Literature Review
2.4 Research Gaps and Objective
A comprehensive review of the development of the hybrid solar dryer, the kinetics analysis of the thin layer solar and sun drying processes of the food and agricultural products, and the energy and exergy analyses of the solar drying process has been carried out. The recent studies on the hybrid solar dryer have focussed on the application of the latent heat storage as the thermal energy storage in the solar dryer. The heat pump assisted solar drying is another growing area of study on the solar drying. The incorporation of the energy storages, auxiliary heaters, and the heat pump into the solar dryer provide operational flexibility of the solar dryer irrespective of the weather conditions.
Large numbers of studies on the thin layer solar and sun drying processes of the food agricultural products and the mathematical modelling were reported in the literature. The highest study product is the fruit (35.5%) followed by the herb (29%) and the vegetable (25.8%). The remaining percentage is the fish and other products. No study was reported on the thin layer drying process of the meet. The food and agricultural products can be dried in the solar dryer in the temperature range between 35 °C and 60 °C. A high quality dried product can be obtained by drying the food and agricultural products in the solar dryer in contrast to the open sun drying. Although a great deal of efforts has been made to develop the solar dryers for drying food and agricultural products, the following areas still demand the attention of the researchers.
The paraffin wax was the widely applied latent heat storage material in the solar dryer.
The application of the other phase change materials in the solar dryer need to be investigated.
Most works reported on the solar dryer were experimental in nature. The future direction of study in the solar dryer lies in the development of the mathematical model of the existing dryers, optimization of operating parameters, and the economic study.
The application of the software packages such as COMSOL multi - physics, ANSYS, and FLUENT in the simulation of the hybrid solar dryers are very limited.
The thin layer kinetics studies of the solar dried products like fish, meat, and some other products are still unexplored. Limited information is available about the modelling of the thin layer drying process of the products dried in the solar dryers with the energy storage and the heat pump assisted solar dryers.
The main disadvantage of the natural convection solar dryer is the low air velocity and the reverse thermo - syphoning during the night - time when it is operated with the biomass heater. Therefore, further investigation is required to improve this drawback.
It is observed from the literature review of the exergy analysis of the solar drying system that many investigators presented exergy analysis of the drying processes of the agricultural products dried mainly in the indirect - type forced convection solar dryer.
Complete exergy analysis of the solar drying system integrated with the auxiliary heat sources and the thermal energy storage are not reported in the literature.
The exergy analysis of the heat pump assisted solar dryer and the mixed - mode type solar dryer was also not reported in the literature.
Most works focused only on the exergy analysis of the drying process. The detailed exergy analysis of the system including the components needs to be carried out.
In addition to these general research gaps, it has been observed that
The thin layer kinetics and energy and exergy analyses of the dying process of Ghost chilli are not reported in the literature.
The studies on the exergy analysis of the solar drying system focused only on the exergy analysis of the drying chamber and the drying process of a specific product,
The drying of chilli in a solar dryer integrated with a latent heat storage unit is not reported in the literature.
The detailed energy and exergy analyses of the solar dryer integrated with latent heat storage have not been performed by the researchers.
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Therefore, the main objective of this study is to develop a forced convection solar dryer integrated with a paraffin wax based latent heat storage module to dry some of the valuable agricultural products available in the subtropical climate of the North - Eastern region of India.
The following are the sub - objectives of the present study;
Kinetics studies of the solar and sun drying processes of the Ghost chilli: In this study, the kinetics of the drying process of the Ghost chilli will be studied by drying it in the solar dryer and the open sun. The best drying models representing the drying process of the Ghost chilli in the solar dryer and the sun drying will be identified applying non - linear curve fitting.
Energy and exergy analyses of the drying processes of the Ghost chilli and the sliced ginger: In this part of this study, Ghost chilli and sliced ginger will be dried in the solar dryer without energy storage. Then, energy and exergy analyses of the drying process of the two products will be performed with the experimental data.
Performance investigation of the complete drying system with thermal energy storage: The solar dryer with thermal energy storage will be tested by drying red and green chilli. The energy and exergy efficiencies of each component of the solar dryer will be evaluated by energy and exergy analyses.
Economics analysis: In the economic analysis, the cost of drying per kg of dried product in the solar dryer will be compared with the electric dryer by annualized cost method.
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