Design, fabrication and testing of a parabolic trough solar collector air heater for a greenhouse solar dryer.

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Njeri, Eric King'ori
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The University of Zambia
In an effort to mitigate post-harvest losses (PHL) of agricultural products from farmers and the markets, greenhouse solar dryers (GSD) have been extensively employed by various stakeholders. The operation of these conventional GSD is though allowing cool or cold ambient air to enter them during drying. Consequently, temperature and relative humidity of the air inside the GSD is lowered and increased respectively thereby increasing the drying time which is undesirable, and reducing the overall efficiency of the GSD. The research study aimed at designing, fabricating a parabolic trough solar collector (PTSC) air heater for a GSD and testing its performance. Additionally, the performance of the modified GSD with and without the PTSC was investigated. Quantitative research design was employed in carrying out of this research study. The specific methodological aproaches of the research study included, design of the PTSC air heater, fabrication of the PTSC air heater, testing of the PTSC air heater, experimentation for data collection, data analysis using Microsoft Excel and reporting. Key geometrical parameters for the designed PTSC air heater were rim angle of 98o, focal length of 0.2608 m, height of 0.3451 m, aperture width of 1.2 m and PTSC length of 2m giving a collector area of 2.4m2. The PTSC air heater was tested and attached to the GSD that acted as the drying chamber with a total volume of 35.625 m3. With a PTSC air heater thermal efficiency of 5.3 percent, the results showed that the maximum temperature at the absorber tube was 86.7 oC and temperature of the heated air was 80.1 oC. The maximum temperature recorded inside the GSD with and without the PTSC connected was 62 oC and 44 oC respectively. The new modification of the GSD with a PTSC increased the temperature of the GSD to the optimum and stable levels of 45 oC to 60 oC required for drying tomatoes. Performance analysis of the GSD that were done included initial and final moisture content determination of tomatoes, found and reported to be 94 percent on average and 14 respectively. Drying rates of the GSD with and without the PTSC was found to be 1.738 kg/hr and 1.227 kg/hr respectively. Percentage saved time of drying 15kg equal amounts of tomatoes was found to be 27.3 percent when the PTSC air heater was connected to the GSD compared to unmodified GSD respectively. Specific energy consumption of the GSD with and without the PTSC air heater was found to be 5.54 kWh/kg and 7.627 kWh/kg respectively, specific moisture extraction rate was found to be 0.1778 kg/kWh and 0.1311 kg/kWh for the GSD with and without the PTSC air heater respectively. The thermal efficiency of the GSD was found to be 11.3 percent at 15 kg loading capacity, comparable to other hitherto similar work done by various researchers. These findings showed the significance of modifying the GSD with a PTSC air heater in improving the drying technologies of agro-based products in mitigating PHL.
Thesis of Master of Engineering in Renewable Energy.