Innovation in Marine Renewable Energy
Mr Mike Todman – Tidal Stream.
This lecture presented the range of marine renewable energy options. Approximately
15% of the UK’s total energy requirement could be supplied by marine energy.
To set the scene, a review of the competing types of power generation was presented.
The field is still new so innovation in the field is strong. Approximately 150 design
concepts have been prepared By way of illustration, some of the leading contenders
were presented.
The main groups are:(i) Wave power – near shore
(ii) Wave power – far shore
(iii) Tidal range
(iv) Tidal stream
Wave Power – near shore.
Examples of near shore wave power concepts presented were the Voith oscillating
water column see http://voith.com/en/products-services/hydro-power/oceanenergies/wave-power-plants-590.html for a video illustrating the concept, ie the rising
and falling waves push / suck air enclosed in a chamber through a turbine. These were
said to be noisy. A small unit has been installed in Scotland. Another concept
described was Aquamarine’s “Oyster” oscillating wave-surge converters, a
description can be found at http://www.aquamarinepower.com/technology/ . A third
was CETO’s “buoys on bike pumps”
http://www.carnegiewave.com/index.php?url=/ceto/ceto-overview and fourth concept
shown was the OPT power buoy http://oceanpowertechnologies.com/technology.htm .
It was explained that near shore has the advantage of good access, but a random
energy source.
Wave Power – far shore.
In contrast, far shore is difficult of access, but does offer reliable waves. The
examples of far shore wave power generation presented included the Pelamis seasnake http://www.pelamiswave.com/pelamis-technology. The caveat with this method
is that thus far demonstrators have not delivered power to their design specification
being approximately a factor of 10 down. These worked by exploiting the articulation
of the 4-part snake to direct hydraulic fluid through turbines. Another concept was the
overtopping barge, where water filled a barge and flowed out through power turbines,
and another an upturned bedstead!
Tidal range.
The earliest example of a tidal range installation is the 1966 La Rance tidal power
station in Brittany, France https://en.wikipedia.org/wiki/Rance_Tidal_Power_Station .
This facility continues to develop 240MW of power.
Tidal stream.
This offers substantial potential power generation capability around the UK (and
elsewhere) Values of 10GW annually for the UK, representing about 50% of Europe’s
tidal energy potential. It was suggested that across the globe, there is a potential for
the generation of up to 100GW / year.
The illustration above shows the tidal power around the UK, more detail can be found
at:
http://www.renewables-atlas.info/downloads/documents/Renewable_Atlas_Pages_A4_April08.pdf
The most notable location is the Pentland Firth stream between mainland Scotland
and the Orkney islands. This has stream velocities up to 12mph, with an average
stream speed of 5mph and is approximately 70m deep. The flow is equivalent to
moving 3million tonnes of water a second, or emptying / re-filling the North Sea each
tide!
A number of designs were discussed, the more promising designs were horizontal axis
pointing into the flow. Two examples are http://www.swanturbines.co.uk/ and
http://www.marineturbines.com/SeaGen-Products/SeaGen-S . A feature of these two
concepts is the use of very large rotors (up to 20m diameter, which would give
approximately 1MW output per rotor) and need large vessels to deploy them and
deployment requires a calm sea state – often at odds with the chosen location for
maximum power potential. Installation via bespoke heavy-lift shipping could cost
£100-£300k / day for vehicle hire!
Another general marine energy risk mentioned was the mechanical integrity of a
system. We were told of one installation where the stream proved too much for the
GRP covering of the spars which was stripped off by the flow! The current thinking
regarding maintenance was to plan for 3 scheduled services / year and expect up to 3
unscheduled services. These estimates are based on current offshore wind turbine
experience. These are important reasons to consider mechanical integrity and
deployment.
The marine environment imposes huge demands on mechanical integrity. For a 1MW
output, torque and speed for different power generator types are shown in the table
below.
Tidal Stream’s response to the mechanical integrity challenges of deployment and
operation in the marine environment was to use a set of modest size (up to 5m
diameter) horizontal axis machines in a large array. This was a route that the German
company Schottel wished to pursue and the two companies are now working together
to develop and implement the technology.
After development work, including 1/20th and 1/10th scale testing they settled on a
catamaran frame that could be floated out – without the need of a specialist vessel and
positioned by simple chamber flooding. The frame structure is tethered to a base with
an articulating joint so that it could follow the tide and stream. Established slip ring
technology was sufficient for up to 10MW power offtake.
TidalStream, with their partners Schottel have won funding to deploy a full size
prototype (see below) in Nova Scotia’s Bay of Fundy. The plan is for a phased
deployment of all the 36 rotors over 3 years.
Their website, http://www.tidalstream.co.uk/index.html provides useful details of
their systems, including a description of their design evolution and fact sheets of their
Bay of Fundy system. A fast moving presentation was followed by a multitude of
answers to the interested questions of the audience.
Neville Pollard, April 2014