9.6 Can tidal power be harnessed as a source of energy?

  • during high tide, water can be trapped in a basin and then harnessed to do work as it flows back to sea

    • ex, water wheels driven by tides were used to power gristmills and sawmills in 12th century

    • 17th and 18th centuries - much of boston’s flour was produced at a tidal mill

  • advantages:

    • initial building cost of tidal power plant is higher than conventional thermal power plant, but operating costs are lower bc it doesn’t use fossil fuels or radioactive substances to generate electricity

    • tidal power = clean, renewable source w/ vast potential

  • disadvantages

    • periodicity of tides - power can only be generated during a portion of the 24-hour day

    • people operate on a solar period but tides operate on a lunar period, so energy available from tides would coincide with need only part of the time

    • power would have to be distributed to the point of need at moment it was generated which could be a great distance away = expensive transmission problem

    • power could be stored but it’s expensive and technically difficult

    • to generate electricity effectively, electrical turbines need to run at a constant speed, which is difficult to maintain when generated by the variable flow of tidal currents in two directions (flood tide and ebb tide)

      • specially designed turbines that allow both advancing and receding water to spin their blades are necessary to solve this issue

    • environmental concerns i.e. change of habitat and harm to wildlife

      • most tidal plants have a dam that alters the ecology of estuaries

    • disturb normal flow of tidal currents negatively affecting marine organisms that depend on the currents for food or migration

    • marine organisms can be trapped or injured by moving tidal power devices

    • underwater turbines produce noise that can disturb animals

    • interferes with traditional human uses of estuaries i.e. transportation and fishing

Tidal Power Plants

Tidal power can be harnessed in 2 ways: (1) tidal water trapped behind coastal barriers in bays and estuaries during high tide can be released at a later time to turn turbines and generate electrical energy and (2) tidal currents that pass through narrow channels can be used to turn underwater pivoting turbines which produce energy

  • (1) is more common

  • Ex. 1966, Saint-Malo France, on estuary of the La Rance River

    • estuary has surface area of approx 23 square km, tidal range of 13.4m

    • power-generating barrier built across estuary over 3km upstream to protect it from storm waves,

    • barrier - 760m wide

    • water passing through barrier powers 24 electricity-generating units that operate beneath the power plant

    • peak operating capacity = 10 megawatts per unit = 240 megawatts total

    • to generate electricity, needs sufficient water height between estuary and ocean, which only occurs half the time

worldwide, only 6 other tidal power plants besides La Rance exist, with small power generation of less than 2 megawatts

**general rule of tidal power = usable tidal energy increases as the area of the basin increases and as the tidal range increases

CONCEPT CHECK 9.6

(1) discuss at least two positive and two negative factors related to tidal power generation.

  • positives:

    • clean, renewable energy

    • lower operating costs than thermal energy plants

  • negatives:

    • can harm marine organisms

    • generate power w/ lunar day, not solar day, and humans function on the solar day

(2) explain how a tidal power plant works, using as an example an estuary that has a mixed tidal pattern. why does potential for usable tidal energy increase with an increase in the tidal range?