It is important to put some thought into which targets you aim your heliostat toward if you want to maximize the amount of energy you get from it. This page will give you some guidelines which should help with this task.
It might not be immediately obvious when you first start thinking about making a heliostat, but you can’t just reflect the sun’s light any direction you please. Although there is a fairly large range of altitudes and azimuths you can use as a target that will reflect at least some of the sun’s light, you will have to use a somewhat narrower range if you want to maximize the amount of light collected.
In the pictures below, we have some simple 2D examples which should make things easier to understand. These pictures contain four different objects, the sun, a heliostat (the blue line), a target, and a line which represents how the sun’s rays reflect from the heliostat’s mirror.
The amount of energy the target receives increases from left to right in the three pictures below. This is because the angle between the sun and the target decreases. Put simply, the closer the heliostat is to pointing directly towards the sun, the more energy it gets.
Let’s look at this another way. On the left picture below, you can see how the extreme angle of the heliostat causes most of the sun’s rays to miss. In the right picture though, the angle is smaller, so the same rays of light which missed the heliostat in the left picture are now able to hit it.
What all this really comes down to is that you have put some thought into where you put your targets (and also the heliostat itself) if you want to get as much energy from your heliostat as you possibly can.
If you can only have one target, the best place for it would be directly in front of the heliostat when it’s facing geographic south (or geographic north if you are in the southern hemisphere). This helps avoid extreme angles, but, depending on your latitude, they may be unavoidable because the sun can travel a considerable distance across the sky in the summer months.
You can further maximize the amount of energy gathered if you are able to set up multiple targets. The pictures below are an example of this. In the first one you see two targets. The target on the left will gather a decent percentage of the available energy during the morning hours.
Once you start moving into the afternoon hours though, the percentage of energy gathered for the left target starts to drop considerably. If you were to switch to the target on the right though, you would again collect a much better percentage of the sun’s energy Note: Check out the -45 vs. +45 degree graphs toward the bottom of this page for less vague percentage values.
Choosing targets with higher altitudes is another way you can gain more energy from your heliostat. The reasoning is the same as above. Higher altitudes give smaller angles between the sun and the target and therefore yield better energy collection.
The picture below shows a situation where you have two choices for targets. Choosing the window with the highest altitude is better in terms of the most heat energy gained.
To help illustrate things a little more scientifically, I have made a few graphs which show some of the same above situations in a different way.
The x-axis shows the time of the day in hours.
The y-axis shows the percentage the sun’s energy gathered by the heliostat. Note that these graphs do not tell you how much* energy the heliostat gathers. Right now, we are only concerned with minimizing the losses associated with target placement.
For these examples, the azimuth equals zero when pointing south.
If you want to try making these graphs yourself, you can do it with the program on this page Heliostat Target Efficiency Simulator.
*There is a program available on this page on this site’s forums which will allow you to use Solar Radiation from NREL to answer the “how much” question.
Winter vs. Summer Solstice
Note: All of these graphs use a latitude of 45 and a longitude of 0. Unless otherwise stated, the target’s altitude is 0 and so is its azimuth.
This first graph shows the percentage of energy that the heliostat gathers on the day of the winter solstice. Because the sun’s altitude stays low in the sky, the heliostat is particularly efficient during the winter months. For this graph the heliostat’s efficiency ranges between 86% and 98%.
Here is the same situation as above only this time it’s during the day of the summer solstice. Since the sun is so much higher in the sky and also because it travels so much further across the azimuth, the heliostat’s efficiency is less than it is in the winter months (for this target at least). During the early morning hours and late evening hours it can get particularly low. Of course, because of the longer days you will still probably gain more energy overall.
-45 Degrees Azimuth vs. +45 Degrees Azimuth
Here is another example taken during the summer solstice. For this one, I used -45 degrees for the target’s azimuth. Notice that the heliostat is much more efficient during the morning hours than it was in the previous graph. During the late evening hours though, it drops considerably.
The graph below is similar to the one above only this time I used +45 degrees for the target’s azimuth. Here the heliostat is the most efficient during the evening hours. This shows how using multiple targets can greatly increase the overall efficiency of your heliostat if you switch from one to the other as the day passes.
Notice that switching from one target to the next at noon will keep the heliostat’s efficiency at ~80% and above.
Higher Altitude vs. Lower Altitude
These next two graphs compare a target altitude of 0 degrees (top graph) with a target altitude of 25 degrees. Overall, using a higher altitude for your target should result in an overall increase in the heliostat’s efficiency. When using the higher altitude, the heliostat is somewhere between 3% and 10% more efficient as the sun makes its way across the sky.
We have reached the end of this quick overview on how to optimize you heliostat’s efficiency by choosing good targets. Hope you have found it useful!
This documentation is part of the Open Sun Harvesting Project.