M. Meir*, J. Gjessing, J. Rekstad, N. Rumler[4]

University of Oslo, Department of Physics, P. O. Box 1048, N-0316, Oslo, Norway
Corresponding author: mmeir@fys. uio. no

Abstract

Passive ventilation was experimentally studied as overheating protection for polymeric collectors. The dimensions of the slit apertures, tilt angle and distance between collector cover and absorber were varied and the effect on the maximum temperature reduction was investigated. The longitudinal thermal expansion of the polymeric absorber was applied as trigger to open a ventilation flap if the absorber temperature exceeds a certain limit; sensitivity studies with the triggering of the flap were performed. A simplified method to determine the temperature dependent heat loss coefficient and the heat capacity of a glazed polymeric solar collector is presented. With this method the collector does not need to be connected to a heat removal system with circulating heat carrier. The method uses experimental data from the whole day, and is therefore not based on steady state conditions.

1. Introduction

Recently the interest has increased in using polymeric materials for solar thermal collector components, as cover sheets or absorbers (e. g. IEA-SHC Task 39). One reason among several is to utilize the flexibility offered by polymeric sheets and to design modular building units. These can substitute conventional roof or facade covers and simultaneously convert solar to thermal energy. Due to direct building integration the collector and the building share the thermal insulation and constructive parts of the building (support for the collector frame).

A serious limitation for this approach is the fact that if low-cost, low-/medium temperature performance plastics are chosen as absorber material in glazed solar collectors, the temperature of the absorber during thermal stagnation might be critical for the material and its service life. Thermal stagnation occurs for solar heating systems when no more heat is removed from the collectors (heat store is charged and controller stops the solar pump). If applied as absorber material in glazed collectors most actual polymers would therefore require a save and reliable overheating protection in the case of stagnation.

In the present work, the passive ventilation between collector cover and absorber is studied as overheat protection during thermal stagnation. Kearney et al. [1] and Roberts [2] have modelled corresponding passive ventilation processes, additionally with ventilation between absorber and thermal insulation. Kearney concluded that passive ventilation alone was not sufficient. This conclusion implied extreme desert climate with solar irradiance of 1100 W/m2 and ambient temperatures of 43 °C. Kearney and Roberts found that passive ventilation gave a reduction of the maximum temperature of 25-30 K for different absorber temperatures. Harrison et al. [3] have performed experimental studies with metal absorbers, where the backside of the absorber was cooled by ventilation. The authors measured a temperature reduction of approximately 30 K and propose thermally activated valves, which trigger the ventilation during stagnation and are closed

when the collector is operative. Another principle for a thermally activated ventilation flap is suggested in the patent [4] based on a hydraulic mechanism with low-boiling fluids. A different approach for overheating protection are thermotropic coatings, e. g. applied on the collector glazing; these coatings should switch from transparent to opaque at a critical temperature for the absorber material. Thermotropic coatings are under development and considerable R&D has been done, e. g. [5, 6, 7]. Another principle for the overheating protection is proposed in the patent by Griessen and Slaman [8]. The refraction index of the collector glazing, a prismatic structured optical layer, which is hollow inside, can be changed by a simple mechanism and reduce the transmittance for solar radiation. It should be mentioned that overheating is not only an issue for glazed polymeric collectors but also for glazed metal-based collectors [9].

If polymeric collectors are part of the building, sharing thermal insulation, the stagnation temperature and the collector efficiency vary from case to case. The present experiments allow a simplified method to determine the temperature dependent heat loss coefficient and the heat capacity of glazed polymeric collectors. The present work is based on the master thesis projects of Gjessing, 2006 and Rumler, 2007 [10, 11].