Convective heat transfer analysis of direct steam generation in parabolic trough collectors

Abstract

The direct steam generation in parabolic trough collectors (PTCs) is a recommended application for the economic development of PTC technology for electricity generation (large PTC) and industrial processes (small PTC). This process consists of circulating water under pressure in the receiver tube subjected to concentrated solar radiation by the mirror of the PTC. Along the tube, steam is generated which gives place to a two-phase liquid-vapor flow, under several configurations. Thus, control of such a flow is complex and remains a hot topic for proper PTC design. In the present work, we are interested in the convective exchanges between water and the receiver tube, which is a preliminary step in the simulation of a PTC with direct steam generation. A bibliographic study of the usual models describing heat transfer during convective boiling and a comparison of these models was carried out in a previous work [1] : the models of Chen-Cooper [2], Shah [3], Gungor and Winterton [4] and that of Kandlikar [5] may be suitable for modeling diphasic PTCs [1]. In order to analyze the effects of the water input parameters (mass flow and inlet pressure) and the solar flux density on the liquid-vapor convective heat exchange, the model of Gungor and Winterton for large PTCs and Kandlikar for small PTCs are adopted. The results of this analysis show that increasing the mass flow and decreasing the water pressure promote the convective transfer in the receiving tube of large and small PTCs, while the solar flux density has no great influence on the convective heat exchange particularly for high quality.