Behavior of the compound wall copper-steel receiver with stratified two-phase flow regimen in transient states when solar irradiance is arriving on one side of receiver

Direct steam generation (DSG) in parabolic trough concentrators can cause some instabilities in steel absorbers. These instabilities are caused mainly along the stratified two-phase flow related to the boiling process of water. These instabilities are manifested as a transient state and shown as a bending of the absorber, causing a rupture of the glass envelope of the receiver. This bending has been associated with the thermal gradient produced by the boiling of water, and the effects are related to the thermal conductivity of the absorber, the flow pattern and heating of the absorber from below around solar noon. When solar beam irradiance is arriving on one side of the absorber, the bending, which is always upwards, can be up to 50 mm. This is the worst condition because the temperature difference is at its maximum at a transient state. This paper presents the results of a bimetallic absorber (copper-steel), which should reduce such deflections due to the high thermal conductivity of the copper and the mechanical resistance of the steel. In order to compare the results of the theoretical model with experimental results, it was necessary to build a bimetallic absorber 3 m long with a 31.7 mm internal diameter and a thickness of 2.6 mm. The bending obtained experimentally was between 5 and 10 mm, depending on the mass flow and the temperature difference in the wall receiver. As a main conclusion, the bending of the receiver is acceptable, eliminating the rupture of the glass envelope, and it may be used in DSG of parabolic trough collectors at low flows. The objective of this work is to apply DSG to low power generation with parabolic trough plants. (C) 2003 Elsevier Ltd. All rights reserved.