| The novel and efficient concentrating theory and optimization of the system structure is particularly important to improve conversion efficiency of the beam-down(BD) solar thermal tower system. In order to optimize optical efficiency and solar thermal conversion performance of the central receiver, it is necessary to explore the heliostat with the high efficient optical performance and low cost that can match with the structure of the efficient BD reflector. This paper focuses on a novel point-line-coupling-focus(PLCF) solar thermal collecting system with linear Fresnel reflectors, according to the optical and thermal theory, the MCRT optical analysis model, MCRT and CFD coupling three-dimensional calculation model, and the multi-parameter heat removal factor model. In this paper, the solar thermal performance of the PLCF system has been carried out.Firstly, the geometric and optical structures of the PLCF system are introduced. The focusing concept of the PLCF system is described. The sun-tracking angles of the linear Fresnel heliostat(LFH) are derived. Considering the effects of solar angle model and optical surface errors, the MCRT model is established. Based on the MCRT model, the optical performance indexes are summaried. The comparison study of the traditional BD tower system and PLCF system has been performed.Secondly, line Fresnel heliostat with two axis tracking mechanism inherits the advantages of traditional line Fresnel concentrator with simple structure and good wind resistance. Compared with the conventional heliostat(CH), LFH has smaller size of the focal spot, and higher concentrating ratio. Affected by the rotational symmetry of CPC, the optimal aspect ratio of LFH is 1:1. The shading and blocking efficiency of LFH is slightly lower than that of the rectangular and square plat heliostats, however, spillage efficiency of LFH has obvious advantages. The optical efficiency of the PLCF sytem with LFH is much better than that with CH. For the small scale heliostat field, the tracking error, the image size and the optical efficiency of the cylindrical mirror system are superior than the hyperboloic system. In addition, the cylinder system can be able to eliminate CPC, improving the optical performance. Thus, the cylinder system is more suitable for the small scale PLCF system proposed in this paper.Thirdly, the establishment of two solar thermal analysis models are implemented, namely, one-dimensional model based on the modified heat removal factor and 3D analysis model based on the coupling model of MCRT and CFD. The modified heat removal factor can improve the traditional heat removal factor model. In addition, compared with the traditional heat removal factor model, the modified model is more sensitive to the variation of the related parameters. To test the solar thermal performance of the PLCF system, a closed testing system based on a conical cavity receiver is built. Thermal performance of the conical cavity receiver has been experimental analyzed. The normalized thermal efficiency with the optical efficiency and heat loss parameters is obtained. Based on the experimental data, under the same initial and boundary conditions, validation of the experimental and theoretical data for the heat removal factor model and 3D thermal analysis model is carried out. The comparison results show that the relative deviation of the three-dimensional model is within the range of 0% ? 15%, relative deviation range of the heat removal factor model is 3.8% ? 25%. Two models can be both feasible to analyze solar thermal performance of the PLCF system.Forthly, compared with the conventional heliostat, LFH can save the occupied land area of 1-2/π. The sun-tracking mode of the LFH is different from that of CH, under the parameters of Δθ = 0°, the optimal optical performance of the LFH can be reached. Through the analysis of unique layout structure of LFH, the cosine efficiency, atmospheric efficiency, the shading and blocking loss of the adjacent mirrors, shading and blocking loss of the adjacent heliostats, shading efficiency of the BD mirror, and spallage loss of the cavity receiver are analyzed in this paper. A GA algorithm combined with the bionic layout is developed to optimize the structure and parameters of the heliostat field. Under the parameters of a = 7 and b = 0.55, the optimal optical efficiency of 0.543 can be obtained for the heliostat field composed of 780 LFHs and 15600 m2 mirror area.Fifthly, based on the modified heat removal factor model with multi parameters, and the analysis model coupled MCRT and CFD, solar-thermal-fuild-solid conversion performance of the cavity receiver for different heliostat fields are analyzed in detail. The optimal conical angle of the spiral-tube receiver with the small-scale heliostat field is 100°. From the analysis of the cavity receiver with CPC matched the large-scale heliostat field, it can be found that thermal efficiencies of the cylinder and conical cavity receiver are gradually increasing with increasing withd-height ratio, and further reducing with increasing inlet air temperature. Under the same conditions, the heat removal factor of conical cavity receiver is larger than that of the cylinder type, while thermal efficiency of the conical cavity receiver is slightly higher than the cylinder type. Compared with the temperature field distribution of the cylindrical and hemispherical receivers, it can be found the temperature distribution of the absorber surface of the hemispherical receiver is more uniform than that of the cylindrical receiver. Compared plat and shaped cover glazing, the shaped structure can effectively reduced the natural convection heat loss. This reduction can be significantly enhanced with the increase of the air inlet temperature, and thermal efficiency can be improved 6%. For the split cover, thermal efficiency decreased gradually with cover spacing increases. Compared the hemisphere, cylinder and conical receivers, the hemisphere receiver has obvious advantages in thermal efficiency. With the increase of air inlet temperature, the thermal efficiency of three types of cavity receivers are reduced. Thermal efficiency of the hemisphere receiver is higher about 5% and 9% than that of the conical receiver and that of the cylinder receiver, respectively. The modified heat removal factors of three cavity receivers represent an exponentially decreasing trend. The heat removal factor of the hemispherical cavity receiver is superior to the others. With the increasing flow rate, thermal efficiencies of three cavity receivers increase. The optimum flow rate is 41 m3/s. The modified heat removal factor decreases with the increasing flow rate. Within the range of 25?41 m3/s, the heat removal factor is more sensitive with the variation of flow rate. The thermal efficiency of the three cavities increases with the increase of DNI, while the modified heat removal factor increases which is gradually deviated from the ideal state. The hemispherical cavity receiver has the best performance of uniform flux distribution, which leads to the superior thermal performance. Therefore, the hemispherical receiver has the best matching behavior for the solar thermal mechanism of the large-scale heliostat field proposed in this paper. |
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