Page 155 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Other Forms of Ocean Energy Chapter | 6 145

persistence of ocean currents and their lack of lunar influence (because they
are indirectly governed by the Sun, i.e. they are driven by winds) could make
them attractive from a daily/weekly and seasonal electricity demand perspective.
However, there are clearly significant challenges in attempting to convert the
theoretical ocean current resource into practical electricity.

6.2.2 Technology Types
Although there is clearly a significant ocean current resource, including a
‘technical resource’ where the currents pass relatively close to coastlines and
population centres, designing a device (or an array of devices) to economically
convert energy within such environments will be challenging. Due to increased
water depths (of order several hundred metres) compared with typical tidal
energy environments (Chapter 3), it is likely that any device that is to exploit
the ocean current resource will be tethered to the sea bed with cables, with
the relatively constant current interacting with the turbine(s) used to maintain
position and stability [10]. The ‘Kuroshio power plant’ concept proposed off the
coast of Taiwan consists of three clusters of turbines, each individually anchored
to a floating platform that can deform with the variable ocean currents, and
anchored to the sea bed using a large number of cables [11]. It is recommended,
to avoid damage to the power plant from large typhoon-generated waves in
this region, that the turbines be placed at least 30 m below the sea surface.
Shirasawa et al. [12] propose an ocean current design based on a horizontal
axis turbine that is tethered 100 m (hub height) below the sea surface. Although
a 2.3 m diameter prototype has been tested using towing experiments at sea, it
is claimed that an array configuration of 300 × 80 m diameter turbines based on
this design could produce up to 1 GW of electricity from the Kuroshio Current
off Japan; however, clearly the technical challenges associated with such large-
scale energy conversion are immense.
From commercial and R&D perspectives, two devices have shown promise
for harvesting the ocean current resource. Again, both are tethered designs.
The Aquantis 2.5 MW C-Plane device, developed through the US Department
of Energy Aquantis project, was specifically designed to harness the energy
resource of the Gulf Stream. Although no sea trials have yet been conducted,
the device is rated at 1.6 m/s [7], and so operates at considerably lower current
speeds than typical tidal stream applications. The Minesto 500 kW Deep Green
technology (Fig. 3.26) is also suited to lower current environments and is
again a tethered device with a rated speed of 1.6 m/s. Minesto have themselves
expressed an interest in adapting the Deep Green turbine (originally intended
for low-speed tidal energy environments) for ocean current applications (www.
minesto.com).
6.2.3 Environmental Impacts

In the 2004 film ‘The Day After Tomorrow’, a slowdown in the major ocean
currents (caused by melting polar ice) results in a new ice age. Therefore, people