Getting a rough idea of how much energy you can get from a heliostat is a fairly straightforward process; however, there are more steps involved than most people might typically guess. I’ve seen instances both on the web and in books where it has been quoted that there is about 1KW/m^2 available from the sun. Although this may be true enough when considering instantaneous power on a clear, cloud free day, there are a few more factors that need to be taken into consideration when you try and figure out the total energy received over a given period of time.
To get a more accurate picture, you will first need to find average daily solar radiation data for your area.
Once you’ve done that, you will need to find out how much of that solar radiation is actually being intercepted by your heliostat based on the angles between it, the sun, and the target.
Finding Average Daily Solar Radiation
To get a realistic idea of how much energy you can collect, it is best to get solar radiation data obtained from real world measurements. It is, in fact, possible to either build or buy a device called a pyranometer and record these measurements yourself; however, it would be very time consuming to do accurately. If you want to obtain this data quickly, your best bet is to simply find a solar radiation map for your location.
Finding a Solar Radiation Map
There are many different types of solar radiation maps which show data for different types of collectors and also for different collector orientations.
A heliostat is closest to a two axis tracking concentrator, so that’s the type of map you need to find. One of the key words here is “concentrator”. There are other types of two axis trackers which do not “concentrate” the light to a fixed location. These are generally known as flat plate collectors.
A solar panel would be an example of a flat plate collector. Light that strikes its surface from any and all directions is counted toward the total amount of KWh/m^2/day.
For a heliostat, most all of the light which strikes it from angles other than those of the sun will never reach the target.
If you compare a two axis flat plate collector to a two axis concentrator, you will see that the average number of KWh/m^2/day is significantly lower for the two axis concentrator because of this.
You can see this for yourself by generating different graphs at the site http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/
Links to Solar Radiation Maps
Here are two sites which I personally use that have good solar radiation maps. Sadly, they only seem to give data for the U.S.
http://www.nrel.gov/gis/solar.html
http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/
If you are located in another country, you hopefully shouldn’t have too much trouble finding a map for it just by doing a quick Google search. (I have yet to find a site which has maps for all the different countries. I’m still looking though.)
Remember to find one that gives data for a two axis tracking concentrator as explained above.
It also wouldn’t be a bad idea to try and find a site which has different maps for each month of the year.
If you are interested, you can find more information about how these solar radiation maps are created at this link http://rredc.nrel.gov/solar/pubs/redbook/HTML/interp.html#solrad
Example of a Solar Radiation Map
Note: The screenshot below comes from the site http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/ You may want to open it in a new window and generate an annual map for a two axis tracking concentrator so that you can follow along better.
Reading a solar radiation map is a piece of cake. All you have to do is just look and see what color the map shows for your location and match it with the appropriate range of KWh/m^2/day (not shown in screenshot below).
I am located in Virginia (far right and about in the middle of the map vertically) which is a greenish color. Matching it with the range of values, I see that I get somewhere between 3-4 KWh/m^2/day annually.
Remember this value because we are going to use it in the next part below.
Finding the Amount of Available Sun Energy Intercepted by the Heliostat
Above, we discovered how to find the average daily solar radiation collected by a 1m^2, 2-axis solar concentrator. This isn’t quite what we want though.
Using my own location as an example, a 1m^2 collector would gather an average of 3-4 KWh/day if it always pointed directly at the sun (ignoring losses due to various factors). A Heliostat doesn’t point directly at the sun though and is instead always turned away by a certain amount in order to reflect the light to the desired target. The fact that it doesn’t directly face the sun decreases how much energy it collects.
Note: You might want to check out the pages How to Maximize Your Heliostat’s Efficiency by Choosing Good Targets and Heliostat Target Efficiency Simulator so that you can understand this section better.
In the graph below, the percentage of the total energy the heliostat collects is shown on the y-axis while the hours of the day is shown on the x-axis.
For the most part, the percentage of energy intercepted stays between 80% and 95%. This means that I would get somewhere between 80% and 95% of the 3-4 KWh/m^2/day given by the solar radiation map.
Note: This graph was generated with the Heliostat Target Efficiency Simulator.
Assuming 3.5 KWh/m^2/day and 85% as the amount of the total energy intercepted, 3 KWh/day is roughly how much I might expect to get from a heliostat with a 1m^2 mirror.
Keep in mind that the amount of energy for your own location and situation will likely vary depending both on how much sun you get and the altitude and azimuth you use for your target.
If you can find a solar radiation map for each month of the year, try experimenting to see how much energy you will receive on average for each month. It will no doubt be different than the yearly average.
Remember that you will also have to recreate the graph using a different date for each month.
Since everything is always changing (i.e. weather, day length, and the angles between the sun, heliostat, and target), it is impossible to know exactly how much energy you will receive over a fixed period of time, but hopefully you now understand how to at least get a rough idea.
