Parametric Study of Thermal Performance of Cylindrical Parabolic Trough Solar Collector in Ogbomoso Environs.

Transcription

1 Parametric Study of Thermal Performance of Cylindrical Paraolic Trough Solar Collector in Ogomoso Environs. Emmanuel O. Sangotayo, M.Tech. * ; Waheed M. Adekojo, Ph.D. 2 ; and Jelili O. Alamu, Ph.D. 3 Department of Mechanical Engineering, adoke Akintola niversity of Technology, Ogomoso, Nigeria. 2 Department of Mechanical Engineering, College of Engineering, niversity of Agriculture, Aeokuta, Nigeria. 3 Department of Mechanical Engineering, Osun State niversity, Osogo, Nigeria. olemsangotayo@yahoo.com * ABSTRACT This paper presents a numerical investigation on the enhancement of thermal performance of solar air heater having cylindrical paraolic trough solar collector with twisted tape in Ogomoso weather conditions (lat.8 o ', long. 4 o ). The parametric studies were conducted to investigate the effects of all the operating parameters on the system performance in order to otain the optimum performance of the system. The energy equation for heat transfer of two dimensional fully developed fluid flow of the cylindrical paraolic trough collector have een considered and susequently collector efficiency factor, F, collector heat removal factor, Fr and collector overall heat loss coefficient, are used to analyze the thermal performance and to study the effect of mass flow rate, from to. kg/s for the fixed value of incident solar asored flux, I of 86 w/m 2. The results revealed that the optimum design parameters are length,.3m, mass flow rate,.36kg/s, concentrator aperture width,.6 m, concentration ratio, 3.667, asored flux, 96.39, tilt angle, 8.2, fluid temperature,.553 K and instantaneous efficiency, 47.4%. It is oserved that performance of the cylindrical paraolic trough collector with twisted tape was enhanced apprecialy. These results can guild practicing engineers and designers in the evaluation of the existing real systems and design of future system. (Keywords: thermal performance, cylindrical paraolic solar collector) INTRODCTION Flat-plate solar collectors are vastly employed in low temperature energy technology and have drawn the attention of a large numer of investigators. Several designs of solar air heaters have een industrialized over the years in order to improve their system thermal performance. The thermal performance of solar air heater is generally low ecause of low value of the convective heat transfer coefficient etween the asorer plate and the air, leading to high asorer plate temperature and greater heat losses to the surroundings. It has een found that the main thermal resistance to the heat transfer is due to the formation of a laminar su-layer on the heat transferring surface, (Prasad and Mullic, 983; Momin, 2). Efforts of improving the heat transfer rate have een directed towards artificially destroying the su-layer. The suitale design of solar air heaters for high temperatures applications have een the suject of many theoretical and experimental investigations. Recently, researchers have employed wire screen matrices, expanded metal mesh, finned, corrugated, and packed ed as asoring porous media for directly incident solar radiation in the solar air heater due to improve its performance ecause as the surface area and turulence producing air flow path through the ed increase, heat transfer rate increases. The high heat transfer area to volume ratio for the air flowing through these matrices enhances the heat transfer capaility. Bhagoria, et al. [22]; Han [988]; Taslim [996]; au [99]; Park [988]; Mital [25]; Saini and Saini [996] have The Pacific Journal of Science and Technology 59 Volume 3. Numer. May 22 (Spring)

2 carried out investigation on the performance of solar air heater. These studies have indicated that such air heaters have superior performance as compared to that of flat plate collectors. T p T c Recently, much attention has een given to concentrating solar collectors, which are capale of reaching higher temperatures compared to flat plate collectors. Hong [976]; Kumar and Prasad [2], Sharma [23], and Togrul and Pehlivan [25] have carried out investigations on the twisted tape; it was reported for enhancement in performance. These works need investigations of the heat and fluid flow phenomena in the asoring tue y evaluating friction factors, pumping power and convective heat transfer coefficients and comparisons for the alternative cases and may ring into light the factors to construct an economically viewale solar air collector for high temperature applications. This paper aims at investigating the optimal values of the key design parameters and effect of twist tape factor on the system thermal performance in Ogomoso weather conditions. ANAYSIS OF CYINDRICA PARABOIC CONCENTRATING COECTOR T f dx Figure 2: Energy Flow Diagram of Cylindrical Paraolic Concentrating Collector (Sukhatme, 27). PERFORMANCE ANAYSES T f +dt f The structure of a cylindrical paraolic concentrating collector of glass envelope asorer tue with selective surface of such system having concentrator width aperture W and length as shown in Figure. The performance of a cylindrical paraolic concentrating collector is assumed for the same radiation flux all along the length and the negligile temperature drop across the asorer tue and the glass cover. The energy alance under steady state conditions is shown in Figure 2 and the total heat gain rate can e expressed as: dq u I r = + I O πdo ( W Do ) ργ ( τa) r D ( τa) ( T T ) p a dx () Figure: Cross Section of Cylindrical paraolic Concentrating Collector. The left side term represents the useful heat gain rate for a length, dx. The first term on the right side represents the incident eam radiation asored in the asorer tue after reflection, while the second term represents the asored incident eam radiation which falls directly on the asorer tue and the third term represents the loss y convention and radiation. Also ργ(τα) represents the optical properties of the system. The intercept factor, γ is defined as the reflected fraction of the incident radiation on the asoring surface of the receiver. τ is the transitivity of The Pacific Journal of Science and Technology 6 Volume 3. Numer. May 22 (Spring)

3 transparent cover. α and ρ are the asorptivity and the reflectivity of the asorer and concentrator, respectively. The asored flux S can e given as: S = I r ργ + I r D O ( τa) DO (τa ) W D Equation () thus ecomes: dq u o ( T T ) ( W D )dx (2) = S p a o C (3) where C is the concentration ratio of the collector which is defined as the ratio of the effective aperture area and asorer tue area and is written as: C ( W D ) ( W D ) o o = = (4) πdo πdo The useful heat gain rate dq u can e given as: dq = h π D ( Tp T dx (5) u f i f ) where h f and T f are the heat transfer coefficient on the inside surface of the tue and the local fluid temperature, comining Equations (3) and (5) to eliminate the asorer tue temperature (T p ), it yields: dq u = F S ( T f Ta ) ( W Do )dx C ' (6) where F is the collector efficiency factor and written as: F = DO + D h i f ' (7) Therefore, The inlet fluid temperature (T fi ) and outlet fluid temperature (T fo ) are estalished y applying oundary conditions at inlet (x =. T f = T fi ) and at outlet (x=, T f = T fo ). They are written as: T T f i fo F'πD O x = exp (9) mc p F' D O x = exp () mc p Thus the useful heat gain rate can e expressed as: Q u = F = mc r p ( T T ) ( W D ) S ( T T ) O fo fi C fi a () where Fr, the heat removal factor is expressed as: F r = πd mc O p F' πd O exp (2) mc p Equation () is the equivalent of the Hotel- Whillier-Bliss equation for the flat plate collector, (Sukhatme, 27). The instantaneous collector Efficiency (h i ) is given y this equation: Qu η i = (3) ( I r + I r )W d d The instantaneous efficiency can also e determined on the asis of eam radiation alone, if the ground reflected radiation is neglected and given in Equation (4): Qu η i = (4) I r W HEAT TRANSFER COEFFICIENTS dt f dx F' πd CS ( T T ) O = f a mc p (8) In order to determine the performance of cylindrical paraolic concentrating solar collector, correlations are required for calculating the values of convective heat transfer coefficient etween The Pacific Journal of Science and Technology 6 Volume 3. Numer. May 22 (Spring)

4 the asored tue and cover, outside surface of the cover and inside surface of the asored tue, respectively. The following correlations are used to calculate the natural convection heat transfer coefficient (h p-c ) for the enclosed annular surface etween the horizontal asorer tue and the concentric cover. / 4 K eff / K =.37( Ra) (5) where K eff is the effective thermal conductivity. The convective heat transfer coefficient (h w ) on the outside surface of the cover may e calculated y this Equation (6): Nu n = C Re (6) The convective heat transfer coefficient (h f ) on the inside surface asored tue can e calculated using Dittus-Boelter equation:.8.4 Nu =.23Re Pr (7) The heat transfer coefficient for twisted tape may e determined using the correlation given y Hong and Burgles equation. + Nu = (8).5484{ Pr( Re/ X ) } where X is the tape twist ratio, it is expressed as:.5 (9) A simple and flexile program, writing in C++ language was developed ased on models aove (Equations -9) to otain results under different design and operating conditions. The flowchart for implementing the program is shown in Figure 3. RESTS AND DISCSSIONS The results otained from the program developed are presented as profiles in Figures 4 7. The effect of variale mass flow rate ( ) ranges from. to. kg/s on the system thermal performance parameters were investigated for different values of tape twist ratio, X (, 3, 6, 9, 2, and 5) for a fixed constant value of incident eam solar flux (I = 86 W/m 2 ) in Ogomoso climatic conditions. Start Get input design parameters, insolation OOP STARTS CACATE HEAT REMOVA FACTOR, FR CACATE INCIDENT ABSORBED FX CACATE SEF HEAT GAIN CACATE OTET FID TEMPERATRE CACATE COECTOR EFFICIENCY FACTOR CACATE COECTOR INSTANTANEOS EFFICIENCY OOP ENDS PRINT THE CACATED RESTS Stop Figure 3: Flow Chart for Solar Collector Calculations. Figures 4 7 show the plots of the collector instantaneous efficiency as a function of concentration ratio (C), concentrator aperture width (W), tilt factor (r ) and length (). Figures 8 and 9 show the plots of fluid inlet and outlet temperature distriution as a function of mass flow rate ( ) and length (). It is deduced that the optimum design parameters are: length () is.3 m, mass flow rate is.36 kg/s, outlet and inlet fluid temperature is.55k with instantaneous collector efficiency of 47.38%. The Pacific Journal of Science and Technology 62 Volume 3. Numer. May 22 (Spring)

5 Instantaneous Efficiency 2.5 Instantaneous Efficiency Concentration ratio, C Figure 4: Effect of Concentration Ratio, C on Instantaneous Efficiency ength (m) Figure 7: Effect of Variation of ength, on Instantaneous Efficiency Instantaneous Efficiency 2.5 Temperature distriution Tfi Tfo Concentrator Aperture Width, W Figure 5: Effect of Concentrator Aperture Width, W on Instantaneous Efficiency Figure 8: Effect of Mass Flow Rate, m on Fluid Inlet and Outlet Temperature Instantaneous Efficiency Fluid Temperetures Tfi Tfo Tilt Factor, r Figure 6: Effect of Tilt Factor r on Instantaneous Efficiency ength (m) Figure 9: Effect of ength, on Fluid Inlet and Outlet Temperature. The Pacific Journal of Science and Technology 63 Volume 3. Numer. May 22 (Spring)

6 Figures,, and 2 present the effect of mass flow rate with variale tape twist factor, X on inlet fluid temperature distriution (T fi ), collector efficiency factor (F ) and collector heat removal factor (Fr). It was found that the system losses reduce as the twist tape factor inserted in the asorer increases. Figures 3, 4, 5, and 6 show the plots of fluid outlet temperature distriution, T fo and collector overall heat loss coefficient ( ), useful heat gain rate (Q u ) and instantaneous efficiency as a function of mass flow rate ( ) with variale tape twist factor, X. It was found that the system thermal performance parameters increases as the twist tape factor inserted in the asorer increases. Hence it enhances the thermal performance of the system. Collector Efficiency Factor, F x = x = 2 x = Figure : Effect of Mass Flow Rate Coupled with Twisted Tape Factor on Collector Efficiency Factor..2 Figure 7 show the plot of Nusselt numer, Nu as a function of Reynolds numer, Re with variale tape twist factor, X and Prandtl numer for air, Pr =.7. It is evident that, higher value of Nusselt numer is otained with corresponding lower value of tape twist ratio, ecause high twisted tape increases the friction factor and pressure drop leading to higher pumping power. Collector Heat Removal Factor, Fr x = x = 2 x = Figure 2: Effect of Mass Flow Rate Coupled with Twisted Tape Factor on Collector Heat Removal Factor. Fluid Inlet tem p erature x = x = 2 x = Fluid Outlet temperature x = x = 2 x = 5 Figure : Effect of Mass Flow Rate Coupled with Twisted Tape Factor on Inlet Fluid Temperature Figure 3: Effect of Mass Flow Rate Coupled with Twisted Tape Factor on Fluid Outlet Temperature. The Pacific Journal of Science and Technology 64 Volume 3. Numer. May 22 (Spring)

7 Collector Overall Heat Coefficient, Figure 4: Effect of Mass Flow rate coupled with twisted tape factor on Collector Overall Heat Coefficient. x = x = 2 x = 5 Figure 7: Effect of Reynolds Numer Coupled with Twisted Tape Factor on Nusselt numer, Pr =.7. 8 CONCSIONS seful Heat Gain Rate, Qu x = x = 2 x = 5 The following conclusions are drawn from the parametric studies conducted investigating the effect of design and operating conditions on the system performance. The optimal values of the key design parameters were estalished for length is.3 m and mass flow rate is.36 kg/s with collector instantaneous efficiency of 46.47% Figure 5: Effect of Mass Flow Rate Coupled with Twisted Tape Factor on seful Heat Gain Rate. The insertion of twisted tape in the asorer tue reduces the losses in the system thermal performance and it increases the heat transfer leading to increase in the system thermal performance. The significant increase in heat transfer coefficient is oserved with high value of Nusselt numer. Instataneous Efficiency Mass Flow Rate ( kg/s) Figure 6: Effect of Mass Flow Rate Coupled with Twisted Tape Factor on Instantaneous Efficiency. x = x = 2 x = 5 REFERENCES. Bhagoria, J.., J.S. Saini, and S.C. Solanki. 22. Heat Transfer Coefficient and Friction Factor Correlations for Rectangular Solar Air Heater Duct having Transverse Wedge Shape-Ri Roughness on the Asorer Plate. Renewale Energy. 25: Han, J.C Heat Transfer and Friction Characteristics in Rectangular Channels with Ri. ASME Journal Heat transfer. : Han, J.C. and J.S. Park Developing Heat Transfer in Rectangular Channels with Ris. International Journal Heat mass Transfer. 3: Hong, S.W. and A.E.Burgles Augmentation of aminar Flow Heat Transfer in The Pacific Journal of Science and Technology 65 Volume 3. Numer. May 22 (Spring)

8 Tues y Means of Twisted Tape Inserts. Journal of Heat Transfer. 98: Kumar, A. and B.N. Prasad. 2. Investigation of Twisted Tape Inserted Solar Water Heaters, Heat Transfer, Friction Factor and Thermal Performance Results. Renewale Energy. 9: au, S.C. 99. Heat Transfer Characteristics of Turulent Flow in a Square Channel. ASME Journal Turo. 3: Mazen, M. and Au-Khader. 26. Effect of Twisted Tape as Tue Inserts in Shell and Tue Heat Exchanger. Journal of Heat and Mass Transfer. 8. Mital, M.K. 25. Effective Efficiency at Solar Air Heaters Having Different Types of Roughness Elements on the Asored Plate. Energy. 9. Momin, A.M. 2. Heat Transfer and Friction in Solar Air Heater Duct with V-shaped Ri Roughness an Asorer Plate. International Journal Heat & Mass Transfer. 45: Momin, A.M., J.S. Saini, and S.C. Solanki. 22. Heat Transfer and Friction in Solar Air Heater Duct with V-Shaped Ri Roughness on Asorer Plate. International Journal Heat and Mass Transfer. 45: Prasad, B.N. and J.S. Saini Effect of Artificial Roughness on Heat Transfer. Solar Energy. 4: Prasad, K. and S.C. Mullic Heat Transfer Characteristics of a Solar Air Heater. Applied Energy. 3: Saini, R.P. and J.S. Saini Heat Transfer and Friction Factor Co-Relations for Artificially Roughened Ducts, with Expanded Metal Mesh as Roughness Element. International Journal Heat and Mass Transfer. 5: Sharma, P.K. 23. aminar Convective Heat Transfer with Twisted Tape Insert in a Tue. International Journal of Thermal Sciences. 42: Sukhatme, S.P. 27. Solar Energy. Tata Mcgraw- Hill: New Delhi, India. 6. Taslim, M.E Experimental Heat Transfer and Friction in Channels Roughened with Angled V-Shaped. ASME Journal Tur. 8: Togrul, I.T. and D. Pehlivan. 25. Effect of Packing in the Air Flow Passage on the Performance of a Solar Air Heater with Conical Concentrator. International Journal Applied Thermal Engineering. 25: NOMENCATRE Fr Collector Heat Removal factor h p-c Heat transfer coefficient etween asorer and cover (W/m 3 K) hf Convention heat transfer coefficient on the inside surface of the tue (W/m 3 - K) X Tape twist ratio I Incident eam radiation (W/m 3 - K) Overall heat transfer coefficient (W/m 3 - K) Re Reynolds Numer Nu Nusselt Numer Ra Rayleigh Numer η Instantaneous Collector efficiency δ Stefan-Boltzmann constant (W/m 3 - K) τ Transmissivity of glass α Reflectivity of glass ρ Asortivity of glass ε Emissivity of the plate γ Intercept Factor ABOT THE ATHORS Emmanuel O. Sangotayo, is a ecturer at the Department of Mechanical Engineering. He is a registered Engineer (COREN) and is a memer of the Nigerian Society of Engineer, (NSE). He holds a Master of Technology (M.Tech.) in Mechanical Engineering from the adoke Akintola niversity of Technology, Ogomoso, Nigeria. He is a Ph.D. degree student at Federal niversity of Agriculture, Aeokuta, Nigeria. His research interests are in thermofluid/energy studies. Dr. Waheed, M. Adekojo, is a Professor in the Department of Mechanical Engineering, Federal niversity of Agriculture, Aeokuta, Nigeria. He is a registered Engineer (COREN) and is a memer of the Nigerian Society of Engineer, (NSE). He holds a Ph.D. degree in Mechanical Engineering and currently serves as the Director, Academic Planning, FNAAB. His research interests are in the areas of thermofluid/energy studies. Dr. Jelili O. Alamu, is a Reader in the Department of Mechanical Engineering, Osun State niversity, Osogo, Nigeria. He is a registered Engineer (COREN) and is a memer of the Nigerian Society of Engineer, (NSE). He holds a Ph.D. degree in Mechanical Engineering and currently serves as the Director, academic planning, Osun State niversity. His research interests are in the areas thermofluid/energy studies. The Pacific Journal of Science and Technology 66 Volume 3. Numer. May 22 (Spring)

9 SGGESTED CITATION Sangotayo, E.O., W.M. Adekojo, and J.O. Alamu. 22. Parametric Study of Thermal Performance of Cylindrical Paraolic Through Solar Collector in Ogomoso Environs. Pacific Journal of Science and Technology. 3(): Pacific Journal of Science and Technology The Pacific Journal of Science and Technology 67 Volume 3. Numer. May 22 (Spring)