THERMAL PERFORMANCE OF NANOFLUID FILLED SOLAR FLAT PLATE COLLECTOR

Transcription

1 INTERNATIONAL JOURNAL OF HEAT AND TECHNOLOGY Vol.33, No.2, THERMAL PERFORMANCE OF NANOFLUID FILLED SOLAR FLAT PLATE COLLECTOR Rehena Nasr* and M. A. Alim Department o Mathematics, Bangladesh University o Engeerg & Technology, Dhaka-1000, Bangladesh. raity11@gmail.com ABSTRACT A numerical study has been conducted to vestigate the orced convection through a lat plate solar collector (FPSC). The water Cu nanoluid is used as the workg luid side the riser pipe o the solar collector. The governg dierential equations with boundary conditions are solved by Fite Element Method usg Galerk s weighted residual scheme. The computation doma is discretized by triangular element with six nodes. The eects o major system parameters on the orced convection heat transer are simulated. These parameters clude the the solar irradiation (I) and diameter (D) o the riser pipe. Comprehensive average Nusselt number, mean temperature, mean velocity, percentage o collector eiciency, output temperature or both nanoluid and base luid through the absorber tube are presented as unctions o the pertag parameters mentioned above. The numerical results show that the highest heat transer rate is observed or both the highest I and lowest D. Percentage o collector eiciency enhances or growg I and allg D. Keywords: Thermal perormance, Flat plate solar collector, Fite element method, Water-Cu nanoluid, Solar irradiation. 1. INTRODUCTION The lat-plate solar collector is commonly used today or the collection o low temperature solar thermal energy. It is used or solar water-heatg systems homes and solar space heatg. Because o the desirable environmental and saety aspects it is widely believed that solar energy should be utilized stead o other alternative energy orms, even when the costs volved are slightly higher. Solar collectors are key elements many applications, such as buildg heatg systems, solar dryg devices, etc. Solar energy has the greatest potential o all the sources o renewable energy especially when other sources the country have depleted. The luids with solid-sized nanoparticles suspended them are called nanoluids. Applications o nanoparticles thermal ield are to enhance heat transer rom solar collectors to storage tanks, to improve eiciency o coolants transormers. Lund [1] analyzed general thermal behavior o parallellow lat-plate solar collector absorbers. Nag et al. [2] analyzed parametric study o parallel low lat plate solar collector usg ite element method. Piao et al. [3] studied orced convective heat transer cross-corrugated solar air heaters. Kolb et al. [4] experimentally studied solar air collector with metal matrix absorber. Tripanagnostopoulos et al. [5] vestigated solar collectors with colored absorbers. Kazemejad [6] numerically analyzed two dimensional parallel low lat-plate solar collectors. Temperature distribution over the absorber plate o a parallel low latplate solar collector was analyzed with one- and twodimensional steady-state conduction equations with heat generations. Lambert et al. [7] conducted Enhanced heat transer usg oscillatory lows solar collectors. They proposed the use o oscillatory lamar lows to enhance the transer o heat rom solar collectors. The idea was to explore the possibility o transerrg the heat collected rom a solar device to a storage tank by means o a zeromean oscillatg luid contaed a tube. Selected nanoluids might improve the eiciency o direct absorption solar thermal collectors. To determe the eectiveness o nanoluids solar applications, their ability to convert light energy to thermal energy must be known. That is, their absorption o the solar spectrum must be established. Struckmann [8] analyzed lat-plate solar collector where eorts had been made to combe a number o the most important actors to a sgle equation and thus ormulate a mathematical model which would describe the thermal perormance o the collector a computationally eicient manner. Azad [9] vestigated terconnected heat pipe solar collector. Perormance o a prototype o the heat pipe solar collector was experimentally examed and the results were compared with those obtaed through theoretical analysis. Tyagi et al. [10] vestigated Predicted eiciency o a low-temperature nanoluid- based direct absorption solar collector. It was observed that the presence o nanoparticles creased the absorption o cident radiation by more than ne times over that o pure water. Accordg to the results obtaed rom this study, under 17

2 similar operatg conditions, the eiciency o a DAC usg nanoluid as the workg luid was ound to be up to 10% higher than that o a lat-plate collector. Generally a direct absorption solar collector (DASC) usg nanoluids as the workg luid perorms better than a lat-plate collector. Much better designed lat-plate collectors might be able to match a nanoluid based DASC under certa conditions. Otanicar et al. [11] studied nanoluid-based direct absorption solar collector. They reported on the experimental results on solar collectors based on nanoluids made rom a variety o nanoparticles carbon nanotubes, graphite, and silver. They demonstrated eiciency improvements o up to 5% solar thermal collectors by utilizg nanoluids as the absorption mechanism. In addition the experimental data were compared with a numerical model o a solar collector with direct absorption nanoluids. Álvarez et al. [12] studied ite element modellg o a solar collector. A mathematical model o a serpente latplate solar collector usg ite elements was presented. Karanth et al. [13] perormed numerical simulation o a solar lat plate collector usg discrete transer radiation model (DTRM) a CFD Approach. Dynamics (CFD) by employg conjugate heat transer showed that the heat transer simulation due to solar irradiation to the luid medium, creased with an crease the mass low rate. Also it was observed that the absorber plate temperature decreased with crease the mass low rate. Martín et al. [14] also analyzed experimental heat transer research enhanced lat-plate solar collectors. To test the enhanced solar collector and compare with a standard one, an experimental side-byside solar collector test bed was designed and constructed. Enhancement o lat-plate solar collector thermal perormance with silver nano-luid was conducted by Polvongsri and Kiatsiriroat [15]. With higher thermal conductivity o the workg luid the solar collector perormance could be enhanced compared with that o water. The solar collector eiciency with the nano-luid was still high even the let temperature o the workg luid was creased. Taylor et al. [16] analyzed nanoluid optical property characterization: towards eicient direct absorption solar collectors. Their study compared model predictions to spectroscopic measurements o extction coeicients over wavelengths that were important or solar energy (0.25 to 2.5 μm). Modelg o lat-plate solar collector operation transient states was conducted by Saleh [17]. This study presents a one-dimensional mathematical model or simulatg the transient processes which occur liquid latplate solar collectors. The proposed model simulated the complete solar collector system cludg the lat-plate and the storage tank. Amrutkar et al. [18] studied solar lat plate collector analysis. The objective o their study was to evaluate the perormance o FPC with dierent geometric absorber coniguration. It was expected that with the same collector space higher thermal eiciency or higher water temperature could be obtaed. Karuppa et al. [19] experimentally vestigated a new solar lat plate collector. Experiments had been carried out to test the perormance o both the water heaters under water circulation with a small pump and the results were compared. The results showed that the system could reach satisactory levels o eiciency. Zambol [20] theoretically and experimentally perormed solar thermal collector systems and components. Testg o thermal eiciency and optimization o these solar thermal collectors were addressed and discussed this work. Sandhu [21] experimentally studied temperature ield lat-plate collector and heat transer enhancement with the use o sert devices. Various new conigurations o the conventional sert devices were tested over a wide range o Reynolds number ( ). Comparison o these devices showed that lamar low regime, wire mesh proved to be an eective sert device and enhanced Nusselt number by 270% while turbulent low regime. While Reynolds number range , concentric coil sert signiicantly creased the heat transer and an crease o 460% Nusselt number was witnessed. Mahian et al. [22] perormed a review o the applications o nanoluids solar energy. The eects o nanoluids on the perormance o solar collectors and solar water heaters rom the eiciency, economic and environmental considerations viewpots and the challenges o usg nanoluids solar energy devices were discussed. Natarajan & Sathish [23] studied role o nanoluids solar water heater. Heat transer enhancement solar devices is one o the key issues o energy savg and compact designs. The aim o this paper was to analyze and compare the heat transer properties o the nanoluids with the conventional luids. Dara et al. [24] conducted evaluation o a passive lat-plate solar collector. The research vestigated the variations o top loss heat transer coeicient with absorber plate emittance; and air gap spacg between the absorber plate and the cover plate. Iordanou [25] vestigated lat-plate solar collectors or water heatg with improved heat Transer or application climatic conditions o the mediterranean region. The aim o this research project was to improve the thermal perormance o passive lat plate solar collectors usg a novel cost eective enhanced heat transer technique. This work ocused on the process o energy conversion rom the collector to the workg luid. This was accomplished by employg an alumium grid placed the channels o a collector to duce a gradient o heat capacitance. The numerical simulations ocused on the thermal and hydrodynamic behavior o the collector. Conduction convection radiation processes o a solar collector usg FEA was perormed by Mongi [26]. Radiation domated the other two processes. It beg nonlear phenomena required an iterative procedure to solve problems analytically, which was quite diicult. So, he tried to d the temperature distribution o the solar collector usg FEA. Chabane et al. [27] studied thermal perormance optimization o a lat plate solar air heater. Experimentally vestigates o sgle pass solar air heater without s; present the aims to review o designed and analyzed a thermal eiciency o lat-plate solar air heaters. The received energy and useul energy rates o the solar air heaters were evaluated or various air low rates were (0.0108, , , and kg.s -1 ) are vestigated. Optimum values o air mass low rates were suggested maximizg the perormance o the solar collector. Nasr and Alim [28-29] studied Soret and Duour eects on double diusive natural convection usg nanoluid with dierent nanoparticlesilled solar collector. In the light o above discussions, it is seen that there has been a good number o works the ield o heat transer system through a lat plate solar collector. In spite o that there is some scope to work with luid low, coolg system and enhancement o collector eiciency usg nanoluid. 18

3 In this paper, the thermal perormance o a lat plate solar collector is studied numerically. The objective o this paper is to present heat transer system through a solar collector or the eect o solar irradiation and diameter o the riser pipe. 2. MATHEMATICAL MODELING I I is the tensity o solar radiation, cident on the aperture plane o the solar collector havg a collector surace area o A, then the amount o solar radiation received by the collector is where T and T out are let and outlet mean temperature o luid, respectively. The maximum possible useul energy ga a solar collector occurs when the whole collector is at the let luid temperature. The actual useul energy ga (Q usl), is ound by multiplyg the collector heat removal actor (F R) by the maximum possible useul energy ga. This allows the rewritg o equation (4): Qu F sl R AI h T Tamb (7) Q i I. A (1) However, a part o this radiation is relected back to the sky, another component is absorbed by the glazg and the rest is transmitted through the glazg and reaches the absorber plate as short wave radiation. Thereore the conversion actor dicates the percentage o the solar rays penetratg the transparent cover o the collector (transmission) and the percentage beg absorbed. Basically, it is the product o the rate o transmission o the cover (λ) and the absorption rate o the absorber (к). Thus Q recv I A (2) As the collector absorbs heat its temperature is gettg higher than that o the surroundg and heat is lost to the atmosphere by convection and radiation. The rate o heat loss (Q loss) depends on the collector overall heat transer coeicient (h) and the collector temperature Q loss col amb ha T T (3) Thus, the rate o useul energy extracted by the collector (Q usl), expressed as a rate o extraction under steady state conditions, is proportional to the rate o useul energy absorbed by the collector, less the amount lost by the collector to its surroundgs. This is expressed as Q u sl recv loss A hat T Q Q I (4) col amb where T col and T amb are collector temperature and ambient temperature outside the collector respectively. It is also known that the rate o extraction o heat rom the collector may be measured by means o the amount o heat carried away the luid passed through it, which is Q usl p out mc T T (5) Equation (4) may be convenient because o the diiculty deg the collector average temperature. It is convenient to dee a quantity that relates the actual useul energy ga o a collector to the useul ga i the whole collector surace were at the luid put temperature. This quantity is known as the collector heat removal actor (F R) and is expressed as: F R mcp Tout T AI h T T amb (6) Figure 1. Schematic diagram o the solar collector Equation (7) is a widely used relationship or measurg collector energy ga and is generally known as the Hottel- Whillier-Bliss equation. The heat lux per unit area q is now denoted as Q u sl A amb q I h T T (8) A cross section o the system considered the present study is shown Fig. 1. The system consists o a lat plate solar collector. The numerical computation is carried on takg sgle riser pipe o FPSC. The glass cover is at the top o the FPSC. It is highly transparent and anti-relected (called the glazg). The glass top surace is exposed to solar irradiation. It is made up o borosilicate which has thermal conductivity o 1.14 W/mK and reractive dex o 1.47, speciic heat o 750 J/kgK and coeicient o sunlight transmission o 95%. The wavelength o visible light is roughly 700 nm. Thickness o glass cover is 0.005m. There is an air gap o 0.005m between glass cover and absorber plate. Air density = Kg/m 3, speciic heat = J/kgK and thermal conductivity = W/mK. All these properties o air doma represent air o temperature at 298K. A dark colored copper absorber plate is under the air gap. Length, width and thickness o the absorber plate are 1m, 0.15m and m respectively. Coeicients o heat absorption and emmision o copper absorber plate are 95% and 5% respectively. The riser pipe has ner diameter 0.01 m and thickness m. The riser tube is also made copper metal. The workg luid the collector is water-based nanoluid contag Cu nanoparticles. The nanoparticles are generally spherical shaped and diameter is taken as 5 nm. The nanoluid is considered as sgle phase low and suractant analysis is neglected. A, L and D are the surace area o the collector, length and diameter o the riser. In the present problem, it can be considered that the low is considered to be lamar and there is no viscous dissipation. The nanoluid 19

4 is assumed compressible. It is taken that water and nanoparticles are thermal equilibrium and no slip occurs between them. The density o the nanoluid is approximated by the Boussesq model. Only steady state case is considered. The computation doma is a luid passg copper riser pipe which is attached ultrasonically to the absorber plate. The luid enters rom the let let and gettg heat orm solid boundaries and ally exits rom the right let o the riser pipe o a lat plate solar collector. The cident radiation is considered to be the comg solar radiation. For the study o the prcipal behavior o the nanoluid based FPSC, atmospheric absorption is neglected this calculation. Once the tensity distributions are evaluated, the energy balance on the solar collector is perormed and the temperature proile with it is obtaed. In order to carry out these steps, some assumptions are made. T at outer surace o riser pipe: 0 y The above equations are non-dimensionalized by usg the ollowg dimensionless dependent and dependent variables: x y u v X, Y, U, V, L L U U Then the non-dimensional governg equations are U V 0 X Y p T T k T T k P,, a U ql ql 2 a a (14) u v 0 x y (9) U U P n 1 U U U V X Y n X Re X Y (15) u u p u u n u v n x y x x y (10) V V P n 1 V V U V X Y n Y Re X Y (16) v v p v v n u v n x y y x y (11) 1 n U V X Y RePr X Y (17) n u T v T T T x y x y x Ta Ta y 0 The thermal diusivity k C The density 1 n n p n n s The heat capacitance Cp 1 Cp Cp n s (12) (13) The viscosity o the nanoluid is considered by the Pak and Cho correlation [30]. This correlation is given 2 as n The thermal conductivity o Maxwell Garnett (MG) model [31] is k n k k 2k 2 k k s s k 2k k k s s The boundary conditions are: at all solid boundaries: u = v = 0 T Ta at the solid-luid terace: kn ksolid y y n solid at the let boundary: T T, u = u at the outlet boundary: convective boundary condition p = 0 at the top surace o absorber: heat lux Ta ka q I ht Tamb y a a 0 X Y (18) ν U where Pr is the Prandtl number, L Re α is the Reynolds number. The correspondg boundary conditions take the ollowg orm: at all solid boundaries: U = V = 0 a at the solid-luid terace: kn ksolid Y Y 0 n solid at the let boundary: θ = 0, U = 1 at the outlet boundary: convective boundary condition P = at the top surace o absorber: heat lux k a Y k T at outer surace o riser pipe: 0 y The non-dimensional orm o local convective heat kn transer at the top surace is Nu. k Y By tegratg the local Nusselt number over the top heated surace, the average convective heat transer along the heated wall o the collector is used by Saleh et al. [32] 1 as Nu Nu dx. 0 The mean bulk temperature and average sub doma velocity o the luid side the collector may be written as dv / V and V V dv / V, where V is the av volume o the collector. av n 20

5 A measure o a lat plate collector perormance is the collector eiciency (η) deed as the ratio o the useul energy ga (Q usl) to the cident solar energy. The stantaneous thermal eiciency o the collector is: Q F A I h T T AI AI T Tamb FR FR h I usl R amb where m is the mass low rate o the luid lowg through the collector; C p is the speciic heat at constant pressure. 3. NUMERICAL TECHNIQUE The Galerk ite element method Taylor and Hood [33] and Dechaumphai [34] is used to solve the non-dimensional governg equations along with boundary conditions or the considered problem. The equation o contuity has been used as a constrat due to mass conservation and this restriction may be used to d the pressure distribution. The ite element method o Reddy [35] is used to solve the Eqs. (19) - (22), where the pressure P is elimated by a constrat. The contuity equation (19) is automatically ulilled or large values o this constrat. Then the velocity components (U, V) and temperature (θ) are expanded usg a basis set. The Galerk ite element technique yields the subsequent nonlear residual equations. Three pots Gaussian quadrature is used to evaluate the tegrals these equations. The non-lear residual equations are solved usg Newton Raphson method to determe the coeicients o the expansions. The convergence o solutions is assumed when the relative error or each variable between consecutive iterations is recorded below the convergence criterion such that n1 n 4 10, where n is the number o iteration and Ψ is a unction o U, V and θ. 3.1 Mesh Generation In the ite element method, the mesh generation is the technique to subdivide a doma to a set o sub-domas, called ite elements, control volume, etc. The discrete locations are deed by the numerical grid, at which the variables are to be calculated. It is basically a discrete representation o the geometric doma on which the problem is to be solved. The computational domas with irregular geometries by a collection o ite elements make the method a valuable practical tool or the solution o boundary value problems arisg various ields o engeerg. Fig. 2 displays the ite element mesh o the present physical doma Thermo-physical properties The thermo-physical properties o the nanoluid are taken rom Ogut [22] and given Table 1. Table 1. Thermo-physical properties o luid and nanoparticles Physical Properties Fluid phase (Water) Cu Cp(J/kgK) (kg/m 3 ) k (W/mK) α10 7 (m 2 /s) Grid Independent Test An extensive mesh testg procedure is conducted to guarantee a grid-dependent solution or Pr = 5.8, = 2%, I = 215W/m 2 and Re = 480 through a riser pipe o a FPSC. In the present work, our dierent non-uniorm grid systems are examed with the number o elements with the resolution ield: 42,010, 99,832, 1,50,472,1,68,040 and 1,92,548. The numerical scheme is carried out or highly precise key the average Nusselt number or water-copper nanoluid ( = 2%) as well as base luid ( = 0%) or the aoresaid elements to develop an understandg o the grid eness as shown Table 2. From the ourth and ith column the Table 2 it is observed that there is no signiicant change the rate o heat transer but time consumg. So considerg the non-uniorm grid system o 1,68,040 elements is preerred or the computation. This element size is taken rom extra e meshg. Table 2. Grid test at Pr = 5.8, = 2%, D = 0.01m, I = 215W/m 2 and Re = 480 Elements 42,010 99,832 1,40,472 1,68,040 1,92,548 Nu (Nanoluid) Nu (Base luid) Time (s) RESULTS AND DISCUSSIONS In this section, numerical results o the average Nusselt number, mean bulk temperature, mean sub doma velocity, percentage o collector eiciency and dimensional outlet temperature o the water-copper nanoluid as well as clear water a lat plate solar collector are shown graphically or various values o solar irradiation (I) and dimensionless ner diameter (D). The considered values o I and D are I = (200 W/m 2, 215 W/m 2, 230 W/m 2 and 250 W/m 2 ), D (= 0.01m, 0.012m, 0.013m and 0.015m) while the solid volume raction o Cu nanoparticle is = 2%. Here, Reynolds number (Re), Prandtl number (Pr), collector area (A), mass low rate per unit area (m) are considered as 480, 5.8, 1.8m 2 and Kg/s respectively. Figure 2. Mesh generation o the FPSC 21

6 4.1 Heat Transer rate The Nu-I and Nu-D proiles or Cu/water nanoluid as well as base luid are depicted Fig. 4-. It is seen rom Fig. 3 that the average Nusselt number creases with risg solar irradiation or both type luids. The rate o heat transer enhances 13% and 9% usg 2% concentrated water-copper nanoluid and water respectively or risg solar irradiation (I) rom 200 W/m 2 to 250 W/m 2 when D = 0.01m. Fig. 3 shows that Nu devalues slowly with growg ner diameter (D) o the riser pipe o a lat plate solar collector. The rate o heat loss or water-cu nanoluid ( = 2%) is ound to be lower than the clear water ( = 0%) due to higher thermal conductivity o solid nanoparticles. Heat loss rate dimishes by 10% and 15% or nanoluid and base luid respectively at the solar irradiance I = 215W/m 2. Growg solar radiance and allg ner diameter enhance the mean velocity o the luids through the riser pipe o a lat plate solar collector. Figure 5. Mean velocity or the eect o I at D = 0.01m and D at I = 215 w/m 2 Figure 3. Mean Nusselt number or the eect o I at D = 0.01m and D at I = 215 w/m Mean bulk temperature Fig. 4- displays mean temperature (θ av) along with the solar radiance and dimensionless ner diameter or both type o luids. θ av rises sequentially or growg I and allg D. It is well known that higher values o solar irradiation (I) dicate higher temperature o luids. Due to escalatg values o I top and bottom suraces o the riser pipe become more heated. As a result luids can take more heat rom hot walls and become hotter. Here base luid has lower mean temperature than the water-cu nanoluid. Figure 6. Collector eiciency or the eect o I at D = 0.01m and D at I = 215 w/m Collector eiciency The collector eiciency (η) is plotted agast the solar irradiation and ner diameter Fig. 6-. It is observed rom Fig. 6 that by troducg greater solar irradiation the collector eiciency creases. More solar irradiance is able to augment heat transer system through the riser pipe o a lat plate solar collector. In this scheme water/copper nanoluid ( = 2%) perorms better than clear water ( = 0%). Thermal eiciency enhances rom 44%-50% or nanoluid and 39%-45% or water. The Fig. 6 shows that the use o small ner diameter o the riser pipe can improve the eiciency o a lat plate solar collector. When the non-dimensional ner diameter creases, the thermal eiciency (η) o the lat plate solar collector dimishes moderately. In other words, the heat extracted rom riser pipe by the luids cannot be creased by growg D. Thermal eiciency devalues rom 45%-35% or nanoluid and 40%-28% or water. 4.5 Outlet temperature Figure 4. Average temperature or the eect o I at D = 0.01m and D at I = 215 w/m Magnitude o average velocity Magnitude o average velocity vector (V av) or the eect o solar irradiation and ner diameter are expressed the Fig. 5-. V av has notable changes with dierent values o solar radiance as well as ner diameter o riser pipe. Clear water moves reely than solid concentrated nanoluid. Fig. 7- displays the dimensional temperature (K) o water-cu nanoluid at the middle height o the riser pipe with the luences o solar irradiation and dimensionless ner diameter o the riser pipe. From the igure 7 it is observed that the mean output temperature o luids creases or the creasg values o I. The output temperature o nanoluid becomes 312K, 313K, 315K, 317K or nanoluid and 309K, 310K, 311K, 313K or base luid or I = 200W/m 2, 215 W/m 2, 230 W/m 2 and 250 W/m 2 respectively. Similarly outlet temperature o Cu/water nanoluid decreases with growg ner diameter (D) o the riser pipe. 22

7 The output temperature o nanoluid becomes 313K, 312K, 312K and 310K or nanoluid and 310K, 309K, 308K and 307K or base luid D = 0.01m, 0.012m, 0.013m and 0.15m respectively. Figure 7. Outlet temperature or the eect o I at D = 0.01m and D at I = 215 w/m 2 5. CONCLUSION The results o the numerical analysis lead to the ollowg conclusions: (1) The Cu nanoparticles with the highest I and lowest D are established to be most eective enhancg perormance o heat transer rate than base luid. (2) Average velocity ield creases due to growg I and dimishg D. (3) Collector eiciency is obtaed higher or risg I and allg D. (4) Mean temperature devalues or both luids with lesseng solar radiance and risg ner diameter. ACKNOWLEDGEMENT The present numerical work is done the Department o Mathematics, Bangladesh University o Engeerg & Technology. The research is supported by the Research Support & Publication Division, University Grants Commission, Agargaon, Bangladesh. REFERENCES 1. K.O. Lund, General thermal analysis o parallel-low lat-plate solar collector absorbers, Solar Energy, vol. 5, 443, A. Nag, D. Misra, K.E. De, A. Bhattacharya, S.K. Saha, Parametric study o parallel low lat plate solar collector usg ite element method, In: Num. Methods Therm. Problems, Proc. o the 6th Int. Con., Swansea, UK, Y. Piao, EG. Hauptmann, M. Iqbal, Forced convective heat transer cross-corrugated solar air heaters, ASME Journal o Solar Energy Engg., Vol. 116, pp , A. Kolb, ERF Wter, R. Viskanta, Experimental studies on a solar air collector with metal matrix absorber, Solar Energy, vol. 65, No. 2, pp , Y. Tripanagnostopoulos, M. Souliotis, T. Nousia, Solar collectors with colored absorbers, Solar Energy, vol. 68, pp , H. Kazemejad, Numerical analysis o two dimensional parallel low lat-plate solar collector, Renew. Energy, vol. 26, pp , A.A. Lambert, S. Cuevas, J.A. del Rı o, Enhanced heat transer usg oscillatory lows solar collectors, Solar Energy, vol. 80, pp , F. Struckmann, Analysis o a Flat-plate Solar Collector, Project Report 2008 MVK160 Heat and Mass Transport, Lund, Sweden, E. Azad, Interconnected heat pipe solar collector, IJE Transactions A: Basics, vol. 22, No. 3, pp. 233, September H. Tyagi, P. Phelan, R. Prasher, Predicted Eiciency o a Low-Temperature Nanoluid- Based Direct Absorption Solar Collector, J. o Solar Energy Engg., vol. 131/ , T.P. Otanicar, P.E. Phelan, R.S. Prasher, G. Rosengarten, and R.A. Taylor, Nanoluid-based direct absorption solar collector, J. o Renew. and Sust. Energy, vol. 2, pp , A. Álvarez, M.C. Muñiz, L.M. Varela, O. Cabeza, Fite element modellg o a solar collector, Int. Con. on Renew. Energies and Power Quality, Granada (Spa), K.V. Karanth, M.S. Manjunath, N.Y. Sharma, Numerical simulation o a solar lat plate collector usg discrete transer radiation model (DTRM) a CFD approach, Proc. o the World Congress on Engg., vol III, WCE 2011, London, U.K. 14. R.H. Martín, A.G. Par, J.P. García, Experimental heat transer research enhanced lat-plate solar collectors, Solar Thermal Applications, World Renew. Energy Congress-2011, pp S. Polvongsri and T. Kiatsiriroat, Enhancement o Flat- Plate Solar Collector Thermal Perormance with Silver Nano-luid, The 2nd TSME Int. Con. on Mech. Engg., Krabi, R.A. Taylor, P.E. Phelan, T.P. Otanicar, R. Adrian, R. Prasher, Nanoluid optical property characterization: towards eicient direct absorption solar collectors, Nanoscale Research Letters, vol. 6, pp. 225, A.M. Saleh, Modelg o lat-plate solar collector operation transient states, thesis o Master o Sci. Engg., 2012, Purdue University, Fort Wayne, Indiana. 18. S.K. Amrutkar, S. Ghodke, Dr.K.N. Patil, Solar lat plate collector analysis, IOSR J. o Engg, vol. 2, No. 2, pp , R.R.T.K., P. Pavan and D.R. Rajeev, Experimental vestigation o a new solar lat plate collector, Research J. o Engg. Sciences, vol. 1, No. 4, pp.1-8, E. Zambol, Theoretical and experimental study o solar thermal collector systems and components, Scuola di Dottorato di Ricerca Ingegneria Industriale, Indirizzo Fisica Tecnica. 21. G. Sandhu, Experimental study o temperature ield lat-plate collector and heat transer enhancement with the use o sert devices, M. o Engg. Sci. thesis, The School o Graduate and Postdoctoral Studies, The University o Western Ontario London, Ontario, Canada, O. Mahian, A. Kianiar, S.A. Kalogirou, I. Pop, S. Wongwises, A review o the applications o nanoluids solar energy, Int. J. o Heat and Mass Trans., vol. 57, pp ,

8 23. E. Natarajan & R. Sathish, Role o nanoluids solar water heater, Int J Adv Manu.Technol. DOI /s J.E Dara, K.O. Ikebudu, N.O. Ubani, C.E. Chwuko, O.A. Ubachukwu, Evaluation o a passive lat-plate solar collector, Int. J. o Advancements Res. & Tech., vol. 2, No. 1, G. Iordanou, Flat-plate solar collectors or water heatg with improved heat transer or application climatic conditions o the mediterranean region, Doctoral thesis, Durham University. 26. M.K. Mongi, Conduction convection radiation processes o a solar collector usg FEA, University o Massachusetts, Amherst. 27. F. Chabane, N. Moummi, S. Benramache, D. Bensahal, O. Belahssen, F.Z. Lemmadi, Thermal perormance optimization o a lat plate solar air heater, Int. J. o Energy & Tech., vol. 5, no. 8, pp. 1 6, R. Nasr and M.A. Alim, Soret and Duour eects on double diusive natural convection a chamber usg nanoluid, Int. J. o Heat & Tech., vol. 30, no. 1, pp , R. Nasr and M.A. Alim, Duour-Soret eects on buoyant convection through a nanoluid with dierent nanoparticles-illed solar collector, Int. J. o Heat &Tech., vol. 31, no. 1, pp , B.C. Pak, Y. Cho, Hydrodynamic and heat transer study o dispersed luids with submicron metallic oxide particle, Experimental Heat Transer, vol. 11, pp , J.C. Maxwell-Garnett, Colours metal glasses and metallic ilms, Philos. Trans. Roy. Soc. A, vol. 203, pp , H. Saleh, R. Roslan, I. Hashim, Natural convection heat transer a nanoluid-illed trapezoidal enclosure, Int. J. o Heat and Mass Trans., vol. 54, pp , C. Taylor, P. Hood, A numerical solution o the Navier- Stokes equations usg ite element technique, Computer and Fluids, vol. 1, pp , P. Dechaumphai, Fite Element Method Engeerg, 2nd ed., Chulalongkorn University Press, Bangkok, J.N. Reddy and D.K. Gartlg, The Fite Element Method Heat Transer and Fluid Dynamics, CRC Press, Inc., Boca Raton, Florida, E.B. Ogut, Natural convection o water-based nanoluids an cled enclosure with a heat source, Int. J. o Thermal Sciences, vol. 48, No. 11, pp , S.A. Kalogirou, Solar thermal collectors and applications, Progress Energy and Combustion Science, vol. 30, pp , NOMENCLATURES A Surace area o the collector (m 2 ) C p Speciic heat at constant pressure (J kg -1 K -1 ) d Inner diameter o riser pipe (m) D Dimensionless ner diameter o pipe h Local heat transer coeicient (W m -2 K -1 ) I Intensity o solar radiation (W m -2 ) k Thermal conductivity (W m -1 K -1 ) L Length o the riser pipe (m) m mass low rate (Kg s -1 ) Nu Nusselt number, Nu = hl/k Pr Prandtl number Re Reynolds number T Dimensional temperature (K) T Input temperature o luid (K) T out Output temperature o luid (K) u, v Dimensional x and y components o velocity (m s -1 ) U, V Dimensionless velocities U i Input velocity o luid (m s -1 ) X, Y Dimensionless coordates x, y Dimensional co-ordates (m) Greek Symbols α Fluid thermal diusivity (m 2 s -1 ) Nanoparticles volume raction ν Kematic viscosity (m 2 s -1 ) η collector eiciency, θ Dimensionless temperature ρ Density (kg m -3 ) μ Dynamic viscosity (N s m -2 ) V Dimensionless velocity ield Subscripts av average col collector luid n nanoluid s solid particle 24