GREEN SYSTEMS
SOLAR SOLUTIONS Bristol’s Six Principles for Good Solar Hydronic Design
New life for old flat plate collectors
BY BRISTOL STICKNEY CONTRIBUTING WRITER I 68
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 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
Figure 35-1 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 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
The restoration process The restoration of the Solaron solar thermal hot air
collectors at the Espanola campus involved removing the e Turn to STICKNEY on p 70
Figure 35-2
phc june 2011
www.phcnews.com
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