Modeling and Simulation of an Indirect Natural Convection Solar Dryer with Thermal Storage Bed

The intermittent nature of solar energy limits a 24 hour operation and the effectiveness of solar thermal devices. Affordable and environmentally friendly materials for storing solar energy are currently in search. A natural convection solar cabinet dryer coupled with thermal energy storage bed (gravels) is modeled and simulated for space heating application (tomatoes drying) using TRNSYS 16 software. Performance of the solar thermal system (solar cabinet dryer) with a thermal storage bed will serve as a guide in developing a gravel-pit (GP) and or water-gravel pit storage system (WGPS) on a medium to large scale to facilitate solar thermal storage of heat for space and water heating applications in homes, health care and educational facilities. Thermal storage volume and thickness of gravel bed were determined and an optimized solar collector area obtained using TRNSYS 16 software for drying 6kg of tomatoes slices. A computer program was written to predict the product drying temperature, mass of moisture removed, moisture content and drying rate at two different trays including solar collector efficiency, heat storage bed temperature profile using meteorological data input of dryer location, gravel properties, solar collector parameters and solar cabinet dryer chamber variables. The month of August was used as the design month bearing in mind that it has the least solar radiation in Bauchi and thus, predicted the least drying performance while, the month of March with the most solar radiation predicted the optimum drying performance. The maximum predicted gravel bed temperatures were 44 and 59.3°C for the months of August and March respectively. Predicted performance of the solar cabinet dryer was compared to a similar cabinet dryer without thermal storage bed. Predicted maximum product drying temperatures of 48 and 69°C were obtained for solar cabinet dryer with thermal storage bed as against 46 and 66°C for solar cabinet dryer without thermal storage bed in the month of August and March corresponding to solar intensity value of 575.4 and 1049.2W/m2 respectively. To attain 4.5% moisture content for 3kg of tomatoes slices placed on each tray containing 94% of moisture, requires 37 (20 hours of sunshine and 7 hours of supplementary heat stored) and 53 (26 hours of sunshine and 6 hours of supplementary heat stored) hours of drying for solar cabinet dryer with thermal storage bed and, 52 (25 hours of sunshine) and 75 (34 hours of sunshine) hours under same weather condition for similar solar cabinet dryer without thermal storage bed for the month of March and August respectively. The average moisture extraction rate is 0.0759 and 0.0531kg per hour in the month of March for solar cabinet dryer with and without thermal storage bed and, 0.0540 and 0.0374kg per hour the month of August respectively. Predicted maximum solar collector efficiency for cabinet dryer with thermal storage bed is 50.12 and 43.85% for the month of March and August whereas, it was 45.83 and 37.66% for cabinet dryer without thermal storage bed respectively. The performance prediction of the solar cabinet dryer with thermal storage bed indicates clearly good potential for storing solar thermal heat collected during the day and effectively utilizing the stored heat during off-sunshine hours for heating applications. It is recommended that a gravel-pit (GP) and or water-gravel pit storage system (WGPS) should be developed and adequately studied for a range of operating parameters based on temperature distribution, thermal energy stored, available energy stored in the bed, energy consumption by blower (for active bed), and thermal efficiency of the collector to give clear guidelines for using the gravels for large scale solar thermal energy storage for space and water heating applications.