Modelling of Air Conditioning Within a Hybrid Desiccant Solar Dryer and Storage Unit

Abstract

In East Africa, maize and other cereal grains are normally sun-dried on mats placed on the ground after harvesting. Post-harvest preservation techniques and conditions greatly contribute to the loss of the produce mainly due to mycotoxin infestation. There is need to innovate affordable and effective solutions for post-harvest preservation of grain. A hybrid desiccantsolar drier was therefore designed in an attempt to mitigate the problems facing farmers. However, it was found necessary to model and optimize the operation of the drier. Therefore, the overall objective of this study was to model the use of SAP Fabric as a desiccant in grain dryers and silos. The specific objectives were to : Undertake a market study on grain drying In Kenya to understand the market needs, Identify physical quantities pertinent to the grain drying system, Establish Determine the adsorption capacity of SAP Fabric and Model Moisture Sorption Isotherms of Superabsorbent Polymer Fabric for Desiccant Drying and Model Optimize the design of the grain drying system using Computational Fluid Dynamics (CFD) and Validate the theoretical models using a fabricated physical model To determine the moisture adsorption capacity of SAP fabric; experiments were carried out in an air conditioning unit, where the inlet air temperatures were 20°C, 30°C and 40°C, the inlet air relative humidity was varied between 50% and 90% in 10 units interval. It was found that the SAP Fabric had high adsorption capacity of up to 105% of its dry weight at an air temperature of 20°C and 90% relative humidity. The adsorption rate trends are best predicted using logarithmic models which showed a high initial moisture adsorption decreasing gradually to the equilibrium moisture content. The pressure drop across 3 layers, 6 layers, 10 layers and 15 layers was averaged at 0.10 millibar, which showed that SAP fabric is a highly porous desiccant. There was no noticeable change in the moisture adsorption capacity of the SAP when oven-dried at 105°C to regenerate it. It was established that SAP fabric is effective for use in drying air. Applying the gravimetric technique the sorption isotherms and the effectiveness of the desiccant in crop drying was determined at temperatures 20, 25, 30, 35, 40, 45, 50 and 55°C. Using R statistical software, eleven sorption isotherm models were fitted to the experimental data using the non-linear regression functions. The coefficients of each model were obtained. The modified Freundlich, modified Henderson and modified Oswin models best predicted the experimental data for the study temperature range. Using Symbolic Regression and Non-Linear Regression in R software; four mathematical models were obtained. R software codes were assembled for the analysis of sorption isotherm models. Compared to existing models the xii developed models were found to give a more statistically accurate association between the temperature, moisture content of SAP Fabric and relative humidity. The design and development of a Plastic Silo/Dryer were carried out with careful consideration of the identified physical quantities relevant to grain drying and storage systems. The dryer which is utilized as a silo after drying is equipped with a removable solar-powered fan to generate the required power for operation. By incorporating suitable properties and components, a prototype of the Plastic Silo/Dryer was fabricated. The Ansys Fluent software within the Ansys Workbench was utilized for Computational Fluid Dynamics (CFD) analysis. This software encompasses a range of features; it includes the ability to handle CFD heat transfer simulations, simulate porous materials, and conduct twophase gas simulations. To solve the governing equations, it applies the Finite Volume Method (FVM) approach, which discretizes the domain into control volumes. Additionally, it employs a hybrid surface modeling method, combining different techniques to accurately represent the geometry and generate the computational mesh. By utilizing the results obtained from experiments, simulations were conducted to model the drying process of maize using SAP Fabric as a desiccant. To validate the results of the CFD modelling experiments we conducted on the fabricated grain dryers. The results of a market survey conducted indicates that majority of the small and medium scale farmers use traditional grain management systems. Despite a lack of information on modern grain drying and storage methods, the grain dryer designs presented to farmers had a high acceptability across all socio-demographic classes. Acceptability of the grain dryer was promising given that 77% of the farmers were willing to accept the new technology and purchase it for use on their farms. The mean price for 10 bag capacity grain dryers was obtained from the average of the price recommendations by the farmers and was Ksh. 15000. The factors influencing the decision by farmers vary from region depending the socio-economic, demographic and cultural practices. The findings of this study make significant contributions to the development of affordable and effective post-harvest preservation techniques for seed and food grains in East Africa. The study enhances our understanding of using SAP Fabric as a desiccant in grain drying and storage systems, providing insights into its adsorption capacity, sorption isotherms, and drying efficiency. The developed mathematical models for moisture adsorption rates and sorption isotherms offer valuable tools for technicians and engineers, enabling more accurate xiii predictions and optimization of grain drying systems. Additionally, the utilization of Ansys Fluent software and the User Defined functions provides a platform for visualizing and optimizing drying processes, allowing for the evaluation of different design configurations and the identification of potential improvements. The developed grain dryer design will benefit many agricultural communities. In conclusion, these contributions have a positive impact on various aspects, including enhancing food security, mitigating losses, and improving the efficiency of post-harvest preservation practices. As a result, agricultural communities and society as a whole stand to benefit from these advancements in the field.