[en] Highlights: • It is to present comparative and sensitive analysis for PTCs with the MCRT method. • A detailed PTC optical model was developed based on a novel unified MCRT model. • Reference data determined by the divergence effect is useful to design a better PTC. • Different PTCs have different levels of sensitivity to different optical errors. • There are no contradictions between accuracy requirements of different parameters. - Abstract: This paper presents the numerical results of the comparative and sensitive analysis for different parabolic trough solar collector (PTC) systems under different operating conditions, expecting to optimize the PTC system of better comprehensive characteristics and optical performance or to evaluate the accuracy required for future constructions. A more detailed optical model was developed from a previously proposed unified Monte Carlo ray-tracing (MCRT) model. Numerical results were compared with the reference data and good agreements were obtained, proving that the model and the numerical results are reliable. Then the comparative and sensitive analyses for different PTC systems or different geometric parameters under different possible operating conditions were carried out by this detailed optical model. From the numerical results it is revealed that the ideal comprehensive characteristics and optical performance of the PTC systems are very different from some critical points determined by the divergence phenomenon of the non-parallel solar beam, which can also be well explained by the theoretical analysis results. For different operating conditions, the PTC systems of different geometric parameters have different levels of sensitivity to different optical errors, but the optical accuracy requirements from different geometric parameters of the whole PTC system are always consistent

[en] Highlights: ► A general-purpose method or design/simulation tool needs to be developed for CSCs. ► A new modelling method and homemade unified code with MCRT are presented. ► The photo-thermal conversion processes in three typical CSCs were analyzed. ► The results show that the proposed method and model are feasible and reliable. -- Abstract: The main objective of the present work is to develop a general-purpose numerical method for improving design/simulation tools for the concentrating solar collectors (CSCs) of concentrated solar power (CSP) systems. A new modelling method and homemade unified code with the Monte Carlo Ray-Trace (MCRT) method for the CSCs are presented firstly. The details of the new designing method and homemade unified code with MCRT for numerical investigations on solar concentrating and collecting characteristics of the CSCs are introduced. Three coordinate systems are used in the MCRT program and can be totally independent from each other. Solar radiation in participating medium and/or non-participating medium can be taken into account simultaneously or dividedly in the simulation. The criteria of data processing and method/code checking are also proposed in detail. Finally the proposed method and code are applied to simulate and analyze the involuted photo-thermal conversion processes in three typical CSCs. The results show that the proposed method and model are reliable to simulate various types of CSCs.

[en] A three-dimensional optical model for a pressurized volumetric receiver (PVR) is developed and corresponding solar radiation propagation process within the PVR is simulated by the Monte Carlo Ray Tracing (MCRT) method. In the computation, the complicated photon transmission process in the SiC porous absorber is simplified as the transmission process in the statistically homogeneous and isotropic turbid medium. Meanwhile, the non-uniform cylindrical coordinate grid is applied in the statistics of energy distribution, which could greatly reduce the number of cells in the computational grid and time compared with normal uniform grid. Based on the above model, the energy distribution in the irregular macro scale porous absorber is determined and then the effects of system parameters, including the incidence angle, the shape of absorber and the optical property of absorber, on the local heat flux of the absorber are investigated. The results show that, under the given operating condition, the radiation heat flux is mostly concentrated at the top area of the absorber and the maximum heat flux value is up to 2.73 × 10⁹ W m⁻³, but it quickly decreases in the sideward locations. The incidence angle and a relative narrow shape of absorber are helpful to reduce the maximum heat flux in the absorber. Furthermore, as the ratio of absorption coefficient/extinction coefficient decreases, the absorbed radiation energy distribution is more uniform and the max heat flux in the absorber decreases greatly. -- Highlights: ► Solar radiation propagation process within a volumetric receiver is simulated by MCRT method. ► The SiC absorber is simplified as the statistically homogeneous and isotropic turbid media. ► Non-uniform cylindrical coordinate grid is applied in the statistics. ► The non-uniform energy flux density distribution in the receiver is calculated. ► The influences of system parameters on local heat flux distribution are also investigated.

[en] This paper presents an axisymmetric steady-state computational fluid dynamics model and further studies on the complex coupled heat transfer combined radiation–convection–conduction in the pressurized volumetric receiver (PVR), by combining the Finite Volume Method (FVM) and the Monte Carlo Ray-Trace (MCRT) method. Based on this, effects of geometric parameters of the compound parabolic concentrator (CPC) and properties of the porous absorber on synthetical characteristics and performance of the photo-thermal conversion process in the PVR are further analyzed and discussed detailedly. It is found that the solar flux density distributions are always very heterogeneous with large nonuniformities, and the variation trends of the corresponding temperature distributions are very similar to these but with much lower order of magnitude. The CPC shape determined by the CPC exit aperture has much larger effects on synthetical characteristics and performance of the PVR than that of the CPC entry aperture with a constant acceptance angle. And a suitable or optimal thickness of the porous absorber could be determined by examining where the drastic decreasing trends occur at the curves of variations of synthetical characteristics and performance with the porosity. - Highlights: ► An axisymmetric steady-state CFD model of PVR is presented with MCRT–FVM method. ► The complex coupled heat transfer and synthetical performance of the PVR are studied. ► The effects of geometric parameters and porous properties are analyzed and discussed. ► Solar flux and temperature in PVR are very heterogeneous with large nonuniformities. ► An optimal absorber thickness can be determined by examining the effects of porosity.

[en] The mass of the τ lepton has been measured at the Beijing Electron-Positron Collider using the Beijing Spectrometer. A search near threshold for e⁺e-→τ⁺τ^- was performed. Candidate events were identified by requiring that one τ decay via τ→eν bar ν, and the other via τ→μν bar ν. The mass value, obtained from a fit to the energy dependence of the τ⁺τ^- cross section, is m_τ=1776.9_-0.5^+0.4±0.2 MeV