J. Scheuren*, M. Kirchner

Institut fuer Solarenergieforschung Hameln (ISFH), Am Ohrberg 1, D-31860 Emmerthal, Germany

Corresponding Author, j. scheuren@isfh. de
Tel: ++49-5151/999-523, Fax: ++49-5151/999-500

Abstract

The main characteristic parameters to describe the stagnation process of solar-thermal collector arrays are the steam-producing power (SPP), the steam spread (StS) in the pipework of the solar circuit and the resulting steam volume (SV). For a proper dimensioning of the expansion vessel and a protection of the temperature-sensitive components in the collector circuit a reliable prediction of both the SV — and StS-value is necessary. Based on a total of 210 outdoor stagnation experiments a model to predict the main characteristic stagnation parameters of collector arrays has been developed. The experiments were carried out between 2003 and 2007 using three different collector types with a total of eight different connection and installation variations. With this empirical model, it is now possible to predict SPP, StS and SV in the solar circuit for collector arrays of up to 30 m2.

Keywords: Solar Thermal Collector, Stagnation, Expansion Vessel, Steam-producing power, SPP, Steam Spread, Evaporation

1. Introduction

Solar heating systems with high solar fractions, which provide both domestic hot water and space heating energy, are regularly under stagnation conditions during summer. The evaporation of the heat transfer fluid can lead to a considerable spreading of hot steam into the solar circuit. As a consequence the collector loop components and the heat transfer fluid are exposed to a high thermal impact, which may lead to an accelerated ageing and damage. For the further development of solar thermal combi-systems with high solar fractions it is necessary to understand and control the stagnation process of large-scale collector arrays. In this paper the impact of the stagnation process to the solar circuit will be quantified and an easy to apply calculation method will be presented.

In 2001/2002 the AEE INTEC (Gleisdorf, Austria) has investigated single collectors and collector arrays with favourable draining behaviour regarding their stagnation behaviour [1]. In the subsequent work, carried out at ISFH in the years 2004 to 2007, collector arrays with critical evaporation behaviour are measured in detail. In addition a vacuum-tube collector array is being investigated. In [2] first results of the outdoor stagnation experiments at ISFH have been published.

The main characteristic parameters to describe the stagnation process of solar-thermal collector arrays are steam-producing power (SPP), steam spread (StS) in the pipework of the solar circuit and the resulting steam volume (SV). For a proper dimensioning of the expansion vessel a reliable prediction of the SV is necessary. In 2007, in the course of a research project supported by the

German Federal Ministry for Environment, Environmental Protection and Reactor Safety (BMU), we have now quantified the impact of the stagnation process on the solar circuit and developed a model to predict the main characteristic stagnation parameters of collector arrays up to 30 m2.