above them, what we engineers would call "normal to" the face of the solar panels. In the math world, "normal" means "mutually perpindicular". Think of this as putting a post in the middle of a flat piece of plywood, then using a carpenter's square to square the post to the plywood, move the square around the post 90 degrees then square it again. When you have the post "normal to" the surface of the plywood, the square will show the post being perfectly square (90 degrees) with respect to the plywood no matter where around the perimeter of the post you place that carpenter's square.
ANY deviation from that situation with the light coming straight in results in less than maximum conversion of the light's energy to electricity by the panel. Note that throughout the day, those angles change, so you get the best production when the sun is directly overhead in most cases. As the sun moves throughout the day, the production ramps up in the morning to a peak, then falls as the sun moves to the West and eventually sets. Seasons also effect this, so in the Northern hemisphere, production peaks around the summer equinox, then gradually declines towards the winter equinox before gradually rising again as you get closer to the summer equinox.
The worst case scenario for North-facing panels is on the day of the winter equinox, and especially in the morning and evening, since that's when the angles are most shallow and furthest away from that ideal "normal" scenario. If you face the panels South, you're immediately improving those angles under ALL time/day/season situations. While you'll always get SOME level of production, the arrival angle of the light is very important to maximize yuor production. It's really worth optimizing placement of the panels for this reason, even if that means putting them on a seperate structure instead of the roof.
Bill
whether they're "worth it" depends. If the home has a net metering agreement and can bank sun hours in the summer when the sun is high in the sky then it doesnt matter much in the winter when the array is producing nothing.
The short answer to "does it ever make sense?" is yes. It depends on several factors:
-how "north" is the orientation? Very few houses are oriented exactly north-south
-what is the latitude where the house is located? Further north, worse the performance
-local considerations such as available roof space, differing shading on sides of the roof, etc.
-what is your primary use for the panels? If you expect to use them to take some of the heating load in the winter, it's probably not going to work out too well.
The link that Malcolm provided is nice, although a little biased towards more southerly locales. It does give some examples of slightly more northerly (and higher roof tilt) locales, but with fewer graphs and data. It would be nice to see the month by month penalty like they did for the NC example.
There are online calculators where you put in your orientation and tilt and it will tell you the annual insolation. Rather than listening to opinions here that will get you some hard facts. My recollection from playing with it was that sub-optimal positioning had a surprisingly low impact, maybe a 25% difference between best and worst. If the panels are shaded that's a different story, they produce nothing when shaded.
I just did a sample calculation using a Buffalo address. Here's what it spat out as losses relative to optimum for a north facing placement, 6/12 roof pitch:
Total annual: -37%
Best month: -13%
Worst month: -70%
If you just look at overall numbers, it's not horrible. Looking at the details tells you that if you want to use it to supplement AC in the summer, it's good. If you think you're going to use it in the winter to supplement heating or plug loads, it's pretty abysmal.
These numbers will get worse further north, and better further south.
Here's one for northern Maine:
Total annual: -42%
Best month: -14%
Worst month: -79%
The National Renewable Energy Laboratory has an excellent calculator called PVWatts. For my location in Southern CT it shows a North-facing array would have less than half the output of a South-facing array. In snow country, a North-facing array could remain covered with snow all winter. On my 10/12 pitch South-facing array, the snow usually slides off in a day or two. The short answer is, a South-facing array tilted at the location's latitude will produce the most output on a yearly basis. As always, your mileage may vary.
https://pvwatts.nrel.gov/index.php
Great thanks to all the responses. It's not my intention to install a north-facing array. My house faces only a few degrees away from solar south with few shading obstacles or roof obstructions. However, I was surprised to see a north-facing installation in a neighborhood near me. For what it's worth the roof on which the array was installed is close to 40 degrees north latitude and I would estimate the roof pitch to be about 8/12.
Interesting to learn that for a lower pitched roof, it's not terrible if a homeowner doesn't have other options.