The Comparison between the Effects of Using Two Plane Mirrors Concentrator and that without Mirror on the Flat- Plate Collector

Transcription

1 ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December 2010 The Comparison beteen the ffects of Using To Plane Mirrors Concentrator and that ithout Mirror on the Flat- Plate Collector S. Himran, B.Sudia,.H. Piarah Universitas Hasanuddin, Indonesia Kampus Tamalanrea, Makassar 90245, Indonesia Phone: , Fax: ,.mail: syukri_h@yahoo.com ABSTRACT: The aim of the study is to compare beteen the effects of using to planes mirror concentrator and that ithout mirror on the flat plate collector. The ork is performed on solar ater heating system ith flat plate collector installed in it. Measuring of the data are taken beteen 9.00 to h over a day. Based on the global and diffuse radiations ith the average of 594 and 119 /m 2, the average efficiencies of the system over a day are % and % for collector ith and ithout mirror respectively Keyords: plate collector, mirrors concentrator NOMNCLATUR A p - absorbing area [m 2 ] I b - hourly beam radiation [/m 2 ] I d - hourly diffuse radiation [/m 2 ] I g - hourly global radiation [/m 2 ] I bm - reflected beam radiation [/m 2 ] I T - flux falling on the surface of collector [/m 2 ] r - tilt factor [-] S - incident solar flux absorbed in the absorber plat[/m 2 ] T - temperature [ o C] U - loss coefficient [/m 2 -K] Greek letters - solar altitude angle [ o ], absorptivity [-] - slope angle of collector [ o ] - declination angle [ o ] - latitude angle [ o ] - incidence angle of reflected radiation on collector surface [ o ] - incidence angle on collector surface [ o ] () - transmissivity-absorptivity product [-] - hour angle [ o ] ξ - azimuth angle [ o ] - mirror inclination angle [ o ] 080-1

2 ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December 2010 Subscripts a - absorption b - beam d - diffuse g - global l - overall coefficient p - absorber plate r - reflection and refraction u - useful - all, - ast M - mirror - est 1. INTRODUCTION From many studies, it has been proved that a flat-plate collector for heating liquids and gasses gives only to a maximum temperature around 100 o C. hen higher temperatures are required, it becomes necessary to concentrate the incoming solar radiation. Concentrating collectors are of various types and can be classified in many ays i.e.: reflecting type utilizing mirrors and of the reflecting type utilizing Fresnel lenses. The reflecting surfaces used may be flat, parabolic or spherical. In this paper e analyzed the performance a flat-plate collector ith adjustable mirrors at the edges of collector to reflect radiation to the absorber plate. An advantage associated ith this type of concentrating collector is that the diffuse component of the incoming solar radiation is not entirely asted. A flat-plate collector ith plane reflectors is a simple non-imaging concentrating collector and represents an effective means of getting slightly higher temperatures than are obtainable ith a flat-plate collector alone. 2. XPRIMNTAL APPARATUS AND PROCDURS: The major components that make the flat-plate collector are: (1) absorber plate made of Alsheet metal painted black, 0.4 mm thickness, size m, (2) tubes fixed to absorber plate ith nine tubes ½ inch diameter ith spacing 9.9 cm, made of brass. (3) to transparent cover glasses 4 mm thick ith spacing 4 cm. The spacing beteen absorber plate and first cover glass is 4 cm, (4) collector box made of plyood. The bottom and sides of the box are insulated by foam cork 4 cm thick. A schematic diagram of the test set-up is shon in Fig.1. It is a closed lop consisting of the solar ater heater is to be tested. ater can be circulated by loering the outlet pipe from collector 10 cm belo the ater surface in the tank. The experiments ere performed for collector ith mirror and that ithout mirror, during 8 hours over a day, from 9.00 AM to PM. The solar radiations in to days give nearly the same value. The collector is equipped by to mirror-reflectors (mirrors), one of hich facing est and the other ast

3 ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December N 1 S 1. ater flat-plate collector 2. ater tank 3. circulating pipes 4. concentrating mirrors Figure 1 Schematic diagram of a closed loop set-up for testing flat-plate collector ith concentrating mirror 3. ANALYSIS: From Figure 2 and 3, the angle of incidence of direct solar radiation on the collector can be related by a general equation by 3 position angles of the sun in the sky i.e.: declination angle, latitude angle, hour angle ω and 2 orientation angles of the plane of the collector: slope of the collector, and surface azimuth angle ξ. The governing equation of incidence angle on the surface is given by: cos sin sin cos cos cos cos sin cos cos cos cos sin cos sin (1) cos sin sin sin The flat-plate collector is facing due south and is inclined at an angle = 0 o. Since Makassar located at latitude 5.1 o S and longitude o. The angle of incidence on the collector, based on equation (1) becomes: cos sin sin cos cos cos (2) The inclination angle of the reflector is adjusted manually every hour. In order to get the optimum solar radiation reflected in to collector, the reflector mirrors are adjusted in such a ay that the sun s rays striking the top edge of the mirrors are reflected to the top edge of the collector. This needs the reflecting mirrors have the same length L as the collector in order to boost the output, Fig. 4. The inclination angle can be calculated from equation (1) by changing to, the incident angle on top edge of the reflector by changing to r, see Fig. 5. The incident angle on top edge of the reflector is given by: 080-3

4 ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December 2010 cos r sin sin cos cos cos cos sin cos cos cos cos sin cos cos sin sin sin sin (3) Figure 2 The celestial sphere = solar altitude; = declination angle; = latitude angle; = hour angle; = solar azimuth angle Latitude for Makassar: = 5.1 o Declination: o sin N Day of the year on 24 October 2009, N = 298 days, gives declination angle = o Surface azimuth angle: = 0 o for facing south; = 180 o for facing north Hour angle: o = (12 t) 15 o, t = local apparent time 080-4

5 ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December 2010 Normal to tilted plane z N Normal to horizontal plane (zenith) Tilted plane Horizontal plane S Figure 3 Diagram illustrating the angle of incidence z =zenith angle, =solar altitude angle, = slope of the plane, = solar azimuth angle, = all azimut angle L Mirror reflector Mirror reflector Flat-plate collector L 1 2 Figure 4 Flate-plate collector ith reflectors Based on equation (3) and Figure 4, the incident angles of beam radiation on reflector θ r are given by : 080-5

6 ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December 2010 for reflector facing est, surface azimuth angle: = 90 o cos r sin sin cos cos cos cos cos for reflector facing ast, surface azimuth angle: = 90 o cos r sin sin cos cos cos cos cos From Fig: 5, e found the relations for,, r are: cos sin sin..(4.a) cos sin sin (4.b) = 90 o r = 2 Then: = 2 r (5) From equations (4.a), (4.b) and (5), e found by trial-and-error, the inclination angle of the reflector and for facing est and ast respectively. r r Reflector= mirror r Flat-plate collector Flat-plate collector r Reflector =mirror (a) N (b) S Figure 5 Flat-plate collector and reflectors facing due est and ast, = 0 o The flux incident on the top cover of collector I T and the flux absorber in the absorber plate S are: Solar radiation flux incident on the top cover of flat-plate collector I T and flux absorbed in the absorber plate S are sum of the beam and diffuse radiation falling directly on the surface and the radiation reflected onto the surface from the surrounding ith the factors r b, r d, and r r. Fluxes I T and S are given by: I T = I b r b + I d r d + (I b + I d ) r r (6) S = I b r b () b + {I d r d + (I b + I d ) r r }() b (7) Tilt factor r b, diffuse radiation factor r d and reflected radiation factor r r are given as: r cos, r 1 cos 2, and 1 cos 2 respectively. b z d r r here, z,, are incidence angle on the surface, zenith angle, surface slope angle, reflectivity respectively. Since = 0 o, then r b and r d = 1, and r r =

7 ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December 2010 Based on equations (6) and (7), solar radiation incidents on the existing flat-plat collector become: (i). Incident flux on the top of the collector: ithout mirror: I I r I r (6.a) T b b d d ith mirror: I I I r I r (6.b) Tm b bm b d d I bm = beam radiation reflected by the to-mirrors = I b + I b I b I b b sin, I sin b cos I cos (ii). Flux S absorbed on the absorber plate: ithout mirror: S I b r b b I d r d d (7.a) ith mirror: Since the reflected radiations are not influenced by r b and r d then, S I I r I r (7.b) M b bm b b d d d The factors () b and () d are transmisivity-absorptivity product for beam and diffuse radiations. The procedures to calculate () can be found in Ref. [2]. The useful heat gain = the rate of heat transfer to the orking fluid: q u =A p S q l (8) The instantaneous collection efficiency is given by: u (9) Radiation Useful incident heat gain on the collector 4. RSULTS AND DISCUSSIONS A q p I T The global (I g ) and diffuse (I d ) solar radiations ere measured by using digital pyranometer over a day from 9.00 to h and the variations are as shon on Fig.6. For performing the calculations, data are taken for the temperatures at: plate-absorber surface, to glass cover surfaces, and ater at the inlet and outlet ports of the collector. The ambient temperature and the hourly ind speed over the surface of the collector are also measured. Based on the equations (1) to (5), e can find the variation of hour angle ω, incident angle on the collector surface θ, incident angle on mirror facing est θ r and facing east θ r, inclinations of the mirrors facing est and facing east. The results obtained are given in Table 1. By considering the calculated inclination angles and for every hour during the day, the slopes of the to mirrors are adjusted. All the data calculated are available in the files. Figure 6 shos the variations of global I g and diffuse I d radiations during a day. It is seen that the values increase a little bit sharp from 0900 to 1100 h, touch a peak around noon and then drop sharply to It is also interest to study the performance of collector h for collector ith and ithout mirrors over a day. The variation of the important items are tabulated and presented on graphs are shon in Table 2 and Figures 7, 8, and 9. The variation obtained indicates the strong independence of these items on the radiation incident on the collector. From Figure 7, it seen 080-7

8 Ig, Id, (/m 2 ) ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December 2010 that the flux incident I T for collector increase from 0900 to 1100 h and has a maximum value at noon then decrease to 1600 h. It is also shon that collector using mirrors give higher value compared to that ithout mirrors. The flux incident I T using mirror facing est has higher values at the 0900 h and the decrease to 1600 h and inversely for collector ith mirror facing ast gives loer values at 0900 h and higher at afternoon 1600 h. The overall heat loss coefficients do not vary much during the day but have a higher value for collector ith mirrors compared to that ithout mirrors. The average efficiencies over a hole period, during hich considering the useful energy q u, flux absorbed S, heat loss from collector q l, flux incident on the top cover I T, could also be calculated. Making the approximation, then the efficiencies average over 8 hours from 0900 to 1600 h for collector ith and ithout mirrors ork out to be 41.9 % and 22.8 % respectively Ig Id Time (h) Figure 6 Variations of global I g and diffuse I d radiations over a day Table 1 The variation of the inclination angles over a day on 24 October. N = 297, δ = o, ξ. = 90 o, ξ. = +90 o Time ω( o ) ( o ) r ( o ) r ( o ) ( o ) ( o )

9 ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December 2010 Table 2 Performance of a flat-plate collector ith and ithout mirrors over a day, T a 32 o C LST(h) I b (/m 2 ) I d (/m 2 ) I bm (/m 2 ) I T (/m 2 ) I TM (/m 2 ) S (/m 2 ) S M (/m 2 ) T p ( o C) T pm ( o C) U l (/m 2 -K) U l. M (/m 2 -K) q u () q u.m () (%) M (%) IT_C, IT_, IT_, IT_tot (/m 2 ) IT_ Collector IT Mirror-est IT_Mirror ast IT_Total Time (h) Figure 7 Variations of flux incident on the top cover of the collector over a day 080-9

10 ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December S_C, S_, S_, S_tot (/m 2 ) S_total S_C S_ S_ Time (h) Figure 8 Variations of Insident solar flux in the absorber plate over a day ff M, ff (%) ff. ith mirrors ff. ithout mirrors Time (h) Figure 9 Variation of efficiencies ith mirror and ithout mirror over a day 5.CONCLUSIONS From the calculations of useful energy q u, flux absorbed S, heat loss from collector q l, flux incident on the top cover I T, and making approximation, the efficiencies average over 8 hours from 0900 to 1600 h for collector ith and ithout mirrors ork out to be 41.9 % and 22.8 % respectively

11 ICCHT th International Conference on Cooling and Heating Technologies, Bandung, Indonesia 911 December 2010 RFRNCS [1]. Sudia B., Himran S., Piarah. H., 2009, Flat-Plate Solar Collector Performance ith to-mirrors Concentrators, S2 Thesis, Mech. ngineering Dept., Unhas, Makassar [2]. Foniman H, et.al, Himran S., Jalaluddin, 2007,2008, The Comparison among the Performance of Flat Plate Collector ith One, To, and Three Covers, S1 Minithesis, Mech. ngineering Dept., Unhas, Makassar [3]. Sukhatme, S. P., Nayak J. K., 2008, Solar nergy, Principles of Thermal Collection and Storage, 3 rd d, Tata McGra Hill Private Limited, Ne Delhi [4]. Duffie J. A., Beckman. A., 1991, Solar ngineering of Thermal Process, J.. & Sons, Inc, Ne York