(393h) Toward Sustainable Liquefied Green Hydrogen Supply Chain: An Optimal Design for Cold Energy Recovery in Liquid Hydrogen Regasification

Hydrogen is being evaluated as a critical resource for global decarbonization because it does not emit greenhouse gases during its use, and can replace fossil fuels in carbon-intensive industries.¹ To meet the growing demand for hydrogen, importing green hydrogen from countries with abundant renewable resources can be the feasible strategy for the countries with poor renewable resources.² Liquid hydrogen is considered one of the primary hydrogen carriers for the following reasons: it is approximately 800 times smaller than that of the same mass of hydrogen in gas form and can be converted to hydrogen by regasification without needing dehydrogenation or separation steps.³ However, since the regasification process of liquid hydrogen operates at much lower temperature conditions than liquefied natural gas, a significant amount of cold energy is dissipated. Therefore, it is crucial to recover the cold energy to improve the value of liquid hydrogen and achieve sustainable development in the liquefied green hydrogen supply chain.

The main purpose of this study is to investigate various options for cold energy recovery of liquid hydrogen and find the optimal design in the thermodynamic and economic aspects. This study proposes and optimizes 3 types of dual-stage power generation configurations with various working fluids: (i) series Brayton and Rankine cycle (SBR), (ii) parallel Brayton and Rankine cycle (PBR), and (iii) integrated Brayton and Rankine cycle (IBR). The process model is constructed using Aspen HYSYS V14 and optimized via python using a Bayesian optimization algorithm. The objective function is set to maximize the net power output in liquid hydrogen regasification. To evaluate environmental effect, the recovered cold energy is calculated as carbon credit which can reduce carbon emission from existing fossil fuel-based power plants. An economic evaluation is performed for each process in consideration of the carbon tax credits, benefits from low carbonization.

The liquid hydrogen regasification is operated at 300 tons per day. The Brayton cycle uses helium, and the Rankine cycle uses R1150, R23, R1270, and R134a as working fluids. The optimal operating conditions of each process for different fluids are explored, and the optimal working fluid is selected.

• • The SBR process has the highest capacity for recovering cold energy from hydrogen by placing the Brayton cycle and the Rankine cycle in series. However, a considerable amount of cold energy is discarded from the helium in the Brayton cycle.
  • The PBR process generates additional power and has the lowest investment cost since the compressed helium exchanges heat with the working fluid of the Rankine cycle. However, it has the lowest capacity for recovering cold energy from hydrogen.
  • The IBR process, combines the characteristics of the SBR and PBR processes, has the largest net power output among them. However, the large flow rate of the working fluid in the Rankine cycle results in the highest investment cost. Nevertheless, considering the benefits of selling electricity and securing carbon credits, the results indicates that the IBR process is the best option economically.

Sensitivity analysis result indicates that various factors, such as discount rates, electricity prices, tax rates, maintenance costs, and carbon taxes, impact the economic feasibility of each process. Specifically, a 40% decrease in the selling price of electricity renders the SBR process economically infeasible, while the PBR process becomes the most economical option. Furthermore, the proposed process can reduce carbon dioxide emissions by over 13.07 kilotons per year in 500MW coal-fired power plants.⁴

This study proposes solutions that can offset the relatively high cost of liquefying hydrogen, as well as enhance the sustainability of the value chain of liquefied green hydrogen. These proposed solutions are economically feasible and have a positive environmental impact by reducing carbon emissions from existing fossil fuel-based power plants. Overall, this study contributes to the movement towards a green hydrogen society. Additionally, expanding the scope of the study to include other cryogenic systems such as liquid air energy storage, can lead to further benefits.

REFERENCES

[1] International Energy Agency, “Energy Technology Perspectives 2020”,
https://www.iea.org/reports/energy-technology-perspectives-2020, (accessed on Feb. 2023).

[2] International Energy Agency, “The Future of Hydrogen – Seizing today’s opportunities”,
https://www.iea.org/corrections, (accessed on Mar. 2023).

[3] Clifford chance, “Focus on hydrogen: Japan’s energy strategy for hydrogen and ammonia”,
https://www.cliffordchance.com/expertise/services/esg/sustainability-and...,(accessed on Mar. 2023).

[4] Jeong, S., et al., “Economic comparison between coal-fired and liquefied natural gas combined cycle power plants considering carbon tax: Korean case,” Energy., 33 (8), 1320-1330 (2008).