Analysis of 5 MW hydrogen power system with thermal energy storage

Abstract

Energy storage for further energy production has become a feasible option to deal with energy fluctuation, energy over production and energy shortcomings caused by the penetration of renewable energies. Hydrogen storage has been studied through mathematical model and simulation to predict its performance and technological feasibility. This thesis presents a model where a 5 MW electrolysis plant is simulated. The power plant consists on an electric input from renewable sources like wind turbines or photovoltaic panels. Electrolysis is done by a solid oxide cell that also produces electric power working as fuel cell. Thermal energy storage is added in order to recover heat released by the cell. The main objective of the present work is to analyse the advantages of implementing thermal energy storage in order to store heat released by the fuel cell, determine the best configurations of the system to achieve high efficiencies and identify those parameter that contribute to significant losses. In general, the model shows an efficiency value between 0.54 and 0.84 against 0.28 and 0.44 in similar models. Electrolysis process is validated with high temperature electrolysis models, which consider solid oxide cells as the electrolyser with heat recovery systems. Power generation process is validated against solid oxide fuel cell models, which use the heat produced by the fuel cell in different applications. Using phase change materials (PCM) as thermal energy storage (TES) can increase the round cycle efficiency of the system from 0.44 without TES up to 84% with the application of TES at high and low temperatures. Efficiencies can increase up to 10% when liquid water is pressurized at the initial stage instead of compressing hydrogen at the final stage. Periods of operation are another parameters that could be modified in order to raise the efficiency. The same system working 12 hours as electrolysis at 1.2 V and 12 h as fuel cell has a power ratio of 0.6886, whereas working 5 hours as electrolysis at 1.2 V and 19 h as fuel cell has a power ratio of 0.7838, showing better heat management. Effective utilization of by-product oxygen is an added value to the system. Energy savings around 70% are achieved respect common technologies of oxygen production, which could justify a new cell design in order to keep oxygen purity.