Once you have identified a suitable position for your solar power system, it’s time to be a little more scientific in ensuring there are no obstructions that will block sunlight from reaching the solar array at different times of the year.
The path of the sun across the sky changes throughout the year. This is why carrying out a site survey is so important – you can’t just check to see what the sun is shining on today, as the height and position of the sun constantly changes throughout the year.
Each year, there are two days in the year when the day is exactly twelve hours long. These two days are the 21st March and 21st September and are known as the ‘solar equinoxes’. On these equinoxes, the sun rises due east of the equator and sets due west of the equator. At solar noon on the equinox (i.e. exactly six hours after the sun has risen) the angle of the sun is 90 deg. Minus the local latitude.
In the Northern Hemisphere (i.e. north of the equator), the longest day of the year is the 21st June and the shortest day of the year is the 21st December. These two days are known as the summer and winter solstices respectively.
On the summer solstice, the angle of the sun is 23.5 deg higher than it is on the equinox, whilst the angle is 23.5 deg lower than the equinox on the winter solstice.
These two extremes are due to the tilt of the earth, relative to its orbit around the sun. In the Northern Hemisphere, the summer solstice occurs when the North Pole is tilted towards the sun, and the winter solstice occurs when the North Pole is tilted away from the sun.
We’ll take London, United Kingdom as an example. London’s latitude is 51 deg. On the equinox, the angle of the sun at noon will be 39 deg (90 deg – 51 deg). On the summer solstice the angle will be 62.5 deg (39 deg + 23.5deg.) and on the winter solstice it will be 15.5 deg (39 deg – 23.5 deg).
Now that you have understood the Path of the sun, your next objective is to learn how to build a solar panel.
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