Environmental Engineering Reference
In-Depth Information
1.0
0.5
0
-
0.5
Ocean level
Pool level
Power
-
1.0
0
0.5
1.0
Tidal cycle time
Figure 7.22
The pool and ocean levels, and power output, of a single effect tidal power plant during one
tidal cycle. (Data from Fay, J., and M. Smachlo, 1983.
J. Energy,
7
, 529.)
pool level reaches
th
at of low tide. The maximum amount of this work is equal to the product of
the mass of fluid
AH
, the acceleration of gravity
g
, and t
he
average distance
h
by which it falls
during outflow to the sea, for a total maximum energy of
ρ
g AHh
. If the emptied po
ol
is then again
closed off from the sea until high tide occurs, an additional amount of energy,
ρ
ρ
gH A
(
H
−
h
)
, may
be reaped for a total energy of
g AH
2
Ideal tidal energy
=
ρ
(7.16)
and total average power of
g AH
2
T
=
ρ
Ideal tidal power
(7.17)
ρ
gH
2
/
.
22 W/m
2
The ideal tidal power per unit of tidal pool surface area,
T
, which equals 0
for
=
1 m, increases as the square of the tidal range, showing the importance of tidal range in an
economical power plant design.
If power is generated both during inflow to and outflow from the tidal pool, the design is called
a
double effect
plant. Because it is difficult and expensive to design a turbine and power house that
operates in both flow directions, most tidal power plants operate in the outflow direction only, called
a
single effect
plant.
24
Because
h
is usually more than half of
H
, more energy can be recovered on
the outflow than the inflow to the pool.
For a typical single effect plant, Figure 7.22 shows the pool and sea surface levels and power
output during one tidal cycle. Beginning at midtide, the sluice gates are opened and sea water flows
into the pool, filling it to the high tide level in the first quarter period, at which point the sluice is
closed. Shortly thereafter, the turbine inlet is opened and the turbine power output rises quickly to
its rated power for nearly half of the tidal cycle. When the difference in water level between pool
and ocean becomes small, near the end of the cycle, the turbine is closed down.
H
24
The La Rance plant (see Table 7.6) was built to operate as a double effect plant, including the possibility of
incorporating pumped storage, but now operates exclusively as a single effect outflow tidal plant for various
practical reasons.
