A linear generator has been modified in a bid to fulfil the promise of cheap wave energy.
Snapper: a low-cost wave power generator
Narec, EM Renewables, Meccanotecnica Riesi, Technogama, SubseaDesign, Ecotricity, Ocean Resource, University of Edinburgh
There is no shortage of superlatives and statistics to describe the potential of the oceans and seas to meet our ever-expanding energy needs. Indeed, one oft-quoted study by Oregon State University in the US estimates that just 0.2 per cent of the oceans’ power, in the form of waves, tides, salinity and more, would be needed to power the entire world.
For the UK particularly, with its extensive and often turbulent coastline, this has added relevance. Indeed, wave and tidal power is frequently hailed as the one truly reliable and predictable source of renewable energy.
Devices to harness the power of tidal streams are now progressing well, with the Scottish government recently approving plans for 10 tidal turbines, which will generate enough electricity to power more than 5,000 homes.
However, the technology is relatively expensive and potentially difficult to maintain – essentially being wind turbines adapted to work under water.
And so, arguably, wave power – which at least partly operates at the surface – is an even better prospect for energy generation.
The preceding decades have seen a number of novel concepts for wave power devices, the simplest of which are linear generators that essentially comprise a floating buoy system that undulates with the sea swells. An electric coil inside the buoy surrounds a magnetic shaft, which is anchored to the sea floor. When waves cause the coil to move up and down relative to the fixed magnetic shaft, voltage is induced and electricity is generated.
However, while such devices are cheap to build, they generally do not generate enough energy, owing to a universal problem with wave power: speed.
’Comparing wind with wave, the fluid you’re dealing with is much more dense and there is much more energy to capture in the same sort of volume of fluid,’ said Paul McKeever of the National Renewable Energy Centre (Narec), a research organisation that supports fledgling renewables technologies, particularly in the marine sector. ’In some ways, that’s an advantage, but also a different challenge.
It is essential to include some form of velocity amplification within a wave power device if it is to be cost effective
’One of the challenges of wave energy is that you’re dealing with quite low velocities and the thing about electrical generators, particularly rotating machines, is that they tend to work much more efficiently at high speed. So if you’re dealing with low velocities, to get the same sort of power index you have to build quite bulky devices because you’re having to create more torque.’
Indeed, wind turbines are generally rated for wind speeds of 10-15m/sec and tidal-stream devices are rated for flow velocities of up to 4m/sec; however, wave power devices must operate with reciprocating motion with velocities of less than 1m/sec.
It is therefore essential to include some form of velocity amplification within a wave power device if it is to be cost effective, according to McKeever.
Narec came across a possible solution to the problem a few years ago, after hearing about the work of a researcher at Durham University, Prof Ed Spooner, who is now at EM Renewables. Dubbed the Snapper, his concept was based on a linear generator, but with an important difference.
Like a typical linear generator, it has a set of magnets mounted in a translator moving up and down inside the multiple coils of wire of an armature. But alongside the armature coils is a second set of magnets of alternating polarity.
When the force exceeds a certain threshold, the magnetic coupling can no longer transmit the force and it snaps, allowing the energy stored in the spring to be released quickly and efficiently.
Therefore, the continuous slow movement of the wave power machine is divided into a sequence of rapid bursts in a ’snapping’ motion.
Narec saw in Snapper the potential for the long-heralded promise of cheap wave energy. ’Effectively, it’s our mission statement,’ said McKeever. ’Narec was set up to help advance renewable technologies, particularly with inventors and developers that might be quite small and don’t have the resources, whether financial or [the] teams of people [needed] to do the development.’
He added: ’The appeal of Snapper was that it wasn’t just another type of technology, but it was building on an existing technology with linear generators, and trying to improve one of the fundamental problems of marine devices, which is effectively cost per kilowatt hour.’
Narec gathered partners from around Europe and managed to secure coveted funding under the European Commission’s Framework Programme 7 (FP7) for a two-year project to develop Snapper.
A total of eight organisations are involved in the Snapper Consortium – six industrial partners (EM Renewables, Meccanotecnica Riesi, Technogama, SubseaDesign, Ecotricity and Ocean Resource) and two research organisations (Narec and the University of Edinburgh).
’The idea of building a consortium was that we wanted to try and put a group of organisations together that could effectively play a part in the overall development,’ said McKeever. ’So some of the SMEs [small and medium-sized enterprises], for example, have subsea-design experience or experience with buoy devices; others would have electrical experience. One of the SMEs was precision manufacturing, which came in very useful for building the prototype. So there was a reasonable spread of skills there.’
“We’ve got the option of developing the entire system from buoy to grid connection”
PAUL MCKEEVER, NAREC
The first challenge for the consortium was the development of a robust and detailed simulation environment of the electrical machine characteristics and its hydrodynamic interface in real sea conditions. Next came the design and build of the individual mechanical and marine components, and electromagnetic and power-conditioning aspects.
These were then tested on a linear rig to simulate marine field conditions within a dry environment. This validated the performance against technical objectives such as the electrical grid connection.
Finally, the team assessed the function and integration of the generator when tested in a wet marine environment. The FP7 project is now complete and the consortium was in the process of filling in the required reports at the time of going to press.
Of course, the ultimate aim is to develop a prototype wave energy capture and conversion system that is appropriate for deployment in near and offshore environments. Water depths of up to 60m are targeted, along with a lifetime cost of energy of €0.15/kWh (£0.13/kWh).
Based on an analysis of materials required for the Snapper device, the capital costs will be significantly less than that for current offshore wind energy devices at around €2,900 per kilowatt and 4.1 tonnes per megawatt.
’The beauty about Snapper is that it’s a power-generation device, so we’ve got the option of developing the entire system from buoy to grid connection or we could actually just develop the generator and maybe incorporate that into another developer’s device structure,’ said McKeever. ’It’s not just a wave energy device; it’s got the potential to be used in other applications.’
The other shortlisted candidates in this category were:
SEARASER: A near-shore buoy device that pumps water on high to be used later in hydroelectric conversion
Dartmouth Wave Energy; Exeter University
The SEARASER is a simple vertical pump that moves up and down with the crest and troughs of waves in open sea, near to the coastline. It pumps water to a high head-holding tank or purpose-built reservoir on shore (anything up to 100m above sea level), then sends it back down to the sea-driving turbines, thus enabling on-demand hydro energy in the form of stored water.
TIDALDESIGN: Delivering low-cost tidal turbines with novel material combinations and manufacturing processes
University of Cambridge; Green-Tide Turbines; TWI; Net Composites; NewPro Foundries; Alpha-Electotech
The project aims to make use of new material combinations and manufacturing processes capable of delivering low-cost and robust turbine blades suitable for mass production. Particular emphasis is placed on the use of materials with low embodied carbon emissions and also the potential use of recycled materials such as PET from drinks bottles.