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    • International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN IN – INTERNATIONAL JOURNAL OF ADVANCED RESEARCH 0976 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME ENGINEERING AND TECHNOLOGY (IJARET) ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 4, Issue 7, November-December 2013, pp. 01-09 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2013): 5.8376 (Calculated by GISI) www.jifactor.com IJARET ©IAEME EXERGY ANALYSIS OF A SINGLE-ENDED GLASS DIRECT FLOW EVACUATED TUBE SOLAR COLLECTOR Hamza Al-Tahaineh1, Rebhi Damseh2 1,2 Department of Mechanical Engineering, A-Huson University College, Al Balqa Applied University, Irbid, Jordan. ABSTRACT Exergy analysis for a single ended glass evacuated tube solar collector system was carried out in this investigation. The second law of thermodynamics was used to obtain the net exergy, exergy destructed, and exergetic efficiency of the Evacuated Tube Solar Collector (ETSC) 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 65.88 % 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 Thermodynamics, Exergy, Evacuated Tubes, Solar Systems. INTRODUCTION 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. Two heating schemes were used, uniform wall heating and differential wall heating [1-3]. 1
    • International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME S.K. Tyagi et al 2007 evaluated the exergetic performance of concentrating type solar collector and the parametric study was made using hourly solar radiation. The exergy output is optimized with respect to the inlet fluid temperature and the corresponding efficiencies were computed. The 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 oC 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]. The 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 EVACUATED TUBE SOLAR COLLECTOR 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. The 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).Thus, (Exergy = Energy Quantity × Energy Quality). The general rate form of exergy balance equation is given by: • • • X 42 4 out 1in − X 3 Rate of net Exergy transfer through the collector • − 1destroyed = ∆ X system X 4 42 3 1 4 42 3 Rate of exergy destructio n (1) Rate of change of exergy The exergy carried by the evacuated tube is given by the following relation: • • (2) X in = η col Q Where: • X in : The rate of exergy transfer to the collector by heat (W) ηcol : Collector efficiency. The 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. 2
    • International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME For a real process the exergy input always exceeds the exergy output, this unbalance is due to unbala irreversibilities (called exergy destruction Xdesroyed). The exergy output consists of the utilized output and the non-utilized exergy of waste output. This latter pan we entitle the exergy waste Xdesteryed. It is utilized 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 irrever irreversibilities have, by definition-no exergy and no environment effects[7]. no effects The exergy destruction (system irreversibility, ) is related to the entropy generation by system irreversibility by: • • • I = X destroyed = To S gen (3) Where To is the environment temperature and Sgen is the entropy generation and governed by the following equation: • • S gen Q  Tsur  1 −  (W K ) = Tsur  Tsys    (4) Where: • Q : Useful energy gain from the collector (W). Tsur: surrounding temperature (equal ambient temperature, Ta= 20 oC). Tsys: system temperature. Substituting equations (2), (3) into equation (1) yields in: • η col Q 13 2 Rate of net Exergy transfer by heat • • − To S gen = ∆ X system 1 4 42 3 1 3 2 Rate of change Rate of exergy destructio n (5) of exergy 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 (Ta), 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]: ( ηcol (T = 0.58 − 0.9271 f − Ta G ) − 0.0067 (T f − Ta ) 2 (6) G TUBE EXERGETIC 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
    • International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME The final expression for exergy balance in the solar collector will be: • •   T  T  & I = Q1 − sur  − mc p (Tout − Tin ) − Tsur ln out  T   T  sys   in    (7) The exegetic efficiency (ηП) of an evacuated tube solar collector system is given by the following relation [7,8]: • • ηΠ =1 − X destroyed • X in = 1− T sur S gen   1 − T sur  T sys  (8)  • Q   Where: Xdestroyed: Exergy destructed or destroyed. INVESTIGATION APPARATUS AND SETUP The results of the current study was obtained by investigated a 20 single-ended evacuated tubes with specifications shown in table (1). The tubes were connected directly to a horizontal storage tank mounted over a diffuse reflector plate, Collector inclination: 45º, Tube 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. The outer tube has very low reflectivity and very high transmisivity that radiation can pass through. The inner tube has a layer of selective coating that maximizes absorption of solar energy and minimizes the reflection, thereby locking the heat. The 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
    • International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME Table 1 Evacuated Tube Basic Specifications Length 1500 mm Outer tube diameter 47 mm Inner tube diameter 37 mm Glass thickness 1.6 mm Thermal expansion 3.3x10-6 oC Material Borosilicate Glass 3.3 Absorptive Coating Graded Al-N/Al Absorptance 93% Emittance 7% (100oC) Vacuum P<0.005 Pa Stagnation Temperature >200oC Heat Loss Coeff. <0.8W/ ( m2 oC ) Tube Life >15 years RESULTS AND DISCUSSION To 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. The hot water is defined that water having a temperature equal to 40oC or exceeds. The convention is to rise the cold water temperature in the water heating systems 50oC, i.e. if the cold water temperature 5oC (like in winter) it will rise to 55oC, while the cold water temperature will not exceed 20oC the decision to rise its temperature 50oC to become 70oC was determined to avoid the formation of Calcium sedimentations [1]. Figure 2: Sunshine and solar radiation in Amman [8] 5
    • International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME Figure (2) show the amount of incident radiation at the location of investigation, 32o north latitude, around the year. The peak insolation was found to be at June with a maximum value of solar insolation 28.32 (MJ/m².day) while the minimum was found to be 9.87 (MJ/m².day) at December. The average values and trend of solar insolation were found to be constant for different years. 1000 900 X_in X_destoyed 800 E xerg (W y ) 700 600 500 400 300 200 100 0 0 10 20 30 40 50 60 Temperature Difference (Tout-Tin) Figure 3: Variation of Exergy Input and Exergy Destructed with Temperature difference The net useful exergy is the difference between transfer exergy (as input exergy, Xin) and the exergy destructed due to irreversibility and entropy generation (Sgen). Figure (3) show that the net useful exergy decreases with increase in water temperature difference (Tout-Tin) and this is due to increase in entropy generation with temperature since the amount of heat transfer to the surrounding (ܳሶ ) will increase. Figure 4: Variation of thermal and exergetic efficiencies with collector temperature difference 6
    • International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME 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 ሺܶ௢௨௧ െ ܶ௜௡ ሻ ؆ 50 Ԩ after which the exergetic efficiency become almost constant. This 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. The 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). 0.60 Present Work 0.58 Gang Pei Work (2012) ETC Therm Efficiency al 0.56 0.54 0.52 0.50 0.48 0.46 0.44 0 10 20 30 40 50 60 70 80 Temperature Difference (Tout-Tin) Figure 5: Comparison of ETC thermal efficiency of present work with Gang work [9] 0.80 0.70 E e e E ie c x rg tic ffic n y 0.60 0.50 0.40 0.30 0.20 Present Work Gang Pei (2012) 0.10 0.00 0 10 20 30 40 50 60 70 80 Temperature Difference (Tout-Tin) 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 ETC’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
    • International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME temperature difference increase above 50 oC the exergetic efficiency of present investigation show good agreement with Gang. This means that despite the model and conditions used for investigation of ETC the exergetic efficiency was found to be at its maximum steady value at a temperature difference above 50 oC and all ETC 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 (Tout-Tin) of the collector on the net exergy, exergy destructed, and exergetic efficiency of the Evacuated Tube Solar Collector (ETSC) system. The 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: • • • • The exergetic efficiency of the ETC 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/m2.K). For larger number of tubes the losses will be bigger. The exergy destroyed increases when the temperature difference between the system and the surrounding increases i.e. when (Sgen) increases. The ETC show good exergetic efficiencies at higher temperature difference, i.e. at higher energy collected and stored through the system. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] Morrison and M. Behnia, Performance of a Water-in-Glass Evacuated Tube 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, 2002. I.Budihardjo, G.L. Morrison and M. Behnia, Development of TRNSYS Models for Predicting the Performance of Water-in-Glass Evacuated Tube Solar Water Heaters in Australia, School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney 2052 Australia – ANZSES, 2003. Budihardjo, G.L. Morrison, Performance of water-in-glass evacuated tube solar water heaters, Solar Energy, 83, 2009, p 49–56. S.K. Tyagi, Shengwei Wang, M.K. Singhal, S.C. Kaushik, S.R. Park, Exergy analysis and parametric study of concentrating type solar collectors, International Journal of Thermal Sciences, Volume 46, Issue 12, 2007, pp 1304–1310. I. Jafari, A. Ershadi, E. Najafpour, and N. Hedayat, Energy and Exergy Analysis of Dual Purpose Solar Collector, World Academy of Science, Engineering and Technology, 57, 2011. Michel Pons, Exergy analysis of solar collectors, from incident radiation to dissipation, Renewable Energy, Volume 47, 2012, pp 194–202. Yunus A.Çengle and Michal A. Boles, Thermodynamics an engineering approach (4th ed., Mcgraw-Hill, 2002). Hamza Abdel-Latif Al- Tahaineh, 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, 2002. 8
    • International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 7, November – December (2013), © IAEME [9] [10] [11] [12] [13] Gang Pei, Guiqiang Li, Xi Zhou, Jie Ji, and Yuehong Su, Comparative Experimental Analysis of the Thermal Performance of Evacuated Tube Solar Water Heater Systems With and Without a Mini-Compound Parabolic Concentrating (CPC) Reflector(C < 1), Energies, 5, 2012, 911-924. Z. Ahmed and D. K. Mahanta, “Exergy Analysis of a Compression Ignition Engine” International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 2, 2012, pp. 633 - 642, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359, Published by IAEME A.Ramanan and P.Senthilkumar, “Heat Transfer Characteristics and Exergy Study Of R744/R1270 In A Smooth Horizontal Tube” International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 4, 2013, pp. 166 - 170, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359, Published by IAEME 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 & Technology (IJMET), Volume 3, Issue 3, 2012, pp. 137 - 149, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359, Published by IAEME Hitesh N Panchal, Dr. Manish Doshi, Anup Patel, Keyursinh Thakor,, “Experimental Investigation on Coupling Evacuated Heat Pipe Collector on Single Basin Single Slope Solar Still Productivity” International Journal of Mechanical Engineering & Technology (IJMET), Volume 2, Issue 1, 2011, pp. 1 - 9, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359, Published by IAEME 9