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3.4 Preventing Overheat Failure of Polymer Collectors Polymer SDHW collectors are currently being developed by Envirosafe Technologies, Davis Energy Group, FAFCO, and NREL. The advantage of polymers is their very low cost. However, the cost advantage can only be fully realized if the lifetime approaches that of competing metal collectors. UV light degrades polymer materials. Fortunately, NREL has identified and tested a UV-protective film that is applied to the top of the collector glazing. Excessive operating temperatures can also rapidly degrade the polymers. Of primary concern is overheating of the absorber during dry stagnation conditions. Dry stagnation is unlikely, but could occur following a collector leak or if the collector was not refilled in the morning after being drained the previous night. SNL has recently developed and tested a protective gel that should protect the collector from overheat failure. Applied to the underside of the collector glazing, the thermochromatic gel (a mix of polystyrene and poly-vinyl-methyl ether) “shuts off the sun” by changing from clear to opaque at a glazing temperature of 80oC. (This temperature was selected because heat transfer calculations indicate the absorber will be ~125oC when the glazing is 80oC; if the absorber exceeds 125oC, rapid degradation will occur.) Results to date are encouraging, but lifetime tests and cost studies must be concluded before the gel could be considered a commercially viable product. 3.5 Freeze-Protected, Passive Collector “Passive” solar hot water collectors are inherently simple and reliable because they do not require the “active” operation of pumps and valves. Two types of passive systems are widely used: the integrated collector storage and the thermosiphon. These collector systems have historically not been freeze-protected, thus limiting their use to southern climates. To expand the market, NREL and SNL studied design modifications that would freeze-protect these systems. SNL studied the roof-integrated thermosiphon (RITH) that was developed under a CRADA with Energy Laboratory, Inc., and the Salt River Project. Two changes must occur to freeze-protect the RITH: 1) move the house insulation from the ceiling to the rafters, and 2) run glycol within the collector and transfer the heat to the tank inside the attic via a heat exchanger. Because the RITH is intended for the new home market, it is possible to specify rafter insulation prior to home construction. Rafter insulation prevents freezing temperatures in the attic, thus protecting the tank and supply/return water lines. Rafter insulation has additional energy-saving benefits and is becoming popular in high-efficiency homes. Fig. 1. Freeze-protected RITH SNL’s systems analysis suggests that a passive freeze-protected RITH could produce hot water for less than 9 ¢/kWh. This is significantly lower than the current active freeze-protected systems on the market (13 to 14 ¢/kWh.) 4. Planned FY 2005 Activities • Issue RFP to demonstrate a freeze-protected, passive collector system within a high- efficiency home (for a U.S. thermosiphon manufacturer (TBD), Artistic Homes, and DOE Building America). • Study combined solar heating and cooling systems (for Salt River Project). • Complete development of thermochromatic film to prevent overheat failure of polymer collector (for Envirosafe Technologies). • Help solar industry and city officials integrate solar heating into new projects (TBD). • Help solar industry resolve manufacturing issues (TBD).PDF Image | FY 2004 ANNUAL REPORT DOE Solar
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