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Solar energy concentration techniques in flat plate collector
1. INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME AND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online) IJMETVolume 3, Issue 3, September - December (2012), pp. 450-458© IAEME: www.iaeme.com/ijmet.asp ©IAEMEJournal Impact Factor (2012): 3.8071 (Calculated by GISI)www.jifactor.com SOLAR ENERGY CONCENTRATION TECHNIQUES IN FLAT PLATE COLLECTOR 1 Pravin N. Gajbhiye 2 Rupesh S.Shelke 1 Student, III rd Semester, M. Tech. Heat Power Engineering 2 Assistance Professor Mechanical Engineering Department, G.H. Raisoni College of Engineering , Nagpur-440016, India Corresponding Author E-mail:- email@example.com ABSTRACT The technology and thermal performance of flat plate solar collectors is summarized and status of technology development in the field of concentrated solar power is reviewed. Concentrated solar power (CSP) systems use mirrors or lenses to concentrate a large area of sunlight, or solar thermal energy, onto a small area. Concentrating technologies exist in four common forms, namely parabolic trough, dish starlings, concentrating linear Fresnel reflector, and solar power tower. Flat-plate collectors are a very useful tool for low to medium temperature heat collection from the sun. They can be used for many purposes including the various thermal desalination methods from low to medium capacities. Flat-plate collectors have simple characteristics: they are easily assembled, and easily operated. The developments are being carried out continuously in the field of cover materials, absorber plate materials, absorber and glazing coating etc. along with the changes in the design, fluid used for heat transfer. Numbers of studies have been carried out on thermal performance of solar flat plate collector and found more increase in the thermal efficiency in comparison to conventional solar flat plate collector. These studies include use of double side absorber plate, honeycomb material, nano-material, more efficient coatings and use of optical lenses. Analysis given in this paper will help to create the best design and operational conditions with the best economic characteristics for solar flat plate collectors. KEYWORDS: Flat plate collector, solar concentration, Optical lenses, wire-coil inserts, Translucent glazing, transparent conductive oxides, nano- fluid. 450
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME1. INTRODUCTION Solar power is the flow of energy from the sun. The primary forms of solar energy areheat and light. In recent years solar energy has been strongly promoted as a viable energy source.One of the simplest and most direct applications of this energy is the conversion of solarradiation into heat. The cost of these energy systems depend on the construction andmaintenance of the plant, the source of energy is free and unlimited. The environmental impactof these systems is practically zero. Hence effective way that the domestic sector can lessen itsimpact on the environment is by the installation of solar flat plate collectors for heating water.Although it should be said that some of these collectors have been in service for the last 40-50years without any real significant changes in their design and operational principles. Theimportance of flat plate collectors are that their thermal performance can be predicted and treatedin considerable detail. Purpose of Flat plate collector is to convert the solar radiation into heat tosatisfy energy needs but with some limitations it is not being used on grid scale because of itspoor efficiency and higher initial cost. So there is a requirement of advancement in the flat platecollector to overcome its limitations so that it can be used as a replacement of conventionalheaters and electric power consuming devices. To match demand and production of energy, the thermal performance of the collectormust be evaluated. The instantaneous useful energy collected is the result of an energy balanceon the solar collector. The term ‘flat plate’ is slightly misleading in the sense that surface maynot be truly flat it may be a combination of flat, grooved or of other shapes as the absorbingsurface with some kind of heat removal device like tubes or channels. A flat-plate solar collectorconsists of a water proof, metal or fiberglass insulated box containing a dark colored absorberplate, with one or more translucent glazing. Absorber plates are typically made out of metal dueto its high thermal conductivity and painted with special selective surface coatings in order toabsorb and transfer heat better than regular black paint. The glazing covers reduce the convectionand radiation heat losses to the environment. A heat-conducting fluid, usually water, glycol, orair, passes through pipes attached to the absorber plate. As the fluid flows through the pipes, itstemperature increases. This is the energy to be utilized for productive activities. The amount ofthe energy taken by the working fluid corresponds to a fraction of the useful energy collectedafter the heat losses. Objective of this paper is to presents extensive studies of the researchcarried out in order to enhance the flat plate collector performance using solar thermalconcentration techniques in this paper both experimental and theoretical developments in thefield of solar water heater have been reviewed thoroughly.2. CONSTRUCTION ELEMENTS OF A FLAT PLATE SOLAR COLLECTOR a. Absorber Plate or Selective Surface Is a metal, glass or plastic surface, mostly black in color. It absorbs and converts radiation into thermal energy. b. The Transparent Cover Is the upper part of the collector covering the tide absorber plate. It is made from glass or transparent plastic sheet to permit penetration of solar beams. c. The Collector Insulation Consists of a material with very low thermal conductivity. It is installed in the bottom and around the sides of the collector, in order to minimize heat loss. 451
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME d. The Heat Transfer Medium Flowing through the collector to transfer the heat from the absorber to the utilization system. Can be either air or a liquid, usually water.3. OPERATIONAL CHARACTERISTICS OF THE COLLECTOR a. Collector efficiency (η) Is the ratio of useful gained thermal energy for period of time t to the incident solar radiation onto the collector for the same time period. b. Thermal Capacity of the Collector (C) Is the amount of heat that can be stored per surface collector area and per unit of temperature change. c. Pressure Drop (∆P) Is the difference in pressure between the inlet to the collector and the outlet due to circulation friction. d. Stagnant Conditions is characterized by no fluid circulation inside the collector during the period in which the absorbing surface area receives a considerable incident radiation. e. Incidence Angle Coefficient (kθ) The ratio of the optical efficiency of a solar collector with a fixed beam angle of incidence to the optical efficiency of the collector at its normal. f. The cover reflectance (ρc) g. Cover Transmittance (τc) h. Cover Absorptance (αc) i. Coefficient of cover Emissivity (εc) j. Coefficient of Absorber Emissivity k. Collector Efficiency Factor (F)Is the ratio of the real energy output of the collector to the energy output in the case when the total absorber area was at the average fluid temperature with the same fluid quantity of flowing water. l. Collector Flow Factor (F″)Is the ratio of the energy that the collector can deliver at the average temperature of the fluid to the energy that the collector can supply at the inlet collector temperature. m. Collector Heat Removal Factor (FR) Is the ratio of the energy collector output to the energy output of the collector in temperature of the inlet fluid. It is temperature dependent. n. Collector Heat Loss Coefficient (UL) The coefficient of thermal loss of a collector is defined as the ratio of the temperature difference per unit area of the cover to the ambient temperature. o. Incidence Angle Coefficient (kθ) The ratio of the optical efficiency of a solar collector with a fixed beam angle of incidence to the optical efficiency of the collector at its normal. 452
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME 3.1. Main Components of flat plate collector Fig:- Advanced Flat Plate Collector4. THE LIMIATIONS OF CONVENTIONAL FLAT PLATE COLLECTOR ANDTECHNICAL AVANCMENTS TO INCREASE ITS EFFECTIVENESS The conventional Flat Plate collectors installed last 40-50 years are stationary deviceswith limited solar radiation absorption area. Observed Practical Limitations of conventional flatplate collector (FPC) are:-1. Require large installation space hence difficult to install on small roof area.2. As these are at constant angular inclination with surface it is difficult to utilize effective solarradiations for long day hours.3. Surface heating require more time to heat water.4. Top front surface exposed to solar radiation hence only some part of solar heat is absorbed.4. Operating temperature limits are inefficient.5. Very low efficiency due to heat loss.6. Installation cost is more as compared to performanceHence unwillingness of costumers to handle such bulky and costly device. In order to increase the efficiency and performance of flat plate collector varioustechnical advancements are proposed. To improve heat transfer in (FPC) metal wire coils ofhelical structure inserted inside the water carrying and heating pipes. Nono fluids are used toincrease heat transfer rates. This increases the turbulence and heat transfer rate some extenteffectively. This advancement is not sufficient to improve overall performance. To reduceradiation losses double glazing glass cover with more absorptive absorber plate is used whichimprove thermal efficiency. Still effective utilization of solar energy is not achieved duestationary FPC installation. All advancement provide surface heating of water carrying metaltubes which increase required time to heat water. It need such arrangement to transfer heat 453
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMEdirectly into to the water inside tube instead of heating surface. To increase temperature solarconcentration techniques can be utilized along with flat plate collector. To increase solarabsorption side faces should be exposed to sun light. Heat loss can be reduced using green houseeffect i.e. green transparent membrane along with transparent glazings. A small compacteffective solar flat plate collector can improve conventional solar utilization approach.5. DEVELOPMENT IN FLAT PLAT COLLECTOR WITH SOLAR CONCENTRATIONTECHNIQUES The developments are being carried out continuously in the field of cover materials,absorber plate materials, absorber and glazing coating etc. along with the changes in the design,fluid used for heat transfer. R. Herrero Martín proposed an enhancement technique applied to flat plate liquidsolar collectors for more compact and efficient design. The design consists of tube-sideenhancement passive techniques which are incorporated into a smooth round tube (twisted tapes,wire coils). This type of inserted device provides better results in laminar, transitional and lowturbulence fluid flow regimes. To test the enhanced solar collector and compare with a standardone, an experimental side-by-side solar collector test bed was designed and constructed. Arelevant improvement of the efficiency up to 5% has been reported and quantified through theuseful power ratio between enhanced and standard solar collectors. Edward K. Summers, John H. Lienhard V, provided collector using highlytransmissive polymer films or low iron glass with double glazing, and using a very absorptiveabsorber, which is inexpensively accomplished by including a carbon black coating. .Doubleglazing reduces radiative losses as glass is opaque to infrared radiation. Addition of roughsurface on the absorber plate provides 12% thermal efficiency increase. The collector wasdesigned for heating air. Madhukeshwara. N, E. S. Prakash presented the performance offlat plate collector with three different coatings for solar flat plate collectors where temperaturesup to 70°C are easily attained by flat plate collectors. With very careful engineering using specialsurfaces, reflectors to increase the incident radiation and heat resistant materials, higheroperating temperatures are feasible. Otanicar et al.  proposed a direct absorption solarcollector operated on nanofluids. They demonstrate efficiency improvement up to 5% byutilizing nanofluids as the absorption mechanism. Groenhout et al. suggested a novel designof a double-sided absorber with low emissivity selective surface coupled with high reflectancestationary concentrators to reduce the radiative and conductive losses through the back of thecollector. This particular design reduce the net heat loss to be 30–70% lower than conventionalsystems. Martin et al.  presented heat transfer enhancement in a tube-on-sheet solar panel withwire-coil inserts, using TRNSYS as the simulating tool and found thermal efficiency increasesup to 4.5% . N. Ehrmann and R. Reineke-Koch used integration of double glazing with lowe- coating and Transparent conductive oxides (TCO) coating into a flat-plate collector.Transparent conductive oxides (TCO) coatings are used as low e-coatings due to their optical 454
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMEselectivity with high solar transmittance. These coatings provide high efficiencies attemperatures above 100 °C as well as at low solar irradiation. Jason H. Karp*, Eric J. Tremblay and Joseph E. Ford present a new approach to solarconcentration where sunlight collected by each lens in a two-dimensional lens array is coupledinto a shared, planar waveguide using localized features placed at each lens focus. This geometryyields a thin, flat profile for moderate concentration systems which may be fabricated by low-cost roll manufacture. Jason H. Karp and Joseph E. Ford provided micro-optic slab concentrator integratesmultiple, focusing apertures with a common, multimode waveguide equipped with optical lensesarray to direct solar energy to a single PV cell. Using the hybrid, imaging/non imaging approach,the system becomes essentially planar while opening a new design space for flat plateconcentrating photo voltaic cell. Otanicar et al. proposed a direct absorption solar collector operated on nanofluids. Theydemonstrate efficiency improvement up to 5% by utilizing nanofluids as the absorptionmechanism. E. AlShamaileh proposed a selective coating composed of a nickel–aluminium (NiAl)alloy into the black paint having higher solar absorption efficiency compared to the commercialblack paint coating. Optimum composition was 6% NiAl alloy by mass. E. Natarajan and R. Sathish  suggest the use of nano-materials in the solar devices toincrease the heat transfer and that can be useful in energy saving and compact designs. 6. SUMMARY OF PRACTICAL ADVANCEMENTS AND ITS RESULTSTable: 5.1 Comparison between developed and conventional Flat plate collector (FPC)(Experimental studies)S.No Developed FPC Conventional FPC Effects of Development Reference Design consists of tube-side Tubes without Efficiency optical factor R. Herrero enhancement using a smooth round insertion device like increased by 15% Martín  1 tube and twisted tapes, wire coils twisted wires insertion inside carrying tubes. Double glazing highly transmissive Single transmissive Provide 12% thermal Edward K. polymer film and a very absorptive cover plate with efficiency increase Summers et. al. 2 absorber plate with rough surface conventional  used. absorber plate used. Coating of metallic particle composed Selective coating Increment of 5°C Madhukeshwar 3 of nickel-aluminium (NiAl) 6% alloy without embedding a. N ,E. S. into the black paint metal Prakash  Double sided absorber with low Single sided absorber Heat loss 30-70% lower N.K. Groenhout 4 emissivity and high reflectance plate. than conventional et.al.  Honeycomb material inserted between Air between glass Reduces heat loss A.A. Ghoneim. 5 the glass cover and absorber. cover and absorber. effectively  integration of double glazing with low Conventional black Provide high efficiency N. Ehrmann 6 e- coating and Transparent conductive paint coating above 100°C as well as and R. Reineke- oxides (TCO) coating at low solar irradiation. Koch  Two-dimensional lens array fitted on Single transmissive Provide high temperature Jason H. Karp 7 planar glass cover using localized cover plate used. concentration for photo et. al.  features placed at each lens focus. voltaic cell 455
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMETable: 5.2 Comparison between theoretical development and there result for flat platecollector (FPC) (Theoretical studies)S.No Proposed theoretical advancement Effect of advancement Reference1 Tube-on-sheet solar panel with wire- Thermal efficiency increases up to 4.5% Martin et al. coil inserts, using TRNSYS as the simulating (CFD) tool.2 Direct absorption solar collector Efficiency improvement up to 5% by T.P. Otanicar et al. operated on nanofluids. utilizing nanofluids as the absorption  mechanism3 Use of nano-materials in the solar Energy loss reduce and compact designs E. Natarajan and R. devices obtaibed. Sathish 4 Trapezoidal profile for absorber Give optimum efficiency B. Kundu  plate5 Indirect force circulation system Increase in hot water supply to demand in A. Hobbi and K. using TRNSYS as the simulating winter conditions. Siddiqui  tool.6 1. If Teflon film used as second 1.Increase in performance estimated to B. Hellstrom et.al. glazing 5.6%.  2. If Teflon honeycomb used. 2. Increase in performance estimated to 3.Antireflection treatment of cover. 12.1%. 4. With external booster reflectors 3. Increase in efficiency estimated to 6.5%. used. 4. Increase in efficiency estimated to 19.9 to 29.4%.Table: 5.3 Practical advancements in flat plate collector and its effect summaryPractical Advancements :- Design consists of tube-side enhancement using a smooth round tube and twisted tapes, wire coils insertion inside carrying tubes. Fig 2:- Thermal Efficiency curves forFig. 1.:- The helical-wire-coil fitted in the raisers of the conventional and enhanced flat platemodified solar collector. collector with insertion wire coil and smooth tubes.  456
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMEPractical Advancements :- Double glazing highly transmissive polymer film and a very absorptive absorber plate with rough surface used.Fig. 3. :- Double glazing highly transmissive polymer film Fig.4. :- Comparison of baseline designand a very absorptive absorber plate with rough surface „double glazed, rough, nonselective absorberused.  with existing air heatersPractical Advancements: - Two-dimensional lens array fitted on planar glass cover using localized features placed at each lens focus. Fig.: Analysis of a 2mm diameter lens array andFig.5. :- The main components of the micro-optic slab 1mm slab waveguide symmetrically couplingconcentrator with focusing lens array for Photo voltaic C light to both edges.cell Flat plate concentrator . 7. CONCLUSIONS This paper highlights the advancements in design configurations and component materialinvestigation to enhance efficiency and performance of flat plate collector. It has been found that flat platecollector enhancement widely investigated both analytically and experimentally. Overall, improving thetransmissivity of the glazing by using highly transmissive polymer films or low iron glass, and using avery absorptive absorber, which is inexpensively accomplished by including a carbon black coating,would have the largest impact on performance of flat plate collector. Advancement like inserting devices,double glazing polymer films, metal additives in absorber black coating, use of nano-material and fluidsprovide improvement in flat plate collector performance lead to increase the solar flat plate collector 457
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMEapplication worldwide. Solar radiation concentration using optical lens arrays make possible to achievehigh temperature using conventional flat plate collector will provide cost effective performance.Optimized study of various operating parameters of flat plate collector proved improved efficiency withcost reduction. The information presented here will be beneficial for beginners in this area of research.REFERENCE 1 1 11. R. Herrero Martín , A. García Pinar , J. Pérez García* World Renewable Energy Congress 2011–Sweden 8-13 may 2011.” Experimental heat transfer research in enhanced flat-plate solar collectors”.2. Edward K. Summers, John H. Lienhard V1, Syed M. Zubair (2011) “Air-Heating Solar Collectorsfor Humidification-Dehumidification Desalination Systems”, Journal of Solar Energy Engineering,ASME FEBRUARY 2011, Vol. 133 / 0110163. Madhukeshwara. N1, E. S. Prakash2 (2012), “An investigation on the performance characteristicsof solar flat plate collector with different selective surface coatings”, INTERNATIONAL JOURNAL OFENERGY AND ENVIRONMENT, Volume 3, Issue 1, 2012 pp.99-108.4. T. P. Otanicar, P. E. Phelan, R. S. Prasher, G. Rosengarten, and R.A.Taylor, (2010). “Nanofluid-based direct absorption solar collector” journal of renewable and sustainable energy, 2, 033102.5. N. K. Groenhout, M. Behnia, G.L. Morrison, (2002). “Experimental measurement of heat loss inan advanced solar collector”, Experimental Thermal and Fluid Science, 26, 131–137.6. R. H. Martin, J. Perez-Garcia, A.Garcia, F.J. Garcia-Soto, E. Lopez-Galiana, (2011). “Simulationof an enhanced flat-plate solar liquid collector with wire-coil insert devices” Solar Energy, 85, 455–469.7. N. Ehrmann, R. Reineke-Koch, (2011). “Selectively coated high efficiency glazing for solar-thermal flat-plate collectors” Journal of Thin Solid Films, Paper in press.8. Jason H. Karp*, Eric J.Tremblay and Joseph E. Ford (2010) “Planar micro-optic solarconcentrator” , 2010 Optical Society of America, 18 January 2010 / Vol. 18, No. 2 / OPTICS EXPRESS1122.9. Jason H. Karp and Joseph E. Ford “Planar micro-optic solar concentration using multipleimaging lenses into a common slab waveguide”, High and Low Concentrator Systems for Solar ElectricApplications IV, edited by Lori E. Greene, Proc. of SPIE Vol. 7407, 74070D.10. A. Hobbi, K. Siddiqui, (2009). “Optimal design of a forced circulation solar water heating systemfor a residential unit in cold climate using TRNSYS” Solar Energy, 83, 700–714.11. T. N. Anderson, M. Duke, J. K. Carson, (2009). “The effect of color on the thermal performanceof building integrated solar collectors”, Solar Energy Materials & Solar Cells, 94,350-354.12. B. Kundu, (2002). “Performance analysis and optimization of absorber plates of differentgeometry for a flat-plate solarcollector: a comparative study” Applied ThermalEngineering, 22, 999–1012.13. E. AlShamaileh, (2010). “Testing of a new solar coating for solar water heating applications”,Solar Energy, 84, 1637–1643.14. E. Natarajan, R. Sathish, (2009). “Role of nanofluids in solar water heater”.15. B. Hellstrom, M. Adsten, P. Nostell, B. Karlsson, E. Wackelgard, (2003). “The impact of opticaland thermal properties on the performance of flat plate solar collectors” Renewable Energy, 28, 331–344.16. E. Zambolin, D. Del Col, (2010). “Experimental analysis of thermal performance of flat plate andevacuated tube solar collectors in stationary standard and daily conditions” Solar Energy, 84, 1382–1396.17. D. Rojas, J. Beermann, S.A. Klein, D.T. Reindl, (2008).“Thermal performance testing of flat-plate collectors” Solar Energy, 82, 746–757. 18. A. A. Ghoneim, (2005). “Performance optimization of solar collector equipped with different arrangements of square celled honeycomb” International Journal of Thermal Sciences, 44, 95–105. 458
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