Cryogenic turbine design for a novel hydrogen liquefaction expansion

Abstract

In order to meet the rise in hydrogen demand forecast for 2050, hydrogen liquefaction plants are expected to increase in scale by an order of magnitude to 100 tonnes per day (tpd) of liquefied hydrogen. This gives an opportunity to move from a CAPEX-dominated design to an OPEX-dominated design, with the introduction of more complex liquefaction cycle configurations and more efficient components, which could lead to a doubling of the plant efficiency by 2050. One such modification is to improve the performance of the final hydrogen expansion of the cycle, which is the subject of this thesis. The thesis is divided into four parts, the first being a techno-economic analysis to quantify the need for hydrogen liquefaction plants in Europe by 2050. A hydrogen and liquefaction infrastructure demand analysis was performed using energy system modelling software and the Heat RoadMap Europe scenario data. It is shown that 300 × 100 tpd hydrogen liquefaction plants would be required in Europe by 2050, of which 48 would be used to meet the demand of the UK. The second part of the thesis quantifies the cycle benefit of different final hydrogen expansion configurations by using a thermodynamic cycle analysis. Two novel configurations for the final hydrogen expansion, employing turboexpanders, are compared to the baseline Joule–Thomson (J–T) valve only configuration (Case A). The first alternative configuration (Case B) employs a fully wet turboexpander and the second (Case C) proposes two components in series: first a turboexpander in the sub-cooled liquid region (single-phase), followed by a J–T valve in the two-phase region. It is shown that Case B and Case C improve on Case A in both liquid yield and exergetic efficiency, with Case C demonstrating the highest improvement in exergetic efficiency. Case C is therefore selected for further study. The final two parts describe the development of a viable cryogenic turbine design for the configuration proposed (Case C), for a radial and an axial turbine respectively. A radial turbine design methodology for a sub-cooled liquid hydrogen turbine has been developed. The use of conventional turbocharger non-dimensional parameters and mean-line design, coupled with real gas CFD using the TURBIGEN open-source design code, has been demonstrated. A Baseline radial turbine with a predicted isentropic efficiency of 94.3% has been designed. After modifications for stress and manufacturing constraints, the Final design was found to have a predicted efficiency of 93.9%. The off-design study showed that the Final design turbine would be able to operate in a plant with hydrogen mass flow rate varying between 85 and 105 tonnes per day. If the Final turbine design were to be used in the novel hydrogen expansion configuration, cycle analysis shows that the exergetic efficiency of the hydrogen liquefaction cycle would increase by 3.4 percentage points and the liquid yield by 10.7 percentage points. Partial admission operation would be required in the case of a variable supply of hydrogen in the liquefaction plant. An axial turbine for the Turboexpander + J–T valve configuration has been designed using the same design system as for the radial turbine, achieving an efficiency of 92.9%. Partial admission was simulated using unsteady RANS on the axial turbine geometry. The study revealed that there is an 18 percentage points decrease in isentropic efficiency when going from full admission to half admission area, but that the use of this turbine in the liquefaction cycle is still beneficial to the overall cycle compared to the single J–T valve. When the turbine operates in partial admission (50% inlet area), the exergetic efficiency of the hydrogen liquefaction cycle increases by 2.3 percentage points and the liquid yield by 8.6 percentage points. This shows that partial admission offers a promising solution to increase the mass flow rate operating range of the turbine, such that it can be used even in flexible hydrogen supply conditions.