3.3 Irrigation and processing of tropical fruits
Performance evaluation of a greenhouse solar dryer for
drying longan fruits
Serm Janjaia, Poolsak Intaweea, Busarakorn Mahayotheeb, Pittaya Sruamsiric,
Ekkapob Khajornpaisana, Anusorn Sangjaroena, Yutthasak Boonroda, Bilash Kanti
Balad, Marcelo Precoppee, Marcus Naglee, Joachim Müllere
Faculty of Science, Silpakorn University, Nakhon Pathom, Thailand
Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom, Thailand
Faculty of Agriculture, Chiang Mai University, Chiang Mai, Thailand
Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh,
Institute of Agricultural Engineering, University of Hohenheim, Stuttgart, Germany
Longan (Dimocarpus longan Lour.) The objective of this research work is to
is an important fruit of Thailand with the evaluate the performance of a greenhouse
annual productions of about 500,000 tons. solar dryer for drying longan fruits. This
About 43% of this production is processed dryer has a parabolic shape and it is covered
to be dried longan. More than 20,000 with polycarbonate sheets (Figure 1). The
batch-type static-bed dryers using liquefied base of the dryer is a black concrete floor
petroleum gas (LPG) as heat source are with an area of 8 x 20 m2. The products to
now in operation for production of dried be dried were placed in a thin layer on four
longan fruits in Thailand. In recent years, arrays of trays. These arrays of trays were
the cost of drying longan has increased placed on single level raised platforms with
significantly due to the increase of the price passages between the arrays for loading
of LPG. Thailand is located in the tropics, and unloading the products. Nine DC fans
and there is abundant of solar energy with powered by three 50-W solar cell modules
the annual average daily solar radiation of were used to ventilate the dryer. To ensure
17.4 MJ/m2-day (Janjai et al., 2010). Thus, a continuous drying during night time,
the use of solar drying technology for drying cloudy or raining periods, a 48-kW LPG
longan is an interesting alternative. burner was incorporated into the dryer.
Figure 1. Schematic diagram of the greenhouse solar dryer and positions of the measurement
sensors. T = Temperature, M = Moisture content and It = solar radiation
International Conference “Sustainable Land Use and Rural Development in Mountain Areas” 149
Hohenheim, Stuttgart, Germany, 16-18 April 2012
The dryer was installed at the experimental comparable to that of longan fruits dried
site of the Physics Department, Silpakorn in the batch-type static bed dryer (Janjai
University, Nakhon Pathom in Thailand. et al., 2006). The colour of dried longan
A total of three drying experiments were fruits were measured with a chromometer.
conducted in September, 2011. It was found that the colour of the dried
For each drying experiment, 1000 kg longan fruits were comparable to good
of longan fruits was used. The experiments quality dried longan in markets. The LPG
were started at 8.00 am. During night time, used for drying one batch of longan fruits
cloudy and raining periods, LPG burner varied between 80-100 kg depending on
was operated to supply supplementary weather conditions. The consumption of
heat to the dryer. During drying, product LPG was very low as compared to the LPG
samples were placed in the dryer at various consumption of the static-bed dryer for
positions (Figure 1) and were weighed drying longan fruits (Janjai et al., 2006).
periodically at 3-h intervals. To compare In conclusion, the performance of a
the performance of the dryer with that of greenhouse solar dryer has been evaluated.
natural sun drying, control samples of It was found that the drying air tempera-
longan fruits were spread on a tray near the ture varied in an acceptable range and good
dryer and dried simultaneously under the quality dried longan fruits were obtained.
same weather condition. Additionally, due to the utilization of
The typical results are as follows. Air solar energy, this dryer allow users to save
temperatures at different positions in the significantly LPG consumption for drying
dryer varied within a narrow band (Figure longan.
2a). During day time the drying air
temperature varied between 43 –
75 ºC while at night the drying air
temperatures were 45-50 ºC.
This temperature range was
acceptable for drying longan fruits
(Janjai et al., 2006). Figure 2b
reveals that the moisture contents
of longan fruits decreased more
rapidly, as compared to those
dried with the natural sun drying.
One thousand kilograms of longan
fruits were dried to reach the final
moisture content with 4 days.
The drying rate of longan fruits in
Figure 2. The variation of the temperature (a) and mois-
this greenhouse solar dryer was ture content (b) during a typical experiment
Janjai, S., Intawee, P., Chaichoet C., Mahayothee, B., Haewsungcharern M., Muller, J. 2006.
Improvement of the air and temperature distribution in a conventional longan dryer.
International symposium towards sustainable livelihoods and ecosystems in mountainous
regions. Chiang Mai, Thailand. 7-9 March, 2006
Janjai, S., Lasanaboonsong, J., Masiri, I., Soonklang, T., et al. 2010. Solar radiation maps from
satellite data for Thailand, A research report. Department of Alternative Energy Development
and Efficiency, Bangkok, Thailand