A novel algorithm to investigate conjugate heat transfer in transparent insulation: Application to solar collectors

A theoretical method and novel computational algorithm are presented for th e analysis of radiation heat transfer in transparent insulation (TI) struct ures. The radiation spectrum is separated into short- and long-wave compone nts. 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 la teral channel walk;. In existing models, using multidimensional accurate co mputational methods, these boundaries are usually assumed to be diffuse in the infrared spectrum. This assumption is relaxed here, which is a main inn ovative feature of the algorithm. The method is applied to simulate solar f lat-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 t he temperature field within the TI structure channel and the temperatures a nd heat fluxes on the top glass cover and on the absorber plate. A comparis on with the care of isotropic (diffuse) reflection shows a drastic differen ce in collector performance results: for 100 mm TI depth, the calculated ma ximal plate temperature in the anisotropic model is about 140 degrees C bel ow that of the isotropic model. The ratio of the heat flux on the latter to the incident insolation an the collector is considered as an efficiency of the collector The effects of optical properties, geometry, and operating c onditions on the performance of the solar collector (efficiency and maximal stagnation temperature of the absorber plate) are discussed and compared w ith results for the case without TI and with published experimental data. T he results indicate the expedience of collectors with TI channels of rather large thickness h(c). For the collector studied here, the efficiency incre ases asymptotically with h(c) up to 150-160 mm (corresponding to aspect rat ios of 30-32).