An Experimental Study of Natural Convection Heat Loss from a Solar Concentrator Cavity Receiver at Varying Orientation.

Transcription

1 Solar Concentrator Cavty Recever at Varyng Orentaton. T. Taumoefolau and K. Lovegrove Centre for Sustanable Energy Systems, Department of Engneerng, Australan Natonal Unversty, Canberra ACT 0200, AUSTRALIA. Telephone: +(61) (2) Facsmle: +(61) (2) Emal: Abstract Convecton losses are an mportant determnng factor n the performance of solar thermal power systems wth cavty recevers such as those used n parabolodal dshes. An expermental nvestgaton based on an sothermal electrcally heated model recever, s n progress. The convecton loss of the model recever has been measured at nclnatons varyng from 0 o (cavty algned horzontally) to 90 o (cavty facng straght down). The results show that the maxmum convecton loss occurs at 0 o and s a mnmum at 90 o. Numercal analyss of the problem shows good agreement whle most prevous emprcal correlatons over predct the losses. 1 INTRODUCTION In solar thermal systems, heat loss can sgnfcantly reduce the effcency and consequently the cost effectveness of the system. It s therefore vtal to fully understand the nature of these heat loss mechansms. Wth parabolodal dsh cavty recevers, conducton and radaton can readly be determned analytcally, however the complexty of the temperature and velocty felds, n and around the cavty makes t consderably harder to determne the convecton loss. The Australan Natonal Unversty (ANU) has been nvolved wth the nvestgaton of solar thermal energy converson usng parabolodal dsh concentrators for many years. Currently the team s workng wth a 400 m 2 concentrator ftted wth a monotube boler cavty recever for superheated steam producton and a 20 m 2 concentrator that operates a cavty recever lned wth reactor tubes for ammona dssocaton for energy storage (Johnston et al, 2001 and Lovegrove et al, 2001). To better understand the thermal losses from such recevers, a small electrcally heated, laboratory smulaton of a solar cavty recever has been constructed to measure losses drectly. The results from ths system have then been used n comparson wth the predctons obtaned from Computatonal Flud Dynamcs (CFD) calculatons, whch are descrbed n a parallel paper also presented n these proceedngs (Patoonsurkarn et al, 2002). 1.1 Prevous nvestgatons There have been several prevous nvestgatons of convecton losses from cavty recevers. An analytcal model of large cubcal central recevers was proposed by Clausng (1981), based on the local convectve heat transfer coeffcents nsde the cavty, determned from standard sem-emprcal correlatons and the energy transferred by the ar through the aperture due to buoyancy and wnd effects. Ths model was later refned and verfed by the same author, Clausng (1983) wth expermental results from a 2.7m square aperture recever and good agreement found. Emprcal correlatons proposed by Koeng and Marvn (1981) cted n Harrs et al (1985) and Stne and McDonald (1989) cted n Lebfred et al (1995), ncludes parameters such as nclnaton angle and aperture sze n ther models. Lebfred et al (1995) carred out expermental studes on both upward and downward facng recevers. Ths study used electrcally heated sphercal and hemsphercal recevers wth a dameter of 400mm. The aperture for these recevers ranged from 60 to 195mm n dameter and was adjusted by addng nsulated dsks of dfferent nner dameters. Correlatons developed from ths study were a modfcaton of Clausng s (1981) and Stne and McDonald s (1989) models. Wth all these studes the range of applcablty beyond the recever geometry drectly examned remans unclear and therefore cauton should be used when usng them for dfferent geometres and operatng condtons.

2 Solar Concentrator Cavty Recever at Varyng Orentaton 2 LABORATORY MEASUREMENT OF CONVECTION LOSSES 2.1 Expermental apparatus: An electrcally heated expermental smulaton of a cavty recever has been constructed to allow drect measurement of losses under laboratory condtons. The detals of the model recever are shown n Fg. 1 and the arrangement n the laboratory n Fg. 2. The model recever conssts of a mld steel tube cavty wth a Pyrotenax mneral nsulated electrcal heater cable wound around t as a source of heat nput. The cavty nteror surface has been panted wth hgh temperature resstant black Pyromark 2500 pant. The steel tube s mounted n a framework of Calcum Slcate nsulaton board. A sheet metal casng covers the entre structure and all nternal spaces are flled wth Kaowool ceramc nsulaton materal. The model recever s attached to the end of a support frame on a trolley by a hnged angle adjustment mechansm to enable testng at dfferent angles. There are 7 K- type thermocouples that measure the cavty surface temperature, 8 on the exteror surface of the model, plus a further 8 measurng varous temperatures wthn the model. These thermocouples are logged wth a Datataker 600. The temperature of the cavty s controlled by a self-tuned Eurotherm 808 PID temperature controller that regulates the power level to the heatng col. A host computer, not shown n Fg. 2, acqures both the data from the Eurotherm 808 PID temperature controller and the Datataker 600. Partcle board Sheet metal casng Angle adjustment Eurotherm 808 Temperature controller 320mm Kaowool nsulaton Heatng col 150mm Mld steel cavty Steel bracket Model recever aperture Datataker 600 Power supply 5mm d=70mm Calcum slcate board do=280m Fg. 1 Cross secton sketch of model recever. Fg. 2 Expermental setup. Durng operaton, a tme nterval of approxmately one hour s requred for the system to reach steady state. Temperatures are logged at 30 second ntervals whle the power level s logged every second for a perod of 30 mnutes, to provde the data for a relable steady state data pont. A Fluke 83 multmeter s used to measure the supply voltage V, and heater resstance R, and n conjuncton wth the regulated power level p L, the total heat loss rate q t from the recever can be calculated by Eq. (1). q t 2 V = pl (1) R 2.2 Determnaton of the energy balance The expermental arrangement provdes a drect measurement of overall thermal losses from the cavty. The energy balance of the model recever s shown n Fg. 3, where convecton loss, q conv and radaton loss, q rad are modes of heat loss through the aperture, whle conducton loss, q cond s through the walls of the recever. 2 Proceedngs of Solar Australan and New Zealand Solar Energy Socety Paper 1

3 Solar Concentrator Cavty Recever at Varyng Orentaton q cond q cond q rad q conv q cond Fg. 3: Energy balance of model recever. Snce t s convecton loss that s of nterest, conducton and radaton contrbutons need to be accounted for n Eq. (2). q conv = q q q (2) t cond rad Conducton loss measurement To determne the conducton heat loss, measurements of loss were made wth the cavty at 90 0 and wth an nsulated plug n the aperture. The nternal and external temperatures of the plug were measured and used to determne the plug conducton loss. It s assumed that conducton s the same for all nclnaton angles, wth the thermal resstance assocated wth the boundary layer on the outsde of the casng beng consdered neglgble. Fnte element analyss of the conducton problem usng STRAND 7 release 1.03 has also been carred out. The constructon of the mesh for the conducton model comprses of three dmensonal brck elements, where expermentally measured temperatures were used as boundary condtons. Agreement to wthn 10% was found and the dfference attrbuted to the uncertanty n the actual effectve conductvty of the nsulaton materal Radaton loss calculaton Radaton loss has been determned analytcally wth the network method descrbed by Holman (1997) where the surface s assumed to be grey and radaton s dffuse. The cavty s dvded nto 5 sectons as shown n Fg. 4 and expermentally measured temperatures for each secton are used to determne the net radaton from each secton. End plate 1 Aperture Fg. 4 Cavty dvson for radaton calculaton. A radaton energy balance s carred out for the th secton wth Eq. (3), where J s the radosty, F j s the fracton of radant energy leavng surface and reachng surface j, ε s the surface emssvty and E b s the black body emssve power. J = 1 (1 ε ) Fj J 1 F (1 ε ) j j + ε E b (3) Havng set up an equaton for each surface, a set of 5 equatons s then solved smultaneously to calculate the radostes. Eq. (4) s then used to determne the radaton transfer rate for each th surface havng area A. The radaton loss through the aperture s then obtaned from the sum of ndvdual radaton losses. ( E J ) ε A = (4) 1 q b ε Proceedngs of Solar Australan and New Zealand Solar Energy Socety Paper 1 3

4 Solar Concentrator Cavty Recever at Varyng Orentaton An emssvty of 0.93 was used for the Pyromark panted cavty surface. Wth the hgh emssvty of the pant, the uncertanty n the effectve emssvty at the aperture s neglgble. Therefore the uncertanty n the radaton loss calculaton s manly due to the error from the expermental temperature measurements. 3 RESULTS AND DISCUSSIONS 3.1 Expermental Results Fg. 5 presents the results of heat loss measurements usng the model recever operatng at a set pont temperature of 450 o C, where the average expermental temperature values for the cylndrcal secton s 445 o C and the end plate secton s 408 o C. Conducton losses were measured to be constant at 66.4 ± 6.0 W and radaton losses calculated at 57.9 ± 1.3W. The maxmum convecton loss occurs at 0 o when t represents 45.5% of the total heat loss. Wth ncreasng nclnaton the convecton loss reduces to a mnmum at 90 o representng 4.2% of the total heat loss. Ths trend s to be expected snce as the nclnaton ncreases, more hgh temperature buoyant ar remans stagnant wthn the cavty. Heat loss (W) Total Convecton Conducton Radaton (analytcal) Inclnaton (degree) Fg. 5 Expermental heat loss for a cavty temperature of C. Convecton loss for three cavty temperatures s compared n Fg. 6. They all show a smlar dependence on nclnaton as that descrbed for the convecton loss n Fg. 5. It s also evdent that the losses ncrease wth hgher cavty temperatures throughout all nclnatons, as expected. Convecton loss (W) deg C 450 degc 350 degc Inclnaton (degree) Fg. 6 Convecton loss at varous cavty temperatures 4 Proceedngs of Solar Australan and New Zealand Solar Energy Socety Paper 1

5 Solar Concentrator Cavty Recever at Varyng Orentaton 3.2 Comparson between expermental results and other correlatons. The expermentally measured convectve heat loss from the model recever at a cavty temperature of C s plotted n Fg. 10 together wth the results from the CFD calculatons of Patoonsurkan et al (2002) and the values calculated from correlatons presented by the varous authors dscussed n secton 1.1. The CFD calculatons show good agreement at 0 0 and 90 0 wth the expermental results whle t slghtly over predcts at ntermedate angles. The Clausng (1981) correlaton shows the best agreement wth the expermental results despte ts dervaton for large central recevers. The Modfed Clausng and Modfed Stne and McDonald correlatons proposed by Lebfred et al (1995) are of smlar magntude and slghtly over predct the expermental results. There s a further over predcton of the expermental results by the Koeng and Marvn (1981) and Stne and McDonald (1989) correlatons, whch may be due to the larger scale recevers that ther studes were based on. All the correlatons show a smlar tendency of decreasng natural convecton loss wth ncreasng nclnaton untl there s no natural convecton loss at Ths s physcally unrealstc and t s worth notng that wth the expermental and CFD results that ths s not the case. Convecton loss (W) Koeng & Marvn Stne & McDonald Modfed Stne & McDonald Clausng Modfed Clausng Expermental Numercal Inclnaton (degree) Fg. 6 Comparson of expermental convecton loss at a cavty temperature of C wth other correlatons. 4 CONCLUSION Wth a small-scale expermental recever, t has been proven that a smple experment can be bult to quantfy the convecton losses from a cavty recever. The expermental system has proven to be relable and convecton losses have been determned wth good accuracy. Experments wth a larger sze cavty need to be carred out n order to verfy the CFD calculatons for full sze cavty recevers. The numercal results obtaned are qualtatvely n good agreement wth those predcted by varous prevously proposed correlatons. The Clausng (1981) correlaton shows the closest predcton to both numercal and expermental results despte ts orgnal use for bgger-scale central recevers. The dscrepancy of convectve heat loss at an orentaton of 90 0 between the present results and the predctons of correlatons taken from the lterature requres further nvestgaton. 5 REFERENCES Clausng A.M. (1981). An Analyss of Convectve Losses From Cavty Solar Central Recever, Sol. Energy 27, Clausng A.M. (1983). Convecton Losses From Cavty Solar Recevers-Comparsons Between Analytcal Predctons and Expermental Results, Journal of Solar Energy Engneerng 105, Johnston G., Burgess G., Lovegrove K., and Luzz A. (2001), Economc Mass Producble Mrror Panels for Solar Concentrators, Proceedngs of ISES World Congress, Adelade, Australa. Proceedngs of Solar Australan and New Zealand Solar Energy Socety Paper 1 5

6 Solar Concentrator Cavty Recever at Varyng Orentaton Harrs J.A. and Lenz T.G. (1985). Thermal performance of concentrator/cavty recever systems, Solar energy 34, Holman J.P. (1997) Heat transfer, 8th edton, New York: McGraw-Hll Companes. Koeng A.A. and Marvn M. (1981). Convecton heat loss senstvty n open cavty solar recevers, Fnal report, DOE contract No. EG77-C Lebfred U. and Ortjohann J. (1995), Convectve Heat Loss from Upward and Downward-Facng Cavty Solar Recevers: Measurements and Calculatons, J. Sol. Eng. 117, Lovegrove K., Luzz A., Soldan I., and Kreetz H. (2001), Developng Ammona Based Thermochemcal Energy Storage for Dsh Power Plants Smelly Experments, Good Technology, Proceedngs of ISES World Congress, Adelade, Australa. Patoonsurkarn S. and Lovegrove K. (2002). Numercal Investgaton of Natural Convecton Loss n Cavty Type Solar Recevers. In Proceedngs of Solar Australan and New Zealand Solar Energy Socety, Newcastle, Australa. Stne W.B. and McDonald C.G. (1989), Cavty Recever Heat Loss Measurements, presented at ISES World Congress, Kobe, Japan. 6 Proceedngs of Solar Australan and New Zealand Solar Energy Socety Paper 1