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 B'elidor (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 B'elidor's Architecture hydraulique (1737; hydraulic

architecture) is often cited as initiating the modern era of

tidal power exploitation. B'elidor was an instructor in the French

'Ecole d'Artillerie et du G'enie (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.