• Energy Research

Power-to-methane (PtM) is a prospective solution to the mismatching between the supply and consumption of renewable energy resources (RES) by converting renewable power into methane. However, the continuous fluctuation of RES causes the PtM system to deviate from the design condition in the vast majority of cases, and thus it is significantly vital to study the operating characteristics of the PtM system under off-design conditions. This paper proposes a comprehensive investigation framework from design to off-design steps for the performance improvement of a PtM system combining solid oxide electrolysis cell with methanation reactor, and solar energy is selected as renewable energy input. Firstly, the system with the total exergy efficiency (ηEX,tot) of 11.83% and levelized cost of exergy (LCOE) of 150.76 $/MWh is selected as the optimal design condition based on the homogeneous assessment from both thermodynamic and economic aspects, by means of Non-dominated sorting genetic algorithm-II. Then, based on the optimal design point, the off-design performances are quantitatively investigated under varying solar radiation and key operating parameters, in terms of synthetic natural gas (SNG) yield and ηEX,tot. The results indicate that with the increment in solar radiation, the SNG yield rises, while the ηEX,tot increases first and then decreases. Finally, the multi-objective optimization based on the Artificial Neural Network models is implemented for the system under off-design conditions to acquire the best trade-off between hourly SNG yield and ηEX,tot. The off-design optimization solutions reveal that the hourly optimal SNG yield is located in the range of 275.06–946.53 kW, achieving a total annual SNG yield of 1697 MWh/y, and the hourly optimal ηEX,tot mainly varies in the range of 10.40–11.40%.

The advantages of compressed air energy storage (CAES) have been demonstrated by the trigeneration system with the characteristic of high penetration of renewable energy. However, since the irreversible loss of compression heat occurs during the overall operation processes of CAES, the development of CAES with high energy efficiency has been hindered by the conventional conversion pathway of compression heat. Therefore, a trigeneration system integrated with compressed air and chemical energy storage is proposed in this study to improve energy utilization efficiency. The compression heat is converted into H2 and CO via the endothermic methanol decomposition reaction to improve its energy level during the charging process, and then the syngas production can be used for air preheating during the discharging process. The parametric analysis is first performed to investigate the technical and economic feasibility of the system. Subsequently, the multi-objective optimization is conducted to identify the tradeoffs in the thermo-economic performance of the system and acquire the optimal values of operating parameters. Notably, the proposed system with a computed exergy efficiency of 43.31% and levelized cost of energy (LCOE) of 97.53 $/MWh is selected as the most compromise solution by the decision maker of Technique for Order Preference by Similarity to an Ideal Solution among the Pareto optimum fronts, which are 8.47% higher than the exergy efficiency and 7.39 $/MWh lower than the LCOE under the design conditions.