K. Resch1* and G. M. Wallner2

1 Polymer Competence Center Leoben GmbH, RoseggerstraBe 12, A-8700 Leoben, Austria
2 Institute of Materials Science and Testing of Plastics, University of Leoben, A-8700 Leoben, Austria

Corresponding Author, resch@pccl. at

Abstract

Within this paper thermotropic hydrogels, thermotropic polymer blends and thermotropic systems with fixed domains are reviewed with respect to their capability to prevent overheating of solar collectors. As to their switching ranges (77% for thermotropic hydrogels, 52% for thermotropic polymer blends and 25% for systems with fixed domains) the functional layers are well suited for solar collector applications. For an all polymeric flat-plate collector with twin — wall sheet glazing and black absorber thermotropic hydrogels, thermotropic polymer blends and TSFD would limit maximum absorber temperatures to 75, 90 and 125°C, respectively. This would allow for the use of cost-efficient plastics as absorber materials. However, thermotropic materials have not yet been developed systematically for the application in solar thermal systems with corresponding switching temperatures and properties to resist the demanding environmental conditions especially for prolonged periods.

Keywords: overheating protection, thermotropic hydrogels, thermotropic polymer blends, thermotropic systems with fixed domains

1. Introduction

The application of cost-efficient plastics as absorber materials for all-polymeric solar collectors requires appropriate overheating protection. A feasible way to limit stagnation temperatures is the reduction in optical gain especially by the use of thermotropic layers [1]. Thermotropic glazings are actively switchable layers which permit the light and energy flux to be adapted dynamically to temperature conditions within the collector. In the past various thermotropic systems for active daylight control in transparent facades have been developed and investigated mainly [2,3]. Within this paper existing thermotropic systems are discussed with respect to their capability to prevent overheating of solar collectors.