Coastal conversions16 January 2008
With the first contract underway, Voith Siemens’ subsidiary Wavegen is finding a run of opportunities for its wave energy converter based on oscillating water column technology
Creativity is often driven by inventors, garage type firms, universities and small and medium-sized enterprises (SMEs). In wave and tidal energy technologies, many concepts are being discussed and there is no convergence towards dominant systems as yet. Potential customers face the challenge of deciding between about a hundred different concepts at very different design states. Hence, there is much uncertainty in the market, including over reliability, life-time of the concepts and energy production costs.
However, larger industrial players have now started to become active, and among them is voith-siemens Hydro Power Generation via its Scottish subsidiary, wavegen, located in Inverness. While some utilities are waiting and observing market developments, others are establishing partnerships, such as Basque utility EVE, British npower renewables and Germany’s EnBW, which have linked up with Wavegen to develop their first commercial projects.
The technologies can roughly be viewed as three main concepts: hydrodynamic movement, wave-induced head or overtopping devices, and oscillating water columns (OWC).
Hydrodynamic movement involved a floating body, such as buoy, a nodding or flapping device or a self-articulated raft, being moved by wavesfollows the movement of the waves. The periodic movement is either directly (e.g. with a linear generator) or indirectly (e.g. through a hydraulic system) converted into electrical energy.
Wave-induced head or overtopping devices involve sea water being driven up an artificial ramps, gaining potenial energy to drive low head turbines. – Wave movements drive the water up an artificial ramp which then reaches a level of higher potential energy and is, for example, captured in a basin. This artificially created head is used to drive low head turbines.
The OWC technology uses the rise and fall of the water surface in a sealed chamber which periodically compresses and decompresses the enclosed air volume. An air turbine, such as a Wells turbine, is used to convert the airflow from the pressure variations into electricity. Due to the low density and viscosity of air compared to water, these turbines can be operated at very high speeds (up to 4,000 rpm). Therefore, OWC technology does not need gearboxes and allows for small generators.
The Wells turbine is a fixed pitch machine with only one direction of rotation. To achieve this, the rotor profile has to be symmetrical and the blade may neither be twisted nor oriented under an angle towards the rotation plane. The complete rotor is symmetrical with respect to the rotation plane.
The turbine is not self-starting and has to be accelerated to reach a minimum speed. The resulting velocity of the airflow from rotation and the superimposed air flow from the chamber, generates a lift with a component in the direction of rotation. If the air flow direction changes, the lift vector is mirrored at the rotation plane while the forward driving component of the lift is conserved. Hence, the turbine is accelerated regardless of the direction of the airflow as water levels rise and fall, and air goes in and out.
A simple concept but one that gives a robust machine with few moving parts, which minimises the potential mechanical and other problems there could be if testing in deeper, at least initially, marine environments. While the technology continues to be developed, wavegen is focusing on onshore applications for ease of access without the requirement of support using specialised boats or other marine equipment but such tests and applications helps to mimimise the research budget and may deliver results faster.
The technology is not limited to on-shore applications but that has been the practical focus so far.
Since 2000, Wavegen has operated the grid connected onshore wave power plant, LIMPET, on the Scottish island of Islay. The OWC chamber is made of reinforced concrete and is tilted at 45°. The air above the water surface is guided through several round openings leading to the Wells turbines. Originally, LIMPET was equipped with a 500kW Wells turbine, which was later downgraded to 250kW. The collector has a second opening, which is used to test smaller turbines (again, grid connected), and for experience in optimising and operating different sizes of units.
After seven years of continuous grid-connected operation, the main turbine has been removed, providing additional test space for newer turbine developments. Hence, LIMPET now acts as a huge air pump to test full size turbines under real grid-connected conditions.
The Wells turbine concept has been shown to work under severe weather conditions. For example, in the first winter after installation of LIMPET on Islay, the island faced a 50-year return wave and the device survived without damage.
Unlike conventional hydro turbines, a Wells turbine cannot be designed to one point of operation (i.e. characterised by a head and a flux through the turbine). Due to the irregularity and unsteadiness of the airflow caused by waves, the trajectory a Wells turbine takes in a classical hill chart appears almost random; the point of maximum efficiency is only met coincidentally.
Hence, a broad turbine characteristic is more important than a high peak efficiency and control strategies have to focus on operating the turbine for a high fraction of time as close to the maximum efficiency as possible.
Under those conditions, the Wells turbine currently reaches wave cycle efficiencies of close to 45%. Ongoing computational fluid dynamics (CFD)-based turbine optimisation and improvements in the control strategy are intended to bring the cycle efficiency of the fixed pitch machine above 50%.
Since Wavegen was acquired, most of the development work has been dedicated to the optimisation of the turbine. The experience of a large industrial player and the creativity of a high profile SME have merged, resulting in the 18.5kW Wells turbine.
Moreover, a range of standard turbines of different sizes has been defined and is, in part, engineered. As a consequence, the next size test turbine rated at 100kW is bein started on Islay. By mid-year, the company plans to have a fully engineered and tested set of reliable turbines to serve all realistic conditions required by the market.
Energy production costs are a major driver for the development of wave power – the lower they are the faster the market grows. There are several levers to bring down costs of energy production systematically, including, in order:
• Reliability of the equipment: The degree of reliability of a wave power concept largely drives project risk and thus the rate at which an investor expects a new investment to earn. Therefore, reliability is key for the customer. Wavegen has extensively addressed this issue and therefore made a big step toward commercialisation of the technology.
• Efficiency of the machine: Costs per kWh are, in simple terms, discounted cash-outflows for capital investments and operations and maintenance, divided by discounted number of kWhs produced. Hence, an increase of energy output works on the total investment – not only the turbine – and can bring costs down significantly. Optimisation has made significant progress at Wavegen, Voith Siemens says.
• Reduction of capital investment costs: The largest cost item on an OWC plant is the collector. Bringing this cost down has significant influence on energy production costs. Apart from creative engineering, the best idea is to avoid spending large sums on the collector by sharing it with other applications. This is how the idea of the active breakwater application was born.
For coastal protection, large concrete structures are built in order to protect harbours from waves. If these structures can be modified so that OWCs can be included in the front wall, costs are shared and, according to laboratory measurements, loads on the walls are significantly reduced. For a caisson type breakwater, an empty concrete structure with a rectangular structure of concrete enforcement walls is cast. In this state, the structures are floating and can be towed into position by a ship.
The empty chambers are then filled with stones or sand so that the structure sinks and sits rigidly on the seabed. If the front chambers of the wall are only partly filled and openings under water are included, an OWC chamber is formed. The Wells turbine is placed into the airflow. In this configuration, the breakwater is converted into an active breakwater which not only protects the coast but also produces energy. Harbours usually require a high level of electrical energy and therefore offer good opportunities for grid connection.
The first commercial projects are designed to address the active breakwater opportunity:
• In June 2007, the first commercial contract was signed between Voith Siemens Hydro Tolosa and the Basque Utility Ente Vasco de la Energia (EVE). Under this contract, the Spanish subsidiary will supply and install 16 Wells turbines each rated at 18.5kW for installation in the newly built breakwater of the Basque seaport at Mutriku. The plant will be installed in the winter of 2008/09 and commissioned in early 2009. The project is supported by the EU under the sixth framework programme.
• In the UK, npower renewables and Wavegen are engaged in developing a project on the Scottish Hebridean island of Lewis. This project will consist of a bottom-standing, near-shore device which can be used as a slipway for small scale commercial and leisure craft. The project is currently planned to have a rated power of 3.6MW and will consist of 36 Wells turbines, each rated at 100kW. Installation is envisaged for around 2010.
• The German utility EnBW intend to build and operate Germany’s first wave power plant on the North Sea coast. In many places, the bathymetry of the North Sea is not favourable for wave power, the location search is a tricky task. However, interesting locations have been identified and are currently under detailed examination.
The biggest challenge the industry will face in the future will be an ongoing optimisation of the technology which, through experience and scale effects, will finally lead to reduced energy production costs. In many discussions, there are overly optimistic cost per kWh targets – some triggered by young start-up companies which, in their race for the scarce financial resources that wave power can currently access, tend to draw a distorted picture. Hence, wave power costs of energy production are frequently compared to current costs of fossil fuels or wind.
Interestingly, in many cases it is estimated that wave power, even at its very young age, is already able to beat photovoltaic technology which has globally reached several GWs installed capacity. Looking at the limited energy density of wave power it is obvious that this energy form will never be as cheap as conventional hydro power or fossil fuels.
However, experience and scale effects have not yet been harvested. To achieve that, a feed-in tariff system similar to that which has been introduced for photovoltaic in Germany would be required to stimulate the business and reduce risk for investors.
Wave power has the potential to contribute significantly to the world’s energy supply in the years to come. Voith Siemens Hydro’s OWC technology is now ready to jump the hurdle of commercialisation and the company intends to make a significant contribution to the development of this exciting industry.
For further information, contact Dr Jochen Weilepp, Head of Ocean Energies at Voith Siemens Hydro Power Generation – www.voithsiemens.com