Movements and mechanical stresses in gas-filled flat plate solar collectors

J. Vestlund1*, M. Ronnelid1 and J. O. Dalenback2

1 Solar Energy Research Center, Hogskolan Dalama, Borlange, Sweden
2 Chalmers University of Technology, Goteborg, Sweden
* Corresponding Author, ive@du. se
Abstract

Sealed gas filled flat plate solar collectors will have stresses in the material since volume and pressure varies in the gas when the temperature changes. Several geometries were analyzed and it could be seen that it is possible reducing the stresses and improve the safety factor of the weakest point in the construction by using larger area and/or reducing the distance between glass and absorber and/or change width and height relationship so the tubes are getting longer. Further it could be shown that the safety factor won’t always get improved with reinforcements. It is so because when an already strong part of the collector gets reinforced it will expose weaker parts for higher stresses. The finite element method was used for finding out the stresses.

Keywords: Solar collectors, modelling, mechanical stresses, Inert gases

1. Introduction

One way of getting more energy efficient solar collectors is changing the air inside the collector with a more suitable gas with respect to the unwanted heat losses. Though, a construction with an enclosed gas will cause new challenges. Since the temperature in a solar collector can vary in a range of about 240 to 500 K there will be a volume and/or a pressure variation in the gas. These variations has to be considered, otherwise they can imperil the expected life length of the construction. This study has examined a collector box consisting of an ordinary glass and an ordinary absorber in all respects expect that it is formed as a tub fixed to the glass so a cavity is formed that can host a gas.

2. Method