International Journal of JOURNAL Mechanical Engineering and Technology (IJMET), ISSN 0976 – INTERNATIONAL OF MECHANICAL ENGINEERING 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, SepDec (2012) © IAEME AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 3, Issue 3, September - December (2012), pp. 450-458 © IAEME: www.iaeme.com/ijmet.asp Journal Impact Factor (2012): 3.8071 (Calculated by GISI) www.jifactor.com

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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 protected]

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) © IAEME

1.

INTRODUCTION

Solar power is the flow of energy from the sun. The primary forms of solar energy are heat 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 solar radiation into heat. The cost of these energy systems depend on the construction and maintenance of the plant, the source of energy is free and unlimited. The environmental impact of these systems is practically zero. Hence effective way that the domestic sector can lessen its impact 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 40-50 years without any real significant changes in their design and operational principles. The importance of flat plate collectors are that their thermal performance can be predicted and treated in considerable detail. Purpose of Flat plate collector is to convert the solar radiation into heat to satisfy energy needs but with some limitations it is not being used on grid scale because of its poor efficiency and higher initial cost. So there is a requirement of advancement in the flat plate collector to overcome its limitations so that it can be used as a replacement of conventional heaters and electric power consuming devices. To match demand and production of energy, the thermal performance of the collector must be evaluated. The instantaneous useful energy collected is the result of an energy balance on the solar collector. The term ‘flat plate’ is slightly misleading in the sense that surface may not be truly flat it may be a combination of flat, grooved or of other shapes as the absorbing surface with some kind of heat removal device like tubes or channels. A flat-plate solar collector consists of a water proof, metal or fiberglass insulated box containing a dark colored absorber plate, with one or more translucent glazing. Absorber plates are typically made out of metal due to its high thermal conductivity and painted with special selective surface coatings in order to absorb and transfer heat better than regular black paint. The glazing covers reduce the convection and radiation heat losses to the environment. A heat-conducting fluid, usually water, glycol, or air, passes through pipes attached to the absorber plate. As the fluid flows through the pipes, its temperature increases. This is the energy to be utilized for productive activities. The amount of the energy taken by the working fluid corresponds to a fraction of the useful energy collected after the heat losses. Objective of this paper is to presents extensive studies of the research carried out in order to enhance the flat plate collector performance using solar thermal concentration techniques in this paper both experimental and theoretical developments in the field 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.

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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 Collector 4. THE LIMIATIONS OF CONVENTIONAL FLAT PLATE COLLECTOR AND TECHNICAL AVANCMENTS TO INCREASE ITS EFFECTIVENESS The conventional Flat Plate collectors installed last 40-50 years are stationary devices with limited solar radiation absorption area. Observed Practical Limitations of conventional flat plate 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 solar radiations 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 performance Hence unwillingness of costumers to handle such bulky and costly device. In order to increase the efficiency and performance of flat plate collector various technical advancements are proposed. To improve heat transfer in (FPC) metal wire coils of helical structure inserted inside the water carrying and heating pipes. Nono fluids are used to increase heat transfer rates. This increases the turbulence and heat transfer rate some extent effectively. This advancement is not sufficient to improve overall performance. To reduce radiation losses double glazing glass cover with more absorptive absorber plate is used which improve thermal efficiency. Still effective utilization of solar energy is not achieved due stationary FPC installation. All advancement provide surface heating of water carrying metal tubes 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) © IAEME

directly into to the water inside tube instead of heating surface. To increase temperature solar concentration techniques can be utilized along with flat plate collector. To increase solar absorption side faces should be exposed to sun light. Heat loss can be reduced using green house effect i.e. green transparent membrane along with transparent glazings. A small compact effective solar flat plate collector can improve conventional solar utilization approach. 5. DEVELOPMENT IN FLAT PLAT COLLECTOR WITH SOLAR CONCENTRATION TECHNIQUES 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[1] proposed an enhancement technique applied to flat plate liquid solar collectors for more compact and efficient design. The design consists of tube-side enhancement 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 low turbulence fluid flow regimes. To test the enhanced solar collector and compare with a standard one, an experimental side-by-side solar collector test bed was designed and constructed. A relevant improvement of the efficiency up to 5% has been reported and quantified through the useful power ratio between enhanced and standard solar collectors. Edward K. Summers, John H. Lienhard V,[2] provided collector using highly transmissive polymer films or low iron glass with double glazing, and using a very absorptive absorber, which is inexpensively accomplished by including a carbon black coating. .Double glazing reduces radiative losses as glass is opaque to infrared radiation. Addition of rough surface on the absorber plate provides 12% thermal efficiency increase. The collector was designed for heating air. Madhukeshwara. N, E. S. Prakash[3] presented the performance of flat plate collector with three different coatings for solar flat plate collectors where temperatures up to 70°C are easily attained by flat plate collectors. With very careful engineering using special surfaces, reflectors to increase the incident radiation and heat resistant materials, higher operating temperatures are feasible. Otanicar et al. [4] proposed a direct absorption solar collector operated on nanofluids. They demonstrate efficiency improvement up to 5% by utilizing nanofluids as the absorption mechanism. Groenhout et al.[5] suggested a novel design of a double-sided absorber with low emissivity selective surface coupled with high reflectance stationary concentrators to reduce the radiative and conductive losses through the back of the collector. This particular design reduce the net heat loss to be 30–70% lower than conventional systems. Martin et al. [6] presented heat transfer enhancement in a tube-on-sheet solar panel with wire-coil inserts, using TRNSYS as the simulating tool and found thermal efficiency increases up to 4.5% . N. Ehrmann and R. Reineke-Koch[7] used integration of double glazing with low e- 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) © IAEME

selectivity with high solar transmittance. These coatings provide high efficiencies at temperatures above 100 °C as well as at low solar irradiation. Jason H. Karp*, Eric J. Tremblay and Joseph E. Ford[8] present a new approach to solar concentration where sunlight collected by each lens in a two-dimensional lens array is coupled into a shared, planar waveguide using localized features placed at each lens focus. This geometry yields a thin, flat profile for moderate concentration systems which may be fabricated by lowcost roll manufacture. Jason H. Karp and Joseph E. Ford[9] provided micro-optic slab concentrator integrates multiple, focusing apertures with a common, multimode waveguide equipped with optical lenses array 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 plate concentrating photo voltaic cell. Otanicar et al.[11] proposed a direct absorption solar collector operated on nanofluids. They demonstrate efficiency improvement up to 5% by utilizing nanofluids as the absorption mechanism. E. AlShamaileh[13] proposed a selective coating composed of a nickel–aluminium (NiAl) alloy into the black paint having higher solar absorption efficiency compared to the commercial black paint coating. Optimum composition was 6% NiAl alloy by mass. E. Natarajan and R. Sathish [14] suggest the use of nano-materials in the solar devices to increase the heat transfer and that can be useful in energy saving and compact designs. 6. SUMMARY OF PRACTICAL ADVANCEMENTS AND ITS RESULTS Table: 5.1 Comparison between developed and conventional Flat plate collector (FPC) (Experimental studies) S.No 1

2

3 4 5 6

7

Developed FPC Design consists of tube-side enhancement using a smooth round tube and twisted tapes, wire coils insertion inside carrying tubes. Double glazing highly transmissive polymer film and a very absorptive absorber plate with rough surface used. Coating of metallic particle composed of nickel-aluminium (NiAl) 6% alloy into the black paint Double sided absorber with low emissivity and high reflectance Honeycomb material inserted between the glass cover and absorber. integration of double glazing with low e- coating and Transparent conductive oxides (TCO) coating Two-dimensional lens array fitted on planar glass cover using localized features placed at each lens focus.

Conventional FPC Tubes without insertion device like twisted wires

Effects of Development Efficiency optical factor increased by 15%

Reference R. Herrero Martín [1]

Single transmissive cover plate with conventional absorber plate used. Selective coating without embedding metal Single sided absorber plate. Air between glass cover and absorber. Conventional black paint coating

Provide 12% thermal efficiency increase

Edward K. Summers et. al. [2]

Increment of 5°C

Madhukeshwar a. N ,E. S. Prakash [3] N.K. Groenhout et.al. [5] A.A. Ghoneim. [18] N. Ehrmann and R. ReinekeKoch [7] Jason H. Karp et. al. [8]

Single transmissive cover plate used.

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Heat loss 30-70% lower than conventional Reduces heat loss effectively Provide high efficiency above 100°C as well as at low solar irradiation. Provide high temperature concentration for photo voltaic cell

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME

Table: 5.2 Comparison between theoretical development and there result for flat plate collector (FPC) (Theoretical studies) S.No 1

2

3 4 5

6

Proposed theoretical advancement Tube-on-sheet solar panel with wirecoil inserts, using TRNSYS as the simulating (CFD) tool. Direct absorption solar collector operated on nanofluids. Use of nano-materials in the solar devices Trapezoidal profile for absorber plate Indirect force circulation system using TRNSYS as the simulating tool. 1. If Teflon film used as second glazing 2. If Teflon honeycomb used. 3.Antireflection treatment of cover. 4. With external booster reflectors used.

Effect of advancement Thermal efficiency increases up to 4.5%

Reference Martin et al.[6]

Efficiency improvement up to 5% by utilizing nanofluids as the absorption mechanism Energy loss reduce and compact designs obtaibed. Give optimum efficiency

T.P. Otanicar et al. [4] E. Natarajan and R. Sathish [14] B. Kundu [12]

Increase in hot water supply to demand in winter conditions.

A. Hobbi and K. Siddiqui [10]

1.Increase in performance estimated to 5.6%. 2. Increase in performance estimated to 12.1%. 3. Increase in efficiency estimated to 6.5%. 4. Increase in efficiency estimated to 19.9 to 29.4%.

B. Hellstrom et.al. [15]

Table: 5.3 Practical advancements in flat plate collector and its effect summary Practical Advancements :- Design consists of tube-side enhancement using a smooth round tube and twisted tapes, wire coils insertion inside carrying tubes.

Fig. 1.:- The helical-wire-coil fitted in the raisers of the modified solar collector.

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Fig 2:- Thermal Efficiency curves for conventional and enhanced flat plate collector with insertion wire coil and smooth tubes. [1]

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME

Practical 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 and a very absorptive absorber plate with rough surface used. [2]

Fig.4. :- Comparison of baseline design „double glazed, rough, nonselective absorber with existing air heaters

Practical Advancements: - Two-dimensional lens array fitted on planar glass cover using localized features placed at each lens focus.

Fig.5. :- The main components of the micro-optic slab concentrator with focusing lens array for Photo voltaic C cell Flat plate concentrator . [5]

Fig.: Analysis of a 2mm diameter lens array and 1mm slab waveguide symmetrically coupling light to both edges.

7. CONCLUSIONS This paper highlights the advancements in design configurations and component material investigation to enhance efficiency and performance of flat plate collector. It has been found that flat plate collector enhancement widely investigated both analytically and experimentally. Overall, improving the transmissivity of the glazing by using highly transmissive polymer films or low iron glass, and using a very 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 fluids provide 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) © IAEME application worldwide. Solar radiation concentration using optical lens arrays make possible to achieve high temperature using conventional flat plate collector will provide cost effective performance. Optimized study of various operating parameters of flat plate collector proved improved efficiency with cost reduction. The information presented here will be beneficial for beginners in this area of research. REFERENCE 1 1 1 1. 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 Collectors for Humidification-Dehumidification Desalination Systems”, Journal of Solar Energy Engineering, ASME FEBRUARY 2011, Vol. 133 / 011016 3. Madhukeshwara. N1, E. S. Prakash2 (2012), “An investigation on the performance characteristics of solar flat plate collector with different selective surface coatings”, INTERNATIONAL JOURNAL OF ENERGY 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). “Nanofluidbased 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 in an 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). “Simulation of 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 solarthermal 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 solar concentrator” , 2010 Optical Society of America, 18 January 2010 / Vol. 18, No. 2 / OPTICS EXPRESS 1122. 9. Jason H. Karp and Joseph E. Ford “Planar micro-optic solar concentration using multiple imaging lenses into a common slab waveguide”, High and Low Concentrator Systems for Solar Electric Applications 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 system for 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 performance of building integrated solar collectors”, Solar Energy Materials & Solar Cells, 94,350-354. 12. B. Kundu, (2002). “Performance analysis and optimization of absorber plates of different geometry 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 optical and 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 and evacuated 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 flatplate 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.

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