Vision of an array of wave energy converters forming a plant.

Conversion of wave energy to electricity

Today more than 80 per cent of the world’s electric power production comes from fossil-fuelled plants. As the demand for electricity is forecasted to increase, there is an urgent need to find new methods to extract electric energy from renewable sources. Renewable electric energy supply is today one of the highest priorities in many parts of the world.

The Kyoto declaration 1997 and the last agreement at Marrakech 2002 are significant proof of this. Both the EU and the US have set their targets on future greenhouse emissions. Ocean waves represent a vast unexplored source of renewable energy. The wave energy potential in the EU has been estimated conservatively as 120–190 TWh/year offshore and an additional 34–46 TWh/year at near shore locations.

However, these estimations depend on assumptions of technology and energy cost. The actual resource could be a magnitude larger. In any case, it will be a challenging task to convert the vast energies in the ocean waves into electric energy. When approaching sustainable electric power production for the future, attention must be paid to the economical constraints.

The social, ecological and environmental impacts also needs to be adressed. The need for research and investigations in this area must not be underestimated.
Today, several countries have national efforts within wave energy. The dominating countries in the development of wave power have so far been Denmark, India, Ireland, Japan, Norway, Portugal, The Netherlands, Australia, UK and USA.

The Swedish waters have been estimated to contain too little wave energy and the general opinion has been that it could not be motivated to do research on small 5–50 kW conversion devices. From the mid eighties the area has been considered difficult and uneconomical. Despite this, one of the more tested technologies has been developed in Sweden, the so-called IPS OWEC Buoy with a power of 100 kW or more. It is now further developed in the USA and UK. The device is pumping water up and down, thereby driving a traditional generator.

The ocean waves behaviour have been the objectives for many investigations. However, apart from some tests, mechanical solutions with a traditional rotating generator (1,500 r.p.m.) have been predominant for the conversion. Most of the projects remain in the research stage, but a substantial number of plants have been deployed in the sea as demonstration schemes.

Several ways of classifying wave energy devices have been proposed, based on the energy extraction method, the size of the device and so on. A group of devices, classified as “Point Absorbers”, appears to have the approach a performance where commercial exploitation is possible.

Figure 1. The wave energy converter (WEC) consists of a buoy coupled directly to the rotor of a linear generator by a rope. The tension of the rope is maintained with a spring pulling the rotor downwards. The rotor will move up and down at approximately the same speed as the wave. The linear generator has a uniquely low pole height and generates electricity at low wave amplitudes and slow wave speeds.

Point absorber driven linear generator
Linear generators for wave power conversion have previously been considered but where concluded as impossible, since low velocities were believed to give too slow flux changes and thereby large and expensive electromagnetic converters. However, renewed activities has been reported from England and the Netherlands. Furthermore, recent electromagnetic simulations, revile a neglected opportunity.

We work with a concept that combines Faraday’s law of induction, Newton’s laws of motion, the even older principle of Archimedes with relative recent advancement in materials technology. In the spirit of minimizing mechanics by adapting generator to wave motion a design with a buoy absorbing ocean wave energy at the surface driving a linear generator at the sea floor is studied, see Figure 1.

Wave energy is directly converted into electricity by a linear generator consisting of insulated conductors; NdFeB permanent magnet and steel of different quality like electroplate and construction steel. Detailed modelling and simulations, as opposed to the traditional rule of thumb estimates, with a full account of design in full physics simulation gives detailed data on performance, as illustrated in Figure 2.

The buoy, which drives the linear generator, can be built from different materials and in different forms.

However, a cylindrical shape is preferred as a uni-directional point absorber is desired. Buoy dynamics and its behaviours during ocean wave exposure have been described elsewhere. A buoy connected with a stiff rope will drive the generator piston as the wave is rising. When the wave subsides a spring that has stored energy mechanically will drive the generator. Thus allowing for generation of electricity during both up and down travel.

When the flux from the piston circumvents its coils induction will occur in the generators stator, as the piston ideally moves up and down. Dependent on several parameters, generator design, wave shape, buoy size, weight, load and springs etc., different voltages with varying frequencies will be induced in the stator windings.

For open circuit conditions, the generator AC-voltage starts at zero, when the buoy is momentarily at rest in its lowest position, increases as the buoy accelerates towards the top of the wave, where it again reaches zero as the buoy stops.

For a relatively small wave energy converter (WEC) in the regime of 10–20 kW the buoy will have a diameter of three to five meters depending on wave climate and power rating. The buoy will have a weight in the regime of a few hundred kg to one metric ton depending on size and material. The buoy is connected to the generator with a modern synthetic rope (possible of stretched polyethylene) trade names such as Dyneema and Spectra, with an optional cover for handling of fouling. A housing encloses the generator, as indicted in Figure 1. This could be made of concrete or steel with and integrated bottom concrete slab.

The total weight of the generator is in the range of a few tons whereas the bottom slab must have a weight surpassing the floatation of the buoy, in the range of 10 to 30 metric tons. The slab can be positioned directly at the bottom and kept in place by gravity.

	Figure 2. Design simulation and performance. Winding arrangement on the left, generator in the middle, magnetic flux simulation and the output voltage.

Only active power is converted
From an analytical point of view, introduction of a load necessary to extract energy, poses a new challenge. The load considerably complicates the dynamics of the motion. An electrical current, from the induced voltage in the stator windings, exerts a retarding force on the piston proportional to its speed relative the stator.

Using rectifier with an externally applied DC voltage makes the dynamics even harder. Current passes the diodes when the induced voltages have higher potential than the externally applied voltage.

The retarding force is zero when the current is zero in the windings. Moreover, as induced and rectified voltage excides the applied DC voltage, a retarding force will abruptly be introduced, momentarily reducing the acceleration.

However, only active power transmitted is converted in the rectifier. Hence, the design has to be render the generator insensitive to wave and load variations. This can be accomplished by designing for a load angle close to unity. In practice, the current has to be relatively low at full load securing small variations in load angle versus open circuit.

This strategy has advantages of and widen the range of components used for conversion from stator windings to the grid connection. Simulations show that a working efficiency of around 85 per cent can be obtained.

Gird connection
In a plant a number of WEC’s are interconnected with a underwater substation (UWS) with a three-phase cable on the ocean floor. The UWS features a multi WEC connection and houses components for controlling the individual WEC’s, connecting the power to a common DC-bus and possibly equipment for transforming power before transmission to shore.

The WEC units will be connected in larger arrays ranging from tenths up to thousands of individual converters.

	Figure 3. An example of grid connection with converter and an optional transformer located on shore.

For an ocean with moderate wave climate, like the Baltic, four hundred 10 KW WEC could be interconnected to form a 4 MW plant . The grid connected can be implemented in various ways:
• A number of base units are connected on the DC side, and thereafter a transmission line connects the cluster to land. A converter onshore for grid connection forms a 50 or 60 Hz AC. An optional shore transformer could also include a tap changer in order to compensate voltage variations, see Figure 3.
• Another option, similar to the first, is to move the converter offshore which limits land use. However, this increases the complexity and may decrease the availability as maintenance will be more weather dependent. The converter can be placed on a platform or enclosed in a watertight container on the seabed.
• A further development would be to also install a transformer offshore. This would increase power transmission possibilities since power is proportional to the square of the voltage, i.e. for the same power rating the current is lower with higher transmission voltage.
• A fourth option includes a high voltage DC “HVDC” transmission link. This implies a higher degree of complexity, but transmission losses are kept at a minimum. However, the power components losses will be added. A platform or watertight enclosure is also required for the electrical power components.

Conclusion
Ocean waves represents one of the most densely powered natural fluxes which can be directly used for renewable energy generation.

Furthermore, it can have a relatively large utilisation time as the power flux variations are attenuated when the waves are induced by winds which in turn originates from solar power.

The main challenges
The main challenges have been high investment cost associated with large structures and survivability of the parts exposed to the large powers of the ocean. Mechanical overloads are difficult to handle without excessive over-dimensioning with associated increased costs.

However, electrical overloads can and are routinely handled at moderate cost. Wave energy, absorbed by buoys and transferred via a rope to the linear generators at the ocean floor, can be converted into electricity with varying frequency and amplitude. An array grid connection can be obtained with the use of AC/DC converters together with transformers in a number of topologies. Selection of diodes and power transistors are based on cost, losses and maintenance.

Technology for conversion and transmission from offshore to land has been used in other applications during the last 50 years.

Thus we argue, if raw unsynchronised power can be extracted offshore it can also be converted into practical form and connected to the AC grid. Longitudinal flux three phase generators have previously been regarded difficult or impossible to use for wave power.

However, the use of other types of linear generators have been suggested. A simple conversion scheme, with moderate investment cost and acceptable survivability, is anticipated to give wave energy a competitive potential. n

By Hans Bernhoff and Mats Leijon
The div. of Electricity, dept. of Engineering Science,Uppsala University