Direct Solar Steam Generation in Parabolic Troughs: Thermal Stress due to Variations in Irradiation

Abstract

The Californian parabolic trough power plants belong to the most successful systems for producing solar electricity. In order to improve the efficiency of these power plants, the research activities in direct steam generation (DSG) have been initiated. The aim is to avoid the necessity of a single phase thermooil used as heat transfer fluid (HTF) in the absorber pipes of the solar collectors. Instead, the feedwater of the steam cycle should be evaporated and superheated directly in the absorber pipes, thus eliminating the need for the second cycle and the heat exchanger. This demands the knowledge of the thermohydraulics of a two-phase flow in the absorber pipes. Since the characteristic parameters differ significantly from those typical of conventional steam generators, the basic relations describing heat transfer and pressure losses had to be investigated. Experimental data was gained using a electrically heated test-section. Based on the experiments, simulation tools for the design of DSG-systems have been developed. As a result, the feasibility of direct steam generation in stationary operation mode was proven. The collectors can cope with the thermal stress resulting from a circumferrentially varying solar irradiation provided the mass flow in the absorber tubes is sufficient. The performance can be optimized in order to guarantee an adequate cooling of the absorbersO wall while minimizing the pressure losses. Based on the results of the first stage of DSG-research, the first test facility for steam generation by solar energy was designed. This test facility was completed in 1999 at Almeria, Spain.