Øystein Ulleberg

Modeling of advanced alkaline electrolyzers: a system simulation approach

A mathematical model for an advanced alkaline electrolyzer has been developed. The model is based on a combination of fundamental thermodynamics, heat transfer theory, and empirical electrochemical relationships. A dynamic thermal model has also been developed. Comparisons between predicted and measured data show that the model can be used to predict the cell voltage, hydrogen production, efficiencies, and operating temperature. The reference system used was the stand-alone photovoltaic-hydrogen energy plant in Julich. The number of required parameters has been reduced to a minimum to make the model suitable for use in integrated hydrogen energy system simulations. The model has been made compatible to a transient system simulation program, which makes it possible to integrate hydrogen energy component models with a standard library of thermal and electrical renewable energy components. Hence, the model can be particularly useful for (1) system design (or redesign) and (2) optimization of control strategies. To illustrate the applicability of the model, a 1-year simulation of a photovoltaic-hydrogen system was performed. The results show that improved electrolyzer operating strategies can be identified with the developed system simulation model.

Gas Purge for Switching from Electrolysis to Fuel Cell Operation in Polymer Electrolyte Unitized Reversible Fuel Cells

Unitized reversible fuel cells (URFCs) combine the functionality of a fuel cell (FC) and of an electrolyzer (Ely) in a single device. In polymer electrolyte URFCs, gas purging is crucial in switching from the Ely to the FC operation. A gas purge model was developed here to clarify the relationship between the purge time and the gas flow rate. The model considers evaporation and diffusion of liquid water in the gas diffusion layer and membrane. To validate our model, we compared model predictions of high frequency resistance vs purge time with experimental results for two URFCs of different sizes at different purge flow rates. The simulated curves agree well with the experimental curves.

A simplified battery charge controller for safety and increased utilization in standalone PV applications

This paper presents a new solar/battery charge controller that combines both MPPT and over-voltage controls as single control function. A small-signal model of lead acid battery is derived in detail to design the employed dual-loop control configuration. Two case studies are then conducted, in SIMULINK/SIMPOWER, first to evaluate the performance of the designed controller in terms of transient response and voltage overshoot. Secondly, realistic irradiance data is used to evaluate the performance of the developed charge controller in terms of parameters such as PV energy utilization factor and over-voltage compared to the conventional hysteretic on/of controller. The designed controller is shown to have good transient response with only small voltage overshoot. It is also found that the developed charge controller fares better in terms PV energy utilization and shows at least the same level of over-voltage control.

A combined steady state and dynamic model of a proton exchange membrane fuel cell for use in DG system simulation

This paper proposes a combined steady state (SS) and dynamic model of a proton exchange membrane fuel cell (PEMFC) for use in distributed generation (DG) systems simulation. The SS fuel cell parameters associated with ohmic loss, activation overpotential and concentration over potential which are not easily accessible are estimated using a non-linear fitting of measured V-I fuel cell characteristics. Effect of fuel and air flow dynamics, electrical delay, and delay associated with fuel cell current reference feed back loop on the dynamics of the Nernst potential is modelled. The SS and dynamic models of the FC are then combined to establish a SIMULINK/SIMPOWER terminal model that can easily be interfaced in DG systems simulations. The model SS response is verified using voltage measurements taken by slowly varying the load (current). Simulation results, using SIMULINK/SIMPOWER, are then presented to demonstrate that the model works as desired both for SS and dynamic loads. To demonstrate its applicability in DG Systems simulations, the model is used in long time simulation of an example grid connected FC/Battery Hybrid power system.

Providing Microgrid Resilience during Emergencies Using Distributed Energy Resources

Several incidents reported in the past have shown the inability of the existing power grid to provide reliable services during system failures. Moreover, the communication and control network in the smart grid inherently creates opportunities for the adversaries to launch cyber attacks to the system. Natural disasters may further exacerbate the challenge. Designing resilient (if not robust) solutions for the smart grid therefore, was, and remains a high priority. To this end, we present three solutions towards resilience of a microgrid during emergencies. First, we propose the use of electric vehicles (EVs) as temporary power supplies to support critical infrastructure during emergencies. Second, we recommend to use a combination of distributed renewable energy sources, EVs and a community-level storage unit to further enhance the resilience of the microgrid, by utilizing the locally available renewable energy options, exploiting the electric vehicles moderately, and by investing reasonably on the storage unit. Third, we introduce the software defined networking paradigm as a highly relevant platform for both power virtualization and network function virtualization, to support, coordinate and control the dynamic operation of the virtual power plants for reliable power supply and resilient operations of the microgrid in the disaster mode. Finally, we discuss the feasibility of each of these solutions for implementing them in practice.