Proper layout of passively heated homes, photovoltaic (PV) systems, and solar thermal systems is important for getting the most out of your solar resource. Not only is it imperative to use the proper tools for evaluating shading, but also important is the proper orientation of a home or solar collector relative to true south.
PV systems sited within 10 degrees of true south lose a maximum of only 2 percent of their generating capability. (For math-oriented people: The cosine of 10 degrees is approximately 0.98.) This doesn’t sound like much, but considering the cost of PV modules, finding that extra “free” 2 percent can be worthwhile. Passive solar homes and solar thermal systems can show similar gains from accurate orientation.
But few people I encounter—even those with lots of solar energy experience—understand the differences between true, magnetic, and compass north (or south). And many solar energy system designers use only magnetic declination (a location’s difference between magnetic and true north) to determine true south, without realizing that this gives only part of the picture—and that the solar orientation might be farther off than they think.
As a professional mariner, I work intimately with the difference between true north and magnetic north on a daily basis, and know that, in the northern hemisphere, there is an easier and more accurate way of determining true south than is usually discussed.
Two components determine the difference between true north and what the compass reads: magnetic declination and magnetic deviation. Magnetic declination is the difference between true north and magnetic north based on geographic location, and is approximated in commonly found magnetic declination maps. Current theory is that the spinning, molten iron core at Earth’s center creates an electromagnetic field. Since the magnetic field is not exactly lined up with Earth’s axis (North and South Poles), there is a geographic difference between the true poles and the magnetic poles. Compasses are basically magnets that point as closely as they can toward the magnetic poles.
Magnetic declination changes slightly over time as the magnetic pole moves, but is easy to determine by using the National Oceanographic & Atmospheric Administration Web site (see Access). For example, my house’s magnetic declination is 9 degrees 34 minutes east (E), and has been changing by 0 degrees 7 minutes west (W) per year (1 minute of arc equals 1/60 of a degree). Magnetic declination can either be east of north or west of north, which further complicates the procedure—be sure to get the direction correct or else your orientation could be off doubly far.
Magnetic declination is only one of the potential orientation errors. Magnetic deviation is the difference between magnetic north and what the compass reads, or compass north, and is induced in a compass by local magnetic fields. Deviation must be taken into account along with magnetic declination if accurate bearings are to be calculated.
Just like magnetic declination, magnetic deviation can either be east of north or west of north. And it can be the same or opposite of magnetic declination. Local magnetic fields that can contribute to deviation include:
While geologic variations like iron ore deposits near your site can cause deviation, it is most commonly caused by iron, steel, or magnets near where you are measuring. And deviation calculations can change from measurement to measurement! Most likely, you are causing the deviation. How far away is your vehicle? Are you carrying a wrench or hammer? Most audio speakers have a magnet in them; do you have a cell phone or radio at hand? Is the solar array’s post made of steel? Steel, a magnet, or any magnetic field from electrical equipment can deflect the needle of a compass you are using.
Luckily, it is easy to address most deviation by removing the source or bypassing it. Move your truck farther away. Leave all your metal tools several meters from where you are measuring. Stand at least a few meters from the steel post the array is mounted on.
To determine magnetic north, you must apply deviation effects to what your compass reads. Then you apply magnetic declination to magnetic north to get true north. Again, each correction can be east or west, so be sure to add or subtract correctly. As you can see, determining true south can become quite complicated!