Advancing Tube Receiver Performance by Using Corrugated Tubes

Abstract

Direct introduction of solar energy into a Brayton cycle using Solar Tower systems enables a highly efficient conversion of the solar energy, especially so when ombined cycles are used. One key component of such a solar gas turbine system is the receiver. High and inhomogeneous heat fluxes pose the main challenge for the design of such receivers. One possible design of the receiver uses directly irradiated metallic tubes arranged in an insulated cavity. The paper presents the results of a study comparing thermo hydraulic absorber tube layouts with varying absorber tube dimensions and number of parallel tubes. The more parallel tubes are used the lower is the velocity of the fluid flow and in the same way the heat transfer coefficient is reduced. This leads to higher wall temperatures and therefore to a lower receiver efficiency. Using corrugated tubes instead of smooth tubes gives the possibility to increase the heat transfer coefficient. A thermo hydraulic test bench was developed in order to analyze the influence of different structures of corrugated tubes on heat transfer capability and pressure drop. The geometry of helically ribbed tubes was optimized using CFD modelling. Design goal was a high heat transfer coefficient without exceeding the allowed pressure drop of the turbine. The resulting configurations were used to improve a tubular receiver based on the SOLUGAS receiver design. A thermal FEM model was used to analyze the temperature field and the efficiency of the different receiver designs. Solar radiation, convection to fluid, radiation exchange, convective and conductive losses were considered in the model. It was found that the receiver efficiency of about 0.719 (at design point) could be increased up to 0.835 by using more parallel absorber tubes than needed to fulfill the pressure drop limit.