Michael, Kohl, Volker Kubler, Markus Heck
Fraunhofer-Institut fur Solare Energiesysteme
Heidenhofstr. 2, D-79110 Freiburg
Tel.: 49 (0) 7 61-40 166-82, Fax: 49 (0) 7 61-40 166-81
E-Mail: michael. koehl@ise. fraunhofer. de

Flat-collectors are usually not tight. Therefore they can exchange the air with the environment. Moisture can accumulated in the collector, especially when the thermal insulation material could act as a storage for moisture (like mineral wool, for example). The moisture can increase the corrosivity of the micro-climate in the collector. An optimised ventilation rate which is measure of the air exchange between collector and environment helps to keep the collector dry.

1. Introduction

Sometimes solar collectors exhibit condensed water at the glazing (see figure 1). Usually even identical collectors at the same place in the same field show different degrees of condensation, caused by a unfavourable micro-climate. The micro-climate in ventilated flat — plate collectors is dominated on one hand by the water adsorption behaviour of the thermal insulation material and other components inside collector, e. g. wooden frames of back­planes. The other important property is the ventilation rate, which describes the air-exchange between the collector and the ambient.

Figure 1: Collector field with condensation effects

A high ventilation rate reduces the difference between the micro-climate and the ambient climate and keeps the collector dry, if it is rain-tight. But the thermal losses increase with the ventilation rate.

Simulation of the ventilation by computational fluid dynamic (CFD) showed that the air entering the collector through a relatively small ventilation hole in a lower corner of the collector is moving in several buoyancy driven loops towards the upper exit hole (figure 2a). There is much time for humidity exchange with humidity adsorbing components during this relatively long residence time.

Simulation of the condensation and re-evaporation of the humidity at the cold glazing showed that the drying by sunshine in the morning starts near the ventilation holes because of the dry air entering at the bottom and the warm air leaving at the top (figure 2b).

Figure 2: Computational fluid dynamics for ventilated flat-plate collectors:

a) Flow lines of air through a diagonally ventilated collector resulting from. The colours indicate the velocity of the test volumes (Blue means low velocity).

b) Effect of the ventilation on the drying of a glazing covered with moisture. The colours indicate the

load of humidity (blue-green means dried areas).________________________________________________________________________