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Freshwater scarcity and insufficient sanitation are global urgent problems, affecting billions of people. In this review paper, the process of desalination powered by wave power has been investigated as a potential sustainable solution to water shortage. The different desalination techniques suitable for this type of combined system, i.e. reverse osmosis, electrodialysis and mechanical vapor compression, have been outlined, as well as the different wave energy converters possible to power the desalination process, i.e. oscillating water columns, oscillating bodies (wave activated bodies) and overtopping systems. Some necessary considerations for this type of project are identified. The different wave power/desalination projects and how they have proceeded are presented. The most common design of a wave energy and desalination system includes a wave activated body to pressurize seawater; the seawater flows through a reverse osmosis membrane, resulting in freshwater. Some successful (freshwater producing) wave energy/desalination projects were identified: Delbuoy, the oscillating water column in Vizhinjam, CETO Freshwater, SAROS and Odyssée. It is concluded that wave power and desalination can be combined in a sustainable and autonomous system, generating freshwater from the ocean waves. However, questions regarding cost of produced water, variations in power production due to intermittency and environmental effects still remain. [ABSTRACT FROM AUTHOR]

All the papers selected for this Special Issue have shown important insights and contributed to making this Special Issue successful in demonstrating the recent advances in the field of power electronics for electric drives and renewable energy sources. This Special Issue was intended to consolidate the most recent advances in the field of power electronics for renewable energy sources and electric drives. In this scenario, the power electronics will play a key role in conditioning the power coming from highly variable renewable energy sources and help transition to fully renewable electric grids or smart grids. [Extracted from the article]

Power generation from wave power has a large potential to contribute to our electric energy production, and today, many wave power projects are close to be commercialized. However, one key issue to solve for many projects is to decrease the cost per installed kW. One way to do this is to investigate which parameters that have a significant impact on the wave energy converters (WEC) performance. In this paper, experimental results on power absorption from a directly driven point absorbing WEC are presented. The experiments have been carried out at the Lysekil research site in Sweden. To investigate the performance of the WEC, the absorbed power and the speed of the translator are compared. The result confirms that the buoy size and the translator weight have a large impact on the power absorption from the generator. By optimizing the buoy size and translator weight, the WEC is believed to produce power more evenly over the upward and downward cycle. Moreover, to predict the maximum power limit during normal operation, a simulation model has been derived. The results correlates well with experimental data during normal operation. [ABSTRACT FROM AUTHOR]

A wave energy converter (WEC) should be controlled in order to increase the average output power. In this paper, economic aspects of latching applied to a point absorbing WEC with a linear generator power take-off (PTO) are discussed. The capacity utilisation factor (CUF) is suggested to be used along with average absorbed power for control optimisation. Optimum and suboptimum latching controls are assessed for the WEC and compared with a constant damping PTO force control. The WEC performance is simulated using monochromatic waves for the wave conditions of the Wave Hub test site, UK. The linear wave theory is used in a hydro-mechanical two-body simulation model. It is shown that the latching controls possess considerable practical challenges significantly increasing the return of investment time periods. [ABSTRACT FROM AUTHOR]

The Lysekil wave energy converter (WEC), developed by the wave energy research group of Uppsala University, has evolved through a variety of mechanical designs since the first prototype was installed in 2006. The hundreds of engineering decisions made throughout the design processes have been based on a combination of theory, know-how from previous experiments, and educated guesses. One key parameter in the design of the WECs linear generator is the stroke length. A long stroke requires a taller WEC with associated economical and mechanical challenges, but a short stroke limits the power production. The 2-m stroke of the current WECs has been an educated guess for the Swedish wave climate, though the consequences of this choice on energy absorption have not been studied. When the WEC technology is considered for international waters, with larger waves and challenges of energy absorption and survivability, the subject of stroke length becomes even more relevant. This paper studies the impact of generator stroke length on energy absorption for three sites off the coasts of Sweden, Chile and Scotland. 2-m, 4-m, and unlimited stroke are considered. Power matrices for the studied WEC prototype are presented for each of the studied stroke lengths. Presented results quantify the losses incurred by a limited stroke. The results indicate that a 2-m stroke length is likely to be a good choice for Sweden, but 4-m is likely to be necessary in more energetic international waters. [ABSTRACT FROM AUTHOR]

The modelling of a three‐phase four‐leg four‐wire grid‐forming inverter in a low voltage distribution system 18‐bus European Cigré under unbalanced conditions in an autonomous distribution network is presented. The case study has two types of inverters control strategy: (i) grid‐forming to supply all the system demand in the interval of the intentional supply interruption and (ii) grid‐following to integrate photovoltaic renewable energy resources into power systems. The model suggests a control scheme with two loops: An inner current loop with a proportional‐integral controller and an outer voltage loop with a proportional controller, both in the synchronous reference frame (dq0), in which dq‐axis are decomposed in positive and negative sequences. Simulation results, carried out using the PSCAD software, showed the effectiveness of the suggested control strategy with smooth synchronization where the grid‐forming inverter was able to form a network with an unbalanced degree lower than 2%, sinusoidal voltage and frequency within standard limits 49.5–50.5 Hz. [ABSTRACT FROM AUTHOR]

In the past decade, inverter-integrated energy sources have experienced rapid growth, which leads to operating challenges associated with reduced system inertia and intermittent power generation, which can cause instability and performance issues of the power system. Improved control schemes for inverters are necessary to ensure the stability and resilience of the power system. Grid-forming inverters dampen frequency fluctuations in the power system, while grid-following inverters can aggravate frequency problems with increased penetration. This paper aims at reviewing the role of grid-forming inverters in the power system, including their topology, control strategies, challenges, sizing, and location. In order to facilitate continued research in this field, a comprehensive literature review and classification of the studies are conducted, followed by research gaps and suggestions for future studies. [ABSTRACT FROM AUTHOR]

Abstract: Renewable energy techniques are now gaining more and more attention as the years pass by, not only because of the threat of climate change but also, e.g. due to serious pollution problems in some countries and because the renewable energy technologies have matured and can be depended upon an increasing degree. The energy from ocean waves bares tremendous potential as a source of renewable energy, and the related technologies have continually been improved during the last decades. In this paper, different types of wave energy converters are classified by their mechanical structure and how they absorb energy from ocean waves. The paper presents a review of strategies for electrical control of wave energy converters as well as energy storage techniques. Strategies of electrical control are used to achieve a higher energy absorption, and they are also of interest because of the large variety among different strategies. Furthermore, the control strategies strongly affect the complexity of both the mechanical and the electrical system, thus not only impacting energy absorption but also robustness, survivability, maintenance requirements and thus in the end the cost of electricity from ocean waves. [Copyright &y& Elsevier]

As the penetration of inverter-based distributed generations into microgrids continues to rise, the significance of power electronic inverters in modern power systems becomes increasingly pronounced. They are designed to have fault ride-through capabilities, enabling them to sustain operation even during and after fault conditions. This paper presents a novel voltage compensation strategy based on the line impedances addressing both positive and negative-sequence aspects, for a three-phase three-wire inverter deployed to regulate the low-voltage network during unsymmetrical faults, thereby preserving voltage stability. The control strategy is carried out in a synchronous reference frame (dq) to govern the positive and negative-sequence voltages and currents of the inverter. By synthesizing AC currents, the inverter restores the positive-sequence voltage to its rated value while mitigating negative-sequence voltage at the point of common coupling arising from unsymmetrical faults. Through MATLAB/Simulink, are investigated three unsymmetrical fault scenarios (single line-to-ground, line-to-line, and double line-to-ground faults) within the low-voltage distribution network of the 18-bus European Cigré, incorporating three inverters equipped with the proposed voltage support capability. The results confirm that inverter-interfaced distributed generators with local voltage support can improve the system's fault ride-through capability by reducing voltage drops and unbalances. • Voltage resilience in the face of short-circuit challenges. • Balancing voltage fluctuations during grid faults. • Grid-supporting inverters optimize fault response. [ABSTRACT FROM AUTHOR]

The damping effect, induced inside the linear generator, is a vital factor to improve the conversion efficiency of wave energy converters (WEC). As part of the mechanical design, the translator mass affects the damping force and eventually affects the performance of the WEC by converting wave energy into electricity. This paper proposes research on the damping effect coupled with translator mass regarding the generated power from WEC. Complicated influences from ocean wave climates along the west coast of Sweden are also included. This paper first compares three cases of translator mass with varied damping effects. A further investigation on coupling effects is performed using annual energy absorption under a series of sea states. Results suggest that a heavier translator may promote the damping effect and therefore improve the power production. However, the hinder effect is also observed and analyzed in specific cases. In this paper, the variations in the optimal damping coefficient are observed and discussed along with different cases. [ABSTRACT FROM AUTHOR]

• Distributed generation based on PV array and battery system to make flexible operation modes. • Multiple auxiliary functions (control strategies) in the same interface converter. • Implementation and testing in the real-time simulator (RTDS) with control hardware in the loop (dSPACE). • Proposal of the operation on an isolated microgrid, providing nominal voltage and frequency by using the multi-functional inverter. This paper proposes the implementation of a multi-functional inverter, which was simulated with three ancillary functions. The first function is a Shunt Active Power Filter (SAPF) for harmonic content mitigation, the second is a Virtual Synchronous Machine (VSM) to show the effective regulation of voltage and frequency, and finally, power management in AC microgrids. The multi-functional converter is responsible for the integration of a PV-Battery system (distributed generation) on the AC grid. Furthermore, Real Time Digital Simulator (RTDS) and dSPACE have been used to analyze the performance and control of such multi-functional inverter when added to an AC microgrid. Results demonstrate the operation of the system and can be used as validation of the proposed control strategies through a Hardware In the Loop (HIL) simulation. [ABSTRACT FROM AUTHOR]

Freshwater scarcity is one of humanity’s reoccurring problems that hamper socio-economic development in many regions across the globe. In coastal areas, seawater can be desalinated through reverse osmosis (RO) and transformed into freshwater for human use. Desalination requires large amounts of energy, mostly in the form of a reliable electricity supply, which in many cases is supplied by diesel generators. The objective of this work is to analyze the wave power resource availability in Kilifi-Kenya and evaluate the possible use of wave power converter (WEC) to power desalination plants. A particular focus is given use of WECs developed by Uppsala University (UU-WEC). The results here presented were achieved using reanalysis—wave data revealed that the local wave climate has an approximate annual mean of 7 kW/m and mode of 5 kW/m. Significant wave height and wave mean period are within 0.8–2 m and 7–8 s respectively, with a predominant wave mean direction from southeast. The seasonal cycle appeared to be the most relevant for energy conversion, having the highest difference of 6 kW/m, in which April is the lowest (3.8 kW/m) and August is the peak (10.5 kW/m). In such mild wave climates, the UU–WEC and similar devices can be suitable for ocean energy harvesting for water desalination systems. Technically, with a capacity factor of 30% and energy consumption of 3 kWh/m3, a coastal community of about five thousand inhabitants can be provided of freshwater by only ten WECs with installed capacity of 20 kW. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Introducing an energy management system with a dynamic rate limiter control. • Exemplified by using a wave energy-park as an input power source. • Battery and controller losses are reduced with improved state-of-charge control. • Analysis and verification of adequate control for power-sharing over the hybrid energy storage. • A steady and improved power quality is ensured. Abstract The concern for climate change and energy consumption has increased the demand for renewable energy production considerably. Marine energy sources attract attention because of their high energy density. Wave energy is an attractive renewable energy source with large potential. Due to the nature of the ocean waves, a linear wave energy converter generates intermittent power. It is therefore crucial to regularize the power before connecting to the grid. Energy storage systems present effective ways to minimize the power fluctuations and deliver a steady power to the grid. In this paper, we present an energy management control system with a dynamic rate limiter. The method is applied to control a hybrid energy storage system, combining battery and supercapacitor, with a fully active topology controlled by the power converters. The results show that the method is able to control the charging and discharging states of the battery and the supercapacitor, and minimize the power fluctuation to the grid. The algorithm ensures low losses by shifting the required power and the stored power smoothly over the energy storage system. [ABSTRACT FROM AUTHOR]

Heat sources, such as power electronics for offshore power, could be cooled passively-mainly by conduction and natural convection. The obvious advantage of this strategy is its high reliability. However, it must be implemented in an efficient manner (i.e., the area needs to be kept low to limit the construction costs). In this study, the placement of multiple heat sources mounted on a vertical plate was studied experimentally for optimization purposes. We chose a regular distribution, as this is likely to be the preferred choice in the construction process. We found that optimal spacing can be determined for a targeted source density by tuning the vertical and horizontal spacing between the heat sources. The optimal aspect ratio was estimated to be around two. [ABSTRACT FROM AUTHOR]

A wave energy converter (WEC) based on a linear generator and a point-absorbing buoy has been developed at Uppsala University. Interconnecting an array of WECs in parallel requires a point of common coupling, such as a common dc-bus. The dc voltage level seen by the generator is directly linked to the electromagnetic damping of the generator. A lower dc-level results in a higher damping factor and is important for increased absorption of the wave power. The drawback is increased losses in generator windings and cable resistance. There will be an optimal dc-level for maximum power output. This is a function of not only generator and buoy characteristics, but the current sea state. Experimental results of the full-scale system have been carried out, and used as validation of a simulation model of the system. The model is then used to evaluate how the dc-level seen by the generator influence the power output. The results indicate that higher dc-levels should be used at higher sea states, and power output may vary by up to a factor five depending on which dc-level is chosen. [ABSTRACT FROM AUTHOR]

Wave energy converters (WECs), which are designed to harvest ocean wave energy, have recently been improved by the installation of numerous conversion mechanisms; however, it is still difficult to find an appropriate method that can compromise between strong environmental impact and robust performance by transforming irregular wave energy into stable electrical power. To solve this problem, an investigation into the impact of varied wave conditions on the dynamics of WECs and to determine an optimal factor for WECs to comply with long-term impacts was performed. In this work, we researched the performance of WECs influenced by wave climates. We used a permanent magnet linear generator (PMLG)-based WEC that was invented at Uppsala University. The damping effect was first studied with a PMLG-type WEC. Then, a group of sea states was selected to investigate their impact on the power production of the WEC. Two research sites were chosen to investigate the WEC's annual energy production as well as a study on the optimal damping coefficient impact. In addition, we compared the WEC's energy production between optimal damping and constant damping under a full range of sea states at both sites. Our results show that there is an optimal damping coefficient that can achieve the WEC's maximum power output. For the chosen research sites, only a few optimal damping coefficients were able to contribute over 90% of the WEC's annual energy production. In light of the comparison between optimal and constant damping, we conclude that, for specific regions, constant damping might be a better choice for WECs to optimize long-term energy production. [ABSTRACT FROM AUTHOR]

An analysis of reverse osmosis powered by ocean wave power is provided. A commercially available desalination system is connected via a DC/AC converter to a variable DC source and the input voltage is altered to emulate the response of a renewable energy system. Specifically, wave data from Kilifi in Kenya during 2015 is used. The wave resource variations provide variations in estimated power output from a wave energy converter, as well as in estimated freshwater production from a wave powered desalination system. Up to three wave energy converters for desalination are investigated for Kilifi. Also, a hybrid system including solar and wave power is proposed. The experiments show that reverse osmosis desalination systems can function at power levels below the rated values, but with lower freshwater flowrates. It is concluded that wave power, or wave power combined with PV systems, may be considered as power sources for desalination, with or without battery storage. Unlabelled Image • Wave powered desalination has been investigated for Kilifi, Kenya • The work includes experiments with small-scale desalination system and simulations • Hybrid systems, with wave power and solar PV, could be interesting for desalination [ABSTRACT FROM AUTHOR]

Some countries are facing issues on freshwater and electricity production, which can be addressed with the use of renewable energy powered desalination systems. In the following study, a reverse osmosis desalination plant powered by marine current energy converters is suggested. The marine current energy converters are designed at Uppsala University in Sweden, specifically for utilizing low water speeds (1–2 m/s). Estimations on freshwater production for such a system, in South Africa, facing the Indian Ocean was presented and discussed. It is concluded that the desalination plant cannot by itself supply freshwater for a population all the time, due to periods of too low water speeds (<1 m/s), but for 75% of the time. By using ten marine current energy converters, each with a nominal power rating of 7.5 kW, combined with a reverse osmosis desalination plant and water storage capacity of 2800 m3, it is possible to cover the basic freshwater demand of 5000 people. More studies on the hydrokinetic resource of the Western Indian Ocean, system cost, technology development, environmental and social aspects are necessary for more accurate results. [ABSTRACT FROM AUTHOR]