Effect of Once-through and Recirculated Fluid Flow on Thermal Performance of Parabolic Dish Solar Receiver

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1 Indin Journl of Science nd Technology, Vol 9(33), DOI: /ijst/2016/v9i33/89084, September 2016 ISSN (Print) : ISSN (Online) : Effect of Once-through nd Recirculted Fluid Flow on Therml Performnce of Prbolic Dish Solr Receiver Rmlingm Senthil * nd Mrimuthu Cherlthn Deprtment of Mechnicl Engineering, SRM University, Kttnkulthur , Tmil Ndu, Indi; rsenthilsrm@gmil.com, cherlthnm@gmil.com Abstrct Objectives: The effect of mss flow rte of het trnsfer fluid (HTF) during once-through (OTF) nd recircultion (RF) opertions, through its solr receiver ws nlyzed in prbolic dish (PD) collector. Methods/Sttisticl Anlysis: Inlet temperture of HTF nd mss flow rte re the most importnt operting prmeters in solr concentrting collectors long with locl solr irrdince. The solr receiver ws tested on sunny dys (solr bem rdition intensity in the rnge of W/m 2 ) in Februry nd Mrch 2016 t Chenni, Indi (Ltitude: 13 N, Longitude: 80 E) with HTF mss flow rtes of 25, 50, 75, 100 nd 125 kg/h. Findings: The instntneous temperture gin by the HTF ws observed to be round 100 C for the OTF with 25 kg/h due to effective het bsorption thn tht of 40.5 C for the RF due to more het loss. The instntneous energy efficiency during the RF mode is round three times more thn the efficiency of OTF mode. Exergy efficiency of OTF is 3.28% more thn the RF due to lower exergy destruction. Appliction/Improvements: The once-through flow hs n dvntge over recircultion by the wy of less therml stress effect on the receiver nd piping. Keywords: Exergy Efficiency, Once-through Flow, Prbolic Dish, Recircultion, Solr Receiver 1. Introduction Reserchers hve been working for severl decdes on efficient technologies to collect, store nd utilize solr energy. The prboloidl reflectors re well suited for pplictions like preheting of furnce oil for boiler or cptive power genertion, preheting of biodiesel for engines, LPG vporiztion before burner, pre-tretment of metls, cooking nd lundry pplictions in the temperture rnge of C. The wter or thermic oil or ir cn be used s HTF. Preheting of fuel using solr collectors reduces the viscosity of fuel oils. A storge tnk of 20 litres cpcity ws kept t the focl point of 8 m 2 Scheffler reflector nd investigted by 1 for the temperture sensitive process industry with the verge therml efficiency of 21.61% to rech 98 C of wter. Ellipticl het pipe solr collector nd flt plte collector performnce were nlyzed compred the energetic nd exergetic efficiencies of the box type solr cooker, niml feed solr cooker nd PD solr cooker. 6 crried out chrging experiments on three solr therml oils. The more effective oil ws found with the highest density nd specific het cpcity for solr cooking. In 7 formed correltions to determine the nturl convection het loss from the V-trough solr concentrtor for the Ryleigh number rnge of 1 x 10 8 to 2 x 10 8 through experimentl study. In 8 demonstrted cylindricl cvity receiver with n opticl efficiency of 52%, with the het loss fctor of the receiver t 4.6 W/K.. In 9 found tht n increse in the HTF inlet temperture of PDC leds to significnt increse in collector exergy efficiency, but cuses reduction in the collector energy efficiency. Energy conservtion ws described by 10,11. Modelling of trcking of solr cell ws done by enhnced the effficency of solr cell by cooling. Bsed on literture, the HTF inlet temperture nd mss flow rtes re determined to be the importnt operting prmeters ffecting the therml performnce of solr receivers. However, comprison of OTF nd RF for PDC hs not been reported thus fr in ny literture * Author for correspondence

2 Effect of Once-through nd Recirculted Fluid Flow on Therml Performnce of Prbolic Dish Solr Receiver for the working fluid remins liquid without phse chnge. The energy nd exergetic nlyses of the receiver for OTF nd RF with different mss flow rtes in 16 m 2 Scheffler PDC re discussed in this pper. The preheting of furnce oil in the rnge of C before entering to boiler, furnce/oven nd considerble therml performnce vritions re reported for the OTF nd RF modes of opertion. 2. Experimentl Work The prboloidl dish concentrtor ws fbricted with hrdened steel in n ellipticl frme of minor nd mjor xis s 3.8 m nd 5.3 m respectively by Thermx Ltd, Pune, Indi. The sme prboliodl dish ws described by 14,15. The cylindricl tnk type receiver ws fbricted with mild steel plte of 5 mm thickness. The externl dimeter nd width of the receiver were 406 mm nd 80 mm respectively. The ctul concentrtion rtio vried from The fixed focl length of the receiver ws 2.5 m. The wter flow direction is upwrds through rectngulr fins, inside the receiver. The receiver is kept inclined 13.5 towrds the South s per Chenni ltitude s the optiml position for the sesonl movement of the Sun. The schemtic lyout of the PDC experimentl test set up is illustrted in Figure 1. Figure 1. Schemtic of the PDC setup. The outdoor tests were conducted in Februry nd Mrch The solr rdition intensity (Kipp nd Zonen pyrnometer with shded ring, ± 3% ccurcy), wind speed (Cup type nemometer, rnge 0.3 to 30 m/s, ccurcy ± 1%), HTF flow rte (Glss rotometer, rnge 0 to 150 kg/h, ccurcy ±1%), HTF temperture nd mbient temperture (K-type thermocouples, ccurcy ± 1%) were mesured by bove mentioned qulity instruments during the test periods. The operting prmeters of HTF flow rte, inlet nd outlet temperture of HTF, HTF temperture in storge tnk nd receiver surfce temperture t multiple predetermined points re mesured from the experimentl setup. Cold-strt opertion ws considered in ll the cses. Mss flow rte of HTF vried in increments, 75, 100 nd 125 kg/h for both OTF nd RF cses. The instntneous energy nd exergy efficiency of the receiver for OTF nd RF modes were compred to specify the effective mss flow rte for specific preheting ppliction by PDC. 3. Therml Anlysis The energy nlysis focuses minly on the het gin from the overll input energy to the system. The incident solr energy entering the PDC is dependent on the solr bem rdition (I b ) nd perture re of the concentrtor (A c ); it is expressed s Eq. (1), Q i = A c I b (1) Het bsorbed by the HTF in the receiver is dependent on liquid specific het (C p ), mss flow rte ( m ) nd entry nd exit temperture of fluid (T i nd T o ). The useful het gin by the HTF (Q u ) is given by 16, Q u = m C p (T o - T i ) (2) The Nusselt correltion for the nturl convective het loss from Ryleigh number (R) (Incroper nd DeWitt, 2002) from the dish receiver is expressed in Eq. (3), 1/4 0.67R (3) 5 10 Nu free = ,10 R ³ 10, 9/16 4/9 [1 + (0.492/Pr) ] The Nusselt correltions for the forced convective het loss from Reynolds number (Re) nd Prndtl number (Pr) (Incroper nd DeWitt, 2002) from the dish receiver is expressed in Eq. (4), 1/2 1/3 Nu forced, lminr = 0.322Re Pr, 0.6 < Pr > 50, Re < 5x105 (4) The combined free bsed on Grshff number (Gr) nd forced convection occurs on the verticl surfce of the receiver owing to Gr/Re 2 lies between 0.1 nd 10, the combined free nd forced convection coefficient is expressed s: n n 1/ n (5) Nu = ( Nu ± Nu ) convection focred free In Eq. (5), the plus or minus sign indictes the ssisting or opposing nture of forced convection on free 2 Vol 9 (33) September Indin Journl of Science nd Technology

3 Rmlingm Senthil nd Mrimuthu Cherlthn convection respectively. The vlue of n cn be considered 3 to 4 for verticl to horizontl surfces. Convective het loss coefficient (h) cn be found from the combined Nusselt number. The convective het losses re clculted from the following expression: Q convection = ha r (T w - T ) (6) The rditive het loss from the receiver is given by Eq. (7), 4 4 Q = sae( T - T ) (7) rdition r w where, A r - surfce re of the receiver, T w - verge receiver surfce temperture, σ - Stefn-Boltzmnn constnt, ε- emissivity of receiver surfce nd T - mbient temperture. The instntneous efficiency of the receiver is defined s the rtio of useful het gined by the HTF to the incident solr energy on the PDC nd is given in Eq. (8), Q (8) u h = AI c b The ssumptions mde for exergy nlysis re stedy stte, stedy flow energy eqution, negligible potentil nd kinetic energy effects, no internl het genertion s well s no chemicl or nucler rections. Exergy received by the collector by expression 17 in terms of solr energy input, mbient nd Sun temperture is given s: 4 é 4 T æ T ö ù (9) Exi = Q i 1 ç - +ç 3 T ç sun T ê çè sun ë ø úû where, Q i is the het input to the concentrtor, T is mbient temperture nd T sun is the Sun s temperture (1580 K). The exergy gined by HTF in the receiver cn be clculted bsed on the inlet/outlet tempertures (T i, T o ), mbient temperture (T ), specific het of HTF (C p ) nd mss flow rte of HTF ( m ) is expressed in the following equtions by 18 :. é T ù (10) Ex ( ) ln o u = mc p To-Ti -T ê T ú ë i û Exergy efficiency is the rtio of exergy gined by HTF in the solr receiver to solr rdition exergy input nd expressed s: Exu (11) h x = 100 Ex i 4. Results nd Discussion During the outdoor experiments, the mss flow rte of wter to the solr receiver ws mintined constnt (75, 100 nd 125 kg/h) with flow control vlve nd the liquid phse of HTF only considered. Sunny dys with similr solr bem rdition intensity ( W/m 2 ) were lone ccounted for the energy nd exergy nlyses. The temperture increse of HTF, wind speed (vried from 0 to 1.75 m/s) nd the mbient temperture (34 to 37 C) ws mesured during the tests. The highest therml gin by HTF ws observed for the lower mss flow rte of 75 kg/h flow rte nd the highest receiver wll temperture ws observed for the higher mss flow rte of 125 kg/h from Figure. 2. The HTF outlet temperture ws mintined t 25 C, 16 C nd 12 C during the once-through flow for 75, 100 nd 125 kg/h respectively. Receiver wll temperture ws higher for the lrger mss flow rte of wter. The increse in temperture gin of HTF is higher for the lower mss flow rte of HTF. A single reflector is cpble of preheting oil oncethrough mss flow rte of 25 kg/h (bout 140 kw output) round 100 C. Figure 2. Receiver temperture nd HTF temperture gin (OTF). The highest therml gin by HTF ws observed for the 75 kg/h flow rte. The highest receiver wll temperture ws observed for 100 kg/h due to the lower wind speed nd reduced convective het losses (Figure 3). The verge difference between receiver exit nd entry temperture of HTF re mintined t 24 C, 20 C nd 16.6 C for 75, 100 nd 125 kg/h respectively during the recircultion. The convection nd rdition losses were determined bsed on the receiver surfce temperture nd the lest het loss ws observed for the OTF due to incresed residence time of HTF inside the receiver nd effective het bsorption. Vol 9 (33) September Indin Journl of Science nd Technology 3

4 Effect of Once-through nd Recirculted Fluid Flow on Therml Performnce of Prbolic Dish Solr Receiver Figure 3. Receiver temperture nd HTF temperture gin (RF). The RF ws mintined t higher surfce temperture so the het loss ws comprtively higher thn the oncethrough mode. The het loss ws round 500 W for oncethrough mode nd 800 W for the recircultion mode. The convective nd rdition loss during the once-through mode decresed by 37.5% when compred to recircultion of HTF (Figure 4). Figure 4. Receiver het losses. The increse in HTF mss flow rte increses the energy efficiency of the receiver significntly in Recirculted Flow (RF). Higher exergy efficiency of 8.7% ws obtined during the Once-Through Flow (OTF). A little vrition in exergy efficiencies is observed during the increse in the HTF mss flow rte in recircultion mode, s illustrted in Figure 5. The exergy destruction is higher in the recircultion mode of HTF by mixing of HTF in the storge tnk, but this effect my be llevited by constnt fluid temperture (once-through mode) or by vrying the flow rte for prtuculr solr rdition intensity level. However, the exergy utiliztion is roiund 3% higher for the once-through mode flow thn recircultion mode of HTF. Figure 5. Instntneous energy nd exergy efficiencies of solr receiver. The uncertinities in the mesurement s follows, temperture (± 0.5 C), solr rdition (± 3 W/m 2 ), wind speed (±0.1 m/s), mss flow rte (± 1 kg/h) nd re mesurement (± 2%). The uncertinty in the experimentl efficiency mesurement is well below 5% through soot men squre method nd it indictes tht the instruments nd mesurements re in the sufficient rnge of relibility. 5. Conclusions The solr receiver in PDC cn be operted t oncethrough nd recircultion mode bsed on the ppliction requirements like preheting of fuel oil or combustion ir or domestic hot wter. The working fluid of once-through flow ws reched round 100 C during the HTF flow of 25 kg/h. The highest instntneous energy nd exergy efficiency of once-through mode flow were 46.2% nd 8.7% nd for the recircultion mode flow, 42.9% nd 4.66% respectively. The exergy efficiency of OTF ws more thn the RF of HTF which ws due to exergy destruction in the recircultion mode by mixing (poor strtifiction) in storge tnk nd higher surfce temperture in receiver nd piping. The once-through flow hs n dvntge over recircultion by the wy of less therml stress effect on the receiver nd piping. 6. References 1. Ptil RJ, Awri GK, Singh MP. Experimentl nlysis of Scheffler wter heter. Therml Science. 2011; 15 (5): Sivkumr K, Krishn Mohn N, Sivrmn B. Performnce Anlysis of Ellipticl Het Pipe Solr Collector. Indin Journl of Science nd Technology Jn, 4(1): Kou KB, Gbh P, Koffi EPM, Fssinou WF, Toure S. Modelling of therml behviour of direct solr drier possess- 4 Vol 9 (33) September Indin Journl of Science nd Technology

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