Environmental Engineering Reference
In-Depth Information
addition some designs have emerged which combine tidal current turbines and wind turbines
mounted on the same floating platform. Not all areas with good tidal currents will necessarily
have good wind speeds, but there could be cost advantages in sharing platforms and marine
power cable links to shore. In some locations, combining tidal, offshore wind and wave
energy collection may also be an option, as has been proposed by a UK developer (Freeflow
2008)
Since getting access to the devices will be limited by the weather, and using divers will
be costly and risky, a key design issue is ensuring ease of access to blades, gearboxes and
generators for maintenance. Some devices have gearboxes and/or generators mounted above
water, and some of the floating systems can be towed back to harbour, but for those with
fixed installations, means have to be provided to lift the blades and generators out of the
water. Most devices so far have mechanical systems to do this using their support towers or
frames. But low maintenance designs will obviously have major attractions.
Another key issue is environmental impacts. As indicated earlier, this is a major concern
for tidal barrages and may also prove to be of some significance for tidal lagoons. By
contrast, most studies of tidal current turbines so far have suggested that impacts will be low.
Even large arrays will not impede flows significantly and the rotor blades will turn slowly,
slower than wind turbines, and much slower than the turbines in barrages and lagoons, and so
should not present a hazard to marine life. However all structures put in the sea will have
some impact, and this needs to be carefully assessed when considering possible locations.
Perhaps the key issue is cost. Once built, tidal barrages may be able to generate at
reasonably competitive generation costs, but their large capital costs makes it hard to see
them as viable, except as public sector led projects. By contrast, tidal lagoons might be more
viable as private investments.
At present generation costs for tidal current turbines are relatively high, but this is for
prototype devices. Some of the device teams have claimed that they can get prices down
relatively quickly, if given the right support, with talk of getting to 3-4p/kWh for serially
produced devices and then lower once the commercial market expanded.
In its 2006 review, the Carbon Trust used learning curve analysis to identify 'learning
rates' for tidal current turbines, based on the fact that costs fall as experience in installing
increasing amounts of capacity grows and as economies of volume production are achieved.
The slope of the so-called 'learning curve' produced when kWh price reductions are plotted
against kW capacity installed, with both put on logarithmic scales, was 5-10% for tidal
current turbine technology. On this basis, they suggested that generation costs might reach
2.5p/kWh, competitive with current gas-fired combined cycle turbines, by the time around 2.8
GW of capacity was in place (Carbon Trust 2006).
The large potential of tidal energy has been talked about for many years, but the
economics have often been seen as challenging. Now, with increasing concerns about climate
change and energy security, it seems that priorities may be changing and more effort is being
put into developing new technologies. While interest in barrages and lagoons is increasing,
the rapid development of tidal current technology could well mean that it will become the
leader in the field.
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