INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET)

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1 INERNAIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND ECHNOLOGY (IJARE) International Journal of Advanced Research in Engineering and echnology (IJARE), ISSN 0976 ISSN (Print) ISSN (Online) Volume 4, Issue 7, November-December 2013, pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJARE I A E M E EXERGY ANALYSIS OF A SINGLE-ENDED GLASS DIREC FLOW EVACUAED UBE SOLAR COLLECOR Hamza Al-ahaineh 1, Rebhi Damseh 2 1,2 Department of Mechanical Engineering, A-Huson University College, Al Balqa Applied University, Irbid, Jordan. ABSRAC Exergy analysis for a single ended glass evacuated tube solar collector system was carried out in this investigation. he second law of thermodynamics was used to obtain the net exergy, exergy destructed, and exergetic efficiency of the Evacuated ube Solar Collector (ESC) system. According to the mean solar insolation in Jordan and assumptions of calculation in specific region around the year, the results obtained show an exegetic efficiency of % which seems to have a steady value despite the increase in the temperature difference of water in and out of the collector. Keywords: Second Law of hermodynamics, Exergy, Evacuated ubes, Solar Systems. INRODUCION Evacuated tube solar collectors have been commercially available for over 20 years; however, until recently they have not provided any real competition to flat plate collectors. In order to investigate the flow structure and heat transfer within the tube, extensive experimental Investigations have been done on cylindrical open thermosyphon with various tube aspect ratios, heating schemes and Rayleigh numbers. Extensive numerical modeling has been done for a number of Years. A numerical model of the inclined open thermosyphon has been developed using a finite difference algorithm to solve the vorticity vector potential form of the Navier-Stokes equations the geometry considered was an open cylinder, inclined at 45 to the vertical. Steady flow is simulated at various combinations of Rayleigh number, aspect ratio and mode of heating. wo heating schemes were used, uniform wall heating and differential wall heating [1-3]. 1

2 S.K. yagi et al 2007 evaluated the exergetic performance of concentrating type solar collector and the parametric study was made using hourly solar radiation. he exergy output is optimized with respect to the inlet fluid temperature and the corresponding efficiencies were computed. he performance parameters were found to be the increasing functions of the concentration ratio but the optimal inlet temperature and exergetic efficiency at high solar intensity are found to be the decreasing functions of the concentration ration [4]. I. Jafari et al 2011 investigated energy and exergy of air-water combined solar collector which is called dual purpose solar collector (DPSC). Analysis is performed for triangle channels. Parameters like the air flow rate and water inlet temperature are studied. Results are shown that DPSC has better energy and exergy efficiency than single collector. In addition, the triangle passage with water inlet temperature of 60 o C has shown better exergy and energy efficiency [5]. Michel Pons 2012 investigates the main types of exergy losses that can be identified in solar collector systems in order to minimize the source of exergy losses and maximize the solar energy benefits [6]. he objective of the present investigation is to analyze the evacuated tube solar system from the second law of thermodynamics point of view in order to improve the system performance by investigating the operating conditions where the exergy destruction become minimum and the exergetic efficiency maximum. EXERGY ANALYSIS OF EVACUAED UBE SOLAR COLLECOR Exergy is the maximum amount of work that can be obtained from a stream of matter, heat or work as it comes into equilibrium with a reference environment. he term "exergy" or absolute energy efficiency is also used to define the combination of energy quantity (which is conserved according to the first law of thermodynamics) and energy quality (which is consumed according to the second law of thermodynamics).hus, (Exergy = Energy Quantity Energy Quality). he general rate form of exergy balance equation is given by: X in X out Rate of net Exergy transfer through the collector 1 X destroyed 4243 Rate of exergy destructio n = X system Rate of change of exergy (1) he exergy carried by the evacuated tube is given by the following relation: X in = η col Q (2) Where: in : he rate of exergy transfer to the collector by heat (W) η col : Collector efficiency. he exergy destroyed is another expression for the system irreversibility (I) which is the difference between the heat input and the useful heat obtained by the solar collector ; i.e.: I = X destroyed. System irreversibility which could be also expressed as the system heat losses and it is divided to the tank heat loss and tube heat loss. X 2

3 For a real process the exergy input always exceeds the exergy output, this unbalance is due to irreversibilities (called exergy destruction X desroyed ). he exergy output consists of the utilized output and the non-utilized exergy of waste output. his latter pan we entitle the exergy waste X desteryed. It is very important to distinguish between exergy destruction caused by irreversibilities and exergy waste due to unused exergy flow to the environment both represent exergy losses, but irreversibilities have, by definition-no exergy and no environment effects[7]. he exergy destruction (system irreversibility, ) is related to the entropy generation by: I = X destroyed = o S gen Where o is the environment temperature and S gen is the entropy generation and following equation: (3) governed by the S gen Q = sur 1 sur sys ( W K ) (4) Where: Q : Useful energy gain from the collector (W). sur : surrounding temperature (equal ambient temperature, a = 20 o C). sys : system temperature. Substituting equations (2), (3) into equation (1) yields in: ηcol Q o S 123 gen 123 Rate of net Exergy transfer Rate of exergy by heat destructio n = system X Rate of change of exergy (5) Where the first component of the left hand side of equation (5) is the efficiency of the collector which was modeled experimentally by Budihardjo as a function of ambient temperature ( a ), average film temperature of inlet and outlet water temperatures of the tube, and global solar irradiance at the collector plane (G) as a second order equation [3]: η ( ) f a = G col 0 ( ) f G a 2 (6) UBE EXERGEIC EFFICIENCY Exergy efficiency of the solar collector can be defined as the ratio of increased mass exergy to the exergy of the solar radiation, in other word; it is a ratio of the useful exergy delivered to the exergy absorbed by the solar collector [7,8]. 3

4 he final expression for exergy balance in the solar collector will be: = I sur out Q 1 mc & ( ) p out in sur ln (7) sys in he exegetic efficiency (η П ) of an evacuated tube solar collector system is given by the following relation [7,8]: η Π = 1 X destroyed X in = 1 1 sur S gen sur sys Q (8) Where: X destroyed : Exergy destructed or destroyed. INVESIGAION APPARAUS AND SEUP he results of the current study was obtained by investigated a 20 single-ended evacuated tubes with specifications shown in table (1). he tubes were connected directly to a horizontal storage tank mounted over a diffuse reflector plate, Collector inclination: 45º, ube aspect ratio (length/diameter):1500/34, Absorber diameter: 37 mm, Inter-tube spacing: 70 mm. Each evacuated tube consists of two glass tubes made from extremely strong borosilicate glass. he outer tube has very low reflectivity and very high transmisivity that radiation can pass through. he inner tube has a layer of selective coating that maximizes absorption of solar energy and minimizes the reflection, thereby locking the heat. he ends of the tubes connected to the copper header are fused together and a vacuum is created between them. Figure 1: Evacuated tubes solar collector connected directly to a horizontal storage tank 4

5 able 1 Evacuated ube Basic Specifications Length 1500 mm Outer tube diameter Inner tube diameter Glass thickness hermal expansion 47 mm 37 mm 1.6 mm 3.3x10-6 o C Material Borosilicate Glass 3.3 Absorptive Coating Absorptance 93% Graded Al-N/Al Emittance 7% (100 o C) Vacuum Stagnation emperature P<0.005 Pa >200 o C Heat Loss Coeff. <0.8W/ ( m 2 o C ) ube Life >15 years RESULS AND DISCUSSION o analyze the thermal data, a simplified model was proposed, based on the following assumption: Ambient air temperature 20 ºC, hot water supply to the household 70 C. he hot water is defined that water having a temperature equal to 40 o C or exceeds. he convention is to rise the cold water temperature in the water heating systems 50 o C, i.e. if the cold water temperature 5 o C (like in winter) it will rise to 55 o C, while the cold water temperature will not exceed 20 o C the decision to rise its temperature 50 o C to become 70 o C was determined to avoid the formation of Calcium sedimentations [1]. Figure 2: Sunshine and solar radiation in Amman [8] 5

6 Figure (2) show the amount of incident radiation at the location of investigation, 32 o north latitude, around the year. he peak insolation was found to be at June with a maximum value of solar insolation (MJ/m².day) while the minimum was found to be 9.87 (MJ/m².day) at December. he average values and trend of solar insolation were found to be constant for different years X_in X_destoyed 700 Exergy (W) emperature Difference (out-in) Figure 3: Variation of Exergy Input and Exergy Destructed with emperature difference he net useful exergy is the difference between transfer exergy (as input exergy, X in ) and the exergy destructed due to irreversibility and entropy generation (S gen ). Figure (3) show that the net useful exergy decreases with increase in water temperature difference ( out - in ) and this is due to increase in entropy generation with temperature since the amount of heat transfer to the surrounding ( ) will increase. Figure 4: Variation of thermal and exergetic efficiencies with collector temperature difference 6

7 Figure (4) show that while the thermal efficiency of the collector under specified condition decrease with temperature difference, the exergetic efficiency start to increase until it reach a steady value (o.66) at a temperature difference 50 after which the exergetic efficiency become almost constant. his behavior means that the exergy destruction starts to decrease with temperature difference until it reach its lowest value after which no more destruction in exergy. he exergetic efficiency was found to be constant around the year for the same region and the same temperature difference and its value around (0.66) Present Work Gang Pei Work (2012) 0.56 EC hermal Efficiency emperature Difference (out-in) Figure 5: Comparison of EC thermal efficiency of present work with Gang work [9] Exergetic Efficiency Present Work 0.10 Gang Pei (2012) emperature Difference (out-in) Figure 6: Comparison of exergetic efficiency of present work with Gang work [9] From figures (5) and (6), when comparing the results of present investigation with Gang result [9], it was found that while the thermal efficiency of the both EC s show the same trend there was some difference in the exergetic efficiency at low temperature differences and this may be explained by higher loss in exergy in gang model which was avoided in the present model.as the 7

8 temperature difference increase above 50 o C the exergetic efficiency of present investigation show good agreement with Gang. his means that despite the model and conditions used for investigation of EC the exergetic efficiency was found to be at its maximum steady value at a temperature difference above 50 o C and all EC show higher destruction in exergy at lower temperatures. CONCLUSIONS Carrying out a detailed exergy analysis for a single ended glass evacuated tube solar collector system to show the effect of temperature difference ( out - in ) of the collector on the net exergy, exergy destructed, and exergetic efficiency of the Evacuated ube Solar Collector (ESC) system. he analysis was carried out based on the mean solar insolation in Jordan and assumptions of calculation in specific region around the year. Based on the results of the analysis carried out, one can conclude the following: he exergetic efficiency of the EC seems to be steady with temperature difference especially at higher values while the thermal efficiency decreases with increasing temperature difference. Most of the system exergy destroyed were from the tubes since it has high heat loss coefficient (~0.8 W/m 2.K). For larger number of tubes the losses will be bigger. he exergy destroyed increases when the temperature difference between the system and the surrounding increases i.e. when (S gen ) increases. he EC show good exergetic efficiencies at higher temperature difference, i.e. at higher energy collected and stored through the system. REFERENCES [1] Morrison and M. Behnia, Performance of a Water-in-Glass Evacuated ube Solar Water Heater/I. Budihardjo, G. L.,School of Mechanical and Manufacturing Engineering, University of New South Wales- Sydney 2052 Australia/ Australian and New Zealand Solar Energy Society - Proceedings of Solar, [2] I.Budihardjo, G.L. Morrison and M. Behnia, Development of RNSYS Models for Predicting the Performance of Water-in-Glass Evacuated ube Solar Water Heaters in Australia, School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney 2052 Australia ANZSES, [3] Budihardjo, G.L. Morrison, Performance of water-in-glass evacuated tube solar water heaters, Solar Energy, 83, 2009, p [4] S.K. yagi, Shengwei Wang, M.K. Singhal, S.C. Kaushik, S.R. Park, Exergy analysis and parametric study of concentrating type solar collectors, International Journal of hermal Sciences, Volume 46, Issue 12, 2007, pp [5] I. Jafari, A. Ershadi, E. Najafpour, and N. Hedayat, Energy and Exergy Analysis of Dual Purpose Solar Collector, World Academy of Science, Engineering and echnology, 57, [6] Michel Pons, Exergy analysis of solar collectors, from incident radiation to dissipation, Renewable Energy, Volume 47, 2012, pp [7] Yunus A.Çengle and Michal A. Boles, hermodynamics an engineering approach (4 th ed., Mcgraw-Hill, 2002). [8] Hamza Abdel-Latif Al- ahaineh, Second law analysis of solar powered absorption refrigeration system, Research for the degree of Doctor of Philosophy in Mechanical Engineering, University of Jordan, Amman, Jordan,

9 [9] Gang Pei, Guiqiang Li, Xi Zhou, Jie Ji, and Yuehong Su, Comparative Experimental Analysis of the hermal Performance of Evacuated ube Solar Water Heater Systems With and Without a Mini-Compound Parabolic Concentrating (CPC) Reflector(C < 1), Energies, 5, 2012, [10] Z. Ahmed and D. K. Mahanta, Exergy Analysis of a Compression Ignition Engine International Journal of Mechanical Engineering & echnology (IJME), Volume 3, Issue 2, 2012, pp , ISSN Print: , ISSN Online: , Published by IAEME [11] A.Ramanan and P.Senthilkumar, Heat ransfer Characteristics and Exergy Study Of R744/R1270 In A Smooth Horizontal ube International Journal of Mechanical Engineering & echnology (IJME), Volume 4, Issue 4, 2013, pp , ISSN Print: , ISSN Online: , Published by IAEME [12] Dinkar V. Ghewade, Dr S.N.Sapali, Quantification of Energy Losses and Performance Improvement In DX Cooling By Exergy Method International Journal of Mechanical Engineering & echnology (IJME), Volume 3, Issue 3, 2012, pp , ISSN Print: , ISSN Online: , Published by IAEME [13] Hitesh N Panchal, Dr. Manish Doshi, Anup Patel, Keyursinh hakor,, Experimental Investigation on Coupling Evacuated Heat Pipe Collector on Single Basin Single Slope Solar Still Productivity International Journal of Mechanical Engineering & echnology (IJME), Volume 2, Issue 1, 2011, pp. 1-9, ISSN Print: , ISSN Online: , Published by IAEME 9