After learning about solar power, you may be wondering if your property is a good site for taking advantage of solar. While it may seem simple to look outside during the day and say to yourself, “Yeah, my roof seems to get a lot of sunshine throughout the year,” there’s a lot more involved in determining the potential solar resource of your site.
As solar is a daily and annually variable resource, there are a number of factors that go into determining the solar resource of your site, including:
• Orientation – What direction your roof or array faces. Orientation is typically measured in the degrees like a compass, and often referred to as the “azimuth.”
• Tilt (or pitch) – Where, between horizontal and vertical, your array or roof surface has been tilted. Tilt is also often measured in degrees between 0˚ (horizontal) and 90˚ (vertical).
• Shading – Measurements that take into account current visual obstructions and how it will impact a system as the sun rises and dips in the sky with the seasons. Shading is often referred to as the “solar window.”
While orientation, tilt and shading are interconnected and interact together to determine your solar resource, there is a lot of information to cover, so we’ll start this lesson with orientation.
It is crucial that the orientation of the array (the system of solar panels) is correct in order to maintain the most perpendicular (i.e., most direct) path for photons in the Sun’s rays to reach your system.
Imagine the Sun’s rays as a bucket of paint being thrown at your array. To get the paint on it, you’d want to hit it as square as you could, right? Tilt of the array also comes into play here, but more on that later. First we need the paint to hit the right side of our array.
The key concepts to understand in the relationship between orientation, tilt and solar resource is that the Earth is not only curved (sorry, Flat-Earthers), but it also sits on a tilted axis as it orbits the Sun. This geometry of orbit, seen in the figure below, is what causes the seasons and seasonal variability of solar energy as a resource.
Understanding of the Earth’s orbit and curvature leads us to a two basic rules of thumb for orienting your array for optimal solar productivity:
1. If you live in the Northern Hemisphere, arrays that face the south will gather the most solar. If you live in the Southern Hemisphere, arrays that face the north will gather the most solar.
This rule is based on the principal of Earth’s curvature. If we think back to our bucket of paint example and imagine the paint leaving the Sun as a solid wall headed towards Earth, and our house is built perpendicular to our location on the globe, it becomes a lot easier to imagine which roof surfaces would be completely coated. In the North, you would come out to find your southern-facing surfaces evenly coated with some running over the backside onto the northern surfaces, but in more of a streak or dribble. In the South, you would find the opposite is true.
2. Arrays that face north or south will produce more energy in their respective hemispheres than arrays that face east or west.
This rule can again be explained through tossing buckets of paint at a home in the Northern Hemisphere. This time we will send one wall of paint every hour throughout the day. As the sun rises in the East, it will start coating your home on the east-facing surfaces, then on to the southeast surfaces, then south, southwest, and finally west-facing surfaces as it arcs across the sky in its daily routine. In the early hours, coating will be focus on the eastern surfaces, with only some spillage over to the north and south sides, but none reaching as far as the west-facing surfaces. In the later hours, coating would focus on the southwest, then the south-facing surfaces, again, with some spilling over the north and south, but none reaching the east side by evening. Which side got the most paint throughout the day? South side, right? Replace the paint with photons from the Sun, and you have your solar resource.
Now that you know orientation, read the next part of the series to learn about tilt.
By: Ian Berg
Photo courtesy of Pexels.com