Turbulence model applied to a fluid in a modified evacuated solar collector

L. Crema1*, G. Cicolini1, A. Bozzoli1

1 Renewable Energies & Environmental Technologies research unit (REET), Fondazione Bruno Kessler (FBK),
Via alla Cascata 56/C, I-38050 Povo (Tn), Italy
* Corresponding Author, :rema@,fbk. eu

Abstract

In this work, we propose a solution to convection and conduction heat losses on an solar thermal collector. We have based our work starting from the technology of the evacuated solar tubes, the technology actually with the highest thermal efficiency in the market. The tubes have got a CERMET absorbing layer sputtered on the surface of a glass tube, in the evacuated area. A modified structure for the evacuated tube has been analysed in order to investigate the heat transfer and the thermal resistance from the cermet layer to the vector fluid in different fluid dynamic conditions than the actual used. Some models have been created and their behaviour has been verified using Finite Element Modelling software. The computed results show a lower thermal resistance for the proposed geometries. A higher convective heat transfer has been obtained providing the vector fluid of a turbulent flow, using special shapes (turbulators) applied to the evacuated tubes. The temperature gradient between the cermet layer and the vector fluid has been decreased from 20 to 1,06 K in the better case. The thermal efficiency of the panel has an improvement of about 10% from such modified geometry.

Keywords: solar collector, finite element modelling, fluid dynamic

1. Introduction

During last years the development of new technologies have taken solar thermal collectors to new frontiers [1]. Collectors based on evacuated solar tubes, cermet layers and other applied technologies have taken good results. Many studies had proposed solutions to the evacuated solar tube development [2]. Nevertheless commercial collectors frequently suffer from some problems related to heat transfer phenomena, radiation loss at high temperatures or vector fluid choice. A relevant heat loss comes from conduction/convection phenomena between the absorber material and the vector fluid and from both of them to the environment. In the actual work it has been performed an analysis on a real evacuated solar tube with a relevant presence on the international markets and particularly good characteristics. The aim is to analyse aspects related to heat losses and to propose both a partial solution to the problem and a technical improvement of the system. The analysis will be performed under an analytical point of view, by the use of FEM (Finite Element Modelling) simulation and by an experimental setup for a direct verification of modelling.