Designing capillary systems to enhance heat transfer in LS3 parabolic trough collectors for direct steam generation (DSG)

Direct steam generation (DSG) has been the main research line in parabolic trough technology for many years now, due to its potential to improve the global efficiency of the power generation process. Based on the research performed, there is sufficient knowledge to design a pre-commercial power plant between 3 MWe and 5 MWe. In this design, the process parameters (flow rate, pressure and temperature) are being chosen to assure a reliable and flexible operation. The present point in time, with preparations being made for the actual construction of such a plant, seems an ideal moment for proposals in order to optimize the process for future plants. This paper proposes to incorporate capillary systems on the inner surface of the absorber pipes for DSG. A capillary system enhances the solar energy transfer to the internal fluid flow because, on one hand, it supports the wetting of the inner surface of absorbers under a stratified environment – due to capillary pumping – and, on the other hand, it promotes vaporization – nucleate boiling in the case of a porous medium or thin film evaporation in the case of a micro-channeled pipe. This paper presents a description of the required properties and characteristics of a capillary system (etched micro-channels or a porous coating) to be integrated in LS3 receivers in accordance with operating temperature limitations and the imposed solar heat flux.