Tidal power plant

The invention:

Plant that converts the natural ocean tidal forces
into electrical power.

The people behind the invention:

Mariano di Jacopo detto Taccola (Mariano of Siena, 1381-1453),
an Italian notary, artist, and engineer
Bernard Forest de Bélidor (1697 or 1698-1761), a French engineer
Franklin D. Roosevelt (1882-1945), president of the United States



Tidal Energy

Ocean tides have long been harnessed to perform useful work.
Ancient Greeks, Romans, and medieval Europeans all left records
and ruins of tidal mills, and Mariano di Jacopo included tidal power
in his treatise De Ingeneis (1433; on engines). Some mills consisted of
water wheels suspended in tidal currents, others lifted weights that
powered machinery as they fell, and still others trapped the high
tide to run a mill.
Bernard Forest de Bélidor’s Architecture hydraulique (1737; hydraulic
architecture) is often cited as initiating the modern era of
tidal power exploitation. Bélidor was an instructor in the French
École d’Artillerie et du Génie (School of Artillery and Engineering).
Industrial expansion between 1700 and 1800 led to the construction
of many tidal mills. In these mills, waterwheels or simple turbines
rotated shafts that drove machinery by means of gears or
belts. They powered small enterprises located on the seashore.
Steam engines, however, soon began to replace tidal mills. Steam
could be generated wherever it was needed, and steam mills were
not dependent upon the tides or limited in their production capacity
by the amount of tidal flow. Thus, tidal mills gradually were abandoned,
although a few still operate in New England, Great Britain,
France, and elsewhere.

Electric Power from Tides

Modern society requires tremendous amounts of electric energy
generated by large power stations. This need was first met by
using coal and by damming rivers. Later, oil and nuclear power became
important. Although small mechanical tidal mills are inadequate
for modern needs, tidal power itself remains an attractive
source of energy. Periodic alarms about coal or oil supplies and
concern about the negative effects on the environment of using
coal, oil, or nuclear energy continue to stimulate efforts to develop
renewable energy sources with fewer negative effects. Every crisis—
for example, the perceived European coal shortages in the
early 1900’s, oil shortages in the 1920’s and 1970’s, and growing
anxiety about nuclear power—revives interest in tidal power.
In 1912, a tidal power plant was proposed at Busum, Germany.
The English, in 1918 and more recently, promoted elaborate schemes
for the Severn Estuary. In 1928, the French planned a plant at Aber-
Wrach in Brittany. In 1935, under the leadership of Franklin Delano
Roosevelt, the United States began construction of a tidal power
plant at Passamaquoddy, Maine. These plants, however, were never
built. All of them had to be located at sites where tides were extremely
high, and such sites are often far from power users. So
much electricity was lost in transmission that profitable quantities
of power could not be sent where they were needed. Also, large
tidal power stations were too expensive to compete with existing
steam plants and river dams. In addition, turbines and generators
capable of using the large volumes of slow-moving tidal water that
reversed flow had not been invented. Finally, large tidal plants inevitably
hampered navigation, fisheries, recreation, and other uses
of the sea and shore.
French engineers, especially Robert Gibrat, the father of the La
Rance project, have made the most progress in solving the problems
of tidal power plants. France, a highly industrialized country, is
short of coal and petroleum, which has brought about an intense
search by the French for alternative energy supplies.
La Rance, which was completed in December, 1967, is the first
full-scale tidal electric power plant in the world. The Chinese, however,
have built more than a hundred small tidal electric stations about the size of the old mechanical tidal mills, and the Canadians
and the Russians have both operated plants of pilot-plant size.
La Rance, which was selected from more than twenty competing
localities in France, is one of a few places in the world where the
tides are extremely high. It also has a large reservoir that is located
above a narrow constriction in the estuary. Finally, interference with
navigation, fisheries, and recreational activities is minimal at La
Rance.
Submersible “bulbs” containing generators and mounting propeller
turbines were specially designed for the La Rance project.
These turbines operate using both incoming and outgoing tides,
and they can pump water either into or out of the reservoir. These
features allow daily and seasonal changes in power generation to be
“smoothed out.” These turbines also deliver electricity most economically.
Many engineering problems had to be solved, however,
before the dam could be built in the tidal estuary.
The La Rance plant produces 240 megawatts of electricity. Its
twenty-four highly reliable turbine generator sets operate about 95
percent of the time. Output is coordinated with twenty-four other
hydroelectric plants by means of a computer program. In this system,
pump-storage stations use excess La Rance power during periods
of low demand to pump water into elevated reservoirs. Later,
during peak demand, this water is fed through a power plant, thus
“saving” the excess generated at La Rance when it was not immediately
needed. In this way, tidal energy, which must be used or lost as
the tides continue to flow, can be saved.

Consequences

The operation of La Rance proved the practicality of tide-generated
electricity. The equipment, engineering practices, and operating
procedures invented for La Rance have been widely applied. Submersible,
low-head, high-flow reversible generators of the La Rance
type are now used in Austria, Switzerland, Sweden, Russia, Canada,
the United States, and elsewhere.
Economic problems have prevented the building of more large
tidal power plants. With technological advances, the inexorable
depletion of oil and coal resources, and the increasing cost of nu-
clear power, tidal power may be used more widely in the future.
Construction costs may be significantly lowered by using preconstructed
power units and dam segments that are floated into place
and submerged, thus making unnecessary expensive dams and reducing
pumping costs.



See also : Compressed-air-accumulating power plant; Geothermal power; Nuclear power plant; Nuclear reactor; Solar thermal engine; Thermal cracking process.