Conclusions and Scope for the Future Work
8.1 Conclusions
This study concerns with the development and performance investigation of a forced convection hybrid solar dryer consisting of two double-pass solar air heaters, a paraffin wax- based shell and tube latent heat storage module, a parallel flow drying chamber, and a blower.
The components of the drying system have been sized based on the mass of air and the energy requirements and fabricated with the locally available materials. It has been tested by drying Ghost chilli, sliced ginger, a local variety of red and green chilli. The Ghost chilli and sliced ginger were dried in the dryer without filling the storage with paraffin wax. The kinetics of the drying process of the Ghost chilli was studied. Energy and exergy analyses of the drying process of Ghost chilli and the ginger were performed. The dryer was then filled with the paraffin wax, red chilli, and green chilli of a local variety were dried in the dryer. The performance of each component of the drying system was evaluated by the energy and exergy analyses. Eventually, economic analysis of the dryer is carried out by annualised cost method.
The summary of the drying kinetics analysis of the Ghost chilli, the energy and exergy analyses of the drying process of the Ghost chilli and the ginger, and the performance investigation of the dryer with the latent heat storage are given in the following sections.
8.1.1 Thin layer drying kinetics analysis of Ghost chilli
Two samples of Ghost chilli were dried simultaneously in the solar dryer and in the open sun to study its kinetics of drying. In the solar dryer, the air temperature varied between 44 C and 66 C, and the sun dried samples were dried in the ambient temperature ranging from 29 C to 37 C. The moisture content of the chilli samples dried in the solar dryer and the open sun was reduced from 85.5% (w.b.) to 10.5% (w.b.) in 123 h and 193 h, respectively. Eleven thin layer drying models were fitted to the moisture ratio data and regression analysis was performed to select the best model representing the drying process of the chilli. The best model was selected based on the criteria of the highest value of the coefficient of determination (𝑅2), the lowest
values of the reduced square (𝑥2) and root mean square error (RMSE). The important conclusions of this study are as follows.
The drying process of the Ghost chilli occurs in the falling rate period.
The drying rate of the solar dried sample is faster than that of the sun drying one.
The Midilli and Kucuk model is the most suitable thin layer drying model to describe the drying process of the Ghost chilli dried in the dryer.
The Page and the Modified Page models are the two most suitable models for the open sun drying process of the Ghost chilli.
The colour of the solar drying sample is better than that of the open sun drying one. The original colour is preserved in the solar dried sample.
8.1.2 Energy and exergy analyses of the drying processes of the Ghost chilli and ginger
Nine kg of freshly harvested ripe Ghost chilli and 13 kg of sliced ginger of the thickness varied between 8 mm and 10 mm were dried in the dryer. The Ghost chilli and sliced ginger were successfully dried in the dryer in 42 h and 33 h in the drying air temperature varying between 42 °C and 61 °C and 37 °C and 57 °C, respectively. Energy and exergy analyses of the drying processes of the two products were performed applying the first and the second laws of thermodynamics. The following conclusions are drawn from this study.
The thermal efficiency of the solar air heater is affected by the inlet air temperature. It deceases with increase in the inlet air temperature. The thermal efficiency of the first solar air heater (SAH - 1) was found to be higher than that of the second solar air heater (SAH - 2).
The thermal efficiency of the SAH - 1 was in the range of 22.1‒40.2% with an average of 32% while the thermal efficiency of the SAH - 2 varied between the 8.5% and 19.5%
with an average of 14%. The average overall efficiency of the air heater panel was in the range of 22.9‒23.3%.
The energy and exergetic efficiencies of the drying process may change during successive drying and also from product to product. The exergetic efficiency of the drying processes of both the products was found to be high in the latter stages of the drying periods
The exergy efficiency was in the range of 21‒98% with an average of 63% for the Ghost chilli and 4‒96% with an average of 47% for the sliced ginger. The sliced ginger drying
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process exhibited higher thermal efficiency and lower exergetic efficiency during the initial stage of drying. The exergetic efficiency increased with progressing in drying time. The overall exergetic efficiency of the drying process of the Ghost chilli was higher than the sliced ginger.
The specific energy consumption depends on the type of product dried, drying time and the loading of the dryer. The specific energy consumption of the Ghost chilli (18.7 kWh/kg) was higher than that of the sliced ginger (8.8 kW h/kg).
The higher exergy and lower thermal efficiencies of the drying chamber manifest considerable energy loss in the exhaust of the drying chamber.
8.1.3 Performance tests on the solar dryer with the latent heat storage
Twenty kg of red chilli of the local variety was successfully dried in the hybrid dryer in four consecutive sunny days in the air temperature ranging from 34 ºC to 60 ºC with an average of 50 ºC. The performance of each component the drying system was evaluated by energy and exergy analyses. The following conclusions are drawn from this study.
The integration of the energy storage reduces fluctuation in the drying air temperature and also extends the drying time beyond the sunshine hours.
The exergy efficiency of the solar air heater increases with increase in the solar radiation intensity. The exergy efficiency of the SAH - 1 and the SAH - 2 were 0.9%
and 0.8%, respectively.
The average energy and exergy efficiencies of the energy storage were in the range of 36.4‒42.2% and 13.7‒17%, respectively.
The exergy efficiency of the drying chamber increases with increase in the drying time.
The exergy efficiency of the drying chamber was in the range of 24.2‒98% with an average of 52.2%.
The specific energy consumption of chilli and the overall efficiency of the drying system were 6.8 kW h per kg moisture and 10.8%, respectively.
The electrical energy consumption is very less compared to the total energy required for drying the product. The electrical energy consumption was 0.7 kW h per kg of moisture which was only 10.3% of the specific energy consumption of the product.
The cost of drying per kg of dried product in the solar dryer is found to be Rs. 42.7 that of the product dried in the electric dryer is Rs.81.4.
It is always desirable to have high energy and high exergy efficiencies of the drying chamber. The low energy efficiency is the indication of more energy losses from the drying chamber. The high exergy efficiency means low exergy losses while drying the product and high exergy outflow of the drying chamber. The high exergy outflow of the drying chamber indicates that the work potential of the drying air is still available which can be used further.
The exergy outflow of the drying chamber goes to the environment and is wasted. If the outflow air can be used some way, the energy efficiency of the drying system can be improved.
However, only the outflow air with high exergy can be used further. Therefore, the exergy analysis plays an important role in the thermodynamics analysis of the drying system.
In this study, it was observed from the exergy analysis of different products that the exergy efficiency of the drying chamber increased gradually and became the maximum towards the end of the drying period. The exergy efficiency was minimum at the beginning of the drying process. Initially, the product contains more moisture on the surface and therefore, the moisture evaporation rate is high. The moisture content of the product decreases gradually and the drying air leaves the drying chamber with very low relative humidity at a high temperature. The exergy outflow of the drying chamber also increases gradually and becomes the maximum towards the end of the drying period. Therefore, based on the value of the exergy analysis, one can plan to utilize the exhaust of the drying chamber by recirculating in the same drying chamber or utilizing it for drying fresh product in another drying chamber. This may improve the energy efficiency of the overall drying system.