Design, Fabrication and Evaluation of Solar and Biomass Hybrid Dryer with Trackable Solar Collector

Newly constructed solar and biomass hybrid dryer consists of solar collector with solar tracking option, biomass backup heater and a drying chamber. The absorber of the trackable solar collector was made of black painted corrugated GI sheet and clear glass for the top. Saw dust was used as the fuel for the biomass burner. The drying chamber was built of plywood and insulated by aluminium foil. The performance was evaluated using five drying treatments with three replicates; namely, only with solar power (T 1 ), solar power with tracking (T 2 ), only with biomass (T 3 ), with hybrid mode (T 4 ), and direct sun drying (T 5 ; Control). Invented hybrid dryer reduced the initial moisture content of Musa spp. (banana), Artocarpus heterophyllus (jackfruit) and Pleurotus ostreatus (oyster mushroom) from 80%, 74% and 90% to 12%, 12% and 10%, respectively. Under no-load condition, the average temperature of drying chamber reached to 45.04 0 C with solar drying and 55.65 0 C with solar tracking option, while the ambient temperature was 33.52 0 C. The solar collector efficiency obtained from the no-load test was 34.07%. Results revealed that the dryer can reduce the high initial moisture contents up to a safe storage moisture content within a day. With the support of the backup heater, the dryer can be used even under the inadequate solar radiation. Improper drying chamber insulation was observed as a limitation.


Introduction
In Sri Lanka, agriculture represents a considerable part of the economy. Bischoff (1996)

Designing of the solar collector
At the initial stage, the dryer was designed only for the experimental purpose and expected to dry only 3 kg of fresh banana in a batch. The size of the solar collector was designed accordingly.

Determination of moisture content to be removed
The equation 1 proposed by Hernandez et al. (2000) was used to calculate the total amount of moisture to be removed. Where, MW -amount of moisture to be removed

WWinitial total mass
Mi -initial moisture content on wet basis Mf -final moisture content on wet basis In the drying process, the initial and the required final moisture content of dried banana considered for the study were 77% and 14% (wet basis), respectively (FAO 1997). Therefore, amount of moisture to be removed from 3 kg of banana was 2.19 kg.

Determination of amount of heat energy required for drying
The heat required to remove water from a product was calculated using the equation 2 provided by Mercer (2007 Where,

WW-initial weight
Cp -specific heat capacity of the product (kJ kg -1 C -1 ) Considering the heat losses as 15%, the area of the collector was decided as 1 m 2 . Forson et al. (2007) suggested the aspect ratio of a solar collector to be 1:3.
Therefore, the length and width of the collector were determined as 1.7 m and 0.6 m, respectively.

Collector orientation and tilt angle
Since the experimental location

Results and Discussion
The

Results of evaluation tests
The results of the no-load test carried out to find the actual variation of temperature in the drying chamber during the day time is shown in Fig. 2   Drying time was significantly lower when applying tracking option compared to solar drying without tracking (T1).

Moisture removing rates of different drying methods
The moisture removing rates for the different drying methods are presented in Fig. 7.
According to the Fig. 3, when solar is used with tracking and backup heater as hybrid mode the highest drying rates for banana, jackfruit and mushroom were 0.781 kg of water in 1 kg of product per hour, 0.109 kg of water in 1 kg of product per hour and 0.127 kg of water in 1 kg of product per hour, respectively.
According to the Ringeisen (2014) drying rate of a cabinet type solar dryer directly associated with the chamber temperature, humidity and the ventilation system that remove moisture air inside the dryer.
As shown in Fig. 4, the exponential trend line equations were derived. The time taken to obtain the final moisture content when dryer operate in hybrid mode with tracking option were calculated and presented in Table 3.

Efficiency of the biomass burner
The Heat losses from the air ventilation system and duct system has identified as the major reasons and suggested to insulate the side walls and the duct system of the air circulation system.