Assessment of the possibility of unglazed transpired type solar collector to be used for drying purposes: a comparative assessment of efficiency of unglazed transpired type solar collector with glazed type solar collector

摘要

Energy, which has become a key issue in global scale, has already posed a threat for the existence and development of mankind. Due to environmental degradation associated with energy consumption in all sectors, it is required to focus on utilization of sustainable energy sources. Solar energy, which is an environment friendly and sustainable energy source, is a promising alternative for fossil fuels. Drying processes in industrial sector accounts for about 12 to 20% of the total energy consumption. In developing countries, solar drying is utilized as a decentralized thermal application for the food preservation. Though natural sun-drying does not involve any cost, there are many disadvantages such as long drying time, contamination and intrusion of insects and rodents, which will result in low quality dried material. Currently, unglazed transpired type solar collectors are commonly used for building heating purposes but rarely for agricultural purposes. To check the possibility of unglazed transpired type solar collectors to be used for drying purposes, this study was conducted to compare the efficiency and performance of unglazed transpired type solar collector with conventional glazed type solar collector in local conditions. Dimensions of both solar collectors were designed as 1m x 1m in size. 5mm thick transparent glass sheet and perforated black painted metal sheet were used for solar radiation transmission in glazed type solar collector and unglazed transpired type solar collector respectively. Inlet and outlet temperature of both types of solar collectors, solar radiation, ambient temperature, wind speed and surrounding relative humidity were monitored. When compare the two types of solar collectors, there were no significant difference of solar collector's temperature at their outlets with various levels of ambient temperature (28-32 °C), solar radiation (95.21-504.28 W/m~2) and wind speed (0.43-1.05 m/s). The efficiency of unglazed transpired type solar collector was observed as 38.41 % under 0.0077 m /s airflow rate, with average solar radiation of 304.24 W/m~2. During lower solar radiation levels, comparatively higher collector's efficiency was observed in unglazed transpired type solar collector than in glazed type solar collector. In addition, unglazed transpired type solar collector is a low cost