Tidal energy
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Description
In suitable locations, tidal energy may be harnessed to generate electricity. It is the most reliable and predictable of all the variable renewable energy sources, but suffers from two or, occasionally, four "dead" periods per day.
Detailed description
There are three possible technologies for the exploitation of tidal energy. They all require locations where the tidal range is important.
Barrage
A barrage requires to built across a tidal estuary.[1] Water enters through the barrage, turning a turbine, at high tide, and is allowed to escape again at low tide, again turning a turbine. There is only one major installation, across la Rance estuary in France.[2] This has a peak capacity of 240 MW and, in the 40-plus years of existence, the heavy capital costs are well amortised. There are a number of disadvantages, though:
- high capital cost
- severe bi-monthly, as well as daily, variations of capacity
- shipping is obstructed with locks slowing traffic
- the tidal characteristics of the littoral cause environmental changes, especially in salt-water marshes which are favoured by migratory and resident birds (these may be completely destroyed)
- sediment that would otherwise be taken out to sea is dropped in the basin, reducing capacity and obstructing shipping; this requires extensive and costly dredging.
Proposals have been made over many decades to construct a barrage across the Severn estuary in England. This has met with considerable opposition from a number of organisations. It would obstruct shipping to the major ports of Bristol and Cardiff; it would destroy a major conservation area run by the Severn Wildfowl Trust; it would reduce tourism as the famous Severn bore would disappear etc. This project has recently been renewed.[3]
Caisson
Multiple concrete caissons, typically several tens of metres in diameter, are inverted into the tidal waters. At high tide, the air is compressed and can turn a turbine. At low tide, air is drawn back into the caisson, again turning the turbine. This is a reasonably low-cost technology but the capacity of a single caisson is limited to tens of kW, depending on the tidal range and the caisson dimensions. The advantage is that many caissons along a coastline could harness almost constant power, because the tidal highs and lows vary in time from place to place.
Tidal flow
This depends on natural high tidal flows where a submarine turbine turns in it, analogue to a wind turbine in an air flow.
