Dynamic Modeling of Hydrogen Systems for Grid Operations: PEM Electrolyzer

Abstract

While solar and wind as sources of energy have always had no cost, in the past the cost of the energy conversion technology was prohibitive. Today, due to improved efficiency in the technology and substantial increases in manufacturing volume the cost to generate electricity from the sun and wind and the cost to store this electricity in lithium-ion batteries (4 hours) has made these technologies more than competitive with traditional sources. As higher renewable energy penetrations occur the variability and intermittent nature of solar photovoltaic (PV) electricity can cause ramping issues, so longer term energy storage (days, weeks, instead of hours) will be needed to increase the reliability of grid operation. A PEM electrolyzer can serve as a utility controllable load that can be available at all times and the stored hydrogen can be converted back into electricity directly through a PEM fuel cell. The hydrogen can also be mixed with natural gas to produce electricity through a gas turbine, used for transportation fuel and/or used as a chemical feedstock. The limit on energy production of a PEM fuel cell is a function of the volume of the tank. To understand the transient response of an electrolyzer under varying PV input, a thermal electrochemical dynamic model has been developed.We developed one-dimensional ("through-plane") and two-dimensional ("through-plane" and "in-plane") models. Firstly, a steady state model for a single cell was developed. The model considers mass, energy, momentum and current balance equations. Maxwell-Stefan diffusion equation is used to calculate the gas-phase species molar concentration in backing and catalyst layers. The electrochemical reactions are modeled using Butler-Volmer equation with varying surface molar concentrations of components. In addition, the thermal-electrochemical model considers the heat transfer through the backing layer, plate and the flow. Many of the kinetic parameters in the models were determined using parameter estimation with the data of single cell steady state polarization curves. These single cell steady state models were extended to a 510 kW electrolyzer stack model by adding cells in series and parallel to achieve a hydrogen flowrate of 90 Nm3/hr. To study the transient response of the electrolyzer stack a dynamic model was developed using a real-time PV input data. Results of the dynamic electrolyzer stack model under various changes with respect to the PV input will be presented. Monte-Carlo method was used to study the sensitivity of the electrolyzer design for given PV inputs. The available PV input data is scaled to meet the power requirement of a maximum of 510 kW and to meet the required hydrogen output, all while smoothing the PV output to the grid. The developed novel electrochemical dynamic model with varying PV input data shows the effectiveness of the electrolyzer in increasing the grid stability and flexibility.