A novel computational approach to combine the optical and thermal modelling of Linear Fresnel Collectors using the finite volume method

Abstract

A computational approach is presented, which uses the finite volume (FV) method in the Computational Fluid Dynamics (CFD) solver ANSYS Fluent to conduct the ray tracing required to quantify the optical performance of a line concentration Concentrated Solar Power (CSP) receiver, as well as the conjugate heat transfer modelling required to estimate the thermal efficiency of such a receiver. A Linear Fresnel Collector (LFC) implementation is used to illustrate the approach. It is shown that the Discrete Ordinates method can provide an accurate solution to the Radiative Transfer Equation (RTE) if the shortcomings of its solution are resolved appropriately in the FV CFD solver. The shortcomings are due to false scattering and the socalled ray effect inherent in the FV solution. The approach is first evaluated for a 2-D test case involving oblique collimated radiation and then for a more complex 2-D LFC optical domain based on the FRESDEMO project. For the latter, results are compared with and validated against those obtained with the Monte Carlo ray tracer, SolTrace. The outcome of the FV ray tracing in the LFC optical domain is mapped as a non-uniform heat flux distribution in the 3-D cavity receiver domain and this distribution is included in the FV conjugate heat transfer CFD model as a volumetric source. The result of this latter model is the determination of the heat transferred to the heat transfer fluid running in the collector tubes, thereby providing an estimation of the overall thermal efficiency. To evaluate the effectiveness of the phased approach in terms of accuracy and computational cost, the novel 2-D:3-D phased approach is compared with results of a fully integrated, but expensive 3-D optical and thermal model. It is shown that the less expensive model provides similar results and hence a large cost saving. The novel approach also provides the benefit of working in one simulation environment, i.e. ANSYS Workbench, where optimisation studies can be carried out to maximise the performance of linear CSP reflector layout and receiver configurations.

Optimization of insulation of a linear Fresnel collector

“…Moghimi et al [2][3][4][5][6] suggested the more accurate discrete ordinates method (DOM). The DOM is computationally more expensive than the S2S model, but is more accurate and can predicate complex heat transfer phenomena inside the cavity domain.…”

Introducing CFD in the optical simulation of linear Fresnel collectors

“…The first case study seeks to prove the validity of the methods suggested for reducing numerical errors in the Finite Volume method; a more detailed study has been published on this by Moghimi et al [10]. It is conducted on oblique collimated light in a 2-D domain (as shown in Fig.1), with the walls of the domain constrained as perfectly absorbing, with a temperature of 1 K. The incoming radiation that hits the bottom wall therefore effectively creates a heat flux step change, with a sharp discontinuity in the radiation intensity field.…”

“…Moghimi et al [10,12] conducted a study on a LFC with a multi-tube trapezoidal receiver, largely inspired by the AREVA cavity design. The specifications of this model are described in Tables 1-3 and the schematic of the geometry is displayed in Fig.…”

“…The final strategy is highly effective for simple geometries but becomes somewhat impractical for more complex geometries where radiation beam propagation occurs in multiple different directions. Therefore, the practical strategies suggested by Moghimi et al [10] to remove these numerical errors from a FV CFD solution include the following:…”

Combining ray tracing and CFD in the thermal analysis of a parabolic dish tubular cavity receiver

“…The flux distribution is then applied as a volumetric heat source [3,6,7] in the conjugate heat transfer ANSYS Fluent CFD simulation. It is converted from a surface heat flux [W/m 2 ] to a volumetric heat source [W/m 3 ] by dividing by the tube thickness.…”