## 1 Introduction  
Japan is facing a severe challenge regarding the heavy dependence on fossil fuels after the Great East Japan Earthquake [1]. Fossil fuels currently account for 89% of the total energy consumption in Japan, which approaches the level of the oil shock in 1973. To tackle this challenge, the government of Japan needs to develop and utilize renewable energy sources to increase its energy self-sufficiency rate and reduce the environmental impacts of the increased use of fossil fuels [2,3]. Since renewable energies are from the natural environment, they are all weather-dependent, which makes them vulnerable in developing a stable power system. As a solution, hybridization of multiple renewable energy resources (RER) will allow us to provide reliable, durable, and cheaper electricity from using renewable technologies compared to the individual power generators such as solar photovoltaics or wind turbine, continuously and without interruption. Furthermore, when the hybrid renewable energy system (HRES) is connected to a storage system such as battery or hydrogen storage, the excess energy generated by the renewable energies such as solar or wind can be stored and then utilized during the period when there is no sunshine and wind, wherein electricity is not being generated. Therefore, compared to a single renewable technology, the HRES works more efficiently in all weather conditions, indicating the pivotal role of the energy storage in this particular type of power system. Although battery holds the most promise storage technology, there are several drawbacks with its limited lifespan and round-trip efficiency, in addition to the negative environmental impacts through its incorrect disposal. Hydrogen may be considered as an alternative with much higher storage capacity compared to batteries, which can be re-electrified in the HRES configuration. Besides electricity generation, hydrogen can also be burned as a fuel to provide additional thermal load, which results in reducing the levelized cost of energy (LCOE) generated form the HRES. Depending on the design and configuration of a hydrogen-based HRES, manufacturers can produce hydrogen by using a variety of technologies such as solar electrolysis of water, thermochemical water splitting, gasification of biomass, or even metabolic techniques such as biochemical and fermentation processes. Among all technologies mentioned above, the conventional biomass gasification process is the most reasonable one for producing hydrogen in the HRES applications. However, to increase the hydrogen production efficiency, the pre-treatment and drying process should be considered in this method. One of the recent advancements in hydrogen production is the hydrothermal gasification of biomass feedstock, in which water is being used as the gasifying agent over its supercritical condition (374 \({}^{\circ}\)C and 22.1 MPa). The Supercritical Water Gasification (SCWG) has shown significant improvement in the overall yield of the gasification process and hydrogen production [4; 5; 6; 7; 8]. The generated syngas from the SCWG includes a mixture of carbon monoxide and methane, which can be burned in an auxiliary fired heater to provide the amount of thermal energy needed in securing the gasification temperature of 650 \({}^{\circ}\)C. The SCWG decomposes biomass feedstock entirely, without using any pre-treatment system for residue and wastewater [9]. Research that concentrates on the techno-economic analysis of the hydrogen-based HRES has drawn the attention of global scholars. The current and past investigations have demonstrated a variety of analytical methods used to address the optimal design and configuration of the hydrogen-based HRES. [PERSON] et al. discussed the economic analysis of the integration of a biomass fermentation with the HRES, consisting of the Photovoltaic (PV) module, wind turbine, electrolyzer, and fuel cell [10].