THERMAL PERFORMANCE ENHANCEMENT OF SOLAR PARABOLIC TROUGH COLLECTOR USING SECONDARY REFLECTOR

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1 THERMAL PERFORMANCE ENHANCEMENT OF SOLAR PARABOLIC TROUGH COLLECTOR USING SECONDARY REFLECTOR P Sundaram 1, R Senthl Department of Mechancal Engneerng, SRM Unversty, Kattankulathur , Chenna, Inda 1 vpssundaram@gmal.com rsenthlsrm@gmal.com Abstract The slght msalgnment n the focal of a parabolc trough collector (PTC) resulted n more heat losses and lower thermal effcency due to seasonal movement of the Sun. Wth a neglgble change n the ntercept factor, the secondary reflectors are attached over the recever to reflect the concentrated solar rays agan on the recever. Thermal performance of PTC s carred out numercal and expermentally. Use of secondary reflector mproves the thermal effcency by 10% and the heat loss from the PTC has been reduced by 0.5 kj/s. Keyword - Solar PTC, Secondary reflectors, Recevers, Expermental work I. INTRODUCTION In solar thermal energy converson system, whch the collector absorber and concentrator are actng major to affect the performance. PTC had a smple geometry and used for medum temperature solar thermal applcatons. Solar collector performance nvestgated based on the nputs of optcal propertes, heat transfer flud (HTF) propertes, nlet temperature and flow rate of HTF, solar nsolaton, wnd speed, and atmospherc temperature and outputs of collector effcency, HTF outlet temperature, heat gan, and heat and optcal losses. Abhjeet Aud et al. [1] performed the thermal tests on concentrator at varous tme perods and mass flow rate to nvestgate the varaton of optcal effcency and heat loss factor. Umayal Sundar et al. [] desgned and nvestgated the performance solar collector wth varous operatng parameters. Kumaresan et al. [3] nvestgated the storage system ntegrated wth PTC (parabolc trough collector) for ts performance durng chargng process. The absorber tube transfers solar radaton to the HTF (heat transfer flud) whch crculates through the tube whch s connected to the storage tank. The performance of collector system mproved by ncreasng the ntensty of beam radaton. Senthl and cheralathen [4] expermentally nvestgated the effect of temperature dstrbuton on the recever. Based on obtanng results concluded that the collector effcency mproved by the confguraton of solar radaton absorber. Cheng [5] nvestgated a PTC system that uses mrrored surfaces of a lnear parabolc reflector to focus drect solar radaton on the absorber tube. The effects of varous reflector parameters and recever parameters are dscussed. The analyss proves that the PTC system of dfferent actve recever lengths or dfferent glass cover dameters has lttle effects on the optcal performance and lttle senstvty to optcal errors. Rzwan Masood et al. [6] performed the parabolc trough collector desgn and ts feasblty for ndustral applcatons. The smulaton results show that the sgnfcant amount of hghtemperature heat energy were attaned whch can be used for ndustral applcatons. Rcardo et al. [7] studed the effects of workng and envronmental parameters on the performance of PTC usng exergy analyss. Selvakumar et al. [8] presented the evacuated tube collector by usng thermnol D-1 workng flud ntegrated wth PTC. The results concluded that the thermnol D-1 as an HTF gves better performance compare then the water. From the lterature studes, PTC collector desgn and ts confguraton of absorber and heat transfer flud ponted as mportant. The present work performs the expermental performance of PTC wth a secondary reflector. II. MATERIALS AND METHODS The system used s a parabolc trough collector whose functon s to reflect the solar rays fallng on t onto the absorber tube placed on the focus. The absorber tube s made of copper and ol flows through and s heated contnuously over a specfc perod between whch readngs are taken. A secondary reflector of three dfferent confguratons s to be attached to the absorber tube to reflect the rays escapng from the absorber tube back to the absorber tube. The objectve of the project s to mprove the effcency of the trough usng a secondary reflector of dfferent confguratons. The Methodology s used, s that after readngs are taken on the trough wthout any secondary reflector and the effcency and losses are calculated for the readngs. A sheet of a chosen metal s used to cut and shape nto dfferent confguratons and s attached to the absorber tube on the trough. After attachng the dfferent confguratons, readngs are taken, and the effcences and the losses are calculated DOI: /jet/016/v86/ Vol 8 No 6 Dec 016-Jan

2 for the confguratons and are then compared wth the effcency and losses of the trough and graphs are drawn. The PTC had the aperture of 1. m, focal length 0.61 m, depth of parabola 0.1 m, arc length 1.83 m and made up of stanless steel wth mrror flm. The absorber tube s made up of copper havng dameter m and length 1. m respectvely. The trough s tracked n a sngle axs. The ol crculaton pump s 0.5 kw capactes. The supply and storage tanks are 10 and 8 lters respectvely. The expermental test setup s constructed by Ecosense Sustanable Solutons Pvt.Ltd., New Delh, Inda. The secondary reflectors consdered are trangular and curved confguratons. The confguratons are prepared wth a sheet of polshed alumnum and are attached to the absorber tube one after the other as shown n fg. 1. Fg. 1. PTC test faclty wth secondary reflector III. THERMAL ENERGY ANALYSIS A. Thermal analyss of the system Thermal performance nvestgated by measurng the temperature dfference of workng flud through the recever, together wth flud propertes, mass flow rate and solar drect radaton ncdent. In order to evaluate the thermal performance of the PTC collector based on heat ganed by workng flud s expressed as the absorbed energy by the solar recever s gven by the followng equaton, Q AI (1) s c b The useful heat gan by the HTF can be determned from the expermental data s gven by Q mc T T () u p o The Reynolds number can be calculated by the relaton of Re VHTF D vhtf The convectve heat transfer from the nner surface of the recever to HTF can be calculated from the Nusselt correlatons gven by the Dttus- Boelter equaton, NuHTF Re Pr D L for 10 < LD< 400 (3) The convectve heat transfer coeffcent ( h HTF ) between recever nner surface and HTF can be calculated as equaton [4] and Heat transfer to the HTF by convecton can be gven by equaton (5) NuHTF khtf hhtf (4) D conv HTF m Q h A T T (5) Where the nner surface area s defned as: A DL (6) B. Heat losses from the recever The Reynolds number can be calculated by the relaton, Var Do Re (7) v ar Nusselt number for ar flow over the recever tube s gven by the followng equatons DOI: /jet/016/v86/ Vol 8 No 6 Dec 016-Jan

3 0.53 Nu Re for 0.1 < Re< 1000 (8) 0.6 Nu 0.3Re for 1000 < Re< (9) The convectve heat transfer coeffcent h w between recever and ambent ar due to wnd can be calculated as [10], h w Nu. k ar (10) D o The radaton heat transfer coeffcent h r between recever surface to ambent temperature can be calculated from the equaton, h T T T T (11) r r a t a The overall heat loss coeffcent may be determned usng the followng relaton, Uloss hw hr (1) The followng correlatons have been used for determnng the temperature obtanable and heat loss for the gven PTC confguratons. The maxmum obtanable and optmum temperature of the HTF wth reference to solar beam radaton ( I b ) s gven by Tmax 0.793Ib 83.6 (13) T 0.53I (14) opt b Maxmum obtanable and optmum temperature of the HTF wth reference to concentraton rato (CR) s gven by, Tmax 0.009CR 6.318CR 40.4 (15) T CR CR opt (16) Convectve heat loss coeffcent wth respect to wnd speed s gven by the equaton: 0.489V 11.6V (17) hw Convectve heat loss from the recever wth respect to surface temperature s gven by the equaton: Qconv 1.55T r (18) Radaton heat loss from the recever wth respect to surface temperature s gven by the equaton: Q 0.307T (19) rad r Total heat loss from the recever s gven by, Q 1.859T (0) loss r IV. RESULTS AND DISCUSSION The geometrc concentraton rato s an mportant parameter n the solar concentratng collectors and t nfluences the recever surface temperature. The effectve rato s the product of geometrc concentraton rato and optcal effcency. The ncrease n concentraton rato ncreases the maxmum and optmum temperatures of the recever. The concentraton rato of the gven PTC s 68. In the observaton of fg. show that the maxmum obtanable and optmum temperature are 560 C and 300 C respectvely. Fg.. Effect of beam radaton on the maxmum and optmum temperature of PTC DOI: /jet/016/v86/ Vol 8 No 6 Dec 016-Jan

4 The effectve rato s the product of geometrc concentraton rato and optcal effcency. The ncrease n solar beam radaton ncreases the maxmum and optmum temperatures of the recever. The average beam radaton durng the test months (March and Aprl) n Chenna s about 700 W/m. As observed from the fg. 3 show that the maxmum obtanable and optmum temperature are 540 C and 50 C respectvely. Fg. 3 Effect of maxmum obtanable and optmum temperature at 700 W/m The wnd speed n the test ste s around 1 m/s, and t s responsble for the heat loss from the recever surface. It s an mportant parameter n the solar concentratng collectors, and t severely affects the thermal performance of the recever. The ncrease n wnd speed ncreases the convectve heat losses and the heat loss coeffcent ncreases lnearly wth wnd speed and the convectve heat loss coeffcent s around 5 W/m K. As observatons from the fg. 4 show that the convectve heat loss ncreased ten tmes as that of wnd speed beyond the speed of 1 m/s. Fg. 4 Heat loss from the recever at the normal wnd speed of 1 m/s. The overall heat loss coeffcent manly depends on wnd speed and recever surface temperatures. It s an mportant parameter n the solar concentratng collectors to determne the convecton and radaton heat losses from the recever. It severely affects the thermal performance of the recever. The ncrease n wnd speed and recever temperature ncreases the convectve heat losses, and the heat loss coeffcent ncreases lnearly wth wnd speed. Fg. 5 Heat loss from the recever at the normal wnd speed of 1 m/s. DOI: /jet/016/v86/ Vol 8 No 6 Dec 016-Jan

5 The overall heat loss coeffcent s less than 5 W/m K for the wnd speed of less than 1 m/s. The convecton, radaton and total heat loss for the gven PTC are manly depends on recever surface temperatures and wnd speed. It s an mportant parameter n the solar concentratng collectors to determne the convecton and radaton heat losses from the recever. It radaton loss s less than 50 W/m K whereas the convecton loss s domnant and n the range of 75 to 75 W/m K. In fg. 5 depcts the theoretcal thermal effcency of PTC at an optcal effcency of 75% and the ncrease n wnd speed decreases the effcency steeply. The lower wnd speed results n almost n a narrow range and gves about 75%. The practcal wnd speed at the ste s around 1 m/s and the theoretcal maxmum effcency s around 69%. Fg. 6 Effect of mass flow rate on the thermal effcency In fg. 6 resulted n the heat losses and thermal effcency of the PTC wth and wthout a secondary reflector. The effcency of the trough s 41% wthout the secondary reflector. After the secondary reflector, has been attached, the effcency of the trough wth the flat secondary reflector ncreased to 43%. The effcency of the trough wth the angular secondary reflector was ncreased to 48%. The effcency of the trough wth the curved secondary reflector was ncreased to 51%. The heat losses of the trough wth and wthout the secondary reflectors were calculated. Fg. 7 Heat loss and thermal effcency of the PTC wth and wthout secondary reflector The heat loss that occurred n the ppng of the trough wthout the secondary reflector s KJ/s. The heat loss that occurred n the trough wth the flat secondary reflector s KJ/s. The heat loss that occurred n the trough wth angular secondary reflector s 0.1 KJ/s. The heat loss that occurred n the trough wth curved secondary reflector s KJ/s. The beam radaton s one of the most mportant factors that affect the heatng of Ol n the solar trough. The ncrease of radaton could result n an ncrease n the heatng up of the tube whch n turn wll ncrease the temperature of the ol flowng through t. Any ncrease n the ambent temp could drectly result n an ncrease n the temperature of the tube and turn the temperature of the flud- Ol. Wnd velocty, however, could have an nverse effect on the heatng and result n the decrease n the temperature of the tube and turn the temp of the ol. DOI: /jet/016/v86/ Vol 8 No 6 Dec 016-Jan

6 V. CONCLUSION The secondary reflector s attached to the equpment, and the readngs are taken for all confguratons of the secondary reflector. The effcences and heat losses are calculated for the equpment wth the secondary reflector. A numercal analyss also performed for the same parabolc trough collector and few correlatons are formed. The expermental effcences are compared. The followng conclusons are made from the expermental results: The thermal effcency of PTC ncreases 10% wth the help of secondary reflectors. The heat loss from the PTC has been reduced by 0.5 kw wth the help of secondary reflector ACKNOWLEDGEMENT The author s grateful to Ecosense Sustanable Solutons Pvt. Ltd., New Delh and SRM Unversty, Chenna for provdng the test rg faclty for the research work. REFERENCES [1] Abhjeet Aut, TP Sngh, Dlp R Pangavhane, Thermal analyss of parabolc concentrator for fndng optcal effcency by dfferent methods wth varyng parameters", Internatonal Journal of Engneerng and Technology (IJET), 5(): [] Umayal Sundar AR, P. Neelamegam, C.V. Subramanan, Performance Evaluaton of a Forced Convecton Solar Drer wth Evacuated Tube Collector for Dryng Amla, Internatonal Journal of Engneerng and Technology, Vol 5(3): [3] Senthl R, Cheralathan M, Effect of non-unform temperature dstrbuton on surface absorpton recever n parabolc dsh solar concentrator, Thermal Scence, do: 10.98/ TSCI S [4] Govndaraj Kumaresan, Rahulram Srdhar, Ramalngam Velraj, Performance studes of a solar parabolc trough collector wth a thermal energy storage system, Energy, 47: [5] Z.D. Cheng, Y.L. He, F.Q. Cu, B.C. Du, Z.J. Zheng, Y. Xu, Comparatve and senstve analyss for parabolc trough solar collectors wth a detaled Monte Carlo ray-tracng optcal model, Appled Energy, 115: [6] Rzwan Masood, Syed Ihtsham Ul-Haq Glan, Hussan H. Al-Kayem, Thermal output analyss of a desgned parabolc trough solar feld for moderate temperature ndustral load, Internatonal Journal of Engneerng and Technology (IJET), 8(): [7] Rcardo Vasquez Padlla, Armando Fontalvo, Gokmen Demrkaya, Arnold Martnez, Arturo Gonzalez Quroga, Exergy analyss of parabolc trough solar recever, Appled Thermal Engneerng 67: [8] P.Selvakumar, P.Somasundaram, P.Thangavel, An Expermental Study on Evacuated Tube Solar Collector usng Thermnol D-1 as Heat Transfer Flud Coupled wth Parabolc Trough collector, Internatonal Journal of Engneerng and Technology (IJET),6(1): DOI: /jet/016/v86/ Vol 8 No 6 Dec 016-Jan