UC Davis

With a goal to increase the percentage of grid energy produced by renewable sources, research and development efforts in solar, wind, and geothermal resources are progressing rapidly. In this work, the use of concentrated solar thermal power for energy production and thermal energy storage is presented for the supercritical carbon dioxide (sCO2) Brayton cycle. The use of sCO2 as the working fluid for concentrated solar thermal power generation allows for greater power output potential at a smaller footprint compared to steam power cycles. The present work is targeted at the design of next generation solar thermal receivers for such power cycles. The target design conditions for an sCO2 receiver for next-generation utility scale power generation systems include a design pressure of 200 bar, a fluid outlet temperature of 720C, and an incident concentrated flux surface of 100 W/cm2. The receiver must operate at these conditions for a 30 year lifetime. To operate at the needed high temperature and pressure conditions for next-generation sCO2 power cycles, the central receiver of the solar power plant needs to be made from a high temperature and high pressure materials such as nickel superalloys.

The manufacturing method of the solar thermal receiver becomes important as both pressure stresses and thermal stresses act on the internal features of the solar receiver. In prior work by the group, a microscale pin array receiver was developed and fabricated using microlamination methods. This fabrication method restricted the size of the unit cell of the pin array to 2.5 cm, requiring an elaborate and heavy header structure to route fluid in and out of multiple parallel unit cell pin array panels that comprise the central receiver. Headers were brazed to the diffusion-bonded core to bring fluid into and out of the unit cells. Several fabrication challenges were faced in the development of this design, including leaks in the brazed joints. Odele [30] showed, by numerical modeling, that by using metal Additive Manufacturing (AM), one could design longer unit cell pin arrays, that operate at similar efficiencies as the microlaminated design, resulting in a lighter header structure. Furthermore, the pin array core and headers could be built as a single panel monolithically, resulting in the elimination of braze joints.

In this thesis, detailed design of the header and its interface with the pin array core is developed. The design goals are to reduce weight of the microlamination-based headers, reduce stresses due to the high pressure and temperature, and design for maximal receiver life to 100,000 hours. The pin array core and the headers would constitute a unit cell panel. The panel would need to be designed to allow for placement of several panels in parallel without uncooled regions being directly exposed to the intense concentrated solar radiation in a power tower configuration.

The receiver pin array core makes use of correlations from previously validated experimental data. An iterative design campaign is performed using simulations in Ansys mechanical to meet the pressure stress design requirement of 150 MPa. After eliminating stress regions exceeding the design parameters, simulations were performed in Ansys Fluent to ensure that the flow from the headers to the pin array core was uniformly distributed and to confirm that the pressure drop was below 2% through the panel. The design was subsequently printed by the Rollett group at the NEXT Manufacturing Center at Carnegie Mellon University. The prototype was heat treated and pressure tested at operating temperatures at the UC Davis STEEL facility. Results of static and limited cyclic pressure testing at temperature indicate that a leak-free panel of the design can be fabricated using metal AM. Thermal stress simulations were performed on the receiver showing a few locations with high points. To mitigate these stresses, design changes were considered to insulate the outer edges and corners of the receiver from direct insolation.