تجزیه و تحلیل عملکرد و ارزیابی اقتصادی حرارتی ذخیره سازی فله ای برنج

Performance and economic analyses of a prototype paddy bulk storage with R22 thermosyphon for self-heat rejecting have been studied. The unit has a steel cylindrical bin with diameter of 1250 mm and length of 1500 mm that contains 1000 kg of paddy. The evaporation section of the thermosyphon embedded in the paddy bulk is a set of copper tubes with the total heat transfer area of 8.5 m2. The condenser section with the total area of 12.2 m2 is exposed to the ambient air. The analyses have been compared with the unit having a conventional aeration. The experiment shows that there is a high potential for using thermosyphon to control the paddy bed temperature. It could be found that for the paddy with moisture content of 26.9% and 13.5% wet basis, the thermosyphon can maintain the paddy bed temperature at 37–38 °C and 28–29 °C, respectively compared with 62 °C and 31–32 °C for the unit without any control. Moreover very small temperature difference in the bed is also observed in the unit with thermosyphon. The paddy quality in term of head rice yield (38.06%), percentage of brown (72.7%) and white rice (60.14%) for the unit with thermosyphon is very close to those (40.16%, 72.37%, 60.43%) of the unit with aeration. The mathematical model developed could be used to predict the paddy bed temperature accurately and the evaporator area should not be less than 16 m2 for 1000 kg of paddy (Condenser area of 12.2 m2) at Chiang Mai, Thailand. The economic analysis indicates that the payback period of the storage with 16 m2 evaporator area is shorter and higher IRR obtained with the percent of annual fan operation. The payback is about 8 years when 20% of annual fan operation is taken.

In Thailand, deterioration of paddy during storage due to heat liberated from respiration process is normally controlled by ventilating cooled air or ambient air throughout the bed (aeration process) to reduce the bed temperature. However, vapor recondensation on the cold surfaces of the paddy or walls of the storage might occur which transfers moisture back to the paddy. Moreover, the fan runs for a very long period that results in high electrical energy consumption. Maier et al. [1] studied ambient and chilled paddy aeration under Thai conditions. The study showed that continuous ventilation of cooled air decreased paddy temperature down to 15 °C in 110–144 h. While continuous ventilation with ambient air could not decrease paddy temperature down to 25 °C neither in summer nor rainy season. Intermittent ventilation with ambient relative humidity control (<75%RH), the paddy temperature was higher than 26 °C. The simulation had been carried out incase of aeration operation, it was found that the fan should operate 1650–1836 h in dry seasons (November–February), and 1062–1680 h in wet season (July–October). The total operating time is about 30–40% of a year. The energy consumption was 38–64% lower than the continuous ventilation. In Thailand, one-ton thermosyphon paddy bulk storage could serve for Thai family scale in rural area which has no electrical grid. Moreover, small scale could use to store seed paddy and other grain crops. Thermosyphon heat pipe is a simple device of high thermal conductance or low thermal resistance and could be used as a heat exchanger for recovering or extracting heat in thermal processes. Lee et al. [2] reported the result of using a thermosyphon to remove heat of hydration during curing process of a concrete structure. Kiatsiriroat et al. [3] used a set of thermosyphon for extracting heat from ground to warm up water during winter in Chiang Mai, Thailand. Fukada et al. [4] also used a set of thermosyphon tubes for extracting heat from ground in winter and the coolness was used for a potato storage room in summer. The storeroom temperature was kept at 3–4 °C. Utilization of thermosyphon heat pipe to extract heat accumulated in paddy bed is another choice to reduce the bed temperature without external electrical energy consumption. Kiatsiriroat and Dussadee [5] used a set of a thermosyphon to reduce heat of respiration in a small paddy storage. The bed temperature could be reduced uniformly from 31–32 to 26–27 °C within 20 h. The uniform temperature in the paddy bed could also be found and high paddy quality was obtained. Dussadee and Kiasiriroat [6] also used this technique to keep paddy of high moisture content before drying. It could be found that the paddy could be stored at least of 10 days without any deterioration. The paddy quality in term of the head rice yield and the whiteness of the milled rice were satisfied. In this paper, the performance analysis of a one-ton prototype of thermosyphon paddy storage and its economic evaluation have been investigated. The results will be compared with those of the storage with aeration unit.

From this work, it could be concluded that: 1. There is a high potential for using thermosyphon to control paddy bed temperature. It was found that for the paddy moisture content of 26.9% and 13.5% wet basis, the thermosyphon can maintain paddy bed temperature at 37–38 °C and 28–29 °C, respectively compared with 62 °C and 31–32 °C in case of the unit without any control. Moreover, small temperature difference between different bed depths is also observed in the unit with thermosyphon. 2. The average reduced temperature ratio of the thermosyphon unit is higher with the higher moisture content in paddy bed. 3. The paddy quality in term of head rice yield (38.06%), percentage of brown (72.7%) and white rice (60.14%) of the unit with thermosyphon is very close to those (40.16%, 72.37%, 60.43%) of the unit with aeration. 4. The mathematical model developed could be used to predict the paddy bed temperature accurately and the evaporator area should be over 16 m2 for Chiang Mai, Thailand in case of 1000 kg of paddy storage to maintain the bed temperature lower than 28 °C. 5. The minimum evaporator area of thermosyphon paddy bulk storage should be 16 m2. 6. The payback period and the IRR of the thermosyphon paddy bulk storage is 8.2 years and 8.6%, respectively, when it is compared with the unit with 1/8 hp aeration unit and the fraction of annual fan operating time is 20%. Higher return is obtained when more fan operating time is used.