Solar Solutions Bristol Stickney, chief technical director, SolarLogic LLC, Santa Fe, N.M.
Bristol’s Six Principles for Good Solar Hydronic Design New life for old flat plate collectors
I
n 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 col- lector arrays at NNMC before cleaning.
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
Figure 35-1 After more than a quarter century of service, large sec-
tions 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 sur- rounded the glass aged, dust had blown into the collectors through the seals. The mineral stains were a result of rain- water 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 visu- al 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 hydron-
ic 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
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Figure 35-2
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 ther- mal radiator. The surface simply clings to the radiant heat.
Continued on page 30 June 2011
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