This article discusses the diagnosis of the cause of outgassing from a 3-panel solar collector array using EPDM gaskets, silicone caulk, a selective surface, and 700-Series Owens-Corning high-temperature fiberglass insulation.

Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Photo a rooftop mounted site-built solar collectors at page top courtesy of Paul Galow.

How to Diagnose & Clean Outgassing & Deposits on Glazing in Solar Collectors

This question-and-answer article about the overnight accumulation of a gas (probably air) in a solar collector used for swimming pool heating, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

What Is the Probable Source of Gas Accumulating in and Deposits on the Inner Surface of Glazing on a Solar Collector ?

Question: Diagnose the Cause of a Deposit on the Underside of Glazing on a Solar Collector System

A three-panel solar collector array we installed four years ago (1981) recently developed a deposit on the underside of the textured solar collector AFG glazing. The solar collector has EPDM gaskets, silicone caulk, a selective surface, and 700 Series Owens-Corning high-temperature fiberglass insulation.

We think that the outgassing is from the insulation. The solar collector manufacturer has gone out of business.

We want to make sure we have the right solution to clean it with. What do you recommend? - Mike Fabian, Energy Engineering, Fargo ND.

Reply:

According to Frank Gilleland at Owens-Corning (in 1985), Owens Corning 700-Series insulation was not made to go into solar collectors. It is duct insulation made with a binder that can outgas under the high temperatures that occur inside a solar collector.

We think that the deposit on the inside of the solar collector glazing is probably a skin of condensed resin outgas material. Normally you could scrape off a deposit on the inside of the solar collector glazing (if a glass product was used) but that won't work on textured glass.

How to Diagnose the Source of Outgassed Deposits on Solar Panel Glazing

For liability reasons Owens-Corning did not recommend any solar collector glazing solutions. They also would not take responsibility for the solar collector deposit problem, because they said that it could have been caused by any of several materials used on the collector's construction. Gilleland said that with practice [or using forensic microscopy - DF] you can tell residues apart by color.

1. A bluish hue in a solar collector residue is typically from EPDM or from paint.
2. A chalky colored residue inside a solar collector points to a fiberglass insulation binder resin.

How to Clean Outgassed Deposits on Solar Panel Glazing

AFT Industries' Dick Orton recommended (also in 1985) cleaning the solar collectors with very hot water and a detergent such as Tide™. If that does not work, add a half-pint of white vinegar for each gallon of cleaning solution.

If the film is still there on the solar collector glazing inside surface, try an organic solvent such as toluene or xylene, available from chemical supply or hardware stor4es. He stressed that this is a last resort.

Watch out: toluene and xylene are toxic and carcinogenic and must be used with proper procedures, location, and personal protection. Use extreme caution and follow the manufacturer's instructions precisely.

Never use an abrasive substance to clean solar collector glazing.

If these approaches don't work, talk to chemists. See what they recommend that will break down and remove phenolic resin without leaving a new film on the collector surface. By this time, however, you may just want to replace the solar collector glazing (glass).

There is no guarantee that the outgassing inside the solar collector will stop. If you suspect insulation is the source of the outgassing or collector glazing deposit and the collectors haven't stagnated long enough to burn off all of the resin binder used to produce the insulation in the first place, dismantle the collector and lay aluminum foil between the insulation and the absorber.

Then seal the edges with high temperature caulk. Or replace the solar collector insulation with an oil-free binderless insulation.

As far as Gilleland knew, as of 1985 there was no such product on the market since Owens-Corning stopped manufacturing its SI-100 Series insulation in the early 1980's. But there may be a similar product available.

How to Remove Un-wanted Gases or Air From Solar Collector Systems

As we discussed in more detail at SOLAR COLLECTOR AIR or GAS COLLECTION, in addition to conditions that produced a deposit on the interior surface of a solar collector's glazing, other materials used in the construction of the piping, tubing, or solar collector array used to transport water for heating can also produce gases that interfere with proper solar collector operation.

Air accumulating in water in hydronic home heating systems (hot water boilers) is a well-known problem for which at least two components are in common use that may help automatically purge unwanted air or other gases from inside the solar collector plumbing system:

• An automatic float-operated air purge valve is mounted at a high point on the hot water piping (the cylindrical device at the center of our photo at left|) in order to automatically purge any excessive air in the system.
  
  See AIR BLEEDER VALVES for an article about how these automatic air purge valves work, and see Air-bound radiators, baseboards, boilers for examples of air-bound hot water systems.
  
  If your solar heating system does not already contain an air purge valve, and if the system has trouble from becoming air-bound, adding the air purge might solve the problem at very little cost.
• A small air expansion tank absorbs the initial pressure increase in the system when water is heated (and air is drive out of solution) to avoid opening and spilling a temperature/pressure relief valve when it's not appropriate. See EXPANSION TANKS for details. But watch out: this approach won't purge unwanted gases from the system.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Original Article

• Collector Outgassing: Q&A on Source of Gas Outgassing in A Solar Collector - original text. Use your browser's back button to return to this page. Original article, Solar Age Magazine, January 1985, adapted and updated for InspectAPedia.com November 2010.

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