This week was spent validating the ray tracer. I used the 3D printed box and tested and… the simulation was different from the measurements. Narrowing down the source of this was difficult, but the final suspects were the grazing angle response of the apogee sq-120 sensor and the quality of my diffuse light.
I made 3 different diffuse light sources, but the only one that seemed to work was this:
A SAD lamp suspended over a sheet of glass. On the bottom of the glass, stretched taught was a piece of bakers parchment paper for diffusing.
The sensor response was also tricky. Apogee has this nice plot of their sensors vs sun angle but the low angle error goes off the scale. This is probably because at dusk the light is mostly diffuse. But they do state that 45 degrees or less it’s +/- 2%.
I’m ok with 2%, so I 3D printed a tiny ring to fit exactly over the sensor window and block light coming in from grazing angles which you can see in the picture of the box above. The sensor window is 11mm in diameter, so I made the shade 5.5mm tall. I painted it black and tried again. Of course I also had to include the ring or “sideshade” as I named it in the rayctracer simulation but that’s easy enough to do since I already had it in CAD.
I repeated the measurements and simulations and made a heat map based on the intensity of light at the 16 sensor locations in my box. I normalized all plots to have sum of all tiles equal to one.
It’s not quite perfect, but a paired t-test between the simulations and measurements revealed no statistically significant difference. I think the quality of my diffuser is what’s holding me back. The final validation for next week is to compare measurement and simulation of transmission through a piece of greenhouse plastic. If that passes, I will have great confidence in the rayctracer simulations for a full greenhouse.
These sort of test passing I always view as a bit of a minor miracle. I know it should work, ray tracing is a proven concept, and I tested the code as I developed it, but the real world has so many uncontrolled variables. What if I hadn’t thought of the idea of a sideshade? Would I just be stuck or have given up? It all looks well though so praise the Lord!
Sun Aquasystems 