Analysis and optimisation of a gas-cooled pipe for solar thermal energy production using parabolic collectors

In the framework of the design of a solar thermal power plant proposed by ENEA, the activity carried out by CRS4 on the thermal-fluid-dynamic simulation of a gas-cooled pipe irradiated by a parabolic solar collector is described in this paper. Two methods have been adopted in parallel: a simplified one-dimensional approach and a Computational-Fluid-Dynamics (CFD) three-dimensional approach. The first method was used to build a tool able to give quick answers to parameters changes with an acceptable degree of accuracy. The CFD analysis is used both to validate 1D-model results and to study in details all 3D physical phenomena. The multi-zone one-dimensional model developed at CRS4 is described first. The pipe is split along its axis into a discrete number (typically 100) of sub-domains. Each sub-domain is split further on into five different zones, corresponding to the various components of the pipe. All the main energy-exchange mechanisms between the various parts of the pipe have been implemented, resulting in a system of five equations for each sub-domain, solved iteratively within an EXCEL framework. The 3D-CFD model is then described. The model is fully parametric, allowing a quick variation of the geometrical parameters of the system. Convection, conduction and thermal radiation heat exchanges are solved in a coupled way. The model can give any information about field variables of fluids and solid structures as well as all energy balances. The two models have been used for a parametric study of the effect of the pipe diameter variation on the system efficiency. Although not negligible differences between the two models can be noticed concerning local energy balances, the error on the evaluation of the system efficiency is order 1%. The result of the optimization was practically the same for the two models