WAVE TECHNOLOGIES
This section of the report will investigate a number of wave energy devices, looking at the technology, and the potential to contribute to the UK’s energy requirements. The devices investigated here range from being fully working commercial devices such as the LIMPET through to devices in demonstration schemes such as the Wave Dragon. Due the difference in how advanced the technology is the range of information available is very large. This makes direct comparisons of the devices extremely hard. Therefore this section of the report only aims to narrate the current status of the devises. And make general high level suggestions or prediction of how wave energy devices can and should be used in the future to contribute to the UK’s energy requirements.
Oscillating Water Columns
An OWC comprises of a partly submerged structure called the collector, which has an opening below the water level. The column collector contains a column of water that oscillates up and down with the waves. Above the water within the column is some trapped air. The column of water acts like a piston displacing a trapped air as it oscillates. The movement of the air causes a turbine to rotate. The turbine is coupled to a generator to produce electricity. A schematic of an OWC is shown bellow.
Schematic of an OWC device. [1]
There are two main variations of the OWC today, the LIMPET and the OSPREY both will be discussed in the following sections.
Limpet
The LIMPET, shown in the figures below is a shoreline OWC device located on the island of Islay, off the west coast of Scotland. The Limpet was installed in the 2000, by Wavegen and is connected to the national grid. The acronym LIMPET stands for Land Installed Marine Powered Energy Transformer.
The LIMPET. [2]
Technology
The LIMPET comprises of three distinct components:
· a shoreline oscillating water column collector
· a turbo generation unit
· and a control and monitoring station
The LIMPET has an inclined shoreline oscillating water column collector, inclined at an angle of 40 degrees from the horizontal. An inclined collector has two main advantages:
· It offers an easier path for water ingress resulting in less turbulence and lower energy loss.
· It also increases the water plane area of the column for a given chamber cross section. This permits the primary water column resonance, which is influenced by the ratio of the water plane area to the entry area, to be better coupled to the predominant period of the incoming wave [3].
The collector has a width of 21m, this width has been divided up into three columns because:
· As the width of a column increases there is an increased risk of transverse wave excitation within the column. Which would reduce the energy capture performance of the devise.
· The design of the roof of the collector capable of spanning 21m without additional supports would have been too expensive.
The collector is made from BISTEEL a sandwich of steel-concrete-steel. And has been described as containing a higher density of steel than a nuclear bunker [4]. This is all to cope with the hostile environment of the shoreline. To construct the collector they used a technique called protective excavation. This involves excavating an area for the collector just behind the cliff edge, but leaving a protective bund between the mouth of the collector and the sea. This bund is removed once the construction is complete. The figure below illustrates this technique.
Protective excavation technique. [5]
The air exits the collector and enters the turbo generation unit through two 2.6m diameter openings in the back wall of the collector. The turbo generation unit consist of two 500kW counter rotating Wells turbines. Each turbine has a flywheel to smooth out the energy supply, as well as a sluice gate offer protection in stormy seas.
Current Situation
The LIMPET was estimated to have an average net electrical output of 202kW, however only 21kW where being outputted in 2002. This is due to a number of factors being lower than expected as illustrated in the table below [6]. The lower than expected output is not due to any fundamental problems with the concept, but instead with the modelling used to predict the output.
Initially expected / Actually recordedWave Power (kW/m) / 20 / 12
Pneumatic Efficiency (%) / 80 / 64
Turbine Efficiency (%) / 60 / 40
Despite the lower than initially expected power output the LIMPET has been haled as a success. It is important to remember that this is still a developing technology and the lesions learnt here will help improve the next generation of LIMPETs. And it is still believed that this technology has the capability to output approximately 200kW.
Osprey
The OSPREY is nearshore OWC device, designed by Wavegen. The acronym OSPREY stands for Ocean Swell Powered Renewable EnergY.
History
In 1995 a prototype device called OSPREY1 was launched, towed and installed near Dounreay in Scotland. The steel structure comprised of a 20m wide collector, located between two ballast tanks. The tanks focused the waves towards the collector. Unfortunately during the installation phase when the ballast tanks where being filled with sand, a three-meter swell developed. Due to the ballast tanks not being filled the structure failed, and OSPREY1 never became operational.
Current Situation
OSPREY2 is now under development, but its structure differs greatly from OSPREY1. OSPREY2 is made from concrete and the ballast tanks are built into the walls of the collector. Above the collector there will be two stacks each containing a 500kW counter rotating Wells turbine.
OWC UK Potential
In the UK shoreline resources have been estimated to be approximately 2TWh/yeat, and nearshore resources to be between 100-140TMh/year [9].
Below is a table showing the key data regarding both the LIMPET and the OSPREY [10]:
LIMPET / OSPREYWave Power (kW/m) / 32 / 30
Annual Power Output (MWh) / 2300 / 4955
Capital Cost (£k) / 1400 / 275
Annual Operating Cost (£k) / 29 / 19
For the LIMPET the right geographic factors, i.e. the right combination of shoreline topography and geography, together with low tidal ranges and closeness to the grid, have been identified in 72 sites around the UK.
Overtopping
An overtopping device has three stages. The first stage is the absorption stage, this is where the wave energy is focused and the wave is allowed topple over the structure. The second stage is the storage stage, once the wave has toppled over the structure the water is stored in a reservoir above sea level. The third stage is the power take off stage the water is allowed to leave the reservoir via a hydro turbine. An example of an overtopping device is the Wave Dragon.
Schematic of an overtopping device. [11]
Wave Dragon
Technology
The Wave Dragon, a floating overtopping device, can generate up to 7MW. A Wave Dragon power plant would be capable of producing 77MW, electricity for 60000 homes would consist of 11 individual devices [12]. Covering an area of 5.5km2. The mooring would be either by a concrete gravity base or a pile secured to the seabed. A Wave Dragon Prototype has been tested in Danish sea, a photo of which is shown in the figure bellow.
Current Situation
A Wave Dragon demonstration is due to be conducted off the Pembrokeshire Coast.. The demonstration will consist of one full size device. The Wave Dragon demonstration project will be in place for 3-5 years before the site is completely decommissioned.
UK Potential
This technology is very much in the early stages of development. But pending the results of the Wave Dragon demonstration project, an overtopping device has the potential to make a contribution to the UK energy requirements.
Point Absorbers
A point absorber is a device where one section moves with the waves, relative to a fixed sectioned, often secured by mooring. Point absorbers are often uni directional in their ability to pick up wave power, and they will only absorb energy in one plane of motion.
Salter Duck
Introduction
The Edinburgh Duck, or Salter Duck as it is commonly known, is one of the forerunners of wave power devices. It has been developed over many years by Stephen Salter and the Wave Energy Group at Edinburgh University. Falling under the category of a point absorber, the Duck faces into the direction of the waves and the “beak” moves up and down with the wave motion, while the “bottom” section remains stationary, anchored to the sea bed by a mooring device. An array of around 30 ducks would sit interconnected along a spine, all bobbing in differing phases. Since its conception in the 1970s the Duck has undergone various alterations in design and this report will look at some of the technical alterations between the 1982 Duck and the current day Duck. Although the Duck is one of the most famous offshore devices, as well as being the most highly developed, it has never seen any commercial success, although in the 80s it did come close to government funding.
The research on the Duck has so far been performed under lab conditions, with Stephen Salter saying he wants to solve all the problems before putting the device to sea. Some people are critical of this approach and believe it to be counter productive for the development of the Duck commercially.
The tests on the Duck have required the development of highly complex wave tank systems, designed by the Edinburgh Wave Energy Group. Many test tanks in use for other wave energy devices are developed by the Edinburgh Wave Energy Group, or using many of their revolutionary ideas.
There was one high profile use of a duck outside the laboratory environment, but this was infact a fake duck created by XXXX University [4]. This was tested on Loch Ness and showed good performance, until it sank. Steven Salter claims these tests to have damaged the Duck’s reputation, however many believe it highlighted the feasibility of wave power to the general population.
The main idea behind the Duck was for a device to gain the most energy available from the waves [4]. To achieve the highest power the Duck was optimised for use in depths of around 80m, far offshore. This approach meant that many inherent problems of the ocean environment would need to be tackled and it was understood from early stages that it would be a time intensive product development. Because the design was so innovative many new technologies have been developed by the Edinburgh wave research team for testing wave devices and for the functioning of them. If the Duck never manages to become commercially realised, no one can deny the invaluable contribution its development has brought to wave power technologies.
Technology
Gyroscope - The original idea for the Duck met problems in the power take of, because motion was slow and unstable. For the 1983 Duck design Salter had a breakthrough on the power take of in his idea for use of gyroscopes [7].
The main problem with the gyroscopes was one of cost. It was a very complex system, to complex to explain in this report, and the parts were not only expensive, but difficult to protect from the elements, which drove the costs up very high. Also the gyroscopes weighed a great deal, which lowered the efficiency of power conversion.
Ring-cam pump - When Salter heard about a new material called Ceremax, which was a ceramic coating for offshore devices, he discarded the gyroscopes for a simpler Ring-cam pump design [7]. The ceremax prolongs the life of parts before corrosion sets in. With the ring-cam pump, the motion of the duck bobbing on the waves, relative to the stationary spine of the duck, forces a ring to move across a hydraulic fluid filled container, pumping the fluid into a digital hydraulic motor, known as the “wedding cake”. I believe this motor to be an early stage of the hydraulic motors developed for some modern wave technologies. This pump set-up allowed transfer of twice the torque of the gyroscope design, with only half the mass [4].
Current Situation
It is hard to asses the current position of the Duck technology, because there is little information about any recent development. As mentioned previously, there are many aspects of the Duck’s development that have contributed to other technologies. Perhaps the most the Duck it will ever contribute to a UK renewable energy solution is to be a source of ideas, but never more than a concept.
Attenuators
An attenuator works by the motion of one part relative to another moving part. Unlike in a point absorber there is no fixed part, also there is often no restriction to the direction of motion for power take off, although the amount of energy taken from the waves tends to be reduced.
Pelamis
Introduction
The Pelamis could be considered the most commercially developed of the UK’s offshore wave power devices, with a test device contributing up to 750 kW to the national grid. It looks much like a large snake floating on the sea, consisting of 4 sections held by 3 hinged joints, totalling 150m in length and 3.5m diameter. Power is generated through the motion of each section relative to the others as the wave crests roll by. The motion of the sections is resisted by hydraulic rams, which pump high pressure oil through a hydraulic motor, which in turn drives an electrical generator.
The company Ocean Power Development was set up in 1998 with the specific goal of developing the Pelamis wave power device. The concept of the Pelamis can perhaps be attributed back to Sir Christopher Cockerell, the inventor of the hovercraft. In the 1970s he came up with the idea of interconnected rafts, bucking in respect to one another due to the wave’s motion. Sir Cockerell has a long history working with ships and has spent a great deal of time working to stop boats buckling in the waves, which led to the ideas of designing the opposite. Cockerell’s design was the first device falling in the category of a hinged contour device and the design of the Pelamis is clearly along the same lines, although much of the technology and concepts are significantly moved on from Sir Cockerell’s original ideas.