UC Davis

Concentrated solar power is being investigated by the U.S. Department of Energy (DOE) as a renewable energy source for meeting baseload and peak load electric power. The gas receiver pathway, utilizing supercritical CO2 as the working fluid, has been identified as a potentially viable source of implementing CSP technologies because of recent interest in supercritical carbon dioxide Brayton cycles [1]. In past studies, lab-scale experiments and on-sun test done on a micro-pin-array solar receiver utilizing sCO2 as the heat transfer fluid have shown the ability to absorb heat flux up to 100W/cm2 at thermal efficiency above 90 percent[2], [3]. The feasibility of using microscale unit cells, as building blocks for a megawatt-scale (250MW thermal) open solar receiver through a numbering-up approach, where multiple microscale unit cells are connected in parallel, has been explored by Zada et al.[4]. In the previous studies, a microlamination approach (in which the pin array is chemically etched in a sheet and diffusion bonded to a flux plate) is used in the manufacture of these receivers. The limitation of this manufacturing method has a consequence in limiting the pin array length of the receiver. These short pin array lengths would require more unit cells in the receiver, and thus a more complicated header system. Also, a notable assumption from previous studies is that a uniform heat flux is imposed on the receiver module(s) over a period, whereas in a heliostat field, the flux distribution is highly non-uniform and varies temporally and spatially. In this study, additive manufacturing of the receiver is presented as an alternative means of manufacturing the receivers. Additive manufacturing enables longer pin array lengths, thus reducing the complexity and mass of the header network. In order to study the performance of the receivers, and account for the spatial variation of properties throughout the receiver, a numerical code with a 2-dimensional discretization is developed and the performance of the additively manufactured pin array receiver (AM2PAR) and the microlaminated pin array receiver (µLPAR) are compared at their respective design lengths. Furthermore, this work explores the impact of non-uniform heat flux on a AM2PAR central tower receiver. The heat flux data for a select geographical location has been modeled with NREL's SolarPILOT to model hourly heat flux distribution over a central receiver on a typical hot summer day. The flux data from SolarPILOT is used in numerically modeling the thermal and hydraulic parameters in the modules of a central receiver with an area of 250m2 (7.5m radius and 10m height). The non-uniformity effects are compared with the results of a uniform-flux model. The study results present the estimated mass flow distribution for the novel receiver design needed to heat supercritical CO2 from 550℃ to 720℃ with a maximum permissible pressure drop of 4 bar. The central receiver's surface temperature distribution is assessed to highlight non-uniformity in surface and fluid temperature distribution due to the flux non-uniformity. Thereafter, a novel receiver concept of using variable height pin arrays: VPH-AM2PAR is explored. In such a receiver, the modules will have varied pin heights in accordance with the respective flux on the module. It is shown that the VPH- AM2PAR is effective in reducing the peak surface temperature on the receiver. The study addresses the thermofluidic operating adjustments required and the total thermal power generated to ensure the creep life of the receiver meets a 30 year lifetime requirement.