There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

A comprehensive review on PEM water electrolysis

Improving the sluggish kinetics for the electrochemical reduction of water to molecular hydrogen in alkaline environments is one key to reducing the high overpotentials and associated energy losses in water-alkali and chlor-alkali electrolyzers. We found that a controlled arrangement of nanometer-scale Ni(OH)(2) clusters on platinum electrode surfaces manifests a factor of 8 activity increase in catalyzing the hydrogen evolution reaction relative to state-of-the-art metal and metal-oxide catalysts. In a bifunctional effect, the edges of the Ni(OH)(2) clusters promoted the dissociation of water and the production of hydrogen intermediates that then adsorbed on the nearby Pt surfaces and recombined into molecular hydrogen. The generation of these hydrogen intermediates could be further enhanced via Li(+)-induced destabilization of the HO-H bond, resulting in a factor of 10 total increase in activity.

http://science.sciencemag.org/content/sci/334/6060/1256.full.pdf

Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces

http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

https://publikationen.bibliothek.kit.edu/1000050288/18451059

Renewable Power-to-Gas: A technological and economic review

This paper reviews water electrolysis technologies for hydrogen production and also surveys the state of the art of water electrolysis integration with renewable energies. First, attention is paid to the thermodynamic and electrochemical processes to better understand how electrolysis cells work and how they can be combined to build big electrolysis modules. The electrolysis process and the characteristics, advantages, drawbacks, and challenges of the three main existing electrolysis technologies, namely alkaline, polymer electrolyte membrane, and solid oxide electrolyte, are then discussed. Current manufacturers and the main features of commercially available electrolyzers are extensively reviewed. Finally, the possible configurations allowing the integration of water electrolysis units with renewable energy sources in both autonomous and grid-connected systems are presented and some relevant demonstration projects are commented.

Hydrogen Production From Water Electrolysis: Current Status and Future Trends

When no transformer is used in a grid-connected photovoltaic (PV) system, a galvanic connection between the grid and the PV array exists. In these conditions, dangerous leakage currents (common-mode currents) can appear through the stray capacitance between the PV array and the ground. In order to avoid these leakage currents, different inverter topologies that generate no varying common-mode voltages, such as the half-bridge and the bipolar pulsewidth modulation (PWM) full-bridge topologies, have been proposed. The need of a high-input voltage represents an important drawback of the half-bridge. The bipolar PWM full bridge requires a lower input voltage but exhibits a low efficiency. This letter proposes a new high-efficiency topology that generates no varying common-mode voltage and requires the same low-input voltage as the bipolar PWM full bridge. The proposed topology has been verified in a 5-kW prototype with satisfactory results

Transformerless Inverter for Single-Phase Photovoltaic Systems

The use of doubly fed induction generators (DFIGs) in large wind turbines has become quite common over the last few years. These machines provide variable speed and are driven with a power converter which is sized for a small percentage of the turbine-rated power. A drawback of the DFIG is that it is very sensitive to grid disturbances, especially to voltage dips. However, the operation of the machine in these situations has only been studied in the literature by means of simulations. This paper develops a theoretical analysis of the dynamic behavior of the induction machine during three-phase voltage dips. The proposed analysis contributes to understanding the causes of the problem and represents a very useful tool to improve the existing solutions and propose new alternatives. Experimental results are in good agreement with those obtained theoretically and validate the proposed analysis.

/pdf/dynamic-behavior-of-the-doubly-fed-induction-generator-1x3txmk57i.pdf

Dynamic Behavior of the Doubly Fed Induction Generator During Three-Phase Voltage Dips

Many large wind farms employ doubly fed induction generators (DFIGs). The main drawback of these machines is their large sensitivity to grid disturbances, especially to voltage dips. As the penetration of wind energy in the network increases, it becomes more relevant to understand the behavior of these machines under voltage dips. This paper analyzes the operation of the DFIG under asymmetrical voltage dips and explains why such kinds of dips are more harmful than are symmetrical dips. The influence of the dip type and instant of occurrence is also studied.

Wind Turbines Based on Doubly Fed Induction Generator Under Asymmetrical Voltage Dips

The relative weight of the energy generated by means of renewable sources is constantly increasing. Among all these sources, the photovoltaic (PV) systems present the higher and more stable relative growth. However, the PV system is still too expensive and a significant effort is being done to increase the efficiency and reduce the cost. Concerning the PV inverters, this has lead to the elimination of the low frequency (LF) transformer that has been traditionally included. The LF transformer provides isolation from the grid but reduces the PV inverter efficiency and increases its size and cost. However, the elimination of the transformer might generate strong ground currents, which become now an important design parameter for the PV inverter. The ground currents are a function of the system stray elements. However, there is no simple model and procedure to study the common mode behavior of a PV system, which is required to analyze the ground currents. In this paper, a comprehensible model is proposed which provides a better understanding of the common mode issue in single-phase transformerless PV systems. In addition, a procedure is developed to analyze the global performance, efficiency, grid current quality, and common mode behavior of a PV inverter as a function of its particular structure and modulation technique. Copyright © 2007 John Wiley & Sons, Ltd.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pip.761

Pablo Sanchis

Papers

Hydrogen Production From Water Electrolysis: Current Status and Future Trends

Transformerless Inverter for Single-Phase Photovoltaic Systems

Dynamic Behavior of the Doubly Fed Induction Generator During Three-Phase Voltage Dips

Wind Turbines Based on Doubly Fed Induction Generator Under Asymmetrical Voltage Dips

Ground currents in single-phase transformerless photovoltaic systems