Understanding Heliostat Targets


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When using a heliostat, a “target” is basically just the object that intercepts the light reflected from the heliostat. More specifically though, a target is a direction. Obviously, you can’t just tell the computer that controls the heliostat to “point at that window over there” and expect it to have even the slightest clue what you mean. It instead requires two angle values which specify the target’s direction. Throughout this site these values are referred to as the target’s altitude and the target’s azimuth.

Note that some people like to use elevation instead of altitude, but they both mean the same thing in this case.

Target’s Altitude

Of the two, the target’s altitude is the easiest to conceptualize. What may be a little confusing though is that the word “altitude” (and also elevation) might not have the meaning that you expect in this context. Altitude often refers to a height measurement above sea level. In this case though it’s an angle measurement with the horizon as the zero reference. So one word means two things that sort of seem similar but aren’t really. Confused yet?

Don’t be. All you need to know about the target’s altitude is shown in the picture below. You’ll notice the two arrows that are sticking out from the center of the heliostat. You’ll also notice the target in the upper right hand corner. The bottom arrow is parallel with the horizon and is where the target’s altitude is zero. Altitude target angles above this reference direction are positive and angles below it are negative. In this example, the target is above the center of the heliostat and has a target altitude of 15 degrees.

The input range for a target’s altitude is greater than or equal to -90 degrees and less than or equal to 90 degrees. (-90 ≤  target’s altitude  ≤ 90)

Target’s Azimuth

Understanding a target’s azimuth is (in my opinion at least) a little more complicated, but not incredibly so. To avoid confusion you simply need to keep in mind that the target’s azimuth is handled a little differently depending on whether you are in the northern or southern hemisphere.  You can read the Northern and Southern Hemisphere Differences in the Sun Tracking / Heliostat Program page for more information on why this is and how exactly the azimuth is different in each case.

Target’s Azimuth in the Northern Hemisphere

(You may want to skip this section if you live in the Southern Hemisphere)

If you are in the Northern Hemisphere, the direction where the target’s azimuth equals zero is in the same direction as geographic south. The below picture shows a target with an azimuth of zero. Notice that it sits on the imaginary line that goes from the center of the heliostat to the south pole.

As you can see in this next image, negative target azimuth angles point more toward the east (where the sun rises) and positive target azimuth angles point more toward the west. The input range for target azimuth angles is greater than or equal to -180 degrees and less than or equal to 180 degrees. (-180 ≤ target’s azimuth ≤ 180)

This next image shows a target with a negative azimuth. Make note of its location if you read through the Target’s Azimuth in the Southern Hemisphere section.

 

Target’s Azimuth in the Southern Hemisphere

(You may want to skip this section if you live in the Northern Hemisphere)

If you are in the Southern Hemisphere, the direction where the target’s azimuth equals zero is in the same direction as geographic north . The below picture shows a target with an azimuth of zero. Notice that it sits on the imaginary line that goes from the center of the heliostat to the north pole.

As you can see in this next image, negative target azimuth angles point more toward the east (where the sun rises) and positive target azimuth angles point more toward the west. The input range for target azimuth angles is greater than or equal to -180 degrees and less than or equal to 180 degrees. (-180 ≤ target’s azimuth ≤ 180)*

*This is the same as it was in the northern hemisphere examples. The difference though is that we are now using north as the direction where the target azimuth equals zero. This basically just reverses which side the positive and negative angles are located.

This next image shows a target with a positive azimuth. You’ll notice that the target is in the same place in relation to the heliostat as it was in the northern hemisphere example, but this time it is a positive value instead of a negative one.

This documentation is part of the Open Sun Harvesting Project.

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