Design of hydrogen production systems powered by solar and wind energy: An insight into the optimal size ratios

Green hydrogen is expected to play a crucial role in the future energy landscape, particularly in the pursuit of deep decarbonisation strategies within hard-to-abate sectors, such as the chemical and steel industries and heavy-duty transport. However, competitive production costs are vital to unlock the full potential of green hydrogen. In the case of green hydrogen produced via water electrolysis powered by fluctuating renewable energy sources, the design of the plant plays a pivotal role in achieving market-competitive production costs. The present work investigates the optimal design of power-to-hydrogen systems powered by renewable sources (solar and wind energy). A detailed model of a power-to-hydrogen system is developed: an energy simulation framework, coupled with an economic assessment, provides the hydrogen production cost as a function of the component sizes. By spanning a wide range of size ratios, namely the ratio between the size of the renewable generator and the size of the electrolyser, the cost-optimal design point (minimum hydrogen production cost) is identified. This investigation is carried out for three plant configurations: solar-only, wind-only and hybrid. The objective is to extend beyond the analysis of a specific case study and provide broadly applicable considerations for the optimal design of green hydrogen production systems. In particular, the rationale behind the cost-optimal size ratio is unveiled and discussed through energy (utilisation factors) and economic (hydrogen production cost) indicators. A sensitivity analysis on investment costs for the power-to-hydrogen technologies is also conducted to explore various technological learning paths from today to 2050. The optimal size ratio is found to be a trade-off between the utilisation factors of the electrolyser and the renewable generator, which exhibit opposite trends. Moreover, the costs of the power-to-hydrogen technologies are a key factor in determining the optimal size ratio: depending on these costs, the optimal solution tends to improve one of the two utilization factors at the expense of the other. Finally, the optimal size ratio is foreseen to decrease in the upcoming years, primarily due to the reduction in the investment cost of the electrolyser.