Solar air collectors with packed flow passage for curing tobacco: design and performance evaluation.

Air-type collectors are becoming practical and popular in drying systems mainly due to their simplicity which results in low initial and operating costs. This realization prompted many researchers to investigate the possibilities of using air-type collectors for drying. To-date only nonpacked forced convective type collectors are reported. However, for isolated communities which are not connected to electric power supply, it is wise to deploy natural convective collectors. However the major disadvantage of natural convective air-heater systems is their low efficiency. In this research work, the results of a theoretical and experimental evaluation of a solar air collector with packed flaw passage for use in a tobacco curing process are presented and discussed. The objective is evaluation of its performance for suitability to curing tobacco. The experimental collectors consisted of two types of collectors with different geometrical configurations and different types of packing material. Experimental results showed that the collector with double glazing with absorbing packing in the air flow passage below the inner cover were superior to the single glazed one with absorbing packing in the air flow passage below the absorber. Efficiencies as high as 47 percent could be obtained at insulation rates in the region approximately 900 to 1000 Wm-2. Here, the temperature rise above ambient is about 25K to 30K and reported air mass flow rate of about 40 x 10-3 kgs-1. These values are not satisfactory for the tobacco curing process which requires an equivalent of 100 hours at an average temperature of 50°C. Also due to intermittency of solar radiation resulting from weather changes and its total absence at night, solar drying is viable only when used to supplement fuel wood or other sources of thermal energy in a "solar assisted barn". The performance of the collector is however lower than others reported elsewhere. This might be due to losses accruing from improper insulation coupled to non maximization of the collector channel depth. It is therefore recommended that further optimization should be sought before commercialization.