## 1 Introduction
The state of the art Ni-based catalyst is the preferred active catalyst for methanation because of its low cost, highest activity, and highest selectivity for methane formation [4; 5; 6; 8; 9]. CO\({}_{2}\) methanation can demonstrate almost 100% methane selectivity when Ni-based catalysts are utilized. Al\({}_{2}\)O\({}_{3}\), CeO\({}_{2}\), TiO\({}_{2}\), MgO, SiO\({}_{2}\), zeolite, and ZrO\({}_{2}\) are the most preferred Ni-based catalysts used for methanation [5; 8; 10; 11; 12]. Ni-ZrO\({}_{2}\) is an active Ni-based catalyst for CO\({}_{2}\) methanation as a result of enhanced Ni dispersion resulting from the presence of Zr. In all cases, a catalyst is required that is capable of increasing reaction rates within the temperature range of 25 to 500 \({}^{\circ}\)C and when the feed gas is composed of a 1:4 ratio of CO\({}_{2}\) and H\({}_{2}\) gas [13]. Figure 1: Schematic illustration of the fundamental solar to methane (StM) concept of our system. To avoid a thermodynamic barrier of the reaction and catalyst degradation, temperature control is required as a result of the extreme exothermic behavior of the reaction [14]. Thus, reactor temperature control is required to prevent thermodynamic limitation and catalyst sintering. A higher conversion rate to methane results from such control. An adiabatic, fixed-bed reactor in which the reactor is packed with some catalysts is the most common reactor used for catalytic methanation systems. Catalytic methanation is possible at low temperatures only with Ni-based catalysts. Low methane generation and catalyst sintering can occur at higher temperatures, whereas high pressure favors the catalytic methanation [15]. An instantaneous solar to hydrogen (STH) conversion efficiency of 24.4% has been achieved previously. Such conversion was carried out on CPV cells connected with EC cells. The system set the world efficiency record in outdoor test operations at the University of Miyazaki, Japan [16]. Subsequently, a highly efficient solar to hydrogen conversion system with a DC/DC converter was introduced. The resulting system had improved flexibility for variable radiation conditions [17]. We expanded the system from CPV cells to CPV modules and performed the operation outdoors at the University of Miyazaki, Japan. The system achieved highly efficient solar to hydrogen conversion under variable climatic conditions. The sunlight was collected using the highly efficient CPV modules with InGaP/InGaAs/Ge triple junction solar cells operating with DC/DC converters. The latter was operated as per the maximum power point tracking (MPPT) method. The power to gas (P2G) conversion system was analyzed using the PEM electrolysis method for solar to hydrogen conversion. The highest one-day STH efficiency of 18.78% for a subkilowatt-scale photovoltaic (PV) system using highly efficient CPV modules was successfully obtained under outdoor operation [18]. In this study, we discuss the outdoor operation of a solar to methane system. The electrical energy was supplied to EC cells and collected by highly efficient CPV modules under outdoor operation, and H\({}_{2}\) generated from solar energy was utilized in this methanation system. The reaction was performed within the reactors set up in the outdoor condition. Then, CH\({}_{4}\) was formed by reacting CO\({}_{2}\) and H\({}_{2}\) gases. Therefore, the solar-based methane could be utilized in the current infrastructure.