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Optimization and Performance Analysis of Riser Tubes in Solar Flat Plate Collector H.Saravanan 1, S.Vignesh 2, S.V. Alagarsamy 2 1 PG Scholar (Engineering Design), Dept. of Mechanical Engineering, Mahath Amma Institute of Engineering and Technology, Pudukkottai, 622 101. 2 Asst.Professor, Dept. of Mechanical Engineering, Mahath Amma Institute of Engineering and Technology, Pudukkottai, 622 101. Corresponding E-mail-Id: s.alagarsamy88@gmail.com Abstract: Solar energy is the most considerable energy source in the world. Solar energy collectors are special kind of heat exchangers that transform solar radiation energy to internal energy of the transport medium which is widely used in water heating systems. The major component of any solar water heating system is the solar collector, of all the solar thermal collectors, the flat plate collectors (FPC) have the advantage of being simpler in design, having lower maintenance and lower cost. But it is limited only for low temperature applications due its poor riser design, to achieve higher water temperature usually two or more such collectors are to be coupled which increases the cost of the system. Conventionally, riser tubes of all flat plate collectors are straight copper tubes where the contact area of tube is minimum, which limits the heat collection. Thus to overcome this limitation in this work riser tube is optimized by changing its geometry with the same space of conventional FPC. The straight tubes are replaced with helical coil tubes thereby enhancing higher heat collection surface area and with the same collector space higher thermal efficiency is achieved. In this work Design and simulation of optimum riser tubes for flat plate type solar collector was carried out in ANSYS FLUENT 15.0 for two different tube designs, conventional straight tubes and helical coil type. The flow rate is 0.02 kg/s and flow of water is by natural circulation method, in the overall, helical tube design of FPC is analyzed for its better performance over straight tube design, thereby increasing the efficiency of solar water heating system. Keywords: ANSYS, Straight Tube, Helical Tube, Flat plate collector 1. Introduction In the World population and the living standards, the world seems to engulf into major crisis, called energy crisis. If this growth continues with the same pace the condition would go from bad to worse. The reverse of conventional sources of energy like coal, petroleum and natural gas are depleting at a very fast rate to fulfill the demand of the growing population. So there is a need to look for some other energy sources that could meet this growing demand. One such source is solar energy, which is cheap available in abundance. Solar energy has been utilized in many ways. The most important part of these systems is the solar water heating system where the heat transfer from sun to absorber and absorber to fluid occurs. In order to affect the performance of these systems, generally modifications on solar collectors are performed. With the rapid development in civilization, man has increasingly become dependent on natural resources to satisfy his needs. Water heating is one of those indispensable processes that require natural Page 11

resources in the form of fuels. Solar water heater is fast becoming a preferred method of water heating system considering the potential of saving significant amounts of conventional fuel. In solar water heater, solar energy collectors are special kind of heat exchangers that transform solar radiation energy to internal energy of the transport medium. The major component of any solar water heating system is the solar collector. Of all the solar thermal collectors, the flat plate collectors though produce lower temperatures, have the advantage of being simpler in design, having lower maintenance and lower cost. To obtain maximum amount of solar energy of minimum cost the flat plate solar water heaters with thermal storage have been developed. Solar water heater is type of solar collector which is extensively used in many applications such as residential, industrial and agricultural fields. 2. Literature Review Jinbao Huang et al worked on experimental investigation on thermal performance of thermo syphon flat plate solar water heater with a mantle heat exchanger. In this work done the result show that mean daily efficiency of the thermo syphon flat plate solar water heater with a mantle heat exchanger can reach up to 50%, which was lower than that of thermo syphon flat plate solar water heater without heat exchanger but higher than that of glass evacuated tubular solar water heater. Zong Yanbing et.all, 2009, conducted a simulation on heat transfer performance for integral steel fin-tube through Ansys. The purpose of the study is to find the effect of the fin tube parameters and the fluid temperature on heat transfer performance. It is found that the rate of heat transfer increases with the height decreases as the thickness of the fin effective to certain level and the rate of heat transfer drop. While the fin spacing is important as it will influent the heat transfer. Based on Prof. Kumavat Mukesh Manila et al he attempts to present numerical simulation of solar collector developed for drying food products and how to increase its efficiency. Solar drying is much feasible technically and economically. There has been a remarkable achievement in solar drying of food products due to sustained research and development associated with the adoption of advanced technologies. Simulation is an important tool for design and operation control. For designing of a collector plate, simulation makes it possible to find the optimum design and operating parameters. For the designer of the control system, simulation provides a means to device control strategies and to analyze the effects of disturbances. In this paper, the Computational Fluid Dynamics (CFD) tool has been used to simulate the condition for different types of absorber plates having different shapes and configuration to obtain better efficiency than ordinary solar collector. The 3D model of the solar flat plate collector is modeled by UGS NX and then export in STEP format and then it is imported in ANSYS Workbench and then the meshing was created in ANSYS ICEM. The results were obtained by using ANSYS FLUENT software. The objective of this work is to compare CFD solutions of different shapes of absorber plates for flat plate collector. The plate giving best result has been selected for fabrication. After fabrication, test was carried out and practical results were obtained. The results then compared with CFD analysis. Based on Prof. Rabindra Pokhrel et al from Santa Clara University, US 2014, analyzed on flat plate solar water Page 12

heater in terms of efficiency has very important role the results shown in the analysis can be meaningfully interpreted to obtain the maximum efficiency without much increase in the manufacturing cost. This help to create efficient collectors of higher standard to compete with the international market. The salient features of analysis are with the increase in Inlet fluid temperature above the ambient temperature, the efficiency of collector was observed to decrease gradually and this is because of radiation losses from the collector due to higher temperature distribution. With the increase in ambient temperature it was observed that the collector efficiency increases gradually. Collector efficiency increases with the increase in water-flow rate due to absorption of heat energy with high velocity flow rate and less radiation losses. Collector efficiency decreases with the increase in area of the absorber plate due to increased due to increased surface area leading higher rate of heat losses. Nosa Andrew Ogie et al designed and construction of a solar water heater based on the thermo-syphon principle. Solar energy is received by a flat-plate collector consisting of a thin absorber plate integrated with underneath grids of fluid carrying tubes and placed in an insulated in the system. The radiation emitted by the absorber plate cannot escape through the glass, thus increasing its temperature. The water gets heated and flows into a storage tank through thermo-syphon principle. The system designed in this work requires little or no maintenance because of the thermo-syphon principle and was made basically from locally available raw materials and also no moving parts and also the system works automatically. C.C. Chien et al worked on theoretical and experimental investigations of two-phase thermo-syphon solar water heaters. In this work, the performance of this innovative solar water heater at different solar radiation intensities and tilt angles was experimentally discussed. The experimental results show that the best change efficiency of the two phase thermo-syphon solar water heater is 82% and the charge efficiency decreases no more than 5% when the tilt angle of the system was less than 15 C. The objective of present study is to perform simulation for riser tubes used in solar flat plate collector. The results obtained by simulation are been compared for conventional straight tube design and helical tube design. The overall aim of this work is to understand the temperature distribution in the solar collector and compare the outlet temperature of water. 3. Methodology In the present study, the efficiency of collector riser tubes are analyzed using Ansys, at first cad model for conventional straight tubes and modified design of riser tube (helical tubes) is obtained using Solid works. Then the cad model is imported to ANSYS for analyzing the temperature distribution through IGES Format. Bothe Ansys Fluent and CFD tools are used to compare the results obtained for two different types of designs of riser tubes. In FDM - ANSYS technique, the first step involves the transformation of the actual physical domain into the computational grid. Second step is to transform the differential equations into difference equations, which along with the equations obtained by heat balance across the covers and the absorber are the simultaneous non linear algebraic equations. The third step is to solve the equations using computer program. The solution is obtained in the form of nodal Page 13

temperatures for the riser tubes, the water outlet and the absorber. Study has been extend by changing the various governing parameters like the ambient air temperature, and the intensity of solar radiation and finally the performance characteristics have been obtained and compared with existing one. 3.1 Design and analysis Straight tube Design 1. Overall dimensions : 2000 x 1000 mm 2 2. Collector Area : 2.38 m 2 3. Absorber : 100 % Cu 4. No of Cu Header Pipe : 2 Any 5. O.D of Header Pipe : 30 mm 6. No of Cu Riser Tubes : 9 Nos. 7. O.D of Riser Tubes : 15 mm 8. Tube spacing : 85 mm Helical tube Design Fig.1. Cad modeling of straight tube 1. Overall dimensions : 2000 x 1000 mm 2 2. Collector Area : 2.38 m 2 3. Absorber : 100 % Cu 4. No of Cu Header Pipe : 2 Nos. 5. O.D of Header Pipe : 30 mm 6. No of Cu Riser Tubes : 5 Nos. 7. O.D of Riser Tubes : 15 mm 8. Tube spacing : 100 mm 9. No of turns : 8 10. Width of tubes : 100 mm Page 14

Fig 2 Modeling of Helical Tubes 3.2 Geometry and Boundary Conditions Riser tubes are imported in IGES Format in the ANSYS workbench design module. First, the FLUENT module from the workbench is selected. The design modeler opens as a new window as the geometry is double clicked. Boundary conditions are used according to the need of the model. The inlet and outlet conditions are defined as temperature of water at inlet and outlet. The walls are separately specified with respective boundary conditions. The details about all boundary conditions are as given below. These boundary conditions remain to be same for both straight tube model as well as helical tube model. 1. Mass flow rate : 0.02 kg/s 2. Atmospheric temperature : 29 o c 3. Thermal conductivity (tube) : 386 W/mK 4. Specific heat : 385kJ/kg K 5. Density : 8954 kg/m 3 6. Absorptivity : 0.95 7. Velocity : 0.992 m/s 8. Radiation Intensity : 195 (min) - 1050 (Max) 9. Bond Conductance : 30 w/m o C 10. Allowed Pressure : 0.3 Pascal 11. Wind induced heat transfer Coefficient : 10 w/m o c 12. Momentum : 0.7 kg-m/s Page 15

Fig 3 Meshing of Straight tube and Helical Tubes 4. Result and Discussions Based on the boundary conditions applied the simulation results are found below This simulation analysis reveals, for helical coil heat exchanger the distribution of heat and heat flux is more compared to straight tube conventional tubes, the hoop stress inside the wall of helical coil heat exchanger falls within the safe limit and the flow in helical tubes are turbulent in nature. Fig 4 Distribution of Heat in Straight tubes and helical tube FPC Page 16

Fig 5 Heat Flux Helical tubes Fig 6 Contours of Pressure and temperature inside helical tube Fig 7 Contours of Thermal Conductivity Page 17

Fig 8 Contours of Reynolds Number in helical tubes Fig 9 Velocity vectors in m/s Fig 10 Wall shear stress Page 18

195 360 800 603 1047.5 1015 998 TRANSACTIONS ON ADVANCEMENTS IN SCIENCE AND TECHNOLOGY (TASTONLINE) COMPARISON OF CFD RESULTS Table 1 : Comparison of Results Based on Intensity of Radiation Max. Values Avg. values Min Values Solar Intensity (W/m 2 ) Ambient temp. /Inlet water Temp. (T1) 0 C Temp. (T2) O C Straight tube FPC Temp. (T2) O C Helical tube FPC 998 37 46 56 1015 38 46 62 1047.5 38 48.5 63 603 32 38 45.5 800 36 40 52.41 360 35 38 46.46 195 33 36.3 42 70 60 50 40 30 20 10 0 Fig.11 Comparison of Absorber plates Vs Intensity of Radiation 4. Conclusions ANSYS is an effective tool with which we can stimulate the models on various operating conditions without actually fabricating them and can compare their results. The best solution is then selected for fabrication so that we can save time and money. On the basis of the results obtained in this project, the following conclusions can be drawn. Page 19

1. A helical tube solar water heater has been designed and performance is evaluated and compared with conventional straight tube solar water heater. The outlet temperature of helical tube solar water heater is more than the conventional solar water heater. 2. A maximum temperature difference of 24 o C between inlet and outlet water temperature recorded in helical tube collector, which is more than the conventional solar water heater. 3. The efficiency of helical tube solar water heater and straight tube solar water heater was compared and for helical tube collector it is found to be maximum i.e. 38.70 % whereas for straight tube solar water heater it is 25.05%. As comparing this efficiency, the helical tube solar water efficiency is more. 4. Collector outlet temperature varies depending on solar intensity and it is directly proportional with it. References 1. David Luna, Yves Jannot, Jean-Pierre Nadeau. An oriented-design simplified model for the efficiency of a flat plate solar air collector, Applied thermal engineering 30 (2010) 2808-2814. 2. Basavanna S and K S Shashishekar. CFD Analysis of Triangular Absorber Tube of A Solar Flat Plate Collector, Vol. 2, No. 1, January 2013. 3. C.T. Shaw, Using Computational Fluid Dynamics, Prentice Hall, 1992. 4. G. D. Rai. Solar Energy Utilization, Khanna Publishers, page no. 156-199, 5th Edition, Page no. 01-199. 5. H.P.Garg,J.Prakash. Solar Energy Fundamentals & Application, Tata McGraw Hill,Page no. 01-113. 6. Sukhatme S. P. Solar Energy and Renewable Energy Source, Tata McGraw Hill, 2nd Edition, Page no. 01-196. 7. Nosa Andrew Ogie, Ikponmwosa Oghogho,Julius Jesumirewhe. Design and Construction of a Solar Water Heater Based on the Thermo-syphon Principle, Journal of Fundamentals of Renewable Energy and Applications Vol. 3(2013),Article ID 235592,8 pages. 8. C.C. Chien, C. K. Kung, C.C. Chang, W.S. Lee, C.S. Jwo, S.L. chen. Theoretical and experimental investigation of a two-phase thermo-syphon solar water heater, Energy 36(2011 415-423. 9. K.K. Chong, K.G. Chay, K.H. Chin. Study of a solar water heater using stationary V-trough collector, Renewable Energy 39(2012 207-215. 10. Rakesh kumar, Marc A. Rosen. Thermal performance of integrated collector storage solar water heater with corrugated absorber surface, Applied Thermal Engineering 30(2010) 1764-178. Page 20