Abstract

Coastal regions have abundant off-shore wind energy resources, and surrounding areas have large-scale liquefied natural gas (LNG) receiving stations. From the engineering perspectives, there are limitations in unstable off-shore wind energy and fluctuating LNG loads. This article offers a new energy scheme to combine these 2 energy units, which uses surplus wind energy to produce hydrogen, and use LNG cold energy to liquefy and store hydrogen. In addition, in order to improve the efficiency of utilizing LNG cold energy, and reduce electricity consumption for liquid hydrogen (LH2) production at coastal regions, this article introduces the liquid air energy storage (LAES) technology as the intermediate stage, which can stably store the cold energy from LNG gasification. A new scheme for LNG-LAES-LH2 hybrid LH2 production is built. The case study is based on a real LNG receiving station at Hainan province, China, and this article presents the design of hydrogen production/liquefaction process, and carries out the optimizations at key nodes, and proves the feasibility using specific energy consumption and exergy analysis. In a 100 MW system, the liquid air storage round-trip efficiency is 71.0% and the specific energy consumption is 0.189 kWh/kg, and the liquid hydrogen specific energy consumption is 7.87 kWh/kg and the exergy efficiency is 46.44%. Meanwhile, the corresponding techno-economic model is built, and for a LNG-LAES-LH2 system with LH2 daily production 140.4 tons, the shortest dynamic payback period is 9.56 years. Overall, this novel hybrid energy scheme can produce green hydrogen using a more efficient and economical method, and also can make full use of surplus off-shore wind energy and coastal LNG cold energy.

Introduction

As a clean fossil energy, natural gas has advantages of high combustion efficiency, less greenhouse gas emissions, and convenient transportation (Khan, 2018). The liquefied natural gas (LNG), generally has a temperature of 111 K (Kochunni and Chowdhury, 2020), and the volume of LNG with the same mass is only 1/625 of gaseous natural gas (Chen et al., 2021), which has much higher energy density and lower transportation cost during the process of trans-ocean and long-distance transportation (Peng et al., 2021).

In China, about 63.44 million tons of LNG were imported in 2022 (General Administration of Customs, People’s Republic of China, 2023), mainly stored in LNG terminals in coastal areas. Most LNG needs to be vaporized through pipelines to users. During the re-gasification process, the cold energy released from per unit mass of LNG is about 830 kJ/kg (Li et al., 2021). In general, LNG cold energy can be used for power generation (Ghorbani et al., 2023), dry ice production (Sung and Kim, 2017), desalination of sea water (Ghasemi et al., 2018), food processing (Cerceau et al., 2014), etc.

Conclusion

This paper proposes a novel scheme which uses surplus offshore wind power to produce hydrogen, and makes good use of LNG cold energy and liquid air technology to properly liquefy and store hydrogen. As a bridge between LNG and LH2, liquid air can modify the fluctuation of natural gas load, and also provide stable cold energy for LH2. Taking Hainan offshore wind power and Hainan LNG receiving station as the research objects, a new scheme of 100 MW hydrogen production and liquefication has been designed, and the system process has been optimized and evaluated. Finally, the hydrogen gas is able to be liquefied with the optimal liquefaction process. In addition, considering three electrolytic hydrogen production modes, the economic feasibility of the new scheme is verified using dynamic economic modeling. The following conclusions can be drawn: