Keep irradiance on a horizontal plane constant while changing the inclination angle of the source

  • 19 June 2022
  • 2 replies

Dear all,

I´m currently modelling a solar thermal (evacuated tube) collector in Zemax. The aim is to investigate the collector behavior under different angles of incidence. The collector is in rectangle shape, so I´ve modelled the sun as source rectangle with the same dimensions (see attached image).

Figure 1: Vertical case


I want to make sure, that the irradiation onto the collector plane is 1000 W/m² in every incidence angle and thats my problem right now. Let´s say, my collector area is 1 m².

  • In the vertical case it´s simple, I can set the Source Power to 1000 W to get the 1000 W/m² onto the collector plane.
  • With tilting the radiation source against the horizontal, the projetion plane will increase with 1/cos(angle) which will lead to a decreased irradiation onto the horizontal plane and also I have to adjust the position of the source to focus the whole radiation onto the collector plane.
Figure 2: Inclination of 45° without adjustment of the source position

In the past, I wrote a ZPL macro for that issue. With that, I adjusted the source area and power to meet the 1000 W/m² and fixed its position to make sure, all of the source radiation is hitting my collector. As you can imagine, it´s a lot of work to adjust it for every incidence angle and if the geometric dimensions are changing, I have to redo everything. Is there a better way to implement a constant radiation onto a given horizontal plane while changing the incidence angle of the radiation source? 

Thanks for your help, any ideas are welcome.




Best answer by Jeff.Wilde 24 June 2022, 08:09

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Userlevel 6
Badge +2

You might want to consider utilizing a simple optimization step.  First, make your source beam area large enough that it fully illuminates the collector area for all source tilt angles of interest (but not excessively large since that will only waste rays).  Then, for each tilt angle, optimize the source power to yield your desired incoming irradiance of 1 kW/m^2.  As you may already know, this can be done with the NSTR and NSDD merit function operands.  It’s a simple optimization that should run quickly and would be much easier than trying to simultaneously adjust the spatial dimensions of the source (unless there is some specific reason to do so). 


Userlevel 6
Badge +2

I should add that you may be able to avoid the optimization step and simply scale the power of your source by one over the cosine of the angle of incidence (AOI) on the collector, which is trivial to do for source tilts in one direction, and still fairly simple for 2D (tip/tilt) source motion.  Again, the key is to pick a fixed source size that overfills the collector for all tilt angles of interest (and ideally only slightly overfills it for the steepest AOI).