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.

This paper validates a simulation model that couples an electrical model in Simulink with a hydrodynamic vortex-model by comparing with experimental data. The simulated system is a vertical axis current turbine connected to a permanent magnet synchronous generator in a direct drive configuration. Experiments of load and no load operation were conducted to calibrate the losses of the turbine, generator and electrical system. The power capture curve of the turbine has been simulated as well as the behaviour of a step response for a change in tip speed ratio. The simulated results agree well with experimental data except at low rotational speed where the accuracy of the calibration of the drag losses is reduced.

This paper investigates three load control methods for a marine current energy converter using a vertical axis current turbine (VACT) mounted on a permanent magnet synchronous generator (PMSG). The three cases are; a fixed AC load, a fixed pulse width modulated (PWM) DC load and DC bus voltage control of a DC load. Experimental results show that the DC bus voltage control reduces the variations of rotational speed by a factor of 3.5 at the cost of slightly increased losses in the generator and transmission lines. For all three cases, the tip speed ratio \(\lambda\) can be kept close to the expected \(\lambda_{opt}\). The power coefficient is estimated to be 0.36 at \(\lambda_{opt}\); however, for all three cases, the average extracted power was about \(\sim 19\)\%. A maximum power point tracking (MPPT) system, with or without water velocity measurement, could increase the average extracted power.

This paper presents experiments and measurements on a low speed permanent magnet cable wound generator for marine current energy conversion. Measurements were made for no load and nominal load (4.44 Ω/phase) conditions at nominal speed (10 rpm). For either load condition, the magnetic fields in the air gap were also measured. The measurements on the generator were compared with the corresponding finite element method simulations used to design the machine. It is shown in the paper that measurements and corresponding case simulations show good agreement. At nominal speed, the measured and simulated load voltages (nominal load) differ less than 1% for the rms values and less than 5% for peak values. At no load, measured and simulated voltages had larger differences, that is, <9% for rms values and <5% for peak values. Harmonic analyses of measured and simulated phase voltages and currents show only the presence of third harmonics. The percentage of harmonics in the measured data was comparable with the corresponding predictions of the simulations. The discussions and results presented in the paper could be beneficial for future design of efficient and reliable marine current energy converter systems.

The paper proposes a back-to-back 2L-3L grid connection topology for a marine current energy converter. A prototype marine current energy converter has been deployed by a research group at Uppsala University. The concept behind the prototype revolves around a fixed pitch vertical axis turbine directly connected to a permanent magnet synchronous generator (PMSG). The proposed grid connection system utilizes a well known and proven two level voltage source converter generator-side combined with a three-level cascaded H-bridge (CHB) multilevel converter grid-side. The multilevel converter brings benefits in terms of efficiency, power quality and DC-link utilization. The system is here presented for a single marine current energy converter but can easily be scaled up for clusters of marine current energy converters. Control schemes for both grid-side and generator-side voltage source converters are presented. The start-up, steady state and dynamic performance of the marine current energy converter are investigated and simulation results are presented in this paper.