K. Resch1*, R. Hausner2 and G. M. Wallner3

1 Polymer Competence Center Leoben GmbH, RoseggerstraBe 12, A-8700 Leoben, Austria
2 AEE Institute for Sustainable Technologies, A-8200 Gleisdorf, Austria
3 Institute of Materials Science and Testing of Plastics, University of Leoben, A-8700 Leoben, Austria

Corresponding Author, resch@pccl. at

Abstract

The main objective of the present study was to evaluate the potential of thermotropic layers to provide overheating protection for solar collectors by theoretical modeling of an all-polymeric solar panel. The investigations showed that for a collector with twin-wall sheet glazing and black absorber the stagnation temperatures can be limited by the use of thermotropic glazing. For maximizing the collectors efficient working temperature range the thermotropic layer should exhibit switching temperatures between 50 and 60°C. The impact of the thermotropic layer on collector efficiency is low, as long as the solar transmittance in the clear state is above 0.85. A residual solar transmittance ranging from 0.25 to 0.60 in opaque state is effectual to control the stagnation temperatures in the range from 80 to 130°C.

Keywords: all-polymeric flat plate collector, overheating protection, thermotropic layers

1. Introduction

Polymeric materials offer a significant cost-reduction potential for solar thermal collectors and may thus benefit a broader utilization of solar energy for various heating purposes. However, conventional solar thermal collectors reach stagnation temperatures up to 200°C, which exceed the maximum operating temperatures of cost-efficient plastics (~80-130°C). As especially polymeric absorbers made of cost-efficient plastics tend towards irreversible deformation and/or degradation when exposed to elevated temperatures for prolonged periods an appropriate overheating protection is required. The energy flux in all polymeric flat-plate collectors may be controlled by using thermotropic layers [1,2]. Thermotropic glazings change their light transmission behavior upon reaching a certain threshold temperature reversibly by switching from a transparent to a light diffusing state [3].

Within the present study theoretical modeling is applied to evaluate the potential of thermotropic layers to provide overheating protection in an all polymeric flat-plate collector. Specific focus is given to the effect of the thermotropic materials on overall solar collector performance as well as to required material properties, such as solar transmittance in clear and opaque state, switching temperature and switching performance to limit the stagnation temperature to 80-90°C, which is the required maximum operating temperature for domestic hot water and space heating applications. Furthermore the frequency distribution of stagnation temperatures for a collector with thermotropic overheating protection is determined.

2. Modeling