Entropy Optimization of an Additively Manufactured Heat Exchanger with a Dual Stage Gifford-McMahon Cryogenic Refrigerator for Hydrogen Liquefaction

摘要

Small-scale hydrogen liquefaction presents an opportunity to drive down market costs by eliminating the cost associated with transportation. With the decline of fossil fuels and other nonrenewable energy sources, billions of dollars are being invested in large-scale hydrogen liquefaction. Large-scale systems have higher efficiencies, but limit accessibility. Little research has been done to optimize small-scale systems, and the presented options are often not scalable beyond a laboratory setting. The most inefficient system components are the heat exchanger, the nitrogen refrigerator, and the cycle compressor. In this thesis a novel, scalable, heat exchanger design based on principles of entropy minimization is presented. The design utilizes a branching structure with exponentially varying wall thickness and mounts onto a Gifford-McMahon cryogenic refrigerator. Numerical optimization indicates an efficiency increase of 39.21% when compared to a single tube design. The thermal mass is decreased by 43.82% and the length of the optimized design is only 8.61% of that of the single tube design. The heat exchanger is additively manufactured with an aluminum alloy and the interior is coated with a ruthenium-based catalyst to facilitate the ortho-parahydrogen conversion. Hydrogen enters the heat exchanger at 293 K and 653 kPa. It is cooled to 28.1 K and full ortho-parahydrogen conversion is assumed prior to reaching the storage dewar. The experimental measured rate of liquefaction is determined to be 0.003535 g/s, the upper flange of the heat exchanger resides at 58.0523 K, and the lower flange resides at 26.1631 K. The temperature difference between the lower flange of the heat exchanger and outlet fluid flow is less than 0.5 K. The mass flow rate and operating temperatures are lower than predicted. This is likely due to poor thermal properties of the heat exchanger material and poor thermal contact between the heat exchanger and the cryogenic refrigerator. However, the small temperature difference suggests small temperature gradients within the system, indicating minimum entropy generation. Suggestions are given to improve system performance.