1. Energy extraction potential
how much energy might be available?

2. Tidal flow power extraction potential
tidal current flows for around six hours in each direction
tidal flow reverses direction in less than half an hour
this means that power can be generated for around
11½ hours out of each 12 hour, 25 minute tidal cycle
different parts of the country have different flow reversal times
true base load power generation is possible if units
are installed at two or more complementary sites
whenever one barrier is either idling or stopped,
the complementary one is generating at full capacity
electricity from these units is entirely predictable and thus will be much more acceptable to the national supply grid than the electricity produced by wind or wave generators
diagram only
not to scale
© clf anderson
plan view

3. standard “squirrel cage” electric motors can be used as
the geometry of the guide chamber ensures that the rotors always start and rotate in the same direction regardless of the direction of tidal flow
standard “squirrel cage” electric motors can be used as
induction generators when directly connected to the grid
the generators will run ahead as motors (consuming
only 2% of installed power) when the rotor stops
this avoids the need for synchronisation of national
grid phase and alternating current frequency
because there is no switching, no mechanical, thermal
or magnetic shock is applied to the motors
because of this they can be rated considerably higher
as generators than their nominal rating as motors
if their generator rating is doubled, their power
consumption when motoring will be halved to 1%
roller clutches (ratchets) will pick up the drive to
restart generation when the rotor restarts
diagram only
not to scale
© clf anderson
plan view

4. * as used on typical wind turbine generator drives
plan view
Typical tidal flow extraction installation
this guide chamber is 45 metres wide x 110 metres long
the rotor is 34 metres diameter x 34 metres depth
it rotates at around 2½ revolutions per minute
it is supported on 100 bogies which rotate at 55 r/min,
each bogie drives a standard squirrel cage motor
(or motors) which operate as induction generators
multi-pole winding could accommodate speed changes
each generator is driven by a step-up ratio * gearbox
or a hydrostatic positive displacement hydraulic drive
each generator provides 250 kilowatts (330 bhp) †
when running on a 2 metres head difference between
the water levels upstream and downstream
the rotor will provide 25 megawatts to the national grid
for approximately 23 hours per day every day of the year
* as used on typical wind turbine generator drives
diagram only
not to scale
© clf anderson
† helicopter roller clutches can handle 3,000 bhp

5. Calculations
Performance of a typical single rotor tidal flow extraction installation
Assuming a two metre difference in water level across the diameter of the rotor:
Two metres head (2gH) equals 6.26 metres per second flow through the rotor
34 metre diameter wheel, 10.6 m chamber entrance throat width, 34 m deep
Flow equals 10.6 x 34 x 6.2 equals 2,256 cubic metres (tonnes) of water per second
2,256 m3 x 0.02 MPa equals 45.1 liquid megawatts (59,300 liquid horsepower)
x 70% rotor blade efficiency equals 31.5 megawatts shaft power (41,000 bhp)
x 80% transmission/generator efficiency equals 25.2 megawatts to the national grid
Wheel peripheral velocity equals 6.26 x 0.7 equals 4.38 m/sec (8 mph)
34 metres diameter wheel is metres circumference
divided by 4.38 m/s equals 24.3 sec, ie 2.47 revolutions per minute of waterwheel.

6. Calculations
Horizontal force applied to the tidal barrier generator unit:
Assuming a two metre difference in water level across the width of the unit
45 metres width x 35 metres deep equals 1,575 square metres:
1,575 square metres x 2 metres head difference in water level across the barrier
equals 3,150 tonnes horizontal force against the barrier unit

7. Calculations
Mass of each barrier unit base plate is 45 x 110 x 3 x 1.6 equals 23,760 tonnes
Mass of the barrier unit side walls is 110 x 40 x 10 x 1.6 equals 70,400 tonnes
Mass of the generator machinery, equipment and housing at say 5,000 tonnes
Mass of the deck and the rotor is 15,000 tonnes
Total mass of each barrier unit is 114,000 tonnes
Buoyancy force applied to the underwater concrete parts equals 1 tonne/M3
Subtract 52,250 tonnes buoyancy for the base plate and the submerged walls
114,000 minus 52,250 tonnes equals 61,750 tonnes weight on the seabed
3,150 tonnes side force divided by 61,750 tonnes weight of the unit on the seabed
equals 0.05 co-efficient of friction required between the base plate and the seabed
filling the buoyancy chambers with sand will significantly increase the weight
ridges cast onto the base plate will help to grip the seabed
venturi passages may be installed which apply a vacuum to the base plate bottom
if necessary, each unit may be fixed in place using drilled or driven piles

8. plan view of a typical 150 megawatt partial barrier in a sea passage
there are six rotor units
in this 150 MW barrier section
rotors would be built in a
range of standard diameters
and the rotor blade length
and the height of the guide
chamber would be varied
to suit the depth of water
at the installation site
320 metres
230 metres
© clf anderson
diagram only
not to scale
plan view of a typical 150 megawatt partial barrier in a sea passage

9. plan view of a typical 150 megawatt partial barrier in a sea passage
rotor units are installed at 45º to the direction of tidal current flow
this allows more rotor units to be installed in the width of the channel and allows floating objects to be swept safely past the barrier
320 metres
230 metres
© clf anderson
diagram only
not to scale
plan view of a typical 150 megawatt partial barrier in a sea passage

10. plan view of a typical partial barrier in a tidal passage
clockwise rotors
on this section
anticlockwise rotors
on this section
shipping and escape passage
between barrier
sections
flow guides may be
added at each leading
corner to suit the
local geography
Note: Coriolis forces may
influence the performance
of such large rotors
© clf anderson diagram only not to scale

11. each of these six-rotor barrier sections will generate 150 Megawatts
Plan view of a practical and effective alternative tidal flow energy extraction system
230 metres
each single rotor unit will generate 25 Megawatts for around 23 hours per day
each of these six-rotor barrier sections will generate 150 Megawatts
this group of four sections will provide 600 Megawatts
Scotland’s total electricity demand is around 6,000 Megawatts
ten of these four-unit barrier groups installed in tidal channels around the country
will provide all of this demand with no adverse impact on the environment
overlapping generating periods will provide continuous power
excess “off-peak” and spring tide capacity can be used to produce hydrogen
apart from capital and maintenance costs, the electricity is free
© clf anderson diagram only not to scale

12. this group of barrier sections will produce 900 Megawatts
Island of
Stroma
Skerry
Duncansby
Head
St Johns
Point
Gills Bay
Ness of Hunna
Caithness
Pentland Firth
Inner Sound of Stroma
Swilkie Point
Scarton
Langaton
Red Head
the island forms
part of the barrier
900 mw
capacity
barrier 1
kilometre
the Inner Sound of Stroma with six tidal barrier sections in 30 metres depth of water
this group of barrier sections will produce 900 Megawatts
shipping can pass freely around or between the barrier sections