Design considerations for solar collectors with cylindrical glass honeycombs

A properly designed cell structure placed between the solar absorber and outer cover glass can substantially reduce natural convection and infrared reradiation heat losses. Glass has merit for such a cellular structure or honeycomb because it is an inexpensive, abundant and stable material with low thermal conductivity and outstanding optical characteristics. To optimize the design of a honeycomb structure, i.e. to minimize the cost of the solar energy collected Z, requires the determination of the honeycomb solar transmission as a function of incidence angles of the sun, infrared effective emittance, cell Nusselt number, and cell wall conductance as well as an estimate of appropriate costs. For an array of circular tubes, the design parameters are wall thickness b, cell diameter d_i, and cell length L. It is difficult to make b less than about 0.2 mm. Typically, d_i must be no larger than 1.6 cm. Increasing decreases reradiation and conduction losses, but also decreases solar energy transmission. For d_i = 9.53 mm and b = 0.198 mm, optimum values ranged from 3 to 12 for collector temperatures (above ambient air temperature) between 22°C and 100°C. Since the Z vs curves have fairly broad minimums, values less than 9 can be used with less than a 3 per cent penalty in cost at the higher temperatures.A comparison of collector effiency characteristics indicated that cylindrical glass honeycomb collectors with nonselective-black absorbers were markedly superior to single-glazed selective-black and double-glazed nonselective-black collectors, especially at higher collector temperatures. Cost effectiveness studies also indicated honeycomb collector superiority at temperature differences between the working fluid and ambient air greater than about 35°C.