How Much Energy can you get from a Heliostat?

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This documentation is part of the Open Sun Harvesting Project.

Getting a rough idea of how much energy a heliostat collects is a straightforward process. To do this, you will first need to find the average daily solar radiation data for your area. Next, you will need to find out how much of that energy is actually being intercepted by your heliostat based on the angles between it, the sun, and the target.

Update: I have written a PC based program that (among other things) is able to calculate how much energy you can get from your heliostat using the hourly solar radiation data from NREL. It is still in its early stages, but here is the link if you want to check it out.

Finding a Solar Radiation Map

To get a realistic idea of how much energy you can collect, you will need to find data obtained from real world measurements. Although you could technically do these measurements yourself, it is easiest to just use a solar radiation map.

For the U.S. at least, the NREL site should be your first stop. NREL stands for National Renewable Energy Laboratory and is a bit like the NASA of the renewable energy world in that it is funded by your tax dollars. Ultimately this means that you get access to their information for free since you have technically already paid for it.

You can find tons of useful stuff at NREL, but for our purposes we are only interested in the solar radiation maps found at this link.

You will want to use one of the maps listed under Concentrating Solar Power Radiation. I tend to find that the “40km” static maps are the easiest to read.

If you are located in another country, you might have to do some digging on the web, but you should at least be able to find something. If possible, try and find a site which has different maps for each month of the year.

You will also want to make sure that you find a map that is for concentrating solar power, sometimes referred to as Direct Normal Radiation. Other types of maps are typically for fixed solar collectors, i.e. machines that don’t track the sun.

Reading the Solar Radiation Map

Reading a solar radiation map is a easy. 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.

I am located in Virginia (far right and about in the middle of the map vertically) which looks to be a light brownish color. Matching it with the range of values, I see that I get somewhere between 3-4 KWh/m^2/day annually.


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 one meter square, 2-axis solar concentrator. This isn’t quite what we want though.

We will assume that we have a 1m^2 collector that receives and average of 3-4 KWh/day for the month of April if it always pointed directly at the sun (ignoring losses due to various factors). Since a heliostat doesn’t point directly at the sun, we will have to make an adjustment to this value.

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. Since we are interested in the month of April, I have generated this graph  for the middle of the month, April 15.

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 for the month of April.

If I had a 2m^2 mirror, it would be double that. If I had a 0.5m^2 mirror, it would be half that.

Keep in mind that the amount of energy for your own situation will most probably 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 in each case.

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 you can at least get a rough idea.

This documentation is part of the Open Sun Harvesting Project.

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