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, as the long-term service temperature of cost-efficient plastics is limited for potential applications in solar absorbers an appropriate design including overheating protection is essential. The energy flux in all-polymeric flat-plate collectors may be controlled by thermotropic layers. Thermotropic glazings change their light transmission behaviour upon reaching a certain threshold temperature reversibly by switching from a transparent to a light diffusing state.
The overall objective of the present study was to evaluate the capability of thermotropic layers to provide overheating protection in an all polymeric flat-plate collector applying theoretical modelling. Various designs of polymeric collectors with regards to the position of the thermotropic film and to the type of glazing and absorber were considered. Specific focus was 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 in the range of 80-130°C.
The investigations showed that for efficient domestic hot water generation overheating protected collectors with twin-wall sheet glazing and black absorbers are more suitable than collectors with single-wall sheet glazing and/or selective absorber coatings. Stagnation temperatures of the solar collector can be reduced by using thermotropic layers in the glazing or on the absorber. The impact of a thermotropic layer on overall collector efficiency is low as long as the solar transmittance exceeds 85% in the clear state. To provide an excellent limitation of the stagnation temperature to a maximum operating temperature of ~85°C a residual solar transmittance of 0.25 for a collector with twin-wall sheet glazing and black absorber is effectual. The development and design of thermotropic layers exhibiting a solar transmittance up to 0.60 in the opaque state would allow for the application of cost-efficient plastics as absorber materials. For thermotropic layers on the inner side of the glazing switching temperatures between 55 and 60°C are required for stagnation temperatures of about 85 °C. The solar transmittance should decrease between 75 and 80°C for thermotropic materials mounted on the absorber. The switching temperature should be adjusted higher if higher stagnation temperatures are allowed.
Polymer Competence Center Leoben GmbH (PCCL)
Kunststoffkollektoren besitzen aufgrund eines einfachen und weitgehend automatisierbaren
Fertigungsablaufs signifikante Kostenreduktionspotentiale. Allerdings ist bei Kollektoren aus kostengünstigen Polymerwerkstoffen ein kunststoffgerechtes Design vorzusehen, das auch eine Begrenzung der maximalen Kollektortemperaturen erfordert.
Das wesentliche Ziel dieser Arbeit war es, das Potenzial thermotroper Schichten für den
Überhitzungsschutz von Vollkunststoff-Flachkollektoren mittels theoretischer Modellierung,
zu ermitteln. Dafür wurden Berechnungen durchgeführt, die den Einfluss thermotroper
Schichten auf die allgemeine Kollektorleistungsfähigkeit aufzeigten und die notwendigen optischen Eigenschaften der thermotropen Schichten wie ihre solare Transmission im klaren und im opaken Zustand und die Schalttemperaturen bzw. Schaltcharakteristiken ableiten ließen.