Solar, Biomass and Hybrid Dryers - BioEnergy Discussion Lists

Solar collectorUnglazed transpired solar collectors have lower cost, and higher reliabilitycompared to flat plate collectors. As the name indicates, UTC has no glazing, andconsists of a low porosity perforated absorber plate fixed close to a back wall, whichcan be a vertical wall of the drying chamber. A circulating fan in the air collectionduct draws ambient air near the front surface of the absorber plate through theperforations. Air is heated while passing through the perforations in the hotabsorber plate. Although UTCs have slightly higher radiation losses, these are offsetby other advantages such as low reflection losses, low cost, simple construction, andeasy maintenance. The working principle of UTC is illustrated in Figure 10.1.Figure 10.2 shows a photograph of the collector used in the dryer. The collector ismade of two parts: an aluminium absorber plate, and a galvanized iron (GI) boxwhich houses the absorber. The perforated absorber is constructed using plainaluminum sheet of 20 SGW (0.81mm) thickness, measuring 2.44m (length) x 1.27m(width). The absorber plate has circular perforations, with a hole diameter of 1.6mmand a pitch of 14.5mm.The box is constructed using 20 SWG (0.91mm) GI sheet,measuring 2.44m (length) x 1.27m (width), and a plenum depth of 0.10m. The totalaperture area of the collector is 6.20m 2 .The collector is tilted at a slope of 45° to maximize the amount of solar radiationincident on the absorber plate. The collector is connected to the drying chamberusing mild steel ducts. During operation, ambient air is drawn in by a blowerthrough the perforations in the absorber plate. The hot air is delivered to the dryingchamber through the connecting ducts.The design parameters of the unglazed transpired solar collector are given in Table10.2.Heated AirCollector Air FlowAmbient AirPlenumPerforated AbsorberFigure 10.1: Working principle of an UnglazedTranspired Solar Collector (UTC)Figure 10.2: The Unglazed Transpired SolarCollector used in the Dryer51