Comparison of different types of secondary mirrors for solar application
Introduction
Concentrating is an efficient way to improve the solar energy density [1], [2], [3]. Two-stage concentrators are introduced into the concentrated solar power (CSP) systems for more flexible structures, e.g. with an upward-facing receiver [4] or convenient heat storage arrangement [5], for higher concentration ratios, e.g. compound parabolic concentrator (CPC) [6], [7], or for efficient power delivery [8] and so on.
Two-stage concentrators in CSP have attracted an increasing attention since 1976. Rabl [9] proposed a flat Fresnel “tower reflector” in a power tower system to avoid excessive thermal losses and conducted the optical analysis mathematically. Mauk et al. [5] simulated the performance of a Cassegrain type solar collector for chemical energy storage using an off-axis optics method and found that the focal length of the hyperboloid was not an important factor. Feuermann et al. [10] characterized a purely imaging two-stage solar concentrator using a complementary Cassegrain concentrator and evaluated the potential improvements with secondary concentrators. A solar fiber-optic mini-dish concentrator using a flat mirror to redirect rays was also designed and demonstrated experimentally by Feuermann et al. [4], [8]. The min-dish concentrators were also designed and estimated in concentrating photovoltaic systems [11]. Karabulut et al. [12] conceptually described a system consisting of a parabolic dish and a Stirling engine mounted at the bottom using a double reflection mechanism. Chen et al. [13], [14] simulated a dish system with a hyperboloid or ellipsoid mirror using a ray tracing method and found an ellipsoidal mirror is slightly better than that with a hyperboloidal mirror. Jiang et al. [15] analyzed a nondimensional optical model for a two-stage parabolic trough concentrating photovoltaic/thermal system using a parabolic beam splitting filter to evaluate the local radiation flux density distribution on the elements’ surfaces. Kribus et al. [16], Segal et al. [17] and Suzuki [18] have offered related simulation work on solar tower reflector adopting hyperboloid or ellipsoid, respectively.
A secondary mirror plays an important role in a two-stage system. Representative secondary mirrors include flat mirrors [4], [8], [11], ellipsoidal mirrors (the Gregorian system) [13], [17], hyperboloidal mirrors with upper sheet (the Cassegrain system) [5], [13], [14], [16], [17], hyperboloidal mirrors with lower sheet (the Complementary Cassegrain system) [10] or paraboloidal mirrors [15], [19]. They have been researched individually but less comparison has been done. This work tries to compare five representative types of secondary mirrors based on a dish using Advanced System Analysis Program (ASAP) software provided by Breault Research Organization. ASAP has been widely used in the simulation of optical systems. Effects of geometry parameters and concentrator precisions on the optical performance are discussed and compared. Experiments are carried out using a hyperboloidal mirror with upper sheet as the secondary mirror.
Section snippets
System description
Schematica of two-stage systems with different types of reflectors are shown in Fig. 1. Rays parallel to the optical axis are first reflected by a primary dish (PD) to a secondary mirror and then are redirected by the secondary mirror. If the focal point of secondary mirror is coincident with that of PD (F1), rays reflected from secondary mirror would be redirected and focus onto its second focal point in Fig. 1(a)–(d) or still parallel to the optical axis in Fig. 1(e).
For a flat
Comparison and discussion
Five types of secondary mirrors are discussed and compared at different cases based on a given PD here. The simulation assumptions are made as follows [13], [14]:
- (a)
The incident radiation is 1000 W/m2. All the incident rays are assumed to be carry equal energy with a solar half-angle (δs) of 4.65 mrad.
- (b)
Diameter of PD (D0) is 1000 mm. Reflectivity is 0.95 and absorptivity is 1.
Diameter of the concentrated spot is equal to the diameter of a circle which contains 90% of total flux on the receiver. Flux
Experimental setup
According to the discussion above, GS and CS are suitable to provide both high concentration ratios and low redirected focal points. CS is chosen in this study to validate the ray-tracing model indoor. The experimental setup is illustrated in Fig. 8.
A two-stage concentrator consisted of a primary dish (diameter of 1000 mm, rim angle of 70°) and a hyperboloidal mirror with upper sheet (diameter of 200 mm with NA1 of 0.15). The lower focal point F2 was redirected to 350 mm below the vertex of PD.
Conclusions
A survey on two-stage reflection was given and five types of secondary mirrors, such as a flat mirror, an ellipsoidal mirror, a hyperboloidal mirror with upper or lower sheet or a paraboloidal mirror, were discussed and compared base on a primary dish of 1000 mm using ASAP ray tracing software. The conclusions can be drawn as follows:
- (1)
FS and CCS are more sensitive to rim angle or relative location while GS, CS and PS have stable performances. The secondary mirror is better a convex surface other
Acknowledgments
The authors gratefully acknowledge the support from Important Science & Technology Specific Projects of Zhejiang Province (No. 2012C01022-1), Zhejiang Provincial Natural Science Foundation of China (NO. LY12E06005).
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