8 ms·

How much less efficient are north-facing solar modules? (2016)

72 points by curtis 8y ago

curtis 8y ago
I've been wondering for a while if vertical solar panels on the sides of buildings might make sense. Today (for no reason that I know of) I started wondering if might even make sense to put panels on parts of a building that don't directly face the sun at all. That's what lead me to this particular article, which examines the efficiency of putting panels on north facing roofs. I explored the vertical panel idea in a blog post last year: http://curtisb.posthaven.com/the-solar-garage-door http://curtisb.posthaven.com/the-solar-garage-door. I don't know if that idea would work, but I was trying to imagine an application of the idea so absurdly simple that a startup could easily use it to bootstrap into a more lucrative market.
- Retric 8y ago
  Generally, you don’t run out of roof space before meeting a 2 story buildings energy needs. On taller buildings installation costs would be significant, coupled with lower effects and it’s more viable to put them just about anywhere else. The garage door is a rather complex place to put them as their are a huge range of styles and you would need to either fit the opening and replace the mechanism, or design a door for each existing type sold. On top of that wiring up solar to your house is the tricky part not putting something on a roof.
  - jefftk 8y ago
    Around here (Boston) roofs tend to have a lot of obstructions: dormers, skylights, chimneys, vent pipes. They're also typically very steep (1:1) so North-facing isn't an option. Then you lose some space due to shading from trees and other houses. When we got solar there was only room for panels covering 1/4 of our electricity usage.
    - Retric 8y ago
      That’s an extreme situation as Boston is rather far to the north and has poor weather for solar. Covering a skylight with solar is a net energy win, as would taking down the tree(s). Trees are also going to block the front of your building even more than the southern facing roof. Getting shade from other buildings on your roof is also unusual for 2 story buildings.
      - gumby 8y ago
        Not sure I would call Boston especially north as it is south of, say, Nice.
        Retric 8y ago
        It’s 42.3601° N closer Toronto at 43.6532° N than New York City at 40.7128° N. By comparison Miami is 25.7617° N and still above the Tropic of Cancer 23°26ʹ which is the furthest north you can get where the sun is ever directly overhead. PS: Europe is kind of deceptive as the equator is so far down below the bulk of Africa. But so many maps stop at the top of Africa it’s easy to think of it as some sort of mid point. Further, due to weather patterns it’s warmer than you would expect.
        goodcanadian 8y ago
        It’s 42.3601° N closer Toronto at 43.6532° N than New York City at 40.7128° N. It's relative, I suppose. From a Canadian point of view, Toronto is just about as far south as you can get.
        gumby 8y ago
        What about Turks and Caicos :-) (Couldn’t understand why you guys weren’t enthusiastic for their offer)
      - Johnny555 8y ago
        Covering a skylight with solar is a net energy win Just filling the skylight hole with insulation may be a net energy win but most people don't have the skylight to save on energy bills.
  - curtis 8y ago
    > The garage door is a rather complex place to put them as their are a huge range of styles and you would need to either fit the opening and replace the mechanism, or design a door for each existing type sold. On top of that wiring up solar to your house is the tricky part not putting something on a roof. I think to be practical you'd need a manufacturing line that could build variable sized doors. Since I'm a software engineer and not a manufacturing engineer I don't know if that's economically feasible or not.
- chris_va 8y ago
  > parts of a building that don't directly face the sun at all Bifacial panels (where one side faces the ground) are almost on the market for solar plants. The ground is generally light colored sand, which is great for diffuse light, so they see ~30% extra insolation for the same silicon.
- throwsway2525 8y ago
  I think these panels are called BIPVs.
- specialist 8y ago
  I hope so. The south facade of Phoenix's downtown library has adjustable horizontal louvres. Make them out of solar panels for a power and shade two-fer. https://en.wikiarquitectura.com/building/Burton-Barr-Central-Library/#lg=1&slide=15 https://en.wikiarquitectura.com/building/Burton-Barr-Central... https://en.wikiarquitectura.com/building/Burton-Barr-Central-Library/ https://en.wikiarquitectura.com/building/Burton-Barr-Central... https://en.wikipedia.org/wiki/Burton_Barr_Central_Library https://en.wikipedia.org/wiki/Burton_Barr_Central_Library
Johnny555 8y ago
His numbers are overly optimistic in most of the country where a 2/12 pitch roof is uncommon, 4/12 - 9/12 is more common. Is 2/12 common anywhere? Certainly not in any place that gets snow. (he does give some numbers for other pitches in his charts, but in the teaser paragraph, he uses 2/12)
- Retric 8y ago
  It’s not just the charts, “In Minneapolis, a 10/12 pitched roof that is perfectly north-south will have a 57% penalty between the south-facing and north-facing modules.” Really, you should not complain if you’re badly skimming an article.
  - Johnny555 8y ago
    If he gave some context in the beginning of the article where he exclaimed "Not great, to be sure, but probably not as bad as you might expect!" to say "But a 2/12 roof is very shallow" (and indeed is the minimum pitch suitable for shingles), I might agree. But you shouldn't have to get half way into the article before you find out that he chose the absolute best case for his example and you're not likely to see the same results in the real world.
    - Retric 8y ago
      The very next paragraph starts with ‘The tilt of the roof matters a great deal.’ Including a much worse example at 29% worse, but also a better one 1:12 which are not uncommon for that area. He then adds ordination of the building, and latitude as further examples. Really, if he had started by showing a huge chart it would have been gibberish to most people.
- deleted 8y ago
  [deleted]
thedoops 8y ago
Good rule of thumb is that the best production per panel ratio to get the most utility consumption offset (so you drop to the lowest tier) will return the most savings. Shaded or North facing panels in the northern hemisphere are almost never worth it. Put what you can on the south and look into a better AC unit, new windows, LEDs, and insulation first.
deleted 8y ago
[deleted]
seymour333 8y ago
This probably applies more to homes with an East to West facing pitch, but I find it really odd that it isn't standard issue to have a track system that moves the solar panels from one side of the roof to the other. A simple track and something basic like a garage door motor would allow the panels to be in a more optimal position throughout the day.
tejohnso 8y ago
Slightly off topic, but why is there so much discussion about fixed rooftop solar compared to a ground based system with tracking? Seems to me a ground based system would be easier and safer to set up, easier to keep clean, and easier to keep in an optimal position, even if it didn't have tracking. Why have this tight coupling to the roofing system?
- cglace 8y ago
  Because the roof structure already exists.
  - dashundchen 8y ago
    Expanding on that, built up areas are where the demand and grid already exists, but not necessarily the clear space. Little added transmission capacity is needed. In a neighborhood in my city where a few large electric users have expanded, the utility has been subsidizing rooftop panels for residential users to minimize summer peaks and avoid having to upgrade the old undersized grid. So far it has worked pretty well - peaks usually line up with the hottest sunniest days.
- bradknowles 8y ago
  It develops that tracking systems are expensive, heavy, and take up a lot of space. In fact, so much so that the tracking systems rarely make sense with modern panels. You would usually be better off spending the money, weight, and space on more fixed panels than messing with the tracking systems.
  - tejohnso 8y ago
    That's disappointing. It's too bad the tracking systems are such a burden. But what about a simple wooden structure to hold fixed panels oriented optimally? That's a few thousand dollars at most, for a twenty to sixty percent efficiency improvement. It would kill me to mount panels facing North at a 45deg angle.
- walrus01 8y ago
myroon5 8y ago
Depends on the hemisphere
- oska 8y ago
  You've been downvoted but I also noticed the "northern-hemispherism" (to coin a word) in the title. It does get a bit tedious for those of us who live in the southern hemisphere.
foxhop 8y ago
I live in Connecticut (New England) and most of my panels are positioned slightly north. https://russell.ballestrini.net/fulfilling-childhood-dreams-solar/ https://russell.ballestrini.net/fulfilling-childhood-dreams-...
DuskStar 8y ago
I think it should probably be noted that this analysis is for grid-attached solar, not grid-independent. For the first, you're generally not going to care about the seasonal power profile (you get paid the same for a kW in June or January) and so the north-facing array's production being more heavily loaded into the spring and summer months won't matter. But if you're trying to actually power your house, you probably want all the power you can get in the winter months. Optimizing for minimums, not totals, if that makes any sense...
- coldtea 8y ago
  >But if you're trying to actually power your house, you probably want all the power you can get in the winter months. Even so, if you can't have a south-facing module, the fact that you don't lose as much in many circumstances (roof slopes etc) is good to know. Even when one wants to power their house, there's no need to go in an "all or nothing" approach.
johnhenry 8y ago
Anyone in the southern hemisphere confused about this?
- FooHentai 8y ago
  Nah, most stuff is written with the assumption of being in the Northern hemisphere (after all, ~90% of humanity lives up there). It's the same as having to ROT6 any gardening advice or you'll be planting things at the worst possible time :)
- dugditches 8y ago
  literally in the first sentence '(assuming you are in the northern hemisphere)'
singularity2001 8y ago
With the price for solar-panels falling exponentially (1/2 every ~6 years), north facing tiles even in the most extreme examples will become profitable in a few decades or less. [0] http://blog.comparemysolar.co.uk/wp-content/uploads/2013/03/10-year-solar-panel-price-forecast-2013-2022.png http://blog.comparemysolar.co.uk/wp-content/uploads/2013/03/... etc

72 points by curtis 8y ago

I've been wondering for a while if vertical solar panels on the sides of buildings might make sense. Today (for no reason that I know of) I started wondering if might even make sense to put panels on parts of a building that don't directly face the sun at all. That's what lead me to this particular article, which examines the efficiency of putting panels on north facing roofs. I explored the vertical panel idea in a blog post last year: http://curtisb.posthaven.com/the-solar-garage-door http://curtisb.posthaven.com/the-solar-garage-door. I don't know if that idea would work, but I was trying to imagine an application of the idea so absurdly simple that a startup could easily use it to bootstrap into a more lucrative market.

Retric 8y ago
Generally, you don’t run out of roof space before meeting a 2 story buildings energy needs. On taller buildings installation costs would be significant, coupled with lower effects and it’s more viable to put them just about anywhere else. The garage door is a rather complex place to put them as their are a huge range of styles and you would need to either fit the opening and replace the mechanism, or design a door for each existing type sold. On top of that wiring up solar to your house is the tricky part not putting something on a roof.
- jefftk 8y ago
  Around here (Boston) roofs tend to have a lot of obstructions: dormers, skylights, chimneys, vent pipes. They're also typically very steep (1:1) so North-facing isn't an option. Then you lose some space due to shading from trees and other houses. When we got solar there was only room for panels covering 1/4 of our electricity usage.
  - Retric 8y ago
    That’s an extreme situation as Boston is rather far to the north and has poor weather for solar. Covering a skylight with solar is a net energy win, as would taking down the tree(s). Trees are also going to block the front of your building even more than the southern facing roof. Getting shade from other buildings on your roof is also unusual for 2 story buildings.
    - gumby 8y ago
      Not sure I would call Boston especially north as it is south of, say, Nice.
      - Retric 8y ago
        It’s 42.3601° N closer Toronto at 43.6532° N than New York City at 40.7128° N. By comparison Miami is 25.7617° N and still above the Tropic of Cancer 23°26ʹ which is the furthest north you can get where the sun is ever directly overhead. PS: Europe is kind of deceptive as the equator is so far down below the bulk of Africa. But so many maps stop at the top of Africa it’s easy to think of it as some sort of mid point. Further, due to weather patterns it’s warmer than you would expect.
        goodcanadian 8y ago
        It’s 42.3601° N closer Toronto at 43.6532° N than New York City at 40.7128° N. It's relative, I suppose. From a Canadian point of view, Toronto is just about as far south as you can get.
        gumby 8y ago
        What about Turks and Caicos :-) (Couldn’t understand why you guys weren’t enthusiastic for their offer)
    - Johnny555 8y ago
      Covering a skylight with solar is a net energy win Just filling the skylight hole with insulation may be a net energy win but most people don't have the skylight to save on energy bills.
- curtis 8y ago
  > The garage door is a rather complex place to put them as their are a huge range of styles and you would need to either fit the opening and replace the mechanism, or design a door for each existing type sold. On top of that wiring up solar to your house is the tricky part not putting something on a roof. I think to be practical you'd need a manufacturing line that could build variable sized doors. Since I'm a software engineer and not a manufacturing engineer I don't know if that's economically feasible or not.
chris_va 8y ago
> parts of a building that don't directly face the sun at all Bifacial panels (where one side faces the ground) are almost on the market for solar plants. The ground is generally light colored sand, which is great for diffuse light, so they see ~30% extra insolation for the same silicon.
throwsway2525 8y ago
I think these panels are called BIPVs.
specialist 8y ago
I hope so. The south facade of Phoenix's downtown library has adjustable horizontal louvres. Make them out of solar panels for a power and shade two-fer. https://en.wikiarquitectura.com/building/Burton-Barr-Central-Library/#lg=1&slide=15 https://en.wikiarquitectura.com/building/Burton-Barr-Central... https://en.wikiarquitectura.com/building/Burton-Barr-Central-Library/ https://en.wikiarquitectura.com/building/Burton-Barr-Central... https://en.wikipedia.org/wiki/Burton_Barr_Central_Library https://en.wikipedia.org/wiki/Burton_Barr_Central_Library

Johnny555 8y ago

His numbers are overly optimistic in most of the country where a 2/12 pitch roof is uncommon, 4/12 - 9/12 is more common. Is 2/12 common anywhere? Certainly not in any place that gets snow. (he does give some numbers for other pitches in his charts, but in the teaser paragraph, he uses 2/12)

Retric 8y ago
It’s not just the charts, “In Minneapolis, a 10/12 pitched roof that is perfectly north-south will have a 57% penalty between the south-facing and north-facing modules.” Really, you should not complain if you’re badly skimming an article.
- Johnny555 8y ago
  If he gave some context in the beginning of the article where he exclaimed "Not great, to be sure, but probably not as bad as you might expect!" to say "But a 2/12 roof is very shallow" (and indeed is the minimum pitch suitable for shingles), I might agree. But you shouldn't have to get half way into the article before you find out that he chose the absolute best case for his example and you're not likely to see the same results in the real world.
  - Retric 8y ago
    The very next paragraph starts with ‘The tilt of the roof matters a great deal.’ Including a much worse example at 29% worse, but also a better one 1:12 which are not uncommon for that area. He then adds ordination of the building, and latitude as further examples. Really, if he had started by showing a huge chart it would have been gibberish to most people.
deleted 8y ago
[deleted]

thedoops 8y ago

Good rule of thumb is that the best production per panel ratio to get the most utility consumption offset (so you drop to the lowest tier) will return the most savings. Shaded or North facing panels in the northern hemisphere are almost never worth it. Put what you can on the south and look into a better AC unit, new windows, LEDs, and insulation first.

deleted 8y ago

[deleted]

seymour333 8y ago

This probably applies more to homes with an East to West facing pitch, but I find it really odd that it isn't standard issue to have a track system that moves the solar panels from one side of the roof to the other. A simple track and something basic like a garage door motor would allow the panels to be in a more optimal position throughout the day.

tejohnso 8y ago

Slightly off topic, but why is there so much discussion about fixed rooftop solar compared to a ground based system with tracking? Seems to me a ground based system would be easier and safer to set up, easier to keep clean, and easier to keep in an optimal position, even if it didn't have tracking. Why have this tight coupling to the roofing system?

cglace 8y ago
Because the roof structure already exists.
- dashundchen 8y ago
  Expanding on that, built up areas are where the demand and grid already exists, but not necessarily the clear space. Little added transmission capacity is needed. In a neighborhood in my city where a few large electric users have expanded, the utility has been subsidizing rooftop panels for residential users to minimize summer peaks and avoid having to upgrade the old undersized grid. So far it has worked pretty well - peaks usually line up with the hottest sunniest days.
bradknowles 8y ago
It develops that tracking systems are expensive, heavy, and take up a lot of space. In fact, so much so that the tracking systems rarely make sense with modern panels. You would usually be better off spending the money, weight, and space on more fixed panels than messing with the tracking systems.
- tejohnso 8y ago
  That's disappointing. It's too bad the tracking systems are such a burden. But what about a simple wooden structure to hold fixed panels oriented optimally? That's a few thousand dollars at most, for a twenty to sixty percent efficiency improvement. It would kill me to mount panels facing North at a 45deg angle.
walrus01 8y ago

myroon5 8y ago

Depends on the hemisphere

oska 8y ago
You've been downvoted but I also noticed the "northern-hemispherism" (to coin a word) in the title. It does get a bit tedious for those of us who live in the southern hemisphere.

foxhop 8y ago

I live in Connecticut (New England) and most of my panels are positioned slightly north. https://russell.ballestrini.net/fulfilling-childhood-dreams-solar/ https://russell.ballestrini.net/fulfilling-childhood-dreams-...

DuskStar 8y ago

I think it should probably be noted that this analysis is for grid-attached solar, not grid-independent. For the first, you're generally not going to care about the seasonal power profile (you get paid the same for a kW in June or January) and so the north-facing array's production being more heavily loaded into the spring and summer months won't matter. But if you're trying to actually power your house, you probably want all the power you can get in the winter months. Optimizing for minimums, not totals, if that makes any sense...

coldtea 8y ago
>But if you're trying to actually power your house, you probably want all the power you can get in the winter months. Even so, if you can't have a south-facing module, the fact that you don't lose as much in many circumstances (roof slopes etc) is good to know. Even when one wants to power their house, there's no need to go in an "all or nothing" approach.

johnhenry 8y ago

Anyone in the southern hemisphere confused about this?

FooHentai 8y ago
Nah, most stuff is written with the assumption of being in the Northern hemisphere (after all, ~90% of humanity lives up there). It's the same as having to ROT6 any gardening advice or you'll be planting things at the worst possible time :)
dugditches 8y ago
literally in the first sentence '(assuming you are in the northern hemisphere)'

singularity2001 8y ago

With the price for solar-panels falling exponentially (1/2 every ~6 years), north facing tiles even in the most extreme examples will become profitable in a few decades or less. [0] http://blog.comparemysolar.co.uk/wp-content/uploads/2013/03/10-year-solar-panel-price-forecast-2013-2022.png http://blog.comparemysolar.co.uk/wp-content/uploads/2013/03/... etc