Concentration of fiber transmitted solar energy by CPC for solar thermal utilization

Transcription

1 J Phys. IV France 9 (1999) Concentration of fiber transmitted solar energy by CPC for solar thermal utilization H. Yugami, M. Yano, H. Naito and H. ~ rashi~ Graduate School of Engineering, Tohoku University, Aoba 01, Aramaki, Aoba-ku, Sendai , Japan Abstract. Parabolic dish systems equipped with optical fiber transmission of highly concentrated solar energy can provide a stationary high temperature or high photon density point to us. This is very useful for the applications of solar thermal utilization system, for example solar chemistry on terrestrial or heat source in space. However, the solar energy density sharply decreases form at the end point of the optical fiber, because the transmitted solar light disperses from the exit of the optical fiber according to its numerical aperture. To overcome this problem, we are investigating the CPC (Compound Parabolic Concentrator) coupled to the exit of an optical fiber. This method is capable of increasing the solar energy density emitted from optical fibers. In this paper, we report the experimental results from transmission of solar energy concentrated by a parabolic concentrator through an optical fiber bundle coupled with CPC. Comparing the spatial energy distribution at the CPC exit with that at the entrance of CPC, it is demonstrated that the density of thermal flux is increased about 3.6 times by CPC. 1. INTRODUCTION High density solar energy is useful as a heat or high density photon source for material processing, solar chemistry and laser pumping [1-51. A parabolic mirror is generally employed as a concentrator to obtain high temperature heat source. Solar concentrators are classified according to the method for tracking: single axis or two axis. In the former type, focal point is spatially fixed, therefore, the reactor for material processing must be brought to the fixed focal point. This is a merit for solar chemistry application. However, the temperature obtained single axis tracking system is not sufficient for some chemical reactions. Most of chemical reactions should be performed at an upper temperature as high as possible because the rate of chemical reaction increases with temperature exponentially. On the other hand, while the latter type of solar collector can provide very high thermal energy, however, as the focal point changes its spatial position with time, it is not convenient for material processing etc. From this point of view, heliostat-type dish collectors has been used for many material processing experiments. This type of solar collector needs much more installation area. An optical fiber transmission of highly concentrated solar energy will be the most profitable to resolve these problems [6,7]. The fiber coupled dish solar collector system can provide a heat or light source at any desired place owing to the flexibility of the optical fiber. This technique also provides the microgravity condition for material processing using highly concentrated solar energy as a heat source in space, because of decoupling from the oscillation caused by movement of the solar corrector installed on satellites or space station. In this paper, we have reported the experimental results from transmission of solar energy concentrated by a parabolic concentrator of 1560 mm in diameter through an optical fiber bundle of 18 mm in diameter coupled with CPC. Article published online by EDP Sciences and available at

2 Pr3-546 rl 2. SOLAR CONCENTRATOR SYSTEM EQUIPPED WITH OPTICAL FIBER BUNDLE JOURNAL DE PHYSIQUE IV r&rl Figure 1 shows the schematic diagram of a dish solar collector coupled with a optical fiber bundle. This apparatus has two axis tracking system equipped with a computer. The computer control of sun tracking is performed by monitoring the sun image measured a CCD camera. In addition to the CCD control mode, an auto guide mode referring the Figure 1: Schematic diagram of the parabolic concentrator coupled with a sun position optical fiber bundle. Two axis sun tracking is performed by monitoring CCD is also used in cloudy condition. sun image. The rim angle 8, of a parabolic concentrator is given by 8, = COS-'(~~-~;;/~+~:) where n, is the value of diameter divided by the focal length of a parabolic concentrator. Since the diameter and the focal length of the used in this experiment are 1560mm and 650mm, respectively, its n, value is 2.4. Substituting this value in to Eq. (I), 8, for our concentrator is 62". The optical fiber used in this experiment is that the core and clad are made from silica and polymer, respectively. The maximum angle Om, of converging light which can be taken into this optical fiber is 24". Because of the large difference between 8, and Om,, all solar radiation concentrated by the parabolic mirror can't be taken into the optical fiber when the entrance side is positioned at the focal point. The Cassegrainian type solar concentrator is employed to accord the convergent angle of the concentrated solar radiation with Om,. A hyperbolic mirror, which is used as a sub-mirror, is set so as to its rear focal point coincides with the focal point of the parabolic concentrator as shown in Fig. 1. The calculated concentration ratio is about 36,000 suns, and the diameter of sun image is about 6 mm in the focal plane of the parabolic concentrator. The sun image is magnified three times by reflection from the hyperbolic mirror. Therefore, the diameter of the sun image is 18mm at the entrance of the fiber bundle. The 250 optical fiber elements with core diameter of lmm are bundled to cover this enlarged sun image, as shown in Fig. 2. cooling woter ouffei Air inlei + cooling water inlet t / Element optical f&er of lmm in diameter Figure 2: Entrance of an optical fiber bundle. The 250 optical fiber elements (core :lmm in diameter) are bundled. Polymer clad is removed over locm long from the surface.

3 STCT 9 Pr3-547 In order to avoid thermal damage and to decrease a vacant space among fiber element, polymer clad is removed over locm long from the entrance of optical bundle. Propagating light is expected to be confined by "air clad in this region. The exit side of the optical fiber bundle has the almost same structure. 3. DESIGN AND FABRICATION OF CPC outlet The three dimensional CPC (Compound Parabolic Concentrator) [8] shown in Fig. 3 has been designed and fabricated to reconcentrate the diverging solar radiation emerged from the exit of the optical fiber bundle. The CPC is made from a high purity A1 block, and its surface is protected by Si02 thin film after the production of inner mirror surface. Designing the CPC, the following points are taken into account. (1) The maximum angle of emergent light from the exit is limited - - to 70, because the solar radiation having large emergent angle (normally 90") is not suitable for material processing application. Due to this design, the concentration ratio of the CPC is 4.4. (2) The parameter of acceptance angle is set to 26'. This value is slightly larger than the maximum angle of light emerged from an optical fiber (24") in order to decrease the backward light from CPC. cooling water inlet 4. RESULTS AND DISCUSSION Figure 3: Design of CPC. The hatched area is filled by cooling water. 4.1 Energy transmission efficiency of the optical fiber bundle The solar energy transmission efficiency of the optical fiber bundle was estimated by measuring the heat flux at the exit of optical fiber by a calorimeter. The spatial distribution of heat flux is measured by moving the small sensor head (2mm in diameter) of calorimeter mounted on a XY stage. The distribution of heat flux at the exit of the bundle is shown in Fig. 4. In this figure, heat flux was measured at 150 positions. It is clearly observed that the transmission energy (heat flux) strongly depends on the position in the fiber bundle. This large point to point variation of heat flux in the plane can't be explained by the reflectivity variation of the solar concentrator mirror, in other words, the intensity fluctuation of sun image at the entrance of the optical fiber bundle. It is most natural to attribute this result to large variation of transmission energy in each optical fiber element. x AXIS (mm) Solar energy transmission efficiency q of the optical fiber bundle is estimated by Figure 4: Spatial distribution of heat flux at the exit of optical comparing the heat flux in each position fiber bundle measured by a calorimeter. with the averaged heat flux at the entrance. Corresponding to the large spatial variation of heat flux, the transmission efficiency is also strongly

4 Pr3-548 JOURNAL DE PHYSIQUE IV depends on the position. The maximum and minimum value of 77 is shown in Table 1 besides the theoretically predicted value, where the wavelength dependence of extinction coefficient of optical fiber, the spatial coverage ratio at the surface of the optical fiber bundle, and surface reflective loss etc. are taken into account. For the highest value of 77, the efficiency is nearly 70% of the theoretically predicted value. However the averaged value of is quite low compared with the expected value. Table 1: The energy transmission efficiency 7 of the fiber bundle. Theoretical value Maximum value Minimum value Averaged value 16.4 In order to study the origin of the low 7, the transmission energy of each optical fiber element was measured using a collimated He-Ne laser light. We observed that the averaged value of transmission of optical fiber elements is 63.6%, which is more than 70% of theoretically expected value, if the surface reflection is included. This value is quite higher than the averaged value for solar ray experiment shown in Table 1. From this data, the low 77 can not be attribute to the surface damage of optical fiber elements. This result reveals that the transmission efficiency strongly depends on the incident angle of light. In the case of laser, the light enters optical fiber nearly in parallel, and the low internal reflection will be expected. In contrast to this, light having large incident angle will enter the optical fiber bundle in the solar ray experiment. In this case many internal reflections will occur in the air clad region. If a fiber core touches the adjacent fiber core in air clad region, propagating light will leak from optical core. From these consideration, we have attributed the low 77 to the imperfection of "air clad structure in our optical fiber. This conclusion may be explained the large variation of 77 in the exit plane of the optical fiber bundle. The improvement of optical fiber bundle is now under carrying. 4.2 Re-concentration of optical fiber transmitted solar energy by CPC Figure 5 shows the spatial distribution of heat flux at the exit of CPC measured by the calorimeter. Due to the large spatial variation of heat flux at the exit of optical fiber bundle i.e., at the entrance of CPC, we observed large unsymmetrical variation of heat flux. The averaged heat flux shows a broad peak at around mm measured by the distance from the origin (center of CPC). This feature is well reproduced by ray trace simulation. The concentration ratio of CPC was measured by comparing the averaged heat flux at the exit of the optical fiber bundle with that emerged from the exit of CPC. The result is shown in Table 2. This measurement confirmed that the solar energy density from x Axis (mm) the CPC increased 3.62 times as that of the optical fiber bundle output. This value of 3.62 Figure 5: Spatial distribution of heat flux at the exit of CPC is about 80% of simply calculated value measured by the caloriineter with a s~nall sensor head (dia.=2 from the ratio between ;hi input and output areas of the CPC. mm) mounted on a XY stage. Due to the limitation of our experimental set up, the heat flux at the comers in this figure could not measured.

5 STCT 9 Pr3-549 The number of reflection at the inner CPC surface is estimated to 1.67 by ray trace simulation assuming the spatially homogeneous input ray distribution. Combining this vale with the reflectivity (0.9) of the inner surface of CPC, the expected value of concentration ratio is 3.67 as shown in Table 2. The experimentally obtained value of 3.62 is very close to this value. From these results, it is confirmed that the optical fiber transmitted solar ray, which shows strong divergence at the exit of optical fiber, can be successfully re-concentrated by CPC. Table 2: The concentration ratio (CR) of CPC. CR Theoretical Experimental 3.62 Acknowledgments A part of this work is supported by Grant-in-Aid for Scientific Research (No ) from the Ministry of Education, Science, Sport and Culture. The authors are grateful to Mr. T. Oka (IHI Co. Ltd.), and Mr. M. Chiba (Tohoku Univ.) for their support in this experiment. References [l] R. Palumbo, A. Rouanet, and G. Pichelin, Energy 20 (1995) 587. [2] A. Stainfeld. P. Kuhn, A. Reller, R. Palumbo, J. Murray, and Y. Tamaura, Hydrogen Energy Progress XI (1996) 601. [3] Y. Shimony, and A. Yogev, Opt. Lett. 15 (1990) H. Arashi, D. Cooke, and H. Naito, Jpn. J. Appl. Phys. 34 (1995) [5] A.L. Taylor, and D.W. Carrier, "THE FEASIBILITY OF PROCESSES FOR THE PRODUCTION OF OXYGEN ON THE MOON, in Engineering, Construction, and Operation in Space IU, Proceedings of Third Int. Conf., Denver, (1992) pp [6] B. Jacobson, P. Gleckman, R. Holman, D. Sagie, and R. Winston, Proc. of SPIE-Int. Soc. Opt. Eng (1991) 82. [7] H. Arashi, H. Naito, H. Yugami, and T. Oka, Proc. of Iniersociety Energy Conversion Conference (IECEC-97) Vol. 3 (1997) [8] T. W. Welford, and R. Wiston, "The Optics of Nonimaging Concentrator", (Academic Press, New York, 1978).