Lysekil Project

The main characteristic of the interaction between waves and a WEC (wave energy converter) is that energy is converted at large forces and low velocities due to the characteristic of ocean waves. As conventional generators are designed for high speed rotational motion, traditional wave power take-off system use a number of intermediate steps, for example hydraulics or turbines, to convert this slow-moving wave motion making it suitable for these generators.[1]

Another way to tackle the problem; instead of adapting the waves to the power-take off system is adapting the system to the waves. This can be done by using a direct driven WEC (wave energy converter) with a linear generator. The advantage with this setup is a less complex mechanical system with potentially a smaller need for maintenance. One drawback with this kind of system is a more complicated transmission of the power to the grid. This is due to the characteristics of the generated voltage which will vary both in amplitude and frequency.[1][2]

In the Lysekil project one goal was to develop a simple and robust wave energy system with a low need for maintenance. The approach was to find a system with few moving parts and as few energy converting steps as possible. Because of these requirements, a concept with a direct driven permanent magnet linear generator driven by a buoy that follows the motion at the sea surface was chosen.[1]

The Lysekil research project was established with the purpose to evaluate the chosen concept. The behavior of the WEC is studied both when it works as a single unit and together with several other WECs as a part of a cluster. Other important aspects are the design of the transmission system, in other words, how the power is transported to the grid and how this affects other part of the system, such as the buoy absorption. Another purpose with the project is to determine the WEC’s environmental impact focusing on marine organisms, ranging from small bottom dwelling organism living in the seabed, from organism involving in biofouling to vertebrates.[1]

The test site is located on the Swedish west coast about 100 km (62 mi) north of Gothenburg, close to Lysekil. The site is located 2 km (1.2 mi) offshore and covers an area of 40,000 m² (430,000 sq ft). The seabed in the area has an even surface with a slight incline towards west and the depth ranges from 24 m (79 ft) in the eastern part to 25 m (82 ft) in the west. The seabed consists mainly of sandy silt and some smaller areas are also covered with rougher material. The area will contain ten buoys connected to generators and an additional numbers of buoys for environmental impact studies. The permits allow the studies to go on to the end of 2013, then all equipment has to be removed unless a prolongation is applied for and approved.[1]

The average energy flux at the research site during 2007, excluding August, was 3.4 kW/m. The most frequent sea state is characterized by an energy period, T_E, around 4 sec and significant wave height H_s less than 0.5 m (1.6 ft). The main energy contribution comes from the more energetic (but not that frequent) sea states.[1]

Schematic picture over the WEC with the linear generator at the seabed connected via a line to a buoy at the sea surface

The wave power concept in the Lysekil project is based on a three phase permanent magnetized linear generator placed on the seabed. The generator is connected to a point absorbing buoy at the surface via a line. When the waves move the hydrodynamic action forces the buoy to move in a heaving motion. The movements of the buoy will then drive the translator in the generator, consequently inducing current in the stator windings. The translator is connected to the generator foundation with springs that retract the translator in the wave troughs.[1][3]

The technology of the linear generator is assumed to be somewhat independent of the depth and the unit size of 10 kW is assumed to match a significant wave height in the range of 2 m (6.6 ft). The generator and the mechanical structure around the generator are however designed to be able to handle large overloads in terms of electrical and mechanical strain. In table 1 rating and geometric data of the first experimental WEC is shown.[1]

Because the induced voltage will vary both in amplitude and frequency the generated power cannot be directly transmitted to the grid. Hence several WECs will be connected to a marine substation, where the voltage from each WEC will be rectified and the combined output alternated and transformed before connection to the grid. The sea cable that is used is a 1 kV cable with 4 x 95 mm2 copper conductors with a resistance at 0.5 Ohm per phase.[1][3]

The Lysekil project started in 2002 at the Division for Electricity at Uppsala University. Simulations indicated that there was a potential for harvesting energy with a wave power farm consisting of a number of smaller WEC units in coastal areas. The simulations did also point out the possibility for electrical power production in places with moderately calm seas. Due to these simulations being based on simplified buoy-wave interaction models, further simulations and experimental verifications where needed.[1][3]

During 2003 and 2004 permits to establish the Lysekil Research site were obtained and the first wave measuring buoy was deployed in 2004. The first experimental setup was deployed in March 2005 and the purpose was to measure the maximum line force from a buoy with a diameter of 3 m (9.8 ft) and a height of 0.8 m (2.6 ft). This set up simulated a generator that had been disconnected from the grid and thereby operating without any damping in the system. The results from these experiment were used as input data to the first wave generator and to verify the calculations of the dynamics of non-damped systems.[1][3]

Since the deployment of the first WEC in 2006, it has been in operation in real ocean seas for several months during three different time periods. During these periods measurements of electrical power, buoy motion and mooring line forces have been carried out and analyzed to enhance the knowledge about direct driven linear generator WEC dynamics. In the first test period all electrical power was converted to heat over a three phase delta-connected resistive load. To investigate the impact of a non-linear load on the WEC system, the generator was, during the second test period, connected to a non-linear load consisting of a diode rectifier, capacitors and resistors. The control, load and measurement system for the WEC have successively been expanded and now there is a remotely operated measurement system and control system.[1]

Results from the studies demonstrate how well wave energy converting with this concept functioning in calm as well as rough offshore sea states. Experiments with non-linear loads have increased the knowledge about how the transmission system should be designed. The results also show how the WEC operates when connected to a non-linear load which will be the case when the generated voltage rectifies.[1]

The Lysekil Project has been enlarged with two additional WEC which have been launched in the test site together with a marine substation. These three WEC have recently (June 2009) been interconnected with the substation.[1][4]

The research site was investigated at the beginning of the project and in 2004 sediment samples were taken before the WEC was deployed. This was done in order to investigate and analyze the marine infauna inside the research area and control area and get information about species composition and biodiversity. Biodiversity richness is often used to measure the health of biological systems. The organisms were identified down to family or species level where possible. At Lysekil research area 68 different species were found. There were only small juvenile organisms and no red-listed species were found. Since 2004 sediment sample are taken yearly to follow the development in both the control area and in the research area. Through the initial studies it has become clear that it was a higher species richness and biodiversity in the research area than in the control area. This is explained by the variety of sediment substrate. Sediment in the control area mainly consists of silt, which is mostly relatively oxygen deficient and fewer species are adapted to such extreme conditions. Overall the Lysekil research site is not a unique environment and there was no concern about extinction of sensitive local species.[1][3]

The first experimental setup for marine ecological studies of wave power consisted of 4 biology buoys which were deployed in 2005. The purpose of the buoys is to study the effects an establishment of a wave power farm can have. When placing solid structures on an otherwise rather empty sand bottom the living conditions at the site will change and the consequences of this are studied with the help of the buoys. In 2007 the size and the complexity of the study was increased when additional buoys were installed. The buoys were divided into two different areas inside the research area, with 200 m (660 ft) between the groups. Within the groups the buoys were placed 15–20 m (49–66 ft) apart. Half of the foundations were designed with various holes and the other half without. The holes in the foundations were made to study the difference in colonization between the foundation with and without holes and how the different kind of holes affect the colonization pattern.[1][3]

The biology buoys are also used to study biofouling and its impact on the power absorption. Some preliminary studies suggest that the effect of biofouling on energy absorption can be neglected but this has to be investigated further.[3]