New life for old flat plate collectors

BY BRISTOL STICKNEY
contributing writer

In 1983, Northern New Mexico College (NNMC) built a classroom building that included a “major league” solar heating system integrated into the architecture. A few years later, a new library was built nearby, using the same integrated solar heating system. These systems were originally provided by Solaron, a solar heating company formerly based in Denver, Colorado. Three large groups of flat plate collectors were installed to provide solar hot air to these buildings. A total of 260 glass panels adorned the two buildings, totaling over 4,660 gross square feet of collector area. Figure 35-1 shows one of three large collector arrays at NNMC before cleaning.

After more than a quarter century of service, large sections of the panels were coated with a light brown dust, as well as with streaks of white mineral deposits, covering the black absorber surfaces. As the rubber seals that surrounded the glass aged, dust had blown into the collectors through the seals. The mineral stains were a result of rainwater leaking in through the old glazing seals, flowing down the black surface and evaporating, leaving behind a mineral residue. The dust and minerals offer a clear visual indication that the absorptivity of these collectors had been compromised. And yet, the maintenance staff kept on using collectors, which kept pumping out hot air. Figure 35-2 shows a closeup view of the dust and mineral buildup in the old solar collectors.

In 2009, I became involved in the process to decide whether to remove, replace or repair these panels. The construction work has recently been completed and, as it turns out, the panels were refurbished in place and may continue to heat the buildings for decades to come.

While this system does not resemble the typical hydronic heating systems that I usually cover in this column, there are lessons to be learned from the endurance of a solar installation like this. The technology that was used in the “business-end” of these flat plate collectors is nearly identical to flat plate collectors today, both hot air and hydronic; that is, a copper plate is coated with a black selective surface absorber shielded by tempered glass and sealed with synthetic rubber.

Selective surfaces

A selective surface is characterized by very high absorptivity and very low emissivity. The black surfaces that intercept the solar heat by day are designed to “be selective” by employing some exotic surface coatings that resemble black paint but are often electroplated or bonded to the surface using industrial processes that far surpass a simple paint job. The high absorptivity allows a very high percentage of the available solar energy to be converted to heat, which results in a very high temperature on the black surface. The low emissivity acts like a trap for thermal radiation. Once the surface is hot, it would normally lose heat rapidly by thermal radiation, but is prohibited by the Low E coating, which is engineered to be a very poor thermal radiator. The surface simply clings to the radiant heat.

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 collectors 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 manufactured in the 1980s, their restoration, rather than replacement (or demolition), was deemed to be worthwhile and cost effective.

The restoration process

The restoration of the Solaron solar thermal hot air collectors at the Espanola campus involved removing the 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 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.

FIGURE 35-3	FIGURE 35-4

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 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 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 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 paint reads accurately at 141 F, which is the actual temperature of all the surfaces at that moment. The cleanest selective surface reads near zero, which indicates that it is doing its job as a low emissivity coating, holding on to the thermal radiation. The selective surfaces that are coated with dust and minerals read higher temperatures, showing that they need to be cleaned and restored to take advantage of that Low E benefit.

FIGURE 35-5	FIGURE 35-6

Summary

Figure 35-6 shows one of the solar collector banks after cleaning. Based on the observations, measurements and estimations, the results of this solar collector refurbishment effort can be summarized as follows for the overall surfaces of all the collectors combined.

The low emissivity of the black plates was substantially improved by the cleaning process. The following values are estimates based on comparison and interpolation of the IR temperature readings.

Before cleaning – 0.51
After cleaning – 0.28
Original value – 0.17

This means that the solar heat loss by thermal radiation has been reduced from a rate that was three times higher than the original to a rate that is only 1.6 times higher on average. Another way to say this is that the solar heat loss by radiation has been nearly cut in half.

The high absorptivity of the black plates has been improved as well, which is clearly visible when viewing the reassembled collectors that now have an obvious darker and cleaner appearance. The overall absorptivity values have been estimated as follows, based on the change in color of panels before and after cleaning.

Before cleaning – 0.71
After cleaning – 0.90
Original value – 0.94

Final notes

These articles are targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on pressurized glycol/hydronic systems, since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement.

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.