Pyranometer Mounting Platform: Side by Side Comparison of Multiple Irradiance Measurement Devices

As part of my work in the solar industry, I occasionally get involved with low-cost sort of experiments that need to produce data, but do not justify large engineering outlays. One of these things was to verify and test several different types and brands of Pyranometers at a test facility. Pyranometers are devices used to measure the amount of power that comes from the sun, measured in units of watts per square meter. This value is extremely important in solar plant performance, so it is equally important to be able to verify and measure the accuracy and lifetime behavior of the instruments used to measure irradiance. To do side-by-side comparison between various pyranometers, I built a test plate that allows mounting several instruments at GH (Global Horizontal) and PoA (Plane-of-Irradiance). The unit started as a pencil scribble to a panel shop in town, to which I did the final mounting and wiring in my Denver shop.

Start of the pyranometer platform I started with a reinforced right angle plate, supplied by a local panel vendor, to act as a surface on which to mount the instruments to test. As an experimental platform, this plate would have instruments swapped around frequently, so all the spacings and clearances are general and generous. The drill holes were not done in CAD, but rather by me, using an old-school compass, speed square, ruler, and calculator. The photo shows one of the original instruments sitting on the top of the plate, underneath the speed square. It is nice to have CAD drawings, but it is also nice to pull our a ruler and compass to mark locations with pencil on metal.

Verifying fit and heights of instruments Another view of the pyranoemter plate, playing with rough layouts. The instruments here include a reference cell at left, above a silicon photodiode irradiance sensor, and three other instruments from pyranometer vendors. The cost difference of instruments shown in this photo range from $200 for the silicon diode device to well over $4000 for the unit on the uper right. Our data from this experiment, running continuously for years since this assembly was manufactured, has been used in multiple ways to figure out how to spend pyranometer dollars. I tried several different layouts for the difference sensors, to figure out the most optimal way to mount them, while still allowing ease of swapping instruments as the test requirements change.

The PoA (Plane-of-array) adapter The test platform also needed to have a PoA (Plane-of-array) section added. Originally, the goal was to use all the units in GH (Global Horizontal), but it was decided to add in a PoA section to compare the irradiance output of PoA mounted sensors with some other instruments in the field. This test was to help us characterize the effects of tilt angle differences betwen different instruments, and led to the design of a new pyranometer mounting platform that enables high-accuracy in-plane alignment of pyranometers. This system served as a prototype for the later version, which means the PoA section was added as an afterthought. My shop is not fit for large metal work, but a vise, screwdriver, center punch, drill set, and some blocks of wood are sufficient tools to do this type of small work.

Finished platform, on the bench This is a side view of the Pyranometer plate. This equipment has been installed now for several years, and many of the instruments have been swapped or replaced with others. When originally installed, the unit had eight different irradiacne devices from several different vendors, including some that were alreasdy instaled on live solar sites for some time, as a way to test how they age. The whole thing is about 4 feet long and 2 feet deep.

Finished platform, on sun Behold- On sun. This is the finished unit, installed and operating at dusk. You can see the difference in height between several of these instruments. They were laid out in a manner such that the taller units would not shade the shorter units. The output of pyranometers has a strong dependence on mounting angle, so a great care was taken to verify that the instruments were normal to the planes on which they were mounted, and those planes were oriented correctly to the ground. In the solar industry, the terms of azimuth and elevation are used for solar angles. For global horizontal instruments, the azimuth angle is largely irrelevant. However, in PoA (plane-of-array) mounting, the yaw, pitch, and roll angles of the optical axis on the actual tilted plane is important, and then that tilted plane is oriented according to angle and azimuth.