UK could get a quarter of its energy from tidal power

Tidal energy – the UK’s best kept secret-Carbon Commentary.Tidal energy could provide a quarter of the UK’s electricity, but renewable experts are lukewarm because they are overestimating the cost

Underwater 10 megawatt tidal stream project in the Sound of Islay between the
Hebridean islands of Islay and Jura. Photograph: ScottishPower Renewables
The latest report on Renewables from the Committee on Climate Change (CCC)
offers lukewarm support for electricity generation from tidal streams. The UK
has some of the fiercest tidal currents in the world, but the CCC says the tidal
turbines will deliver energy at a higher cost than PV in 2040. The assumptions
behind this pessimism are questioned in this article.

The tides around Britain’s coasts sweep huge volumes of water back and forth at
substantial speeds. The energy contained in the tidal races off the west of the
UK is as great as anywhere in the world. Because water is a thousand or so times
heavier than air, the maximum speeds of perhaps 6 metres a second are capable of
generating far more electricity per square metre of turbine area than a
windmill. The Pentland Firth, the narrow run of water between the north-east tip
of Scotland and the Orkney islands, is possibly the best place in the world to
turn racing tides into electricity. The challenges are immense: massive steel
structures need to be made that survive huge stresses, day after day.

The rewards for tidal stream developers are commensurate. Unlike other renewable
technologies, tidal power is utterly predictable for the entire life of a
turbine. We know to the minute when the tides on a particular day will be at
their peak. Once installed, the running costs of tidal stream technology will be
low. The environmental impact of tidal turbines appears to be very small. And
the UK could probably provide a quarter of its electricity from tides. (And much
more if an environmentally acceptable means was found of damming the Severn

The CCC might then have been expected to push for a significant programme of
support for tidal. Its reservations appear to be as follows.

a) Tidal generation does not help with the ‘intermittency’ problem of renewables

b) The levels of yield are relatively low. (Yield is the percentage of rated
power that can be delivered in a typical day.)

c) The cost of capital is high for a developer using tidal turbines because of
the risk of the technology not working

d) The relatively small scope for learning curve improvements.


An individual tidal turbine will generate most electricity when the tide is
running fastest. This will be at approximately the mid point between high and
low tides. The CCC therefore says that tidal power will not help deal with
periods of low production.

The cycle of marine power (tidal plus wave) suggests that total output will fall
to zero four times a day. This would only be the case if all the turbines were
sited at the same place. Turbines placed, as they will be, all around the coasts
of Great Britain will generate maximum power at different times of the day. On
the day I looked at the tide tables, the tides in the Channel Islands (where
there are some extremely powerful races) were completely unsynchronised with the
tides in northern Scotland. Two turbines, one off Alderney, one off John
O’Groats, would together produce substantial amounts of (entirely predictable)
power every second of the day. Tidal power is as dispatchable as nuclear.

Yields are low

The CCC offers a view as to the output of a tidal turbine, suggesting that in a
‘high’ case the figure will be 40%. That is, the average electricity output of a
1MW turbine over the course of a year will be 400kW.

Actually, the one piece of reliable data on this number suggests a much higher
figure. The UK’s hugely impressive tidal turbine developer, Marine Current
Turbines (MCT), has had a device in the waters of Strangford Lough for several
years. This early turbine has produced 50% of its rated power. The difference is
important: it means that electricity generation costs are 25% lower than the CCC
would otherwise have predicted.

The cost of capital is high

I think the CCC – normally so forensically rigorous – makes an error here,
guided by its capital markets advisors Oxera. The CCC suggests that capital
projects have to earn a return determined by the ‘riskiness’ of the investment.
The debate over what types of ‘risk’ need to be paid for is complex and almost
theological in its intensity. But I will not argue about this and will accept
that early tidal power projects are ‘risky’ and that investors will therefore
expect high returns to compensate for their exposure.

But let’s dissect what the ‘risk’ of a tidal project actually is. At its
simplest, it is that the technology will fail. And, indeed, most tidal turbines
have simply broken into pieces in the early months of their life in the seas.
But this is the only risk. Once working successfully, the tidal currents will
flow for as long as the moon circles the earth. There are no commodities markets
to disrupt the returns, no risk of increased operating costs once the technology
is proven. To say that tidal has a high cost of capital is wrong: the early
developers take big risks but once the technology matures, the operating risk
disappears. The right assumption to make about tidal is that has huge cost of
capital today, but will have very low rates in the future once the technology is
proven. Instead, the CCC’s advisers weight tidal down with high returns on
capital for ever. This unfairly penalises tidal stream power, and all other
sources of energy in their early stages of development.

Small scope for learning curve improvements

Other renewable technologies have generally reduced in underlying cost by 10 or
15% for every doubling of the output of these devices. (This is an utterly
standard ‘experience’ effect- we’ll assume tidal turbine costs only fall by 10%
for each doubling).

To date, the world tidal industry has probably installed less than 20 full-scale
production devices on the seabed. In fact, you could plausibly say that the MCT
Northern Irish turbine is the only such turbine. Assume nevertheless that
today’s accumulated production experience is 20 units.

But the CCC, advised in this case by Mott McDonald, says that costs today are
about 20.5 pence per kilowatt hour of electricity generated and will only fall
to 15.25 pence in 2040, a reduction of slightly more than 25% (the mid-point of
cost ranges in the CCC report). The learning curve model assumes that a 10%
reduction will typically come after a doubling of total production to 40 units.
A further 10% reduction comes when accumulated volume rises to 80.

The arithmetic is not complex. If Mott McDonald thinks that the costs will only
fall to 15.25, it must believe that the worldwide tidal industry will install
less than 160 turbines before 2040.

The CCC’s analysis locks tidal stream technology into relative failure. Costs
are high, and the technology risk is great. So no developers use the tidal
turbines and costs remain stubbornly high. The cycle continues. Of course this
could indeed be the future. But with sustained effort and support, tidal energy
may become of the UK’s most important industries. In MCT – a business few people
have ever heard of – the country has the most technically advanced marine energy
company in the world. I think it deserves all the backing it can get.
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