Investigation of the drying process of apples by using solar energy and thermal energy storage system

Drying of agricultural products under hygienic conditions has a great significance. In our country, these products are generally dried by laying on the ground, but products dried by this method are prone to harmful affects like aflatoxin which degrades the quality of the products. Driers which are used for this purpose, maintain drying to take place under healthy conditions. In this work, drying system was improved by using solar air heater and thermal energy storage system. An insulated packed bed (flint stone bed) was used as the heat storage medium. So, the parameters affecting the performance of solar drier and packed bed thermal energy storage system were obtained experimentally. Furthermore 50 to 60% of the waste energy was recovered by using a recuperative system. Apple(golden delicious) was dried in this study. Drying behaviour was investigated for six different apple slices(between 5-10mm sliced rings) Measurements of temperature, relative humidity, air speed and pressure into system were recorded with datalogger. The values of the mass of the apple slices were measured every 30 minutes during experiments. During experimental studies, drying times were determined for between 550C- 630C temperature values as a 6 hours. When operating at full capacity of the system, maximum relative humidity difference was found %16, maximum air speed difference was found 1,05 m/s between entry and exit of drying cabin . However, the value of the SMER(Specific Moisture Extraction Rate) was determined 2.502 kgH2O/kWh and the value of the MER(Moisture Extraction Rate) was determined 4.14 kgH2O/h. Some of the test specimens was transmitted Ege University Food Engineering Department for testing of apple slices in color, fiber, taste, determination of moisture and water activity. In the result of the tests, dried apple slices, especially due to determination of moisture values below %10 and water activity values below 0.7, can be accepted dry matter and can be resisted deterioration for a long time. Also, it is observed that the drying is affected positively the quality of the product because the agricultural product dryer is worked between low moisture and temperature values. In this work, the solar energy assisted drying system was compared with electrical drying system which has a similar capacity in energy consumption and initial investment and operation costs. Electrical drying system consumes 35 kWh of energy per day. The agricultural product drying system, due to fans and recuperative unit only, consumes 8.12kWh of energy per day. So, because of developing this system, by 76.8% from the electrical energy was saved up in drying process. In addition, when both the two systems are also operated at similar capacity, the agricultural product drying system can be amortized within 6 months but electrical drying system can be amortized within 12 months.