Performance modelling of a parallel tube polymer absorber

Making use of radiation and convective heat transfer theory the efficiency of a collector fitted with an absorber consisting of spaced parallel tubes is modelled on a personal computer. The absorptance of the tubes for sunlight and the convection coefficient between the back insulation and the tubes are adjusted until agreement between experimentally determined values and calculated values are obtained. The effects of variables not tested experimentally are then investigated. Covering the top of the back insulation with a sufficiently thick metal film to make it isothermal, reduced the dependence of the efficiency on tube spacing from 0.834%/mm to 0.615%/mm for inter tube spacings between 4 mm and 12 mm. A collector with an absorber with an inter tube spacing of one tube diameter and with an isothermal back is only 5.2 percentage points less efficient than one with (twice as many) tubes touching one another at the sides. Efficiency is slightly dependent on wall thickness, decreasing from 64.5% for a 0.5 mm wall to 61.5% for a 3 mm wall. This is because the resistance of the hydrodynamic boundary layer forms a significant part of the total resistance to heat transfer. For outside diameters between 5 and 11 mm efficiency varies by only 0.5 percentage points as long as the diameter/pitch ratio stays the same. For small scale unsophisticated manufacture, the absorber with spaced polymer tubes presents an attractive alternative to more conventional designs. Copyright (C) 1996 Elsevier Science Ltd.