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Open Access Publications from the University of California

Scholarly Works (2 results)

  • Thesis
  • Peer Reviewed
UC Davis Electronic Theses and Dissertations (2024)

Design, fabrication and experimental characterization of the performance of a molten salt (MS)-to-supercritical carbon dioxide (sCO2) heat exchanger (HE) for concentrating solar power (CSP) applications using laser powder bed fusion (L-PBF) additive manufacturing (AM) of nickel-based superalloys is presented in this work. The hypothesis is that such additively manufactured HEs will result in a monolithic, compact, low-pressure drop HE that is durable under cyclic operation at high temperature and high pressure in a corrosive salt environment, making them suitable to withstand the stringent operating conditions of the US Department of Energy’s Generation 3 technologies roadmap for concentrating solar power systems.The primary heat exchanger (PHE) is designed to handle temperatures up to 720 oC on the MS side and an internal pressure of 200 bar on the sCO2 side. Two designs (generation 1 and 2) of the heat exchanger were developed by a co-design effort considering mechanical stresses, thermal and fluidic performance, and AM fabrication constraints such as overhangs, powder removal, and build dimensions. Additional design considerations included creep, fatigue, and thermal stresses. Two Nickel-based superalloys were originally considered to manufacture the PHE- Haynes 230 (H230: 57% Nickel, 22% Chromium, 14% Tungsten, 2% Molybdenum) and Haynes 282 (H282: 57% Nickel, 20% Chromium, 10% Cobalt, 8.5% Molybdenum, 2% Titanium). Ultimately, Haynes 282 was down selected for the HE fabrication based on its better dimensional tolerance, better corrosion resistance to chloride salt, and higher creep strength. An experimentally-validated, correlation-based sectional PHE core thermofluidic model is developed to study the impact of flow and geometrical parameters on the PHE performance, with varied parameters including the mass flow rate, surface roughness, and PHE dimensions. The model results show that a heat exchanger with a power density of 16 MW/m3 (including sCO2 header volume) and effectiveness of 0.93 can be achieved at a heat capacity rate ratio of 0.83. To experimentally demonstrate the performance of the HE, a 20 pair unit was built and characterized using heated air as a surrogate to the chloride salt on the hot side and sCO2 in the 100-200 bar range was used on the cold side. The results of exit temperatures and heat transfer rate were compared against the model. The model was able to predict experimental results within 1.00% percent for heat transfer rate, 1.26% percent for exit sCO2 temperature, and 1.01% percent for exit air temperature. The validated HE model was used to determine the performance of a scaled-up design. With a 20 cm tall build that covers 24 cm x 24 cm of the build plate, a 167.5 kW HE can be built in a single laser machine with a bed size of 25 cm x 25 cm. The impact of system-level variations in inlet temperature on the performance of this HE was established.

    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2022)
    The design and techno-economic performance of a compact additively manufactured (AM) molten salt (MS)-to-supercritical carbon di-oxide (sCO2) primary heat exchanger (PHE) for solar thermal application is described. The PHE design consists of sCO2 flow through an array of microscale pin fins while the MS flows through mm-scale rectangular channels. Constraints imposed by AM using laser powder bed fusion method are considered in the design. Structural and fluid flow simulations are performed to arrive at a viable design of the core and headers. A simplified one-dimensional steady state model for the PHE is developed including the impact of surface roughness from the AM process. A process-based cost model is used to determine the tradeoff between thermofluidic design and manufacturing cost. A parametric study is performed using the thermo-fluidic and cost models to determine the set of geometrical and flow variables that result in high power density and low cost, while restricting the pressure drop on the sCO2 side to less than 2% of line pressure. Flow rates of MS and sCO2 were varied over heat capacity rate ratios ranging from 0.2 to 1. Results indicate that it is possible to design a low-pressure drop AM PHE with an effectiveness of 90% and a power density in excess of 10 MW/m3 (including headers). Fabrication of representative nickel superalloy specimens are shown to demonstrate that low-porosity parts with the requisite dimensional tolerance of PHE core can be generated.
      Cover page: An additively-manufactured molten salt-to-supercritical carbon di-oxide primary heat exchanger for solar thermal power generation – Design and techno-economic performanceCreative Commons 'BY-NC-SA' version 4.0 license
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