Performance Measurements of a Cylindrical Glass Honeycomb Solar Collector Compared With Predictions

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Abstract
A glass tube honeycomb solar collector, composed of a conventional single-glazed flat-plate water cooled nonselective black absorber with a cylindrical glass honeycomb mounted between the plate and cover glass, was designed, fabricated, and tested. The test procedure followed the National Bureau of Standards Method of Testing for Rating Solar Collectors. The honeycomb consists of individual cylindrical thin-wall glass tubes standing on end on the absorber plate in a hexagonal close-packed pattern. The tubes are 9.5-mm ID, 0.2-mm wall thickness, 5.3 length to diameter ratio made of glass with mean solar absorptive index (k), 2.6 × 10−6 μ−1, evaluated from spectral transmittance-reflectance measurements on a tubing specimen. The collector performance equalled or surpassed theoretical predictions for measurements made over a broad range of collector inlet temperatures and environmental conditions. Performance comparisons are made with a baseline double-glazed solar collector tested alongside the honeycomb collector. For applications requiring a working fluid temperature of 65°C above the ambient air temperature, honeycomb collector efficiencies of 35–55 percent may be expected for approximately a six hour period on clear days; for fluid temperature 40° C above ambient air temperature, efficiencies of 50–65 percent may be expected.

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