EXERGY ANALYSIS OF SOLAR DRYER WITH A BACKUP INCINERATOR

Solar dryer with backup incinerator was fabricated with the aim of improving the efficiency of the drying rate of selected agricultural products. The dryer consist of three main parts, the collector, the drying chamber and the incinerator. 1000g of chill pepper was sun dried and 1000g was charged into the dryer for the experiment. Drying using solar drying process was carried out during clear weather while incinerator drying process was carried out during cloudy weather and at nights The collector, dryer and incinerator energy efficiencies were determined and reported elsewhere. Exergy analysis of the dryer was carried out for both solar drying and incinerator drying using the experimental values. The average exergy inflow and outflow during solar drying was found to be 266.97 KJ/Kg and 20.85 KJ/Kg respectively. The average exergy loss at airflow velocity of 2.7 m/s was found to be 269.3 KJ/Kg for incinerator drying. The exergy efficiency of the incinerator fluctuates as it starts from 7.9, 11.1, 5.2, 13.5, 8.0 and 3.6 % for 8.00, 10.00, 12.00, 14.00, 16.00, 18.00 hrs respectively. The result also shows exergy efficiency of 83.1, 85.9, 91.7, 92.4, 89.0 and 73.4 % for 8.00, 10.00, 12.00, 14.00, 16.00, 18.00 hrs respectively during solar drying. The experimental and analytical temperatures values were observed to be solar radiation intensity dependants and are directly proportional with it. Although the heat losses are high for both drying processes, the dryer is suitable for drying agricultural produce during clear, cloudy weather and at nights.


Introduction
Sun drying of agricultural crops in the open air has been in practice over decades. Solar energy has gained acceptance as an alternative source of energy for global utilization. Several attempts have been made to improve the quality of the drying products [1][2]. Most of our crops and grains harvested during raining season, preservation by sun drying proves difficult. These result in wastage of agricultural produce due to unavailability of sunshine.
In order to optimize the efficiency of the system, minimize losses, and reduce the operational and capital investment cost and to improve productivity of the thermal system, energy and exergy analysis of the thermal system need to be investigated [3]. The energy analysis of the dryer was carried out and presented elsewhere [4][5][6][7]. The results of energy analysis can indicate the main The exergy associated with an energy quantity is a quantitative assessment of its usefulness or quality. Exergy analysis acknowledges that, although energy cannot be created or destroyed, it can be degraded in quality, eventually reaching a state in which it is in complete equilibrium with the surroundings and hence of no further use for performing tasks. For energy storage systems, for example, exergy analysis allows one to determine the maximum potential associated with the incoming energy [10]. This paper therefore presents the exergy analysis of a solar dryer with backup incinerator for drying agricultural produce during clear weather, cloudy weather and at night with the aim of identifying heat losses during the drying processes with a view of improving the efficiency of the drying rate of selected agricultural products.

Experimental Setup
Solar drying and incinerator drying processes were studied from the hybrid dryer. No-load test was carried out on the systems for a period of four months. The tests involved measuring the temperature of the air stream and the ambient temperature (Ta) using thermometers. A psychrometer was used to measure the dry and wet bulb temperature (Tdb and Twb) of the drying chambers. A psychometric chart was used to determine the ambient and exit relative humidities (RHa and RHd). The average velocity of air (Va) delivered into the drying chamber was measured using a cup anemometer. The biomass used in the incinerator (charcoal) was burnt and the heat conveying fluid (water) was allowed to flow by gravity. The initial and final temperatures of the fluid were measured and the temperature of the dryer was also measured using a thermometer.
On-load tests were also carried out using chilli pepper as sample. 2000g of the sample was shared into equal parts and charged into the dryers while the other was sun-dried as the control experiment. The dryer was run until the drying samples were fully dried to moisture content suitable for storage. For the solar drying the temperatures of the air stream (Ta), ambient and exit air relative humidity were measured and recorded. The wind velocity (Va) was also measured. For the incinerator drying, same experiment was repeated and the same parameters were measured. The incinerator drying was loaded in a shield / at night and the temperatures of the air stream, ambient and exit air relative humidity and the wind velocity measured to test the efficiency of the dryer. The control was tempered appropriately by sealing it in polythene bag at night to prevent it from rehydrating. Twelve batches of samples were dried to moisture content suitable for storage.
The no-load tests were carried out from 8.00am to 6.00pm. The rate of heat loss and thermal energy output were evaluated and used to compute the efficiency of the collector and dryer respectively. The initial moisture content of the selected farm produce was determined before charging into the dryer.
The exergy analysis of the solar collector and the drying chamber during solar drying and incinerator drying were respectively obtained from equations and used in this study to rate the effectiveness of the hybrid solar dyer with the aim of improving efficiency.

Exergy Analysis
Exergy values were calculated based on the characteristics of working medium from the first law of energy balance. The thermodynamic processes of the drying chamber are illustrated in figure 1. Where, 1) Input of drying air to the drying chamber to dry the products.
2) Input of moist products to be dried in the chamber.
3) Output of the moist air after containing the evaporated moisture removed from the products. 4) Output of the dried products, with moisture content reduced to the desired level.

Balances
Mass, energy and energy balances can be written for the above system, treated as a control volume.

Exergy Balance
The drying unit exergy balance for both solar dryer and incinerator dryer can be written as follows: ṁaex1 + ṁp(exp)2 +(ṁw)2(exw)2 = ṁaex3 + ṁp(exp)4 + (ṁw)4(exw)4 + Ếxq + Ếxd The exergy flow rate due to heat loss: Where, Tav is the average outer surface temperature of the dryer. Typical data for the reference environment are as follows: T0 = 320C, P0 = 1atm, ω0 = 0.0153 and xv0 = 0.024 (mole fraction of water vapour in air), (xv)3 = 0.055, (Cp)a = 1.004kJ/kg0c,(cp)v = 1.872kJ/kg0c, Ra = 0.287kJ/kg0c, Rv = 0.4615kJ/kg0c The results of fig.3 show exergy loss in the drying chamber. The exergy inflow and outflow are also solar dependents. The maximum exergy inflow and outflow during the drying of chilli pepper was found to be 700 KJ/Kg and 4.1 KJ/Kg respectively. Considering the exergy utilization during the drying process the average exergy inflow and outflow was 266.97 KJ/Kg and 20.85 KJ/Kg respectively. The exergy utilization also depends on the solar radiation and increase in ambient temperature the average exergy loss of the dryer was found to be 241.6 KJ/Kg. The average exergy loss of natural convection solar dryer is 255 KJ/Kg [11].

Conclusion
The exergy analysis of the hybrid solar dryer shows increase in exergy from 83,1% to 92.4% then a decrease to 73.4 %. The heat losses for both drying processes were high and the increase and decrease of solar dryer exergy loss and exergy efficiencies depend on timely solar radiation. The exergy efficiency of the incinerator depends on the biomass heat source and it fluctuates with respect to the loading and combustion of the biomass in the incinerator. The exergy efficiency of the incinerator drying process is low when compared to solar drying process. However, due to the humid climate condition and for rural areas the dryer can be used for drying of chilli pepper and other agricultural produce. The research suggest an improvement on the incinerator to incorporate an automated system for continuous loading of the biomass in the incinerator for maximum efficiency.