OPTICAL PERFORMANCE OF CIRCULAR AND ELLIPTICAL 3-D STATIC SOLAR CONCENTRATORS

Transcription

1 OPTICAL PERFORMANCE OF CIRCULAR AND ELLIPTICAL 3-D STATIC SOLAR CONCENTRATORS Imhamed M. Saleh Ali Tadhg S O'Donovan Tapas K Mallick Mechanical Engineering Heriot-Watt University Edinburgh; EH14 4AS E- Mail: isa6@hw.ac.uk T.S.O'Donovan@hw.ac.uk t.mallick@hw.ac.uk K S Reddy Mechanical Engineering Indian Institute of Technology Madras, Chennai , India ksreddy@iitm.ac.in ABSTRACT The present study concerns the development of a 3-D static solar concentrator with wide acceptance angle. This research is conducted to provide better insight into the geometrical-optical performance of four different concentrator configurations: Elliptical Hyperboloid Concentrator (EHC); Circular Hyperboloid Concentrator (CHC); Elliptical conical concentrator (ECC) and Circular Conical Concentrator (CCC). Ray-trace techniques are used to evaluate the optical efficiency of these configurations by performing a parametric study of their profile and geometry for a fixed concentrator height of 0.4 m, concentration ratio of 20, and receiver area of m 2 and an ellipse aspect ratio of five. The flux distribution at the receiver for each concentrator profile is presented. It has been found that the elliptical cross section has a wider acceptance angle than the circular cross section. The EHC design has also been shown to outperform all of the other three solar concentrators with a wider acceptance angle and better optical efficiency. 1. INTRODUCTION optical designs have led to greater concentration ratios [2]. With this design of optical systems, it is now possible to attain or closely come within reach of the greatest geometric concentration that is allowable by laws of physical conversion for a particular angular field of view [2]. This fact has significant implications in the design of solar concentrators, and with non-imaging at hand, it has been possible to come up with designs that deliver moderate levels of concentration with totally stationary concentrators. It is also vital to note that, although nonimaging concentrators, as well as illuminators have a number of tangible, as well as some prospective applications, we will limit this discussion to the broad concept of non-imaging and some concentrators used Renewable energy technology research and development has increased substantially in recent year; solar energy is a significant part of this and can provide high efficiency energy conversion. The available heat flux is not enough for large projects however, and therefore concentration of solar energy is required. The optical and thermal performance of non-imaging solar concentrators depends on the geometry of the concentrator profile and the properties of the material used. Ali et al. [1] describes a non-imaging optical study where instead of the using imaging systems, light collecting systems are employed. This is a new approach to the collection, concentration, and transport of light developed by physicists from the University of Chicago over the past 35 years. Complex under solar energy applications [2]. A concentrator, as described by O Gallagher [2] is therefore, used to amplify the power density to be absorbed to extremely high levels so as to be able to provide sufficient energy for largescale uses or generation of electric or motive power. The notion of a Hyperboloid concentrator originated in geometric optics and was later adopted in solar thermal energy [3] Whether the use of geometrical optics is for 1

2 image forming purposes or not, it has been the main instrument used in the design of nearly all optical systems [3]. The basic notions of a ray of light, defined as the path followed by light energy, and the reflecting or transmitting surfaces are, therefore, employed. Reflection of a ray of light from a smooth surface obeys the known law of reflection, while a change of direction by a ray of light after transmission, follows Snell s law of refraction. The use of non-tracking concentrators is efficient. Such concentrators can remain stationary and yet effective during the day. Elimination of the need to track the sun through change in orientation of the concentrator also eliminates the need for auxiliary mechanisms that facilitate the concentrators tracking ability. These mechanisms are expensive to install and maintain. The present work is carried out based on the unavailability of more information about elliptical hyperboloid concentrators. This work has applied the photometric field theory and elliptic ray bundles method to study the 3D asymmetric and symmetry concentrators. The work observed that the hyperbolic concentrator is an ideal 3-D asymmetric concentrator and the shape of the concentrator is not disturbed in the flow lines of an elliptical disk, so it can be applied for non-tracking concentrators. [4] Studied a two-dimensional truncated wedge. The wedge was based on the union shape between a hyperbola and a tilted parabola. A hyper parabolic surface was obtained, by revolving the profile. It was found that, as the focal length of the designed concentrator approaches infinity, a HPC achieves the thermodynamic limit of concentration. Very limited research has been carried out for hyperboloid concentrator as solar concentrator [5]. This paper aims to compare four different 3-D solar concentrator as elliptical and circular absorber and the shape as conical and hyperboloid. The most important element of these systems is the concentrator whose geometry and optics determine the amount of energy produced. In this study the most commonly used optical elements are compared with some newer designs of concentrators for their optical performance. Detailed analysis is performed using ray tracing technique to identify the important parameters affecting the overall performance of these concentrators. And flux distributions on the receiver area have been presented in this work. 2. GEOMETRIC PROFILE OF 3-D STATIC SOLAR CIRCULAR AND ELLIPTICAL ABSORBER Figure 1 shows the geometry of the Elliptical hyperboloid concentrator (EHC), Circular Hyperboloid Concentrator (CHC), Cone Circular concentrator (CCC) and Elliptical cone concentrator (ECC) respectively. The affect of concentrator geometric profile has been studied; this comparison is based on two different geometric profiles: the hyper parabolic and conical and two different cross-sections: elliptical and circular. The hyperboloid and the conical geometries are symmetrical along the vertical axis. The aperture and receiver of the EHC and ECC designs are elliptical while for the CCC and the CHC, both the aperture and the receiver are circular. In this study the concentration height, the receiver area and also the concentration ratio are the same for the four geometric concentrators. The internal reflectance of the concentrator is 94%. Fig. 2 shows the orientation of the solar source in relation to angle of the incident radiation to the aperture of the solar concentrator. When the source starts from the east (sunrise) the angle is considered to be +90, when the orientation of the source is perpendicular to the receiver that is at noon, when the sun is at zenith the incident angle is considered to be 0. When the source moves to the west the incidence angle is -90. The optical efficiency in section 3.3 is determined from this process. 2

3 (a) (b) (c) ( d) Fig. 1: Schematic diagram of the solar concentrators (a) EHC, (b) CHC, (c) CCC and (d) ECC Fig.2 : Orientation of the solar source in relation to angle of the incidence radiation 3. OPTICALANALYSIS Optis TM ray tracing software [6] has been employed for the performance analysis of the different shapes for the static 3-D concentrating geometries. The ray tracing simulation, flux distribution and the centre line distribution on the receiver area and the optical efficiency of the four different concentrating geometries are presented in the section 3.1 Flux Distribution at the Receiver Area Using the Optis TM ray tracing method the following results were obtained for the four 3-D static geometric concentrators. Fig.3a, 3b, 3c and 3d show the ray tracing diagram and the 3-D flux distribution of the CCC, CHC, ECC and EHC geometric concentrators respectively, when the solar source is perpendicular to the receivers. 3

4 Fig.3ai, 3bi, 3ci and 3di show the ray s path on the concentrator and total amount of rays that were absorbed on the receiver area of the concentrators mentioned above. It can be observed that the CCC and CHC absorb more of the incoming rays than the ECC and the EHC. Fig.3aii, 2bii, 3cii and 3dii show the total flux distribution on the receiver area of the four concentrators. The CCC and CHC geometric concentrators have higher irradiances at the receiver; this is as a result of the circular cross section of both concentrators. However the ECC and EHC geometric concentrators have lower irradiance at the receiver when the angle of the solar source is perpendicular to the receiver area, this is as a result of their elliptical cross section they both have a wider acceptance angles which will be shown in section 3.3 below. i ii a. Conical Circular Concentrator (CCC) (i) Ray tracing and (ii) 3-D flux distribution on the receiver area i ii b. Circular Hyperboloid Concentrator (CHC) (i) Ray tracing and (ii) 3-D flux distribution on the receiver area 4

5 i ii c. Elliptical Conical Concentrator (ECC) (i) Ray tracing and (ii) 3-D flux distribution on the receiver area i ii d. Elliptical Hyperboloid Concentrator (EHC) (i) Ray tracing and (ii) Flux distribution on the receiver area Fig.3: Flux distribution at the receiver area for different concentrator geometrics Flux at the Center Line Fig.4 shows the flux at the centre line for the four 3-D concentrator receivers. Fig 4a and 4b show the flux at the centre line for the CCC and CHC respectively. Because of the circular cross section of these concentrators the flux distribution along both axes are the same. The flux is highest at the centre of the receiver but falls sharply as we move towards the ends. Fig4c and 4d show the flux at the centre line of the ECC and EHC respectively, however, unlike the circular cross section the irradiance is well distributed along the major axes and minor axes of both elliptical cross sections. These figures show clearly the difference between the flux distribution along the centre line of the circular cross section and elliptical cross section. 5

6 a. Circular Conical Concentrator (CCC) b. Circular Hyperboloid Concentrator (CHC) (i) Flux distribution at centre Line of the receiver major axis (ii) Flux distribution at centre Line of the receiver minor axis c. Elliptical Cone Concentrator (ECC) 6

7 (i)flux Distribution at centre line of the receiver major axis (ii) Flux Distribution at centre line of the receiver minor axis d. Elliptical Hyperboloid Concentrator (EHC) Fig.4: Flux distribution at centre line of the receiver area for different concentrator geometrics 3.3 Optical Efficiency From the critical analysis of the observation and result of this study, the elliptical cross section EHC and ECC (with aspect ratio 5) show better performance than the circular cross section with a CHC and CCC. The EHC design has been shown to outperform all of the other three solar concentrator with a wider acceptance angle and better average optical efficiency throughout the day in comparison to the average optical efficiency throughout the day for the four concentration geometries, The optical efficiency of CCC is 32% with acceptance angles is 0, CHC is 24%, acceptances ±15, EHC is 27 %, acceptance angle ±30 and for ECC is 30%, acceptance angle is ±30. But when acceptance angles±45 the optical efficiency of the EHC is 13 %, where the optical efficiency of the ECC is 2.3%, when acceptance angle is ±45. Fig.5 shows the optical efficiency for the four 3-D concentrators. A simple description of the optical analysis is presented in Table1. TABLE1: COMPARISON OF THE OPTICAL PERFORMANCE OF DIFFERENT CONCENTRATORS Solar concentrator Optimum acceptances angle Efficiency (%) Optimum efficiency (%) Acceptances angle ECC ± ±15 EHC ± ±15 CCC ± Zenith CHC ± Zenith 7

8 Fig. 5: Optical efficiency vs. solar incident angles for different concentrator geometric. 4. Conclusions Four different 3-D solar concentrators have been presented and the optimum optical efficiency achieved. The four concentrators have higher optical efficiency when the orientation of the source of incoming radiation is perpendicular to the collector. However the EHC and ECC with the elliptical cross section have a more uniform distribution of the flux in comparison to the CHC and CCC with circular cross sections. The optimum optical efficiency of the EHC and ECC (aspect ratio 5) were shown to be 27 % with an acceptance angle ±30 and 30%, acceptance angle is ±30 respectively. The CHC and CCC had optical efficiencies of 32% with acceptance angles 0 and 24% with an acceptances of ±15 respectively.,. The EHC designs have been shown to outperform all of the other three solar concentrators with a wider acceptance angle and better average optical efficiency throughout the day without the use of any tracking device. This has a great economic advantage over all existing solar concentrators which require the construction of a separate structure to support them and heavy machinery to orient them to intercept and properly reflect sunlight onto a receiver. This is due to the long axis of the ellipse increasing the acceptance angle for a wider incidence angles. Acknowledgment We thank the Libyan government s PhD studentship funding and the British Council UKIERI-DST grant (SA08-061) support for this research. 5. References (1) Ali, I.M.S., Mallick, T.K., Kew, P.A., O'Donovan, T.S. and Reddy KS 2D Ray tracing of Elliptical Hyperboloid Concentrator' Optical Performance Evaluation of a 2D and 3D novel hyperboloid solar concentrator. Proceedings of the XI th World Renewable Energy Congress, September 2010 Abu Dhabi. (2) O Gallagher, J.J., Non-imaging Optics in Solar Energy in Solar Energy. Morgan & Claypool: San Rafael, California. (3) Winston, R., Miñano, C.J., & Benítez, V., Nonimaging optics. Burlington: Elsevier Academic Press. (4) A. Garcia- Botella, A.A. Fernandez-Balbuean, D.Vázquez, and E. Bernabeu, "Ideal 3D Asymmetric Concentrator", Solar Energy 83, (2009a) (5) A. Garcia- Botella, A. A. Fernandez-Balbuean, D. Vázquez, and E. Bernabeu A. Gonzalez-Cano (2009b) Hyper Parabolic Concentrators, Applied Optics 48 (4), pp (6) Joe Wang, Solar Compound Parabolic Concentrator (CPC) modelling using OptisWorks ( 2010, OPTIS North America. 8