A NOVEL ALGORITHM TO INVESTIGATE CONJUGATE HEAT TRANSFER IN TRANSPARENT INSULATION: APPLICATION TO SOLAR COLLECTORS

Abstract

A theoretical method and novel computational algorithm are presented for the analysis of radiation heat transfer in transparent insulation (TI) structures. The radiation spectrum is separated into short- and long-wave components. The former problem is solved by a one-dimensional ray tracing method, and the latter is treated by a three-dimensional discrete transfer method, extended to the case of anisotropic reflection (and transmission) on the lateral channel walls. In existing models, using multidimensional accurate computational methods, these boundaries are usually assumed to be diffuse in the infrared spectrum. This assumption is relaxed here, which is a main innovative feature of the algorithm. The method is applied to simulate solar flat-plate collectors with a TI made of honeycomb or glass capillaries. The heat transfer analysis also includes convection in the air gap between the TI and the absorber plate and conduction. An iterative numerical procedure was developed to solve the nonlinear conservation equations for obtaining the temperature field within the TI structure channel and the temperatures and heat fluxes on the top glass cover and on the absorber plate. A comparison with the case of isotropic (diffuse) reflection shows a drastic difference in collector performance results: for 100 mm TI depth, the calculated maximal plate temperature in the anisotropic model is about 140 C below that of the isotropic model. The ratio of the heat flux on the latter to the incident insolation on the collector is considered as an efficiency of the collector. The effects of optical properties, geometry, and operating conditions on the performance of the solar collector (efficiency and maximal stagnation temperature of the absorber plate) are discussed and compared with results for the case without TI and with published experimental data. The results indicate the expedience of collectors with TI channels of rather large thickness hc. For the collector studied here, the efficiency increases asymptotically with hc up to 150 - 160 mm (corresponding to aspect ratios of 30 - 32).