Performance of greenhouse effect-hybrid type tray dryer in drying cloves

The open sun drying method needs to be improved. This study aimed to evaluate the performance of the greenhouse effect hybrid (GHE-hybrid) type tray dryer using solar and biomass energy in drying cloves as a sustainable dryer for future. The dryer has 10 series of thin layers at gap of 10cm and each layer contains of 4 separated trays by 0.80m x 0.75m cross-section. We had run 2 experiments at capacity of 72 and 244kg, respectively. Results showed that increasing the drying capacity will prolong the drying time. The low solar irradiation will also increase the use of biomass energy and the input energy in the system was dominated by biomass energy at level of 75%. The drying temperature ranged from 37 to 46°C. The humidity was relatively high while the air velocity in drying chamber was lower than 0.68m/s and non-uniformly distributed. The moisture of cloves was reduced below 14% in both experiment after 3 and 6 days drying, respectively. The thermal efficiency was about 14 to 15% and the drying efficiency was 17%. The use of GHE-hybrid dryer is promising to be used by farmers sustainably. However, the dryer needs some improvement on air circulation system.


Introduction
Clove (Syzygium aromaticum (L) Merrill and Perry) is native to Indonesia (North Maluku) and is used as a spice in cuisine in many countries.The local name of cloves in Indonesia is cengkėh.It is one of the potential and valuable agricultural products for the country since Indonesia produces almost 80% of the world's cloves output.Now days, cloves are used to produce essential oil for its high antimicrobial activity against several pathogenic bacteria [1,2] and its anticancer potency [3].
Cloves need an appropriate drying process to preserve its quality and presently, farmers are sundrying this product.In this case, we had faced two unfortunate facts.Firstly, cloves are effective to grow in humid subtropical without a marked dry season with average annual rainfall about 2700mm and average temperature 24ºC; therefore, it experiences unstable, rainy condition [4].Second, nearly 90% of clove plantations are owned by small scale farmers so that they are not capable of buying a dryer that is commonly expensive.Thus, the use of dryer that relies on the sun and biomass as its energy is the best appropriate solution.
Solar dryers continue to struggle to gain acceptance not only by small scale farmers, but also by commercial producers of dried products [5].Greenhouse type solar dryer had succeed in drying 500kg chilli in Thailand in 3 days with completely protecting the products from insects, animals and rain [6].The GHE-dryer also integrated by the use of biomass stove and heat exchanger (HE) to provide stable energy under wet climate area [7].A novel hybrid active greenhouse solar dryer is developed in which a heat exchanger is kept inside the dryer that also acts as a drying bed [8].
Several dryers that used greenhouse effect (GHE) had been designed by combination of a solar collector and a drying chamber in one structure [9].They found that such a structure could reduce the cost by about 75% compared to a dryer with a separate collector, and this model needs less area.A unit of biomass stove and HE to improve the drying temperature constantly at 40ºC had been tested in drying seed of castor oil plant [10].They confirmed that the stove efficiency to HE was 75% and the efficiency of HE to drying chamber was 57.3%.Recently, the development of solar dryer has great potential [11] and the trend development of solar greenhouse dryer is by using integration of thermal storage material, solar air heater, incline reflective north wall, opaque insulated north wall, photovoltaic and so on to maximize the use of solar energy [12].
Previously, we had involved in the study of solar tunnel dryer [13][14][15][16], but this type of dryer is applied for a layer only.In fact, the users had expectation to use a dryer with higher drying capacity.Therefore, the study on solar dryer type tray dryer was important to conduct to contribute the development of solar dryer by increasing its capacity, decreasing drying time, labor and losses, and maintaining the quality of the products, as well as supporting the sustainable development goals.This type of dryer had been evaluated on sweet potatoes [17], red onion [18], chili [19], firewood [20], and banana [21].It is also important to consider that each agricultural product has different characteristics in the drying process.This study aimed to evaluate the performance of the greenhouse effect hybrid (GHE-hybrid) type tray dryer with the additional use of biomass energy in drying cloves for our sustainable future.

Materials
The cloves were bought from a retailer in Sukabumi (West Java).The cloves were placed in plastic sack for one night to allow the fermentation process after hand cleaning.About 72 and 144kg of cloves were dried in 2 separated experiments.The biomass used was the wood charcoal about 80kg obtained from local market.

Dryer's detail
The dryer evaluated was the GHE-hybrid that can be seen in Fig. 1.The dryer has 10 series of thin layers at gap of 10cm and each layer contains of 4 separated trays by 0.80 m x 0.75 m cross-section.Its structure sizes 3.0 x 1.6 x 1.3 m 3 covered by polycarbonate sheet at thickness of 3.2mm and it is built by using steel frame.The whole dryer is covered by transparent polycarbonate sheet functioning as the roof and wall.The absorber area is about 4.39m 2 made from black painted zinc 0.5mm and put above the trays.This hybrid design integrates the use of solar and biomass energy.
The dryer is also supported by 2 axial fans FA-50-y (P=550Watt, V=220volt, f=50 Hz, Ф=60 cm, and Rpm=1400) to generate air into a drying chamber.There are 2 air inlets and 1 air outlet; the outlet 1 and inlet 1 measured 40x20 cm 2 while the inlet 2 measured 40x30 cm 2 .The dryer is backed-up by a biomass stove measuring 150 cm x 52 cm x 26 cm equipped with 99 tubes (Ф=1 inch) as heat exchanger (HE).

Instrumentation and measurements
The solar irradiance was recorded using a pyranometer, type MS-601 Eko-Japan; at interval of 30 minutes starting from 8.00 am to 15.00 pm.The pyranometer was connected to a digital multi-meter, APPA 101 made in Taiwan.Data were recorded in milli-volt and changed to Wm -2 using factor sensitivity of pyranometer at 6.9 µvolt per Wm -2 .A numerical integral (Simpson's rule) was used to calculate the total solar irradiance because the data is available in discrete (Eq.1).Research's parameters were measured at central point of each tray (cross section 0.8 x 0.75 m 2 ), initiated by A, B, C, and D. Since each layer contained 4 separated trays and the dryer had 10 layers, there were about 40 observation points.The layer was accounted from the lowest layer as L1, to the highest layer as L10.The instrument used to measure the air velocity was an anemometer Model A-541 (Kanomax USA, Inc) with an accuracy of ±2%.Additionally, the measurement of air velocity was also done at air inlets and air outlet.Air volumetric is defined as the amount of air that passes a cross section area, in unit of m 3 s -1 [5].To assess the uniformity of drying process, a block of sample was placed at central point of each tray so there were 40 block samples and their wight were measured by using digital balance (EK-1200A) with an accuracy ± 0.1g.In order to measure temperatures distribution in drying chamber, 10 thermometers were used by putting them at the central point of each layer.All thermometers were calibrated using an electric oil batch (Seiwa, OSK).
The data were used to calculate the moisture content in samples and drying rate.The moisture content was calculated in two bases, namely, the moisture content dry basis (MC db ) and the moisture content wet basis (MC wb ) [23].The drying rate was expressed as Equation 5 [17,24,25], where R is the drying rate (kg water/kg solid x h), dMC db is the changes of moisture dry basis (kg water/kg solid), and dt is the interval of drying time measurement (h).
The charcoal was also weighed so the combustion rate could be calculated.The calorific value of charcoal was measured by using an adiabatic bomb calorimeter (made in Japan, OSK, 1984).The calorific value of biomass was expressed by [26] as Eq.6, where C b is calorific value of biomass (MJ kg -1 ), Δt is the increase of temperature in a jacket (°C), W w is the mass of water (2100 g) and E w is the equivalent value of water (592.5 g).
The performance evaluation of dryer was calculated by using Eq 7-15.The mass of evaporated water was calculated by using Eq. 12 [24], where I R is the solar irradiation (Wm -2 ), A ab is the absorber area (m 2 ), ατ) z is the absorptivity black-painted surface, 0.95 [27], t is the drying time (h), P is the electric power input (W), m b is the combustion rate of biomass (kg h -1 ), c b is the calorific value of biomass (MJ kg -1 ), m c is the mass of the products (kg), C pc is the heat capacity of the product (kJ kg -1 0 C -1 ) computing by using equation from Siebel (1982) in [23] as c pc = 0.837 + 3.349 X w , X w is the water content expressed as a fraction, ΔT is the increased temperature ( 0 C), m w is the mass of evaporated moisture (kg), L is the latent heat evaporation of water (kJ kg -1 ), m d is the dry weight pf the product, MCd bi is the initial moisture content dry basis (%), and MC dbf is the final moisture content dry basis (%).All equations used were listed in Table 1.
The initial moisture content of wet cloves and the oil content of dried cloves were analysed by using the Bidwell-Sterling distillation method.The colour performance was also analysed by using RHS-minicolor-chart made by The Royal Horticultural Society.The contamination was analysed by using a simple method by selecting the cloves from its contaminant, weighing, and calculating the percentage of contamination.

The important use of biomass energy
In a preliminary study, we had conducted the study of temperature in the drying chamber with unloading capacity at 2 conditions i.e., by using solar irradiation only and by the additional of biomass energy.The use of biomass energy in addition to the solar irradiation could raise the temperature of drying chamber about 6.7°C.If it is compared to the ambient temperature, without the use of biomass energy, the temperature of drying chamber could be increased about 2.1°C, while by the use of biomass energy the temperature of drying chamber could be improved about 9.2°C.According to these results, the use of biomass energy in this type of dryer is necessary.
The first experiment at loading capacity of 72kg of cloves was conducted for 3days (24hrs) while the second experiment at loading capacity of 144kg was run for 6days (42hrs).The calorific value for wood charcoal was 18.72 MJ kg -1 .It can be seen that the increased use of biomass energy was caused by the low level of solar incident availability.The solar irradiation during the first experiment was higher than the second experiment.Almost all days in the second experiment ended with rain.Therefore, as can be seen in Fig. 2b, the higher the solar irradiation the lower the use of charcoal vice versa.The total wood charcoal used on the first experiment was 19.9kg while the total wood charcoal used on the second experiment was 66.6kg.Thus, the combustion rate is determined about 0.8 and 1.6 kg.h -1 , respectively.

Drying temperature
Trends of temperatures were relatively influenced by solar irradiance (Fig. 2a).The experiment was conducted during the beginning of the rainy season as well as the location of the study was in Bogor known as rainy city.On the first experiment, the total solar irradiation gained was relatively higher than that of on the second experiment.
Figure 3 shows also about the temperature distribution in the drying chamber during across the trays.The uniformity of temperature distribution at each layer was a very important point of view to result the good quality of cloves.The deviation temperatures among the trays were about 1.1 to 1.2°C.The layer number 1 to 3 have high temperature since the position of these layers close to the biomass stove while the layer number 6 to 10 have high temperature as the position of these layers near by the solar collector.
However, the interesting phenomenon happened on tray with layer number 5. At this point, the lowest temperature was obtained during the first experiment, but the highest temperature was observed during the second experiment.It shows that the temperature at this layer was stable in both conditions.

Airflow rate
The air velocity from first air inlet was 1.41 ± 0.20 ms -1 , while based on the spec of fans used, the fan used at the second air inlet could generate the flow around 43.96 ms -1 (velocity = π x diameter x RPM).The air velocity in air outlet was 5.02 ± 0.28 ms -1 .It was found that the air velocity tended to be low in average of 0.68 ms -1 .The reduction of air velocity in the drying chamber was caused by the construction of HE by using 99 pipes.
The results had shown a high variation of air velocities occurred during the experiment ranging from 0.04 to 2.09 ms -1 .It was estimated that these differences were caused by both the distance of observation point to the fans as well as the elevation.Normally the points located closer to the fans higher air velocities than points located far from the fans.In contrast, inverse conditions were also found at several points, i.e., air velocity in point L4-B (0.43 ± 0.13 ms -1 ) was higher than that of at point L4-A (0.24 ± 0.06 ms -1 ).According to tray layers, low air velocities were found in L1, L2, L5, L6, L9, and L10.Thus, the lower and upper part of drying chamber had slow air velocities.Moreover, little or no flow was detected in L1 and L10, namely at point L1-B (0.09 ± 0.05ms -1 ), L1-D (0.08 ± 0.03ms -1 ), L10-A (0.04 ± 0.03ms - 1 ), and L10-C (0.09 ± 0.02ms -1 ).Possibly, the areas were not in front of the fans.In addition, more than 50% of observed points had air velocities below average of 0.68 ms -1 .From these measurements, the average air volumetric of airflow through the dryer at cross section 0.80 x 0.75 m 2 was 0.411 m 3 s -1 .The 1 st air inlet sizing 0.12 m 2 produced air volumetric of 0.169m 3 s -1 , the 2 nd air inlet measuring 0.08 m 2 produced air volumetric of 3.51m 3 s -1 .However, the average air flowrate in the drying chamber was low at 0.411m 3 s -1 .Morad et al. [28] had suggested that the drying is best at air flowrate 2.1m 3 s -1 .Therefore, it is important to consider the design of pipes of HE in order to not reduce the airflow to much.Besides, the design of trays in gap of 10 cm also caused the ununiform airflow distribution.Fellows [29] stated that the increase of gap between trays can decrease the high variation of airflow distribution.In addition to this case, the use of air delivery channel can control the uniformity of airflow distribution [30].Improvement the airflow distribution is necessary for acceptance of solar dryer by users [31].

The humidity
The relative humidity in the drying chamber is shown in Fig. 4. It can be seen that the humidity on the first experiment was lower than the second experiment.The use of biomass energy during the rainy day was still hard to reduce the humidity in the drying chamber on the second experiment as it was occurred in the dryer operation during the night.Important to address that the evidence of this differences would be a significant factor to influence the product quality.For this reason, the next design development is necessary to improve the dryer performance by producing the stable temperature and humidity needed for the drying process.Probably, the operating of fans during rain should be stopped and for this the design of automatic controlled fan's operation is significant.

The quality of cloves
Four parameters taken into account to analyse the quality of cloves were final moisture, oil content, contamination, and colour as can be seen in Table 2.The results of sun-drying method were used as comparison.It can be seen that the moisture content of cloves dried by using sun-drying method was higher than 14%.It is very dangerous since the cloves could be harmed by the growth of microorganism as well as enzyme activity.These data showed that it is necessary to prolong the drying time in order to reduce the moisture until safe storage level.
Oil content of cloves ranged from 16.87 to 19.87.It seems that the method of drying does not influence the oil content.A low contamination was also identified at all samples since the sun-drying method was done on the ground fully covered by grass.However, the contamination of cloves under commonly sun-drying method applied by farmers should be higher as they will conduct the sun-drying method near by the street or on the ground without grass.Last but not least, regarding the color, cloves performed a completely dark-brown colour.Further study on volatile components of cloves dried under these different methods is recommended.
The other benefit of using the dryer was the increase capacity about 10 times and the better working efficiency since during the night the cloves could be left inside the dryer that reduce the energy for loading and unloading process.However, a problem of un-uniformity final moisture had been determined.For example, there was a high variation of moisture, either among tray layers or across tray layers that can be seen from high standard deviation values.In order to improve this condition, the airflow in the drying chamber should be increased to the level of 2 m/s at uniform speed at all cross section of the trays.

Energy and Efficiency
The input energy for this dryer system included the energy from solar irradiance, electricity, and biomass.The solar energy was influenced by the solar irradiance and absorber area, while the input energy from using fans (electricity) was constant at level 27.72 MJ per day.The input energy from biomass was influenced by the combustion rate, calorific value of charcoal used, and also the time.The calorific value of charcoal used was 18.72 MJ kg -1 .The energy used in drying process consisted of energy needed to heat the products and energy needed to remove the water from the product surface.The first energy depends on the heat capacity of the products, mass of the products to be dried, and the gradient temperature performed during the process.Using equation from Siebel (1982), the heat capacity of cloves was found in range of 1.53 to 3.22 kJ kg - 1 0 C -1 .
Generally, energy input was dominated by the use of biomass energy.However, on clear day with high solar intensity, the use of biomass energy could be reduced.Solar energy shared about 34% on clear day and about 15% on rainy day.The electricity shared approximately from10-12%.The electricity is needed for fans since cloves has high moisture.It is necessary to use forced convection in drying high moisture content of crops [12].However, it is important to improve the dryer design by using solar panel to produce electricity for fans operation.
According to the initial moisture, the mass of evaporated water in the first and the second experiment were 48.95 and 99.01kg, respectively, then the total energy used to evaporate water was about 113.87MJ and 230.07MJ, respectively.Therefore, the thermal efficiency was found to about 15% in the first experiment and 14% in the second experiment, while the drying efficiency was 17% in both experiments.
The energy specific needed was 14.1MJ per kg of evaporated water in the first experiment and 16.8MJ per kg of evaporated water in the second experiment.It shows as that the drying capacity and the composition of energy inputs will influence the energy specific needed.In addition, Buschbeck et al. (1967) in [30] stated that high initial moisture content and low drying air temperature cause a comparatively high specific energy, for example it could be about 10 MJ of one kg removed water for the peppermint.They also found that the specific heat energy for Mentha piperita and Hypericum perforatum are 4.84 MJ and 7.694 MJ per kg of removed water, respectively.It can be concluded that the types of products to be dried would also influence the energy specific needed.

Conclusion
The GHE-hybrid dryer as a sustainable dryer could be used for drying cloves at good quality, shorter drying time and higher capacity.The input energy was dominated by biomass energy at level of 75%.Moreover, the incident of low solar irradiation would increase the biomass consumption to about triple.The other benefit is that the dryer can be used during rainy day or at night.However, the humidity in the drying chamber was relatively high and the airflow was relatively low, so that the dryer needs some modification to improve its airflow distribution.

Figure 2 .Figure 3 .
Figure 3. Temperature distribution in drying chamber during the first and second experiment; Mean values of 3 days observation on the first experiment and mean values of 6 days observation on the second experiment

Figure 4 .
Temperature and relative humidity distribution in drying chamber during the first (a) and second experiment (b); Mean values of 3 days observation on the first experiment and mean values of 6 days observation on the second experiment

Table 1 .
The equations used for analysis

Table 2 .
The quality of cloves