GREEN SYSTEMS | SOLAR SOLUTIONS | CONTINUED FROM PAGE 68
single glass covers so that the black flat plate absorber surfaces could be cleaned of dust and mineral deposits. The mineral deposits were loosened by the application of a mild liquid mineral solvent; the dust and loose material was then flushed off the surface with a minimal spray of clean water. The cleaning process was accomplished without the black absorber surfaces being physically touched, except by the
Figure 35-3
on the absorber surface, under the glass cover or with the glass removed, this can be compared to other surfaces under the same conditions. At the Espanola jobsite, temperature measurements were taken with the glass covers removed, so that clean surfaces could be directly compared to dust- covered or mineral-covered surfaces under the same real-time sunny-day conditions.
IR scanning thermometer An infra-red (IR) scanning thermometer was used on
this job as a quick and simple way to make comparisons of the clean absorbers to the dirty surfaces and then to make the same comparisons to the refurbished ones after cleaning. A common IR thermometer measures temperature by sensing (seeing) the infrared thermal radiation emitted from a surface and then converting that thermal radiation to a temperature, using an assumed emissivity typical for common painted surfaces, fixed at 0.95. Keep in mind that many common materials used in
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water spray. Once the water residue had dried off, the glass was re-installed with new gaskets, new sealant and new gasketed screws. The newly sealed frame components can be seen in the closeup in Figure 35-3. The intention of this cleaning process was to restore the black absorber surface to its original optical properties and then to seal it again, making it weathertight. The original black surface was a selective surface coating bonded to copper sheet. This surface is relatively delicate, which is why the cleaning process did not allow touching, wiping or scrubbing. Figure 35-4 shows the progress of the cleaning crew as they work their way through the panels from right to left.
Field observations During the restoration process, simple observations can
offer confirmation that the cleaning process is succeeding and actually improving the selective surface properties of the solar panels. The absorptivity can be observed visually. A surface with a high solar absorptivity will not reflect solar radiation, including the visible spectrum which makes up about 1/3 of the available solar energy. So, a black surface that is non-reflective and shows an absence of color even when well-illuminated will act as a good absorber. The absorptivity can also be confirmed by a simple
temperature measurement on a sunny day. A good absorber (even black paint) will obviously get very hot when exposed to bright sun. When temperature measurements are taken
Figure 35-4
heating systems do not have emissivities anywhere near this common value. For example, shiny metals, including copper, brass and zinc have very low surface emissivity. So, if your IR scanner is calibrated to 0.95, a hot copper pipe will produce a false low temperature reading because the emissivity of copper may be closer to 0.4 (off by a factor of 2). (Our experienced installers are in the habit of wrapping thin plastic tape around a metal pipe before shooting it
Figure 35-5
with an IR thermometer. The emissivity of plastic tape is closer to the normal 0.95.) The emissivity can be roughly evaluated by the same IR scanning thermometer using comparative measurements. The actual emissivity of an original clean selective surface can be expected to be about 0.17. This is less than 1/5 of the thermal radiation that the IR thermometer is
“expecting.” So, when looking at a selective surface, the common IR thermometer will display a false low temperature, because it calculates the temperature based on a higher typical surface emissivity value. Once you know this, the information can be used to your advantage. The false low temperature of one surface can be compared to another to give a relative comparison of the emissivity that the IR scanner “sees.” Under the same temperature conditions, comparatively lower IR readings indicate lower emissivity. Figure 35-5 shows a group of readings taken with an IR scanner calibrated to 0.95. The frame coated with black
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phc june 2011
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