THERMO HYDRAULIC PERFORMANCE OF A FORCED CONVECTION SOLAR AIR HEATER USING PIN-FIN ABSORBER SURFACE

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1 HERMO HYDRAULIC PERFORMANCE OF A FORCED CONVECION SOLAR AIR HEAER USING PIN-FIN ABSORBER SURFACE Balaji.S 1, Saravanakumar.K, Sakthivel.M 3, Siva Kumar.P 4 1 Assistant Professor, 4 PG Scholar, Department of Mechanical Engeerg, Kumaraguru College of echnology (India) Assistant Professor, Department of Mechanical Engeerg, Sri Eshwar College of Engeerg (India) 3 Professor, Department of Mechanical Engeerg, Info Institute of Engeerg (India) ABSRAC he thermo-hydraulic performance of a forced convection solar air heater usg p-f absorber surface was vestigated under the meteorological conditions of Coimbatore city India. he performance was evaluated durg the month of January 015. he contact area of the absorber plate was creased usg p-fs. periments were carried at optimized air flow rate of 0.05 kg/sec. he energy performance parameters such as, thermal energy absorbed, thermal efficiency of the solar air heater, thermal-hydraulic efficiency of the air heater was evaluated. emperature of air at the collector let was varied between 35 0 C and 65 0 C with an average value of 48 0 C. he thermal efficiency of the solar air heater was ab 7% and enhanced optimum efficiency with P-fs absorbed plate. Keywords: Energy Efficiency, P-Fs, Solar Air Heater, hermo-hydraulic Efficiency I. INRODUCION Solar air heaters are the special type of heat exchangers used dryg of agricultural materials, space heatg and desalation (Kalogirou, 004). Solar air heaters maly consist of a thermally sulated absorber plate coated with black pat, one or more glazg plate placed over the absorber by matag 5-30 mm gap for air circulation, fan or blower ( the case of forced convection solar air heater (FCSAH)) for circulatg the air through the passage and sulated divergent duct at entry and convergent duct at exit. One end of the air heater is connected with the blower and the other end connects to specific applications. he major drawback with conventional solar air heaters is the poor heat transfer between the absorber plate and air due to poor thermodynamic properties of air, which results poor energy conversion efficiency. Many researchers improved the energy conversion efficiency of a FCSAH usg recyclable alumum cans (Alvarez et al. 004), obstacles (Esen, 008), roughened surface absorber plates (Bhushan and Sgh, 010), corrugated absorber plates (anda,, 011), wire mesh (El-khawajah et al., 011), fs and baffles (Ho et al., 01) and. A summary of vestigations on FCSAH usg extended surface absorber plate was also consolidated a review work 360 P a g e

2 reported by Alam et al. (014). However, the extended surfaces reported the literature have more pressure drop and more aerodynamic resistance due to the presence of sharp edges (Alam et al. 014). o overcome these drawbacks, p-fs were troduced over the absorber plate to reduce the aerodynamic resistance (Nwosu, 010). he cited literatures confirmed that no major work has been reported with solar air heaters usg p-f absorber plates. Hence, the ma objective of this work is to vestigate the energy performance of a FCSAH by usg p-f absorber plate. II. MAERIALS AND MEHODS perimental observations a FCSAH under the meteorological conditions of Coimbatore (latitude of o N and longitude of o E) were made durg January 015 to March 015. Figure 1 Schematic view of experimental setup.1 Solar Air Heater Set Up A photographic view is shown Fig. 1. he FCSAH consists of a detachable absorber plate made up of 1.mm thick mild steel plate attached to alumum p-fs of diameter 18 mm and length 4mm. he absorber surface area exposed for trackg of solar radiation is m (with dimensions m 1m). Eighty p-fs were provided over the absorber surface to enhance the exit air temperature. A 5mm thick tempered glass was placed above the absorber plate to crease the temperature of the air by the greenhouse effect. A 5mm gap was provided between the glass and the absorber surface for air circulation. A cross flow fan with 180W power capacity was fitted at the entry of a FCSAH to force the air through the gap between absorber plate surface and glazg surface. Both sides of the air heater were connected with the help of convergent and divergent air ducts. One side of the air heater was connected to the blower and the other side of the air heater was connected with an orifice-meter and manometer setup. he solar air heater was tilted to an angle ab 0 o with respect to the horizontal (Shariah et al. 00). he system was oriented towards face sh to maximize the solar radiation cidence on the collector.. Instrumentation Six calibrated thermocouples with ±0.5 o C accuracy were fixed at different locations (as shown Fig. 1) of a FCSAH to measure the temperatures at different locations the absorber plate. Ambient wd velocity was measured with the help of cup type anemometer havg ± 0.m/s. Solar tensity was measured by usg solar 361 P a g e

3 tensity meter with an accuracy of ab ±10W/m. Ambient relative humidity was measured with the help of slg psychrometer (two thermometers, one covered with wetted wick and another with bare bulb) for measurg wet and dry bulb temperatures. Based on the measured dry and wet bulb temperatures, relative humidity was measured by usg psychrometric chart. he stantaneous fan power required for runng the blower was measured by Wattmeter. A U tube manometer was attached the exit air flow path of a solar air heater to measure the air flow rate passg through the solar air heater..3 perimental Procedure he experiments a FCSAH were carried as per the ASHRAE standard. he air blower was switched ON and air flow rate through the flat plate solar air heater was adjusted to 0.05 kg/s with the help of the control valve (Hegazy, 1999). Before startg experimental observations, the FCSAH was warmed up one hour to avoid itial transients. Durg experiments, solar tensity and temperature at typical locations (shown Fig. 1) were measured by usg solar tensity meter and thermocouples, respectively. Similarly the wd velocity and ambient relative humidity were observed by usg a cup type anemometer and slg pychrometer, respectively. All the experimental observations were made at one-hour terval between 9.00 am to 5.00 pm. periments were conducted for ten days contuously and the average value was considered for discussion. III. HERMODYNAMIC ANALYSIS A thermodynamic analysis based first and second law of thermodynamics has been used to analyze the energy performance of a forced convection solar air heater workg with p-fs. For steady state flow processes, three equations such as mass, energy balance equations were used to fd the amount of heat put. 3.1 Energy analysis he mass balance equation can be expressed the form as m m If the effects due to the ketic and potential energy changes are neglected, the general energy can be expressed the form as given below: EX E E (3.) EX EX dest (3.3) he transmitted solar radiation from the transparent glass cover is absorbed by the absorbg plate. he air flows above the absorber plate where it is heated along its path. In order to write the energy balance equations for the systems, the followg assumptions are made: (i) he heat transfer capacities of the glass cover, absorbg plate and sulation are negligible. (3.1) 36 P a g e

4 (ii) here is no temperature gradient across the thickness of the glass cover and the storage material has an average temperature (t) at a time t. his assumption may be achieved by keepg a small thickness of 100 mm storage material. (iii) he system is perfectly sulated and there is no air leakage from the collector. (iv) No stratification exists perpendicular to the air flow. (v) he absorptivity of air is negligible Heat put capacity he amount of heat extracted by a solar air heater is calculated by the followg relation: Q (1 (3.5) 3.1. hermal efficiency of a solar air heater he thermal performance of any type of solar thermal collector can be evaluated by an energy balance that determes the portion of the comg radiation delivered as useful energy to the workg fluid. hermal efficiency of the solar air heater was estimated usg the equation 4 as reported by (Shanmugam and Natarajan, 006). th abs where η th drier thermal efficiency, m a is the flow rate of air through the collector, o and i are the let and let air temperatures respectively. 3. ergy analysis ergy analysis provides a method to evaluate the maximum work extractable from a substance relative to ambient as reference state (dead state). ergy analysis has been widely used to optimize the thermal energy systems and also to identify the efficient components the system.. a ) qu m[( h h ) a ( S S )] s 3..1 General relations ergy is an expression for accountg the loss of available energy due to entropy generation the irreversible systems or processes. An exergy analysis has widely used design, performance evaluation and simulation of energy systems. he exergy destruction a system is determed by multiplyg the ambient temperature by the entropy crease. he exergy balance can be expressed as by the followg equation [63]. m c m c p dest ( A I c ( ) n ) 100 (i) physical exergy, (ii) ketic exergy, (iii) potential exergy p i (3.4) dest (3.5) (3.6) he total exergy of a system can be divided to four components namely 363 P a g e

5 (iv) chemical exergy PH a 1 qu W s m KN m P EX dest CH (3.7) Physical exergy is the major part of the exergy at different locations solar assisted heat pumps. It is defed as the work obtaed when the workg fluid is brought from the reference conditions to the ambient condition. Physical exergy is given by followg equation. phy F G X isi ) G( X ihi F [ i i 0 0 G i i F( X h X s )] Where h is the enthalpy, s is the entropy and X is the molar ratio of each component, F refers to the liquid phase, where as G refers to the vapour phase. he other exergy terms were neglected this work. he assumptions made this study are: Steady flow operation. he effects of potential and ketic energy are negligible. here is no chemical or nuclear reaction. Air is treated as an ideal gas with a constant specific heat and its humidity content is ignored. he directions of heat transfer to the system and work transfer from the system are positive. he general exergy balance can be expressed as follows: (3.9) (3.8) Where, Ψ and Ψ are specific exergy (kj/kg) h h h C p f, s s Ψ =(h -h a )- a (S -S a ) (3.10) Ψ = (h -h a )- a (S -S a ) (3.11) 3.. ergy destruction he changes the enthalpy and the entropy of the air at the collector are expressed by s CpIn f, f, f, P RIn P (3.1) (3.13) he exergy efficiency of a solar collector system can be calculated terms of the net put exergy of the system or exergy destructions the system. he irreversibility dest can also be determed by: dest = a S gen (3.14) 3..3 ergy efficiency he exergy efficiency of SAH system has been evaluated terms of the net put exergy of the system. he second law efficiency is calculated as follows: 364 P a g e

6 (3.15) II as 1 1 gem a s q u 3.3 Uncertaty Analysis Uncertaties of the calculated parameters are estimated accordg to Holman [9]. In the present work, pressures, temperature, solar radiation, power consumption, velocity and mass flow rate were measured with struments as mentioned the previous section. he uncertaty arisg calculatg a result due to several dependent variables is given by equation (3.16) R w x r w R w 1 1 x x n 1 / R... w n (3.16) Here R is a given function, w r is the total uncertaty, x 1 x..x n is the dependent variables, w 1, w w n is the uncertaty the dependent variables. he total uncertaties for the calculated parameters of heatg capacity and thermal efficiency are calculated as 5% and 8%, respectively. IV. RESULS AND DISCUSSION he experiments were carried under metrological conditions of Coimbatore durg the months of January and March 015. periments were carried contuously durg potential sunshe hours (sunshe more than 00 W/m ). he followg data were observed durg experimentation. More number of data observations has been made to have realistic comparison among the different modes. 4.1 emperature at ypical Locations emperatures at different locations a forced convection solar air heater with p-fs are depicted able 1. From able 1 it is observed that let air temperature varies between 8 and 34 o C with an average temperature of 31 o C. Similarly the temperature at the let of air heater is varyg between 3 o C and 58 o C with an average temperature of ab 43 o C. he absorber temperature is varyg between 34 o C to 6 o C with an average temperature of 47 o C. Similarly, the temperature was measured by replacg the absorber plate with p-f tegrated absorber plate. Measured temperatures at different locations a forced convection solar air heater are plotted able 1. From able 1 it is observed that let air temperature varies between 8 and 35 o C with an average temperature of 31 o C. Similarly the temperature at the let of air heater is varyg between 33 o C and 6 o C with an average temperature of ab 46 o C. he absorber temperature is varyg between 41 o C to 68 o C with an average temperature of 54 o C. he absorber plate temperature with p-fs was observed to higher than that of absorber plate with p-fs due to creased heat transfer area. 365 P a g e

7 Outlet temperature ( o C) 4. Variation of Solar Intensity Similarly, the hourly variation of solar tensity durg experimentation of solar air heater with p-fs is depicted able 1. From able 1 it is observed that solar tensity is creasg gradually from 9 am and reach its maximum value of ab 750W/m the noon and reduces to 00 W/m durg 5pm. he average value of solar tensity is around 410 W/m. Sce the solar tensity is the ma parameter fluencg the performance of the solar air heater. 4.3 Optimization of Air Flow Rate he mass flow rate of air passg through the solar air heater tegrated with p-fs is to be optimized to improve the thermal efficiency of a solar air heater. he air flow rate through the collector is changed as 0.0 kg/s/m, 0.05 kg/s/m and 0.03 kg/s/m by usg voltage regulator. he mass flow rate of air was measured by usg orifice meter setup fitted with manometer. he temperature variations and thermal efficiency of solar air heater with reference to mass flow rate durg peak sunshe hours (700 W/m ) are depicted able 1 and able 1. From able 1, it is understood that maximum let temperature was observed for the mass flow Mass flow rate (kg/s/m ) Figure 5.1 Variation of let air temperature with air flow rate ate of 0.05 kg/sm.similarly, maximum thermal efficiency was observed for the mass flow rate of 0.05 kg/sm. Hence, the mass flow rate of air recommended for forced convection solar air heaters is around 0.05kg/sm. 4.4 hermal Efficiency of a Solar Air Heater hermal efficiency of a forced convection solar air heater was evaluated for a constant mass flow rate of 0.05 kg/s/m. he thermal efficiency of a forced convection solar air heater tegrated with p-fs is more when compared to the forced convection solar air heater with p-fs. From the experimental observations with p-fs, it confirmed that p-fs can crease the heat transfer area, there by improved the thermal efficiency of the solar air heater. he efficiency of a forced convection solar air heater with p-fs is varied between 5% and 35%. But the efficiency of a forced convection solar air heater is varied between 41 and 48%. he efficiency of the 366 P a g e

8 hermal efficiency (%) Mass flow rate (kg/sm ) Figure 5. Variation of thermal efficiency with air flow rate forced convection solar air heater tegrated with p-fs enhance the performance of ab 0%. V. CONCLUSION he exergy efficiency of a forced convection solar air heater workg under different ambient conditions usg p-f absorber plate is depicted the followg figure. From able 1, it is observed that more exergy efficiency occurs durg higher peak sunshe hours. More amount of energy is wasted durg peak sunshe hours. he range of exergy efficiency was observed to be similar to previously reported studies. REFERENCES [1] Kalogirou SA. Solar thermal collectors and applications. Progress Energy and Combustion science 004; 30: [] Esen H. perimental energy and exergy analysis of a double-flow solar air heater havg different obstacles on absorber plates. Buildg and Environment 008; 43: [3] Bhushan B, Sgh R. A review on methodology of artificial roughness used duct of solar air heaters. Energy 010; 35:0 1. [4] anda G. Performance of solar air heater ducts with different types of ribs on the absorber plate, Energy 011; 36: [5] Alam, Sai RP, Sali JS. Use of turbulators for heat transfer augmentation an air duct- A review. Renewable Energy 014; 6: [6] Nwosu NP. Employg exergy-optimized p-fs the design of an absorber a solar air heater. Energy 010; 35: [7] Shariah A, Al-Akhras MA. Al-Omari IA, Optimizg the tilt angle of solar collectors, Renewable Energy 00; 6: [8] Hegazy AA. Optimum channel geometry for solar air heaters of conventional design and constant flow operation. Energy Conversion and Management 1999; 40: [9] Holman JP. perimental methods for engeers. New Delhi: ata Mcgraw hill Publishg Company, P a g e

9 [10] Ho CD, Chang H, Wang RC, L CS. Performance improvement of a double pass solar air heater with fs and baffles under recyclg operation, Applied Energy 01; 100: [11] Hegazy AA, hermo hydraulic performance of air heatg solar collector with variable width flat absorber plates. Energy Conversion and Management 000; 41: [1] Gao W, L W, Liu, Xia C, Analytical and experimental studies on the thermal performance of cross corrugated and flat-plate solar air heaters. Applied Energy 007; 84: [13] yagi VV, Panwar NL, Rahim NA, Kothari R. Review on solar air heatg system with and with thermal energy storage system. Renewable and Sustaable Energy Reviews 01; 16: P a g e