Development and performance evaluation of solar greenhouse dryer with desiccant energy storage system for tomatoes
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Date
2021
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Sokoine University of Agriculture
Abstract
Tomato is one of the most important horticultural crops widely grown in the tropical
East Africa countries. It is mostly used as vegetable recipe for food preparation at most
homes or consumed raw as a salad. However, during the rainy season, tomato farmers
experience widespread post-harvest losses due to insect and molds infestation. Also,
during harvesting seasons, most markets in East Africa are flooded with the produce
leading to over- supply against low demand resulting to heavy postharvest losses.
Therefore, it is necessary to use appropriate drying technologies especially solar drying
technology to reduce these losses. The use of solar drying technology is a good
alternative solution to the problem of crop drying and especially the perishable tomato
crop. Literature review show that most solar crop drying technologies developed for
the past 50 years have very small loading capacity and cannot operate during the night.
Therefore, in this study, we developed an integrated greenhouse solar dryer with Clay-
CaCl 2 solid desiccant energy storage system. Solar greenhouse drying systems have an
advantage over other solar drying systems because its structural simplicity combined
with high loading capacity. In addition, they have relatively good thermal crop drying
performance compared to most solar dryers. The system was tested under no-load and
load conditions. The experimental study with no-load condition exhibited the mean
collector temperature of 41.9 °C giving an average temperature rise of 14.7 °C (35%)
above the ambient (27.2 °C) with an average R.H. value of 32.6% at the flow rate of
0.28 m 3 /s on the test date. When the desiccant energy storage was used during night an
average greenhouse temperature recorded within the drying chamber was 26.5 °C
higher than the ambient temperature of 15.9 °C (40 % temperature rise). The results
obtained under desiccant energy storage showed that at a 0.07 m 3 /s air flow rate with
an average rise in temperature of about 13.6 (32.3%) against the average ambient
temperature of 28.5°C. The average relative humidity within the system was found to
be 36.5% lower than the ambient R.H. (84.1%). The collector efficiencies obtained
from no load test was 46.2% and 40.8% for the dryer and desiccant chamber
respectively. The performance of the dryer was evaluated with fresh tomato load during
the month of September - December 2019 at Kenyatta University field site. The dryer
demonstrated capacity to dry fresh tomatoes from 93.9% (wb) to 8.3% (wb) within 27
hours with solar greenhouse drying efficiency of 23% while at night the dryer
demonstrated desiccant drying efficient of 19.9%. The drying rate for the two-day solar
drying was 0.985 kg/h and 0.875 kg/h respectively and that in night drying using
desiccants was 0.34 kg/h. The economic analysis of the drying system shows a payback
period of less than a year (0.54 year) with benefit-cost ratio of 8.4 implying that the
system is economically viable. On the basis of these results, it was concluded that
prototype solar greenhouse dryer with Clay-CaCl 2 energy storage system has great
potential for tomato drying and other high moisture agricultural products in East
African countries.
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Keywords
Solar, Greenhouse, Dryer, Desiccant energy, Tomatoes