Performance Characteristics of Parabolic Trough Solar Collector System for Hot Water Generation

Transcription

1 Intenational Enegy Jounal: Vol. 7, No. 2, June Pefomance Chaacteistics of Paabolic Tough Sola Collecto System fo Hot Wate Geneation A. Valan Aasu and T. Sonakuma Faculty of Mechanical Engineeing Thiagaaja College of Engineeing Maduai INDIA ABSTRACT A paabolic tough sola collecto of apetue aea one squae mete was designed and developed fo hot wate geneation. The collecto s pefomance was tested accoding to ASHRAE Standad 93, The test slope and intecept of the collecto efficiency equation wee found to be and espectively. The collecto efficiency equation obtained in the pesent wok compaes well with the othe epoted liteatue. The collecto s time constant obtained fom the test was 67 seconds. The collecto s half acceptance angle detemined fom the test was 0.5 o, which in combination with the tacking system maximum eo (0.18 o ) implies that the collecto woks continuously at maximum possible efficiency. Keywods - paabolic tough collecto, collecto test, collecto themal efficiency, collecto time constant, acceptance angle, sola hot wate geneation. 1. INTRODUCTION The paabolic tough collecto (PTC) is cuently eceiving consideable attention, despite the fact that this sola concentating device equies some level of sola tacking. The disadvantage inheent with this tacking equiement is appaently offset by the advantage associated with the high ode of concentation available with this collecto. Paabolic Tough Collectos (PTC) ae geneally employed fo a vaiety of applications such as industial steam geneation [1] and hot wate poduction [2]. Paabolic tough collectos ae pefeed fo sola steam geneation because tempeatues of about 300 o C can be obtained without much degadation in the collecto efficiency. Sola themal powe plants based on PTC ae pesently the most successful sola technology fo electicity geneation, as demonstated by the Sola Electic Geneation Systems (SEGS) plant at Kame Junction in Califonia. A feasibility study fo the use of PTC in a hotel fo hot wate poduction was epoted by Kalogiou and Lloyd [2] and it was shown that PTC could be moe cost effective than the conventional flat plate collectos fo lage scale wate poduction. The pesent wok focuses on pefomance study of a new PTC of 1 m 2 apetue aea, designed and developed to poduce hot wate. 2. DESIGN OF PARABOLIC TROUGH COLLECTOR The design of a paabolic tough collecto with 90 o im angle, employed in the pesent eseach wok, is accomplished by consideing the optimized collecto apetue width and the eceive tube diamete. The simulation pogam witten in MATLAB is used to pedict the pefomance of the paabolic tough collecto hot wate geneation system. Detailed design and simulation analysis of pefomance of PTC system is pesented in Ref. [3].

2 138 Intenational Enegy Jounal: Vol. 7, No. 2, June 2006 The value of intecept facto, the most complex paamete involved in detemining the optical efficiency of a PTC, depends on the size of the eceive, the suface angle eos of the paabolic mio and sola beam spead. Accoding to Guven and Banneot [4], these eos o impefections ae of two types, namely andom and non-andom. Random eos ae modeled statistically, by total eflectedenegy distibution standad deviations σ at nomal incidence, Non andom eos ae detemined by the misalignment angle eo β (i.e. the angle between the eflected ay fom the cente of the Sun and the nomal to the eflecto s apetue plane) and the displacement of the eceive fom the focus of the paabola i.e. eceive dislocation distance,. Fo the evaluation of the intecept facto γ a closed-fom expession developed by Guven and Banneot [5] is used, (1) φ 1+ cosφ γ = 2sinφ 0 sinφ (1 cosφ)(1 2 sinφ) ( πβ (1 cosφ ) + d + Ef 2πσ (1 cosφ ) + sinφ (1 cosφ)(1 2 sinφ) ( πβ (1 cosφ ) φ + + d + + d Ef 2πσ (1 cosφ ) + (1 + cosφ) (2) The collecto im angle ( ) is defined as the angle subtended by the edges of the eflecto at the focus. A simple MATLAB pogam, which numeically evaluates the above expession is pesented in Ref. [3]. The paabola tough of the PTC was accuately constucted of fibeglass. Accoding to Guven and Banneot [4], fo caefully constucted collectos, the standad deviation of the distibution of local slope eos at nomal incidence, σ slope = ad and the standad deviation of the vaiation in diffusivity of the eflective mateial at nomal incidence, σ mio = ad. The standad distibution of the Sun s intensity distibution σ sun can be taken as ad. Theefoe, fom equation (1), σ = ad. The misalignment angle eo β = 0.18 o (i.e. the maximum tacking eo) and the displacement of the eceive fom the focus of the paabola, is assumed as 2 mm. Using these inputs to the pogam, togethe with eceive diamete, the intecept facto is evaluated as Using a sola eflecto mateial of eflectance ( ρ m ) (SOLARFLEX foil, Clea Dome Sola) [6], a special black painted coppe tube eceive absoptance ( α ) of 0.9 and a low-ion glass cove tansmittance ( τ e ) of 0.9, a maximum optical efficiency (η o ) of is obtained fom the following expession. γ is intecept facto, defined as the atio of the enegy intecepted by the eceive to the enegy eflected by the focusing device, 3. DESCRIPTION OF PTC SYSTEM A f is geometic facto and θ is angle of incidence In the pesent wok, the PTC system used fo hot wate geneation, which has been developed fo expeimentation, is pesented in figue 1. The PTC system fo hot wate geneation includes a PTC, a hot wate stoage tank (HWST) of well-mixed type and a ciculating pump. The paabola of the pesent collecto with a im angle of 90 o is vey accuately constucted of fibeglass. A flexible sola eflecto (3)

3 Intenational Enegy Jounal: Vol. 7, No. 2, June mateial (SOLARFLEX foil) fom Clea Dome Sola, San Diego with a eflectance of is used in the pesent wok. The sola eceive consists of a coppe tube, a glass envelope and ubbe cok seals at both ends of the glass envelope. The coppe tube is coated with a heat esistant black paint and is suounded by a concentic glass cove with an annula gap of 5 mm. The ubbe coks ae incopoated to achieve an ai-tight enclosue. Wate fom the stoage tank is pumped though coppe tube, whee it is heated and then flows back into the stoage tank. The PTC otates aound the hoizontal noth/south axis to tack the Sun as it moves though the sky duing the day. The axis of otation is located at the focal axis. The tacking mechanism consists of a low speed 12 Vdc moto and an embedded electonic contolled tacking system. The input signals to the contol system ae obtained fom light dependent esistos. The pump fo maintaining the foced ciculation is opeated by an on-off contolle (Diffeential Themostat), which senses the diffeence between the tempeatue of the wate at the outlet of the collecto ( T fo ) and the stoage tank wate ( T l ). The pump is switched on wheneve this diffeence exceeds a cetain value and off when it falls below a cetain value. In the pesent wok, the diffeential tempeatue contolle value ( T fo Tl ) is set as +2 o C. The specifications of the PTC system ae detailed in table PARABOLIC TROUGH COLLECTOR PERFORMANCE TEST In the pesent wok, the themal pefomance of the paabolic tough sola collecto was detemined accoding to ASHRAE standad 93 to 1986 [7]. The themal pefomance of the sola collecto was detemined by obtaining values of instantaneous efficiency fo diffeent combinations of incident adiation, ambient tempeatue and inlet wate tempeatue. All paametes wee measued unde steady state o quasi-steady state conditions. The collecto wate outlet tempeatue ( T fo ), ambient tempeatue ( T a ) and stoage tank wate tempeatue ( T l ) wee ecoded with the help of PT esistance tempeatue device (RTD) sensos. The sola beam adiation intensity was measued by a pyheliomete and the mass flow ate of wate by a otamete. The wind speed was measued by a vane type anemomete. PTC T fo Coppe Tube Hot Wate Out T fi Diffeential Themostat HWST T l Flow Mete Cold Wate In Valve Pump Make-Up Wate Fig. 1. Paabolic tough collecto system.

4 140 Intenational Enegy Jounal: Vol. 7, No. 2, June 2006 Table 1. Paabolic tough Collecto System Specifications Items Value Collecto Apetue 0.8 m Collecto Length 1.25 m Rim Angle 90 o Focal Distance 0.2 m Receive Diamete 12.8 mm Glass Envelope Diamete 22.6 mm Concentation Ratio Wate Flow Rate lpm Stoage Tank Capacity 35 lites Tank Mateial Stainless Steel Tank Insulation Mateial Glass Wool Insulation Thickness 50 mm Wate Pump 0.5 hp 4.1 Collecto Themal Efficiency Test The themal efficiency of a concentating collecto opeating unde steady state conditions can be descibed by ASHRAE 1986 [7], (4) The themal efficiency fom equation (4) is plotted against ( T fi Ta ) I, a staight line will esult povided is constant. The intecept is FRηo and the slope is. The pefomance cuve of the PTC, as deived fom a seies of tests, caied out accoding to ASHRAE standad 93 (1986) on seven days, is shown in figue 2. An equation fo the cuve, established with 28 data points, is obtained using the standad technique of a least squaes fit. The intecept is equal to and the slope is Theefoe, the collecto themal efficiency equation fo this PTC can be witten as, Tfi Ta η = (5) I Using a calculated value of F = and a concentation atio R C = 19.89, we obtain, Optical efficiency, η o = and Heat loss coefficient, = 7.73 W/m 2 K Theoetical calculation of the intecept and slope togethe with the expeimental values ae pesented in table 2. The theoetical values ae estimated by means of a MATLAB compute pogam [3]. It can be obseved fom table 2 that thee is a mino diffeence between the theoetical and the expeimental esults with espect to test slope (5.92 %) and a modeate diffeence is shown fo the test intecept (9.37 %), which demonstates that the design pocedue followed in the pesent wok is coect. The collecto efficiency equation compaes well with those fo othe epoted eseach woks as pesented in table 3.

5 Intenational Enegy Jounal: Vol. 7, No. 2, June Fig. 2. Collecto themal efficiency cuve. Table 2. Collecto Pefomance S. No. Item Test Intecept Test Slope 1 Theoetical Pefomance [3] Expeimental Pefomance [Pesent Wok] % Diffeence Table 3. Compaison of Collecto Efficiency Equations

6 142 Intenational Enegy Jounal: Vol. 7, No. 2, June Collecto Time Constant Test The time constant of a collecto is the time equied fo the fluid leaving the collecto to each of its ultimate steady state value afte a step change in incident adiation. i.e. the time constant of collecto is the time equied fo the quantity ( T fo, f T fo ( τ )) ( T fo, f T fi ) to change fom 1.0 to [7]. whee T fo ( τ ) is the outlet tempeatue of collecto fluid afte time (τ ) and is the final outlet tempeatue of collecto fluid. The time esponse of the paabolic tough sola collecto has to be detemined in ode to evaluate the tansient behaviou of the collecto, and to select the pope time intevals fo the steady state o quasi-steady state efficiency tests. Wheneve tansient conditions exist, the equality defined by equation (4) does not goven the themal pefomance of the collecto, since pat of the sola enegy absobed is used fo heating up the collecto and its components o pat of the enegy lost esults in cooling the collecto. The time constant fo a sola collecto was detemined as follows: The collecto was initially at the defocused position. The collecto was suddenly moved to the focused position and measuements wee continued fo evey 10 seconds until steady state conditions had been achieved (figue 3). Duing the test the collecto wate inlet tempeatue was maintained at the ambient tempeatue. The time constant of the collecto was evaluated as 67s. Fig. 3. Collecto time constant. 4.3 Collecto Acceptance Angle Test An embedded electonic contolled tacking system has been connected to a paabolic tough collecto with apetue width 0.8 m, length 1.25 m, im angle 90 o and eceive diamete 12.8 mm. Since collecto acceptance angle chaacteises the effect of eos in the angula oientation of the tacking mechanism, a test to detemine the collecto acceptance angle was caied out accoding to ASHRAE standad 93 to 1986 [7]. With the tacking mechanism disengaged, the collecto efficiency was detemined at vaious out of focus angles as the Sun taveled ove the collecto s plane. The themal efficiency of the collecto opeating unde steady state conditions is given by [12],[13].

7 Intenational Enegy Jounal: Vol. 7, No. 2, June mc p ( Tfo Tfi ) Themal efficiency, η = (6) A I The inlet tempeatue of wate, T fi was equal to the ambient tempeatue, ( T a = 306 K) and wate was ciculated at the specified flow ate i.e. 0.7 kg/min. Duing the test, the diect beam of sola iadiation on the plane of the collecto apetue was between 800 and 810 W/m 2. The outlet tempeatue of wate, T fo was continuously monitoed fo evey second ove a 30 minutes peiod. The efficiency facto is computed as the atio of themal efficiency at a paticula out of focus angle to the maximum themal efficiency at nomal incidence. The angle of incidence measued fom the nomal to the tacking axis is plotted against the efficiency facto (Figue 4). The collecto acceptance angle is defined as the ange of incidence angles measued fom the nomal to the tacking axis in which the efficiency facto vaies by not moe than 2% fom the nomal incidence angle [7]. Theefoe, fom figue 4, the acceptance angle of the collecto is appoximately 1 o. The tacking system accuacy, i.e. the out of focus angle equied to initialise the system, can be detemined by measuing the peiod between successive opeation of the moto [14]. The tacking system accuacy depends on the Sun s intensity level. The accuacy of the embedded electonic contolled tacking mechanism developed fo the pesent PTC system is 0.1 o with adiation level of 700 W/m 2 and 0.18 o with adiation level of 200 W/m 2. Thus the tacking mechanism maximum eo is 0.18 o, which is much geate than the equied 0.5 o (collecto half acceptance angle value) detemined fom the collecto acceptance angle test. 5. CONCLUSION Fig. 4. Collecto acceptance angle. The pefomance tests of the new paabolic tough collecto designed and developed fo hot wate geneation have been caied out accoding to ASHRAE standad 93 to In the pesent wok, the expeimentally calculated values of test slope and test intecept ae and espectively. The collecto efficiency equation compaes well with the othe epoted eseach woks. The test slope and test intecept values vay only by 5.92 % and 9.37 % espectively fom the theoetically calculated values, which demonstates that the design pocedue followed in the pesent wok is coect. The time constant fo the new PTC is detemined as 67 seconds, which suggests that the collecto has fast

8 144 Intenational Enegy Jounal: Vol. 7, No. 2, June 2006 esponse to maintain quasi-steady state conditions. The embedded electonic contolled tacking mechanism has poved to be accuate fo the pesent sola enegy application fom the collecto acceptance angle test that was caied out accoding to ASHRAE standad 93 to The tacking mechanism maximum eo is detemined to be 0.18 o only. The accuacy of the new tacking mechanism is much geate than the equied 0.5 o, which is detemined fom the collecto acceptance angle test. This implies that the collecto woks continuously at maximum possible efficiency at all times. 6. NOMENCLATURE A Collecto apetue aea (m 2 ) A f Geometic facto (m 2 ) C Concentation atio c p Collecto fluid specific heat capacity (J/kg-K) D,o Receive tube oute diamete (m) d Receive dislocation distance (mm) d Univesal nonandom eo paamete due to eceive dislocation and eflecto pofile eos [d = d/d,o ] F R Heat emoval facto I Beam o diect adiation (W/m 2 ) L Receive tube length (m) m T a T fi T fo T l U L W a α β β φ γ Mass flow ate (kg/s) Ambient tempeatue (K) Collecto wate inlet tempeatue (K) Collecto wate outlet tempeatue (K) Stoage tank wate tempeatue (K) Oveall heat loss coefficient (W/m 2 -K) Apetue width (m) Receive absoptivity Misalignment angle eo (degee) Univesal nonandom eo paamete due to angula eos [β = βc] Collecto im angle (degee) Intecept facto η Themal efficiency (%) η o Optical efficiency (%) θ σ σ σ sun Angle of incidence (degee) Total eflected-enegy standad deviations at nomal incidence Univesal andom eo paamete [σ = σc] Standad deviation of the enegy distibution of the sun s ays at nomal incidence

9 Intenational Enegy Jounal: Vol. 7, No. 2, June σ slope σ mio τ e Standad deviation of the distibution of local slope eos at nomal incidence Standad deviation of the vaiation in diffusivity of the eflective mateial at nomal incidence Tansmittance of envelop mateial 7. REFERENCES [1] Cohen, G.; and Keaney, D Impoved paabolic tough sola electic system based on the SEGS expeience. Poceedings of the ASES Annual Confeence, Sola 94: [2] Kalogiou, S.; and Lloyd, S Use of sola paabolic tough collectos fo hot wate poduction in Cypus. A feasibility study. Renewable Enegy 2(2): [3] Valan Aasu, A.; and Sonakuma, T Design and simulation analysis of a paabolic tough sola collecto hot wate geneation system. The Intenational Enegy Jounal 6(2): In Pess. [4] Guven, H.M.; and Banneot, R.B Detemination of eo toleances fo the optical design of paabolic toughs fo developing counties. Sola Enegy 36: [5] Guven, H.M.; and Banneot, R.B Deivation of univesal eo paametes fo compehensive optical analysis of paabolic toughs. Poceedings of the ASME-ISES Sola Enegy Confeence, Knoxville, USA: [6] CleaDome Sola Systems heating and cooking poducts. San Diego, USA. [7] ASHRAE Standad Method of testing to detemine the themal pefomance of sola collectos. Ameican Society of Heating, Refigeating and Ai Conditioning Enginees: Atlanta, GA. [8] Muphy, L.M.; and Keneth, E Steam geneation in line-focus sola collectos: A compaative assessment of themal pefomance, opeating stability, and cost issues, SERI/TR [9] Hutado, P.; and Kast, M Expeimental study of diect in-situ geneation of steam in a line focus sola collecto, SERI. [10] Kalogiou, S.; Lloyd, S.; Wad, J.; and Eleftheiou P Design and pefomance chaacteistics of a paabolic-tough sola-collecto system. Applied Enegy 47: [11] Kalogiou, S Paabolic tough collecto system fo low tempeatue steam geneation: Design and pefomance chaacteistics. Applied Enegy 55(1): [12] Duffie, J. A. and Beckman, W.A Sola Engineeing of Themal Pocesses- 2 nd edition. New Yok : John Wiley. [13] Sukhatme, S.P Sola Enegy Pinciples of themal collection and stoage- Second Edition. New Delhi: Tata McGaw-Hill Publishing Company Limited. [14] Kalogiou, S Design and constuction of a one-axis sun-tacking system. Sola Enegy 57(6):

10 146 Intenational Enegy Jounal: Vol. 7, No. 2, June 2006