**1. Introduction**

Hydrogen used as a fuel, as an energy source for industrial processes or for generating electrical power can provide an important route to the decarbonization of energy supplies and the integration of renewable energy systems. The study of Acar and Dincer [1], for example, identifies that hydrogen can play "eight significant roles" in the green energy transition. Recent studies have also made the case that achieving a transition to carbon-free energy in the EU is impossible without a large increase in hydrogen production [2], and energy system modeling has found that "hydrogen and synfuels add up to between 20% and 50% of [EU] energy demand in transport in 2050" [3]. This positive view of the role that hydrogen could play in future low-carbon development is also reflected in political intent via the EU hydrogen strategy [4].

Reflecting this political support, research related to hydrogen energy has increased over recent years [5]. Important research topics include energy demand and supply modelling [2,3], the novel integration of renewable energy sources such as solar power [6], the development of enhanced electrolysis based production methods [7], the development of new applications such as use a reductant in steel manufacturing [8] and the assessment of alternative sources such as methanol [9].

In all envisaged future hydrogen-based economies, a significant increase in the transportation capacity for hydrogen is required. It is possible to transport hydrogen as a compressed gas or as a liquid at low temperature, in pipelines as a gas. The optimum transportation strategy will depending on both transportation capacity and the distance [10]. When the distance is significant it is reasonable to expect that shipping of hydrogen will

**Citation:** Jackson, S.; Brodal, E. Optimization of a Mixed Refrigerant Based H2 Liquefaction Pre-Cooling Process and Estimate of Liquefaction Performance with Varying Ambient Temperature. *Energies* **2021**, *14*, 6090. https://doi.org/10.3390/en14196090

Academic Editor: Bahman Shabani

Received: 2 September 2021 Accepted: 21 September 2021 Published: 24 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

be favored, and while researchers such as Ishimoto et al. [11] have studied the economics of shipping liquefied hydrogen, some commercial steps have also been made, with the world's first liquid hydrogen carrier ship launched in Japan in 2019 [12].

If hydrogen is transported at large-scale as a liquid, a key part of the supply chain will be the liquefaction process, which is very energy intensive. The specific energy consumption (SEC) of the most efficient currently operating large-scale hydrogen liquefaction (LHL) plants lies in the range 13 to 15 kWh/kg [13], which is much higher that even the most efficient LNG processes, which have a SEC of around 240 kWh/tonne. Because of this, there is significant interest in the development of new and improved LHL technologies that can help reduced SEC.

Research topics relating to improved LHL technologies include the integration of renewable energy sources, such as solar energy [14] and geothermal energy [15]; the use of mixed refrigerants (MR) for pre-cooling [16–18]; and the use of helium in the cryogenic cooling and liquefaction part of the process [19,20]. Other research has focused on the impact of the conversion of ortho-hydrogen to para-hydrogen on the liquefaction process [21,22] and the relative performance of different heat exchanger types [17,22,23]. The suggested efficiency of the proposed concepts for LHL studied lie in the range 5 to 8 kWh/kg [13], which represents a substantial motivation for the implementation of these technologies in the next generation of LHL plants.

The proposed use of a MR in the pre-cooling part of LHL processes represents a close parallel to the use of MR in the design of some of the largest and most efficient natural gas liquefaction processes and because of this, represents one of the most promising near-future improvements to LHL design. LNG plants based on the use of MRs include the Snøhvit plant located at Melkøya in northern Norway, which uses a cascade of three MR loops and is claimed to be the most efficient LNG plant in the world [24]. While the efficiency achieved by the Snøhvit LNG plant is due, in part, to its advanced design, the plant also benefits from its cold-climate location and subsequent access to a lower temperature heat sink than most other LNG plants.

The study of Rian and Ertesvåg [25] looked at the impact of ambient temperature on the Snøhvit LNG plant, finding that a reduction in the available heat sink temperature from 20 to 4 ◦C gives a reduction in exergy destruction of 10.9%. A small number of other studies have also considered the impact of ambient temperature on the performance of other types of LNG process [26–29] providing similar results. The study of Park et al., for example, finds that specific power consumption of single MR process increases by between 16% and 42% over the temperature range 10 to 25 ◦C, varying with the approach used in process optimization. This significance of this variation in energy consumption with ambient temperature is not only relevant to the design of LNG plant itself, it is large enough to affect the whole energy supply chain. For example, the study of Jackson et al. [29] finds that the CO2 emissions for a power plant supplied by gas from an LNG plant located in northern Norway will be between 0.8 and 1.3% lower than if it were supplied by the same design of LNG plant located in the Middle East. It is therefore logical to expect that the performance of LHL plants using MR pre-cooling and the performance of energy supply chains based on LHL will be significantly affected by the ambient temperature at the liquefaction plant location.

Given the close parallel between MR based LNG processes and MR pre-cooled LHL processes and given the demonstrated impact of ambient temperature the performance of LNG processes, ambient temperature can be expected to have a significant impact on the performance of the type of LHL processes likely to be used in the near future. Although several studies have been made into the performance of MR pre-cooled type LHL process and studies have looked at the impact of ambient temperature on LNG process performance, no studies quantifying this impact of ambient temperature on LHL processes are currently found in the literature. The aim of this study is, therefore, to generate a set of data illustrating the impact of ambient temperature on the performance of MR pre-cooled type LHL process.
