Increasing energy consumption and concern for carbon emissions has boosted the demand for renewable energy production. The focus on renewable energy has gained much attention in wind, solar, hydro and wave power generations. Wave power has great potential due to its high energy density but there are challenges as well. This thesis addresses some of the challenges involved in the grid integration of wave energy and in maintaining power quality. In this thesis a grid connection of permanent magnet linear generator (PMLG) based wave energy converter (WEC) as a renewable energy source is evaluated at the Division of Electricity, Uppsala University.

The grid impact of a wave energy park in terms of flicker, voltage variations and harmonic distortion at the grid-connection point are investigated extensively. The short-term flicker level generated by the WEC and a wave energy park (WEP) related to the rated WEP power and grid impedance angle at the PCC are evaluated.

In this thesis, an improved control for hybrid energy storage is presented, which enhanced the efficiency and increased the battery life while smoothing the intermittent power from the WEP. The thesis, also, contributes to resolve the problem of inertia and power balance by integrating the DC-link capacitor in the control loop which reduce the size and cost of the components at the DC-link.

The work presented in the thesis has contributed for the force control of the PMLG which is predicted and controlled by regulating the stator currents of the generator. A nonlinear, neural, control is evaluated and compared to a linear, proportional-integral, control. The results from the nonlinear control show the good agreement between the referenced and the generated currents. The reduced losses enhanced the accuracy of the system.

A control and grid connection system for a WEC has been designed and installed. The thesis addresses the issue of power quality in low, steady and varying power flows of compliance with the grid code requirements.

This doctoral thesis presents work related to wave powered desalination. Wave power for desalination could be an interesting alternative for islands or coastal regions facing freshwater shortage, and several systems have been proposed in literature. However, desalination is a process which demands a lot of energy. Studies presented in the thesis indicate that the wave energy converter designed at Uppsala University in Sweden could be used for desalination. This wave energy converter includes a floating buoy connected via a wire to a linear generator. The linear generator has magnets mounted on its movable part (the translator). Small-scale experiments have been included, indicating that intermittent renewable energy sources, such as wave power, could be used for reverse osmosis desalination. Moreover, hybrid systems, including several different renewable energy sources, could be investigated for desalination. There may be interesting minerals in the desalination brine. The thesis also includes investigations on the magnetic material inside the linear generator, as well as on control strategies for wave energy converters. An opportunity of including different types of ferrites in the linear generator has been analyzed. The thesis also presents pedagogic development projects for the electro engineering education at Uppsala University, suggesting that including a greater variability and more student-centered learning approaches could be beneficial.

The increasing interest in renewable energy has significantly increased in the last decades. The increasing amount of variable renewable energy resources in the grid, which are connected via power electronics, reduces the total mechanical system inertia. Frequency-regulating resources such as hydropower will become more important in balancing variable renewable energy resources, setting higher requirements on stability and performance to maintain a stable electrical grid. This thesis concerns the decreased mechanical inertia from non-directly electrically coupled generation units. The thesis starts with a description of the grid system inertia situation today and presents two methods for estimating the grid frequency derivative used to provide synthetic inertia and one method used to enhance the mechanical inertia response of a synchronous generator. The synthetic inertia and enhanced inertia methods are tested in a small-scale experimental setup and compared with results from tests in the Nordic grid. A full-scale hybrid energy storage system was designed and built using a split frequency method as a power controller. The results show that a power-frequency derivative controller-based synthetic inertia method achieved an improved grid frequency quality during regular operation in the nano-grid experimental setup. The results are evaluated both via simulations and experimental tests. The results from the hybrid energy storage solution showed the possibility of increasing frequency quality by using a slow run of the river hydroelectric power plants and a battery energy storage system for frequency containment reserve.