Enhanced Heat Transfer and Fluid Flow in a Channel Behind a

Article pubs.acs.org/IECR

Enhanced Heat Transfer and Fluid Flow in a Channel Behind a Photovoltaic Panel in a Hybrid Photovoltaic/Thermal System Syed Mohd Yahya,*,† Syed Fahad Anwer,‡ and Sanjeev Sanghi† †

Department of Applied Mechanics, Indian Institute of Technology, New Delhi,110016, India Department of Mechanical Engineering, Aligarh Muslim University, Aligarh, 202002, India

‡

ABSTRACT: Thermal large eddy simulation (TLES) of air flow in a simple channel is being carried out to understand the insight physics and possibility of enhancing heat transfer in a photovoltaic/thermal system. A photovoltaic panel operating at higher temperature loses its efficiency; to alleviate this situation, a simple channel configuration at the rear of the panel is used to extract maximum heat and keep the electrical efficiency in permissible limits. Forced convection of air is being simulated with different complex internal geometries in a biperiodic channel using a low Mach number approach aiming at enhanced turbulence mixing. Detailed studies of the flow and thermal fields of the air are presented in order to explore the thermal behavior of the air in the channel. Comparison with an empty channel and a classical channel with fins and internal innovative structures are carried out for choosing a suitable configuration for better performance. It has been observed that artificial hindrances in the form of fins and DWVGs (delta-winglet vortex generators) inside the channel is one effective way of improving heat extraction from the channel.

1. INTRODUCTION Reduction in the electrical efficiency of a photovoltaic module due to increase in temperature can be partially avoided by excess heat removal. One impetus for this type of work is the interest in hybrid systems (i.e., the combined generation of heat and electricity). A simple and low cost method for the removal of heat from a PV module is to circulate air through an insulated channel mounted at the rear side of a PV panel. The major applications of solar energy utilization are include solar thermal and solar PV systems. In a PV energy conversion system, the unconverted part from the solar radiation into electricity is transformed into thermal energy. This thermal energy causes an increase in cell temperature and leads to a drop in the efficiency as reported by Park et al.1 Increasing the PV module temperature is the main hindrance to its performance, particularly in hot arid areas as reported by Ali.2 Recently Gan3 used computational fluid dynamics (CFD) to assess the effect of the size of air gap between PV modules and the building envelope on the PV performance. Several studies refer to theoretical and experimental results of air cooled hybrid photovoltaic/thermal (PV/T) systems; most report a maximum thermal efficiency of about 35%. Among the first works on the subject are those of Kern and Russel,4 Hendrie,5 and Raghuraman.6 Later, Bhargava et al.,7 Prakash,8 and Sopian et al.9 studied PV/T system performance regarding air circulation parameters. Such kind of study also deals with electronic equipment in which enhanced heat transfer is also very useful, which otherwise results in damage due to thermal breakdown of circuits.10−12 Tonui et al.13 used a configuration to maximize the heat transfer through channel by inserting a thin metal sheet suspended in air at the middle of the channel with fins on the opposite wall, which improves the electrical as well as thermal efficiency of the PV/T system. Investigation based on energy and exergy evaluation is being reported by Anand et al.14 for studying the PV and PV/T performance and suggesting © 2013 American Chemical Society

the possible improvements for the system. However, in PV/T systems efforts have been made so that this thermal energy can be harnessed by combining both the thermal and photovoltaic in a hybrid system. In such cogenerated system the fluid used for heat transfer is typically air because in hot climatic regions, water resources are scarce. Hence, an effective cooling system is very important to maximize the photovoltaic cell efficiency and to prevent degradation and damage. Computational fluid dynamics (CFD) simulation has been widely used to determine the thermal performance of a cooling system for CPVs (concentrated photovoltaic), computer heat sinks, and others. Yang et al.15 built a photovoltaic wall test rig, which consisted of a massive wall, PV modules in front of the wall, an air duct between the PV modules and the wall, and an air inlet and air outlet of the air duct, to validate the experimental results with the simulation results. The result showed that simulation model could be used to predict the thermal behavior of PV wall and PV roof structures. However, to the authors knowledge, throughout the available literature, the characteristics of forced convective heat transfer and fluid flow in a biperiodic channel with varying fluid properties subjected to large temperature gradient with different complex internal geometries to maintain the efficiency of a PV/T hybrid system have not been analyzed. So the main objective of this paper is to study the flow statistics of the channel with varying thermophysical properties and incorporation of internal structures for enhancing the turbulence mixing and heat removal. Received: Revised: Accepted: Published: 18413

July 30, 2013 November 14, 2013 November 18, 2013 November 18, 2013 dx.doi.org/10.1021/ie402460n | Ind. Eng. Chem. Res. 2013, 52, 18413−18420

Industrial & Engineering Chemistry Research

Article

2. PHYSICAL MODEL AND NUMERICAL SIMULATION The physical model considered in this work along with characteristic dimensions is shown in Figure 1. With reference

Figure 3. Two-dimensional view of mesh in the x−z plane.

4πδ × (4πδ/3) × 2δ where δ = 1 is the half channel width. Simulations are carried out at Reτ = 180 while the Prandtl number is kept constant (Pr = 0.71). The internal geometry of a channel with asymmetric heating is modeled. Several internal geometries studied are the following: an empty channel, a classical channel with fins, and two geometries with the innovative internal structure inside the channel. Regarding the innovative internal structures, the lower wall is smooth whereas the inner face of the upper wall is covered with DWVGs (deltawinglet vortex generators) associated with riblets of length 0.1δ, as shown in Figure 2a−c. These vortex generators are of the form of equilateral triangle having side 0.5δ and width 5% of the channel height making an angle of 20° with the wall. While the classical channel with fins is shown in Figure 2d with height of about 15 mm and width 5 mm. The differences between the two geometries of channels is that the second one has lower riblets with a larger pitch that the first one. Thermal large eddy Simulations are carried out to investigate the thermal performance of such geometry. A comparison of a classical channel with fins and innovative internal structures with an empty channel is carried out. 2.1. Governing Equations and Numerical Modeling. In low-speed turbulent channel flow applications, with low Mach number, the variable-density approximation of the Navier− Stokes equations is a good basis for simulation, as it supports large density variations while eliminating acoustic waves. This eliminates the need for extremely small time steps guided by

Figure 1. Schematic of channel flow configuration.

to the schematics of Figure 1, we consider a weakly compressible and Newtonian turbulent flow of air in a plane channel with differentially heated walls, the hot wall is kept at temperature Thot while the cold wall is kept at temperature Tcold. The flow is driven by a constant pressure gradient along the streamwise “x” direction.The lower wall is fixed at a temperature of 300 K, i.e. Tcold = 300 K. The channel walls are normal to the “z” direction and are at constant temperature. The no-slip velocity condition is applied for the fluid velocity at walls, and the boundaries of domain normal to the x and y directions are periodic. All simulations were carried out at a finer mesh of 96 × 96 × 96. The mesh is nonuniform in the z direction, so the values of Δz+ near the walls are very small, while in the streamwise (x) and spanwise (y) direction, we use uniform mesh (see Figure 3). In the wall normal direction the first point away from the wall is at Δz+ = 0.18. Grid independency can be seen in our paper based on evaluation of models.16 We investigate numerically the physical mechanisms responsible for the interaction between the thermal and kinetic fields. The typical channel dimensions considered are

Figure 2. Sketch of the innovative internal structure. 18414

dx.doi.org/10.1021/ie402460n | Ind. Eng. Chem. Res. 2013, 52, 18413−18420


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Figure 4. (a) Mean velocity profile in semilog scale for model validation. (b) Mean velocity profile for different Rθ at Reτ = 180.

Figure 5. RMS velocity and temperature fluctuation at Reτ = 180 on the hot and cold walls.

the acoustic velocity. This means that the arising velocities are much smaller than the speed of sound, so that density variations due to pressure variations can be neglected. First the low Mach-number approximation of the Navier−Stokes equations is obtained as the low Mach-number asymptotic limit of the compressible Navier−Stokes equations. Details of the derivation of these equations can be found in the literature.17−20 Then, we Favre-averaged and filtered equations using implicit filter to obtained governing equation for channel flow configuration:21

∂ρ ̅ uj̃ ∂ρ ̅ + ∂t ∂xj

∂(ρ ̅ uĩ uj̃ ) ∂(ρ ̅ uĩ ) ∂P(1) 1 ∂τij̃ + Fδi1 =− + + ∂xj ∂xi ∂t Reτ ∂xj

(2)

⎡ ∂ρ T ∂ρ ̅ uj̃ T̃ ⎤ ⎥ Cp⎢ ̅ + ∂xj ⎥⎦ ⎢⎣ ∂t =

(1) 18415

(γ − 1) dP ̅ (0) 1 ∂ ⎛⎜ ∂T̃ ⎞⎟ ( ) + + κ κ t dt Re Pr ∂xj ⎜⎝ ∂xj ⎟⎠ γ

(3)

P ̅ (0) = ρ ̅ T̃

(4)

∂P ̅ (0) =0 ∂xi

(5)



Article

Figure 6. Turbulent heat flux (a) streamwise and (b) normal at Reτ = 180.

where τij = 2(μ + μt)(2S̃ij − (2/3)S̃kkδij), ui is the ith component of the velocity vector, P̅(1) is the fluctuating kinematic pressure, T is temperature, Reτ is the shear Reynolds number, and Pr is the Prandtl number. Re in eq 3 is the bulk Reynolds number which is used for initiating the simulation, based on this we have evaluated Reτ. In our approach, both Reτ and Pr are macroscopic input parameters defined considering the thermophysical fluid properties at reference temperature: Reτ = ρrefμτδ/μref and Pr = μrefCp,ref/λref where μτ = (τw/ρ)1/2 is the shear velocity and S̃ij is the resolved strain rate tensor, here ∼ refers to Favre-averaged quantities. The quantities ρref, μref, and Cp,ref are the values of density, viscosity, and specific heat at reference temperature. In the present work, for modeling subgrid stresses, we have used the wall adaptive layer equation (WALE) model suggested by Nicoud and Ducros,22 while for thermal part, we have used a dynamic Smagorinsky model. In WALE model subgrid scale viscosity accounts for effect of rotation rate and strain rate of the smallest fluctuations. This model has correct wall behavior for sub grid stresses near walls.23,24 The employed scheme is an extension to the pressure correction method proposed for variable density flows under low Mach-number approximation of Anwer et al.25 and Yahya et al.26 2.2. Simulation Results. In this subsection a detailed study of empty channel without extrusion on the inner surface is carried out. First a thermal large eddy simulation model is validated (see Figure 4a) by performing the isothermal simulation of Kim et al.,27 then we studied the impact of thermal gradient on the channel flow by varying the temperature of upper wall (Thot) and fixing lower wall at 300 K. Due to large temperature difference fluid properties like viscosity, the thermal conductivity of air is varied according to the Sutherlands law,28,29 while specific heat is assumed to be constant. The temperature ratio between the walls of the channel is defined by Rθ = (Thot/Tcold). The temperature of the PV module is increased as more and more solar energy is concentrating over it, eventually the temperature of the upper wall of the channel mounted at the back of the panel is increased. There exists a complex thermo-kinetic coupling inside the channel in such a largely anisothermal situation. We have discussed the velocity and temperature statistics, turbulent heat flux, and the modulation in the mean velocity profile due to the large thermal gradient. Due to the temperature stratification, there is a large variation in viscosity. As apparent from Figure 4b, the symmetry of the velocity profile is lost due to the occurrence of lower velocity gradients at the hot wall, favored by the increase of viscosity with temperature, and

higher velocity gradients at the cold wall. The turbulent profile is somewhat changing toward laminar one at the hot side of the channel and the near wall velocity decreases when heat addition increases. While at the cold side the mean velocity profile is almost unaffected but near wall velocity decreases. To appreciate further the effect of temperature-dependent viscosity on turbulence intensities, in Figure 5 the root-mean-square (r.m.s.) of the fluid velocity fluctuations, is shown. Again, profiles are not symmetric and deviations from the constantviscosity situation are observed. Considering the case for Rθ = 1.01, i.e. nearly isothermal simulation, we have found that cold wall and hot wall profile for urms overlap in Figure 5a. However as Rθ approaches 2, there is an underestimation of fluctuation level on the hot half of the domain and an overestimation of its level on the cold half is observed. For further increase up to Rθ = 3, large differences in the peak values of urms are observed. A similar trend is also observed for spanwise, wall normal velocity and temperature fluctuations in Figure 5b−d. Turbulent intensity increases at cold side while at hot side it decreases with increase in the variation of temperature dependent viscosity. Figure 6a and b shows the streamwise and wall normal turbulent heat flux at Reτ = 180. This clearly indicates as the temperature ratio increases both the flux will increase and is more prominent in the near wall region of the cold part of the channel. Simulation parameters are given in Table 1; it is clear Table 1. Effect of Temperature Gradient on Global Parameters of Plane Channel cases with

Reτ

Reb

Reτh

Reτc

⟨Nu⟩

Cf × 103

Rθ = 1.01

180

2787

183

185

20.53

8.45

Rθ = 2

180

2120

92

231

16.48

8.76

Rθ = 3

180

1780

63

268

14.40

9.02

Δx+, Δy+, Δz+ 23.5, 7.8, 0.18 23.5, 7.8, 0.18 23.5, 7.8, 0.18

from Table 1 that Nusselt number decreases at high temperature ratio Rθ. In Table 1, ⟨Nu⟩ is the average Nusselt number, Reb is the bulk Reynolds number, Reτh and Reτc are the shear Reynolds numbers at the hot and cold walls, respectively, and Cf is the coefficient of friction. Figure 7a shows the Reynolds shear stress, due to the large temperature gradient the velocity−velocity correlation decreases at the hot side of the wall while there are no remarkable changes in the vicinity of the cold wall region. In Figure 7b, mean temperature profile shows that the temperature gradient is decreasing near the hot wall for 18416



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Figure 7. (a) Reynolds shear stress profile and (b) mean temperature profile at Reτ = 180.

Figure 8. Evolution of (a) pressure loss, (b) temperature, and (c) Nusselt number along the computational domain for the various geometries.

higher temperature ratio as clearly depicted by the slope. Although we have find that turbulence is promoted in the cold side of the channel, where viscosity is lower and vice versa. The decrease of viscosity near the cold wall, in particular enhances mean kinetic energy. Above observations clearly indicates that when we use an empty channel as a heat exchanger, it does not efficiently removes heat from the channel because of relaminarization on the upper side of the domain at higher Rθ. So as to maximize the heat removal, we should opt for some complex configuration inside the channel in order to enhanced turbulence, which is being sustained even at high thermal

gradient. Some channel configuration with internal geometries like fins and DWVGs (delta-winglet vortex generators) associated with riblets is used to enhance turbulence which inturn surges the heat extraction to best ensure our purpose. 2.3. Performance Evaluation Using Internal Structures. The heat transfer rate for the smooth and the roughened walls is evaluated through the Nusselt number Nu = 2δhc/λ, where hc is the convective heat transfer coefficient of the smooth wall and λ is the thermal diffusivity. Similarly Nu0 is the Nusselt number of the roughened wall projected on the surface of the smooth wall. The correlation30 defining the ratio of 18417



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Nusselt number on the rough wall (with fins and internal structure) to the smooth wall is given by eq 6 and is used for evaluating the convective heat transfer coefficient for extruded geometries in channel. Nu0 /Nu = 1.01 + 8 × 10−6Re − 5 × 10−11Re 2

According to the correlation from the data of Kays, number for a classical channel is given by Nu = 0.0158Re 0.8 =

2hδ λ

a similar behavior. Geometry 2 does not show as much thermal exchanges as geometry 1, due to its smaller exchange surface. Figure 8c shows the variation of Nusselt number. For the fins and the empty geometries, the Nusselt number decreases along with the position in the channel, this is due to the augmentation of the fluid temperature. Regarding geometry 1 and 2, one can notice an initial increase of the Nusselt number until x = 4 m. This is due to the vortices created by the DWVGs whose enhance the local convection coefficient. After x = 4 m, the Nusselt number decreases. However, it shows the potential of the innovative geometry: if we choose to put another DWVG at the end of the computational domain, it will create another vortex, generating a peak of thermal exchange (or Nusselt number) whereas the Nusselt number of the fins geometry will continue to decrease. Performance evaluation of the channel with different complex geometries is given in Table 2. The

(6) 31

Nusselt

(7)

where the symbols have their usual meaning. The Fanning friction factor is evaluated using the formula given below:

f=

2δ ΔP L 2ρU 2

(8)

where ΔP is the pressure loss, L is the length of the channel, ρ is the density, and U is the incoming velocity. In order to evaluate the heat transfer effectiveness of the various simulated combinations, we use the thermal enhancement factor based on the study of Webb and Eckert.32 It is defined as the ratio of the heat transfer coefficient of an augmented surface (hc) to that of a smooth surface (hc0) at the same pumping power: −1/3 2/3 ⎛ S ⎞2/3 λ ⎛ ρ ⎞ Nu ⎛ f ⎞ ⎜ ⎟ ⎜⎜ ⎟⎟ n= ⎜ ⎟ Nu0 ⎜⎝ f0 ⎟⎠ ⎝ S0 ⎠ λ 0 ⎝ ρ0 ⎠

Table 2. Performance Evaluation of Channel with Different Internal Geometries

(9)

internal design

Fanning friction coefficient

turbulent kinetic energy K+

thermal enhancement factor

empty fins geometry 1 geometry 2

0.0227 0.0523 0.1005 0.0796

2.08 2.60 2.32 2.38

1 1.722 1.412 1.338

study of the dimensionless turbulent kinetic energy shows that the highest K+ is obtained with the fins geometry. Although it is not the most efficient geometry for heat exchanges, thanks to the fins the velocity inside the channels is the highest, leading to a rise of turbulent kinetic energy. Geometry 2 has a higher K+ than geometry 1 since the vortices generated are less dissipated at the outlet of the computational domain due to the large pitch of the riblets. The comparison of the Fanning friction factor, as shown in Table 1, shows that geometries 1 and 2 generate more friction than the others geometries. As a matter of fact, the vortices generated by the DWVGs rub against the wall, increasing the friction. The high Fanning friction factor of geometry 1 combined with the high increase of surface causes more pressure loss than the fins geometry. On the contrary, the increase of surface is lower for geometry 2, since it has lower riblets with a larger pitch, and combined with its Fanning friction factor, it causes less pressure loss than the fins geometry. The thermal enhancement factor given by eq 9 shows that the fins geometry remains the most efficient internal geometry. As shown previously, the geometry 1 design causes too much pressure loss in comparison with fins and the geometry 2 design does not generate enough thermal transfer. However, by carrying out simulations of the internal design whose parameters are bounded by those of geometries 1 and 2, it may be possible to find a geometry that performs at least as well as the fins geometry in future studies. Hence, an optimization of pertinent parameters of the geometry, such as the riblet height and pitch, should lead to a more efficient geometry. A higher length of solar exchanger should favor the innovative internal geometry since the DWVGs allow mixing the fluid, heating the center of the fluid volume easily, while the fins geometry has troubles heating the center of the fluid volume. Obviously heat transfer is increased for different geometries and fins as compare to simple channel, however which configuration is suitable for use is depends upon many parameters like angle of attack, channel spacing, number of

where (Nu/Nu0), (f/f 0), and (S/S0) are the ratio of Nusselt number, Fanning friction factor, and surface ratio for augmented surface to that of a smooth surface. In literature, the surface ratio (S/S0) is simplified but in this particular case the longitudinal riblets modify the flow passage surface all along the channel. The reference values for an empty channel are obtained by simulations. A validation of our numerical results of geometry with fins and a smooth channel with experimental/ theoretical work of Kumar and Rosen33 is also given in Figure 8 for Nu and T evolution along the channel. Another criterion is used to compare the various geometries is the dimensionless turbulent kinetic energy (K+), defined as the ratio of the kinetic energy (K) to the square of shear velocity, K+ = K/uτ2. To further optimize the asymmetric heating of the domain, we study an innovative internal design combining the two heat transfer enhancement methods: an association of delta-winglet vortex generators (DWVG) with longitudinal riblets. DWVGs generates longitudinal streamwise vortices, which increases vertical mixing and correspondingly convective heat transfer. The vortices generated by the DWVGs enhance the turbulence level as well as the convective transfers between the fluid and the wall. The riblets canalize the vortices generated by the DWVGs while enhancing the transfer surface area at the same time. The results of the evolution of the pressure loss are presented in Figure 8a. It shows that geometry 1 causes more pressure loss all along the computational domain than the fins geometry. The evolution of the pressure loss of geometry 2 is even more interesting, at first pressure loss is exceedingly high due to the presence of the DWVG and the origin of the vortex at x = 1 m. Then at x = 2 m where the riblets appear on the geometry, the pressure loss increase is proportionally lower than that of the fins geometry Thus, after x = 8 m geometry 2 causes less pressure loss than the fins geometry. The evolution of the temperature are presented in Figure 8b. Geometry 1 presents the best outlet temperature. The fins geometry shows 18418



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(8) Prakash, J. Transient analysis of a photovoltaic − thermal solar collector for co-generation of electricity and hot air/water. Energy Convers. Manage. 1994, 35, 967−972. (9) Sopian, K.; Liu, H.; Kakac, K.; Veziroglu, T. Performance of a hybrid photovoltaic thermal solar collector. Proceedings of the ASME International Mechanical Engineering Congress and Exhibition, Atlanta, GA, USA, Nov. 1996; Vol. 36, pp 341−346. (10) Fabbri, M.; Dhir, V. K. Optimized Heat Transfer for High Power Electronic Cooling using Arrays of Microjets. J. Heat Transfer 2005, 127, 760−769. (11) Kandlikar, S. G. High Flux Heat Removal with MicrochannelsA Roadmap of Challenges and Opportunities. Heat Transfer Eng. 2005, 26, 5−14. (12) Krishna, K.; Ludovic, B.; Yogendra, J. Performance of an AirCooled Heat Sink Channel With Microscale Dimples Under Transitional Flow Conditions. J. Heat Transfer 2013, 135, 111005. (13) Tonui, J.; Tripanagnostopoulos, Y. Air-cooled PV/T solar collectors with low cost performance improvements. Solar Energy 2007, 81, 498−511. (14) Anand, S.; Ibrahim, D.; Bale, V. R. Thermodynamic assessment of photovoltaic systems. Solar Energy 2009, 83, 1139−1149. (15) Yang, H.; Marshall, R. H.; Brinkworth, B. J. Validated simulation for thermal regulation of photovoltaic wall structures. Proceedings of the 25th IEEE Photovoltaic Specialists Conference, Washington, DC, May, 1996; pp 1453−1456. (16) Yahya, S. M.; Anwer, S. F.; Sanghi, S. Performance of different SGS models of LES for low Mach number channel flow. Procedia Eng. 2012, 38, 1192−1208. (17) Majda, A.; Sethian, J. A. The derivation and numerical solution of the equations for zero Mach number combustion. Combust. Sci. Technol. 1985, 42, 185. (18) Rehm, R. G.; Baum, H. R. The equation of motion for thermally driven buoyant flows. J. Res. Natl. Bur. Stand. (U.S.) 1978, 83, 297. (19) Muller, B. Low-Mach number asymptotics of the Navier−Stokes equations. J. Eng. Math. 1998, 34, 97. (20) Paolucci, S. On the filtering of sound from the Navier−Stokes equations; Technical Report, Sandia National Laboratories, 1982. (21) Stephen, B. P. Turbulent Flows, 2nd ed.; Cambridge University Press: Cambridge, U.K., 2000. (22) Nicoud, F.; Ducros, F. Subgrid-scale stress modeling based on the square of velocity gradient tensor. Flow, Turbul. Combust. 1999, 62, 183. (23) Germano, M.; Piomelli, U.; Moin, P.; Cabot, W. A dynamic subgrid scale eddy viscosity model. Phys. Fluids A 1991, 3, 1760. (24) Moin, P.; Squires, K.; Cabot, W.; Lee, S. A dynamic subgridscale model for compressible turbulence and scalar transport. Phys. Fluids A 1991, 3, 2746. (25) Anwer, S. F.; Khan, H. N.; Sanghi, S.; Ahmad, A.; Yahya, S. M. Extension of SMAC scheme for variable density flows under strong temperature gradient. AIP Conf. Proc. 2012, 1440, 683−691. (26) Yahya, S. M.; Anwer, S. F.; Sanghi, S. A conservative pressure correction method on collocated grid for low Mach number flows. World J. Mech. 2012, 5, 253. (27) Kim, J.; Moin, P.; Moser, R. Turbulence statistics in fully developed channel flow at low Reynolds number. J. Fluid Mech. 1987, 177, 133. (28) Serra, S.; Toutant, A.; Bataille, F. Thermal Large Eddy Simulation in a Very Simplified Geometry of a Solar Receiver. Heat Transfer Eng. 2011, 33, 505. (29) Yahya, S. M.; Anwer, S. F.; Sanghi, S. Phenomenological and statistical analyses of turbulence in forced convection with temperature-dependent viscosity under non-Boussinesq condition. Eur. Phys. J. E 2013, 36, 120. (30) Malik, M.; Buelow, F. Heat Transfer Characteristic of A Solar Dryer. UNESCO Congress Sun in the Service of Mankind, Paris, France, July 2−6, 1973; Paper V25. (31) Duffie, J.; Beckman, W. Solar Engineering of Thermal Processes, 3rd ed.; Wiley: New York, 2006.

riblet, and pitch size, etc. which is a further whole field of study. It is now concluded that, by using different internal structures inside the channel, we have maximized the heat removal which is being used to adjust the temperature of the receiver of a solar tower system running simultaneously with a concentrated photovoltaic system. It is observed from Table 2 that the heat transfer increases with fins and innovative internal structure as compared to the classical configuration of channel. Here a situation of symbiosis is achieved in which both the systems (CPV and CSP) benefit, through increasing the efficiency of the PV panel and a stockage of huge thermal surplus by just implementing some sort of heat exchanger at the rear of large PV panel.

3. CONCLUSION The study has been carried out for a thermo-fluid analysis of a thermal−PV cogeneration system using flow simulation in order to increase the efficacy of cogenerated system. Thermal large eddy simulation of a plane channel is carried out to observe the charecteristic of flow and its effect on heat transfer. It is obvious from the results of simulation as the thermal gradient increases between the walls of a channel, flow start laminarizing on the hot side of the wall, and Nusselt number decreases. To enhance the heat transfer inside the channel, some complex geometries are used inside the channel, which maintain the turbulence level in the whole channel and extract maximum heat by dropping down the temperature of PV module. The output thermal energy is being used for domestic purposes or as a preheater in many processes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: yahya_syed@rediffmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS CFD Lab Applied Mechanics Department (IIT Delhi, India), High Performance Computing Lab ZHCET (AMU, Aligarh, India), and Centre for Renewable Energy, EED (AMU, Aligarh, India), initiatives are gratefully acknowledged for generous support and allowance of computer resources.

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REFERENCES

(1) Park, K.; Kang, G.; Kim, H.; Yu, G.; Kim, J. Analysis of thermal and electrical performance of semi-transparent photovoltaic (PV) module. Energy 2010, 35, 2681e7. (2) Ali, A. Characteristics of flow and heat transfer for in-line plate segments inside channel used for photovoltaic modules thermal regulation. Appl. Therm. Eng. 2005, 25, No. 8e9. (3) Gan, G. Effect of air gap on the performance of buildingintegrated photovoltaics. Energy 2009, 34. (4) Kern, E. J.; Russel, M. Combined photovoltaics and thermal hybrid collector systems. Proceedings of 13th IEEE Photovoltaic Specialists Conference, Washington, DC, USA, June 5−8, 1978. (5) Hendrie, S. Evaluation of combined Photovoltaic/Thermal collectors. Proceedings of the International Conference ISES, Atlanta, GA, USA, May 28−June 1, 1979; Vol. 3, pp 1865−1869. (6) Raghuraman, P. Analytical Predictions of liquid and air Photovoltaic/Thermal, flat − plate collector performance. J. Solar Energy Eng. 1981, 103, 291−298. (7) Bhargava, A.; Garg, H.; Agarwal, R. Study of a hybrid solar system − solar air heater combined with solar cells. Energy Convers. Manage. 1991, 31, 471−479. 18419



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(32) Webb, R. L.; Eckert, E. R. G. Application of rough surfaces to heat exchanger design. Int. J. Heat Mass Transfer 1972, 15, 1647−1658. (33) Kumar, R.; Rosen, M. A. Performance evaluation of a double pass PV/T solar air heater with and without fins. Appl. Therm. Eng. 2011, 31, 1402−1410.

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Enhanced Heat Transfer and Fluid Flow in a Channel Behind a

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