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True North vs Magnetic North

  1. Aug 8, 2017 #21


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    Something taught to every sailor is how to read both the declination of magnetic North and how to calculate its movement over the period since the map was published.

    Every nautical map has a compass rose that shows both Mag North and its rate of change.
  2. Aug 10, 2017 #22
    Apart from the good advice note that the sun input to your panels is a cosine funktion of the angle and not super critical to get very accurate.

    Important is that you clean the panels , more frequently if there is a lot of dust in the air , and you should run a test now and then to determine that the efficiency still is OK. It will deteriorate with time and hopefully you have a guarantee that set the max amount.
    Good luck with the project.
  3. Sep 1, 2017 #23
    I just found the true midpoint in the day where I live (halfway between sunrise and sunset). Then drove a stake ahead of where I wanted my solar panels mid point to be then followed the shadow in a northward direction. Lined up a stake with the shadow and first stake at where exactly I wanted the panels and that was the midpoint of the panel or panels. Then pepindicular to the line of first 2 stakes on each side of my midpoint one to the east and on to the west would line up with the 2 ends of my panel or panels.
  4. Sep 1, 2017 #24


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    BTW, a study by some junior whizbang a few years back pointed out that all solar panels today are (or at least were) configured wrong.

    He pointed out the power usage is not symmetrical across the day. More power is used in the early evening than in the early morning. A set of panels that takes this into account (by being turned more westward) is more efficient than one that assumes an even demand of power over the day.

    It was not insignificant; it was at least 10 degrees.
  5. Sep 1, 2017 #25
    I don't think efficient or wrong are the right terms here. The panels are clearly most efficient when they capture and convert the most sunlight over the course of the day. Efficiency is power out over power in.

    In most areas, solar is a small percentage of total power, so matching it to demand isn't an issue, so it's not wrong for them to be symmetrical. If solar is a large percentage of demand, it may be more effective for the grid to shift some solar production to later in the afternoon.
  6. Sep 1, 2017 #26


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    Panels are merely a single component in a system whose ultimate purpose is to get power to where its needed, when it's needed, and do so economically.
    One must prioritize the whole system, and its overall efficiency in meeting its goals, over any single component.

    For example, panels producing electricity when that electricity is not being used is wasted - unless one has a sufficient storage system (and even then they can have losses.) We don't know what the OP's storage system is, and how much power it can store or for how long. So, the system needs to be prioritized above the actual max output of the panels.
  7. Sep 2, 2017 #27
    I agree. The right thing to do is to analyze your power usage during the day, and configure your panels to produce maximum efficiency at those times.
  8. Sep 2, 2017 #28


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    I guess that some suppliers charge more during the time of day when there is higher usage (where I live, there are contract options with free electricity during some hours.) In that case, you would want to turn the panels to work best when you are paying the most.
  9. Sep 2, 2017 #29
    Yes, but the OP mentioned only the panels - no system restrictions were described. And it would be a special case (but not unheard of) to need to optimize the system by reducing the overall efficiency of the panels through time shifting (and thus lowering) their output.

    But I think (especially on a technical forum like this), we should be careful to use the term "efficiency" in it's technical sense, power out / power in. If we need to reduce the efficiency of the panels to match other restrictions of the system, I think that should be referred to as"optimizing" the system, or tweaking its "effectiveness".

    Yes, I covered that in other parts of the post you quoted, but you didn't quote that part.

    Not necessarily. As long as you aren't overproducing (wasting solar), what difference does your demand curve make? You really only care about an "over production" curve, not simply your demand curve. For example, if my peak demand were @ 5 PM, but my demand at mid-day is above my solar power output, shifting my panels to the 5 PM sun would just be reducing their power output with no benefit.

    Yes, but I think it's a little loose to just say you want the panels to "work best" (maximum output?) when your rates are highest. You'd need to look at the entire production and rate curve and optimize them. Your peak output of the panels may still be during the lower rate, it's all about the area under the curves.

    Relative to the OP, I think this post is key. Unless you are really far north (and likely not using solar panels!), the delta between True North and Magnetic North is probably a very tiny effect on total power out.
    Last edited: Sep 2, 2017
  10. Sep 2, 2017 #30


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    Yes, it's a problem of optimizing an objective function. But I think that the real dangers at this point are:
    1) neglecting an important factor or
    2) designing a permanent fixture based on factors that may change.
  11. Sep 2, 2017 #31


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    It behooves us to point out that this may not be the only factor to consider when configuring his panels.
    It is entirely up to him to decide whether it applies to his system, but we would be doing him a disservice to omit mentioning such a salient factor.

    But I agree with your terminology - the better term is optimization.
  12. Sep 2, 2017 #32


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    It's generally on the order of 10 degrees. Roughly, the COS of that is 0.97.
    If the OP didn't care about that 3%, he probably wouldn't be going to the lengths he is - both researching the calcs himself and bringing it to PF for verification.
  13. Sep 2, 2017 #33


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    Good points! I did the maths on PV output vs misalignment a couple of years ago. [ref]

    Code (Text):
    misalignment   percent
    angle °        of rated
    30               86.6%
    25               90.6%
    20               94.0%
    15               96.6%
    10               98.5%
    5                99.6%
    0               100.0%
  14. Sep 2, 2017 #34
    I agree that it it is worth bringing up as a question to the OP, and hopefully they will respond. I disagree with any broad-brush statements that the output needs to be matched to demand - that is system dependent and goal dependent (maximize total output or maximize output at certain times of the day).

    But that is totally dependent upon location (which I don't think the OP provided). This link: https://upload.wikimedia.org/wikipedia/commons/2/28/Mv-world.jpg

    shows a line of "No Variation" roughly from Duluth, MN to New Orleans, LA, and down through part of Central and South America. Another one does a wild curve through Europe, Africa, swooping up through India and beyond.
  15. Sep 2, 2017 #35
    Just as another possibly interesting side note - I was reading about someone who had a little off-grid cabin in the woods that he'd visit on some weekends. He had his solar panels arranged in an extremely inefficient manner, but it was an effective way to achieve his goals.

    They were near vertical, pointing straight East and straight West. He had some small trickle draw that he wanted to keep active while away. His panels provided far more peak power than he needed for this trickle draw, so this arrangement provided that power from sunrise to sunset. That minimized the draw down (and recharging cycles) on his battery storage. I think that he had them set up, so when he was there, he could easily tilt them out flat later in the day, and have more power available for his needs mid-day, or recharge the batteries from draw down from the previous evening.

    The other advantage for him was that a near vertical panel will shed snow. I've seen the panels here in Northern IL have their output drop to near zero for weeks at a time. Panels here are mounted about 20° from horizontal to maximize the annual production (longer summer days), and that leads to the snow clinging to them. So for him, it was more about convenience than getting the most kW-hr out of a panel. Over-sizing the panels was a cheap price to pay for convenience.
  16. Sep 14, 2017 #36


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    Three other ways.
    1. Mark your exact location on a map. Look for a distant visible feature that is also marked on the map. Draw a line on the map between the two. Rotate the map so the line points at the distant feature. The map is then correctly orientated, so the map grid will be N–S at your location.

    2. Use a GPS to read Lat–Long on site and somewhere that can be seen from site. Travel back and forth to check the readings are reliable. The bearing of the line between the two sites can be found from the equations for a great circle passing through those two points.

    3. If you read GPS Longitude, then find a place North or South with exactly the same longitude, the line between the two will be true North–South. The further apart the points, and the more times you check the numbers, the more accurate the N–S line.
  17. Sep 17, 2017 #37


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    That's almost the simplest way. As @Anachronist stated, the apex of the hyperbola will point at the stick at local Noon. This is the same as saying that the shadow will be shortest at Noon. The shortest shadow of stick is your North-South line, and it occurs at local Noon. No need to spend the time creating the other half of the hyperbola.
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