EFFECT OF SOME PARAMETERS ON LINEAR FRESNEL SOLAR CONCENTRATING COLLECTORS

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1 EFFECT OF SOME PARAMETERS ON LINEAR FRESNEL SOLAR CONCENTRATING COLLECTORS Panna Lal Singh *1, R.M Sarviya and J.L. Bhagoria 2 1. Central Institute of Agricultural Engineering, Berasia Road, Bhopal (INDIA) 2. Maulana Azad National Institute of Technology, Bhopal (INDIA) Received August,26, 2009 Accepted December, 02, 2009 ABSTRACT Solar concentrating systems are best suited for medium-grade ( C) thermal applications. The absorber of the linear solar concentrating device play very important roll in collection of solar energy. The non-evacuated solar absorber painted with ordinary black paint yield poor performance. To achieve higher efficiency of the solar collector, there should be minimum thermal losses from the absorber. The overall heat loss coefficient of the absorber includes convection, radiation and conduction heat losses. Surface coating on the absorber surface, glass cover, shape and size of the absorber surface, temperature of the absorber etc. affects thermal performance of the absorber. Transparent glass cover at bottom allows light entry and at the same time reduced heat loss due to wind. Rise in oil temperature with increase in number of mirrors, but the increase was not found to be linear. The time required to reach peak temperature reduced proportionally with increase in number of mirrors. Key Words : Linear Fresnel solar concentrator, Cavity absorber, Selective surface, Thermal efficiency INTRODUCTION The solar concentrating collectors can be used for process heating for low temperature (40 C to 80 C) and medium temperature (80 C to 250 C) ranges 1-3. Solar energy can be used for water heating, drying, space heating, domestic water heating, industrial process heating and electricity generation 4. The collector is placed inclined at suitable angle to receive the maximum solar radiation 5. The glass cover to the absorber reduces convective heat loss from the receiver. Single or double glass cover is * Author for correspondence desirable. The glass cover tube is usually placed around the receiver tube. The reflected light from the concentrator must pass through the glass cover to reach the absorber. However, a glass cover adds a transmittance loss of about 0.1, when the glass is clean. Particularly for high temperature applications, it is to evacuate the space between the glass cover tube and the receiver. However, fabrication of evacuated glass cover for long and non-round tubes may be difficult. The contribution of radiation to the total heat transfer rate is significant even for temperature as low as 47 C temperature 6. The surface coating of 528

2 the absorber plate should be such that it has high absorptivity and poor emmissivity for required temperature range. The surface coating of the plate should be such that it has high absorptivity and poor emmissivity for required temperature range 7. Selective surface coating on the absorber could reduce emission losses and increase efficiency of the solar absorber. Yong Shuaia et al. 8 studied radiation performance of dish solar concentrator with cavity receiver systems. The five cavity geometries were evaluated on the uniformity of wall radiation flux. The results indicated that cavity geometry had a significant effect on overall flux distribution. Based on the concept of equivalent heat flux, the spherical receiver with relatively good radiation performance provides a starting point for the shape optimization. Heat transfer in the cavity absorber is complex phenomena. More study was suggested to better quantify multi-reflection losses and free and forced convection losses from cavity receivers. The bottom and sides of the collector are covered with insulation to reduce the conductive heat loss. Singh et al. 9 found that overall heat loss coefficient of the trapezoidal cavity absorber reduce considerably (20-30%) by use of selective surface coating on the absorber surface, which will help in improving the efficiency of the solar collector. The collector plate should absorb maximum possible solar radiation incident on it through glazing. The glazing, emits minimum heat to the atmosphere and downward through the back of the casing and transfer the retained heat to the fluid 10. The emissive radiation θ n = ½ tan -1 [ { θ n + ( w/2) cos θ n-1 }/{ f - ( w/2) sin θ n-1 }] S n = w. sin θ n-1. tan (2θ n +θ 0 } where, θ 0 = half of the angular distance (=16') of the sun. The recurrence relation for the location of the n th following relation: 529 from the absorber is more due to higher temperature. In this paper, performance of a Fresnel concentrating collector with tubular absorber with respect to number of reflecting mirrors, seasons have been studied and result are presented. MATERIAL AND METHODS A typical linear Fresnel reflector consists of a long narrow flat mirror elements fixed on a horizontal base. Each mirror element is tilted at an angle such that all incident solar rays falling on them are reflected to a common focus. Fig.1 shows the crosssection of linear fresnel reflector with tubular absorber. It was designed as per method described by Negi et al. 11 and Khan 12. An equal width (w) of the constituent mirror elements were considered in each case. The tilt of each constituent mirror element was so adjusted that a ray incident normal to the aperture plane and striking the mid point of each mirror element reached the focus point F after a single reflection. An appropriate distance (called shift) was kept between two consecutive mirror elements so as to avoid blocking of radiation reflected by its adjacent mirror element. Each mirror (say nth) may then be characterized by three parameters, namely; location, Q n, tilt, θ n, and shift, S n, as shown in Fig. 1. The following expressions were obtained for these parameters using simple geometrical optics.(1).(2) mirror element was calculated by the

3 Q n = Q n-1 + w. cos θ n-1 + S n.(3) with θ 0 = 0, S 1 = 0, Q 0 = -w/2, Q 1 = w/2 as initial values for the iteration and n =1, k, where, k is the total number of mirror elements placed on each half of the reflector. The location of the first mirror element, in each case, is chosen such that it is just beyond the shade of the absorber. The size of the absorber is so chosen that it intercepts radiation reflected by each constituent mirror element. With the help of the above, a fresnel reflecting unit was designed. Each mirror was 0.53 m long and 0.1 m wide, fixed on to a square pipe frame (size : 3.2 m x 1.2 m). Each mirror was supported with mild steel sheet on its back. Each mirror was mounted separately on the frame. There was provision to tilt them individually at any angle and adjust height over frame. The absorber consisted of a copper pipe (0.075 m dia and 2.3 m long) painted with ordinary dull black to absorb heat. The pipe was covered with mirrors (4 mm thick) on three sides in a trapezoidal shape to reflect light and thermal radiation back on to the pipe (Fig. 2). The Mirrors were supported with mild steel sheet and insulated with 25 mm thick glass wool. The bottom portion of absorber was provided with 15 cm wide plane glass to allow entry of light and trap infra-red radiation. Eleven litres of Hytherm-500 oil was filled in the absorber tube. Fig. 1 : Linear Fresnel reflecting solar concentrator with tubular absorber 530

4 Fig. 2 : Cross-sectional details of the absorber tube The Fresnel reflecting unit was kept over the supporting structure which was mounted on self aligning ball bearings to facilitate tracking in two axes. The supporting structure was firmly grouted with concrete and fixed with nut and bolts. The mirror elements reflect solar energy on to the absorber kept above the aperture at a distance of 0.75 m. There was provision to change height of the absorber from aperture. The whole reflecting unit with absorber tube was tracked from East to West at the rate of 15 /hr, throughout day light hours with help of an electromagnetic automatic sun tracker. The heat of concentration was trapped in absorber tube. Two type of experiments was conducted. In the first experiment stagnant temperature in the absorber tube with different number of mirrors were studied. The number of mirrors tried were 10, 15 and 20 mirrors. In the second type of study, the experiment was conducted with 12 reflecting mirror sets on typical sunny days in different month of the year. The rise in oil temperature in the absorber tube, ambient temperature and solar insolation were measured. The oil temperature was measured with help of precalibrated thermocouples and solar insolation with help of the Pyranometer. Observations were made during the December (winter) and May (summer) months has been compared. The efficiency of the solar device was computed with the following relations : Efficiency, η (%) = 100.{Heat output (Joule) / Heat input (Joule)}.(4) Heat output = 4.18 m.s.δt Heat input = I.A.t Where m = mass of oil in absorber tube, g s = specific heat of oil, kcal/kg/ C ΔT = Temperature difference, C I = solar radiation, W/m 2 A = collector area, m 2 t = time, second Specific heat of Hytherm-500 (a trade mark of M/s Hindustan Petroleum Corporation Ltd., India) oil is kcal/kg/ C. 531

5 RESULTS AND DISCUSSION The stagnant temperature of oil in the absorber tube and efficiency of the Fresnel solar collector with ordinary black painted absorber with respect to increase in number of mirrors of concentrating collector are shown in the Table 1. The peak stagnant oil temperature during noon in the absorber was 162 C, 206 C, 232 C with 10, 15 and 20 number of the reflecting mirrors respectively during March month. It was found that the time required to reach peak temperature reduced proportionally with increase in number of mirrors. The rise in oil temperature was not directly dependent of the number of mirrors. This variation may be due to following treasons: a) increase in radiation and conduction losses with rise in the temperature, b) reduction projected area of the reflecting mirror with increase in inclination, c) increase in incident angle of the reflected rays on absorber tube. The collector efficiency was found to decreased with increase in reflecting mirrors. This may be due to increase in thermal losses with increase in temperature at higher number of reflecting mirrors. Table 1 : Stagnant temperature of the oil in the absorber tube with different number of reflecting mirrors. S. No. No of reflecting mirrors Stagnant temperature of oil in the absorber ( C) Collector efficiency (%) Further study with 12 number of mirrors indicated that rise in temperature of oil in the absorber tube in the May (summer) month was faster than in December (winter) month (Fig. 3). This may be attributed to the higher solar intensity during the May month. It was found that temperature of oil tended rise rapidly in both cases during start up. However, it reached stagnant value in around two hour. This stagnant temperature may be assumed to be peak for all practical purpose. It was found that temperature of oil tended rise rapidly during start up and reached to almost stagnant value after around two hour. Peak oil temperature in the absorber tube on the typical sunny days was 142 C in the December month and 175 C in the May month. With 12 number of mirrors, the optical efficiency of the solar reflecting system varied from 17.8% to 22.9% in different months with ordinary black painted tubular absorber. 532

6 Temperature ( C) :00 10:30 11:00 11:30 12:00 12:30 13:00 13:30 14:00 Time (h) 14:30 15:00 15:30 16: Solar Insolation (W/m 2 ) Oil temprature in the absorber tube in May (summer) ( C) Oil temprature in the absorber tube in December (winter) ( C) Solar insolation in May (W/sq.m) Solar insolation in December (W/sq.m) Fig. 3 : Temperature of oil in the absorber tube on typical sunny day in the month of May (summer) and December (winter). CONCLUSION The linear Fresnel concentrating collector has been found useful for medium temperature application. The study on the Fresnel concentrating collector shown that there is rise in oil temperature with increase in number of mirrors, but the increase was not found to be linear. The time required to reach peak temperature reduced proportionally with increase in number of mirrors. The peak stagnant oil temperature during noon in the absorber was 162 C, 206 C, 232 C with 10, 15 and 20 number of the reflecting mirrors respectively during March month. It was found that temperature of oil tended rise rapidly during start up and reached to almost stagnant value after around two hour. Performance of a linear Fresnel reflecting solar concentrator with circular absorber was studied and with 12 number of mirrors, the optical efficiency of the solar reflecting system varied from 17.8% to 22.9% in different months with ordinary black painted tubular absorber. REFERENCES 1. Mills D., Advances in solar thermal electricity technology, Solar Energy, 76 (1-3), 19-31, (2004). 2. Dey C.J., Heat transfer aspect of an elevated linear absorber, Solar Energy, 76, , (2004). 3. Reynolds D.J, Jance M.J, Behnai M. and Morrison G.L, An experimental and computational study of the heat loss characteristics of a trapezoidal cavity absorber, Solar Energy, 76, , (2004) 4. Singh H., Singh A.K., Chaurasia P.B.L. and Singh A., Solar energy utilization: A key to employment generation in Indian Thar desert, International Journal of Sustainable Energy, 24 (3), , (2005) 5. Pandey P.C. and Narain P., Solar devices for energy management in Arid

7 region, Journal of Social Policy Research Institute, 2, , (2006) 6. Krishnan A.S., Premachandran B., Balaji C. and Venkateshan S.P., Combined experimental and numerical approach to multi-mode heat transfer between vertical parallel plates, Experimental Thermal and Fluid Science, 29, 75-86, (2004) 7. Tiwari G.N. and Ghosal M.K., Renewable Energy Resources- Basic Principal and Application, New Delhi, Narosa Publishing House, (2005) 8. Yong Shuaia, Xin-Lin Xiaa and He- Ping Tan. Radiation performance of dish solar concentrator/cavity receiver systems, Solar Energy, 82(1), 13-21, (2008). 9. Singh P.L, Sarviya R.M. and Bhagoria J.L., Improvement in thermal performance of the absorber for linear Fresnel solar concentrating collector, Proceeding of the 15 th ISME international Conference on new Horizons of Mechanical Engineering, at RGTU, Bhopal, India, , (2008). 10. Rajullu K.G., Kumar R., Mohanty B., Varma H.K., Enhancement of Nucleate Pool Boiling Heat Transfer by Reentrant Cavity Horizontal Tubes. International Journal of Heat Exchangers, 5, , (2004). 11. Negi B.S., Kandpal T.C. and Mathur S.S., Design and performance characteristics of a linear Fresnel reflector concentrator with a flat vertical absorber, Solar and Wind Technologies, 7, , (1990). 12. Khan Md. K.A., Copper oxide coating for use in a linear solar Fresnel reflecting concentrating collector, Renewable Energy, 17, , (1999). 13. Negi B.S., Mathur S.S. and Kandpal T.C., Optical and thermal performance evaluation of a linear Fresnel reflector solar concentrator, Solar and Wind Technology, 6(5), , (1989). 14. Tripanagnostopoulos Y., Yianoulis P., Papaefthimion S. and Zafeiratos S., CPC solar collectors with flat bifacial absorbers, Solar Energy, 69(3), , (2000). 15. Valan A. and SornaKumar T.D., Performance characteristics of parabolic trough solar collector system, Int. Energy Journal, 7, , (2006). Save Water