Solar Solutions Continued from page 28 In the early 1980s, collector manufacturers discovered
that the use of a selective surface reduces heat loss through the glass with about the same effectiveness as a double glass (thermo-pane) cover. Double glazed collec- tors were widely discontinued when selective surface black absorber materials became commonly available. Single glazed panels with selective black absorbers are by far the most common standard among flat plate collectors available to this day. Because this technology has not changed much since the Solaron panels were manufac-
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
Figure 35-4
Figure 35-3
tured in the 1980s, their restoration, rather than replace- ment (or demolition), was deemed to be worthwhile and cost effective.
The restoration process The restoration of the Solaron solar thermal hot air col-
lectors at the Espanola campus involved removing the sin- gle glass covers so that the black flat plate absorber sur- faces 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 with- out the black absorber surfaces being physically touched, except by the 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 clean- ing 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 visual- ly. A surface with a high solar absorptivity will not reflect solar radiation, including the visible spectrum which
Page 30/Plumbing Engineer
absorber (even black paint) will obviously get very hot when exposed to bright sun. When temperature measure- ments are taken 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 sur- faces 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 tempera- ture 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
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 fac- tor of 2). (Our experienced installers are in the habit of wrapping thin plastic tape around a metal pipe before shooting it 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 measure- ments. 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
June 2011
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