History of Tidal Power
The history of tidal power stretches into antiquity. The earliest evidence of the use of the oceans’ tides for power conversion dates back to about 900 A.D., but it is likely that there were predecessors lost in the anonymity of prehistory. Early tidal power plants utilized naturally-occurring tidal basins by building a barrage (dam) across the opening of the basin and allowing the basin to fill on the rising tide, impounding the water as the tide fell, and then releasing the impounded water through a waterwheel, paddlewheeel or similar energy-conversion device. The power was typically used for grinding grains into flour. Power was available for about two to three hours, usually twice a day.[i] In Hayle, England, tidal power was used to “dredge” a shipping channel by flushing it regularly with a pulse of stored tidally-impounded water.[ii]
Existing Tidal Power Plants
The power requirements of the industrialized world dwarf the output of the early tidal barrages and it was not until the 1960’s that the first commercial-scale modern-era tidal power plant was built, near St. Malo, France. The hydro mechanical devices such as the paddlewheel and the overshot waterwheel have given way to highly-efficient bulb-type hydroelectric turbine/generator sets. The tidal barrage at St. Malo uses twenty-four 10-megawatt low-head bulb-type turbine generator sets. Installed in 1965, the barrage has been functioning without missing a tide for more than 37 years.The second commercial-scale tidal barrage wa
s put in service at Annapolis Royale, Nova Scotia, Canada in 1982 in order to demonstrate the functioning of the STRAFLO turbine, invented by Escher-Wyss of Switzerland and manufactured by GE in Canada. This 16-megawatt turbine had some difficulties with clogging seals necessitating two forced outages, but has been functioning without interruption since its early days. There are approximately 10 small barrages scattered throughout the world, but they are not intended for commercial power generation. For example, there is a 200 kw tidal barrage on the River Tawe in Swansea Bay, Wales that operates the gates of a lock. China has several tidal barrages of 400 kw and less in size.
Numerous studies have been conducted for large-scale tidal barrages in a variety of locations,[iii] but the grandest proposal of all is the 8640-Megawatt Severn Tidal Barrage (“STB”) proposal. A broad range of studies was conducted from 1974 to 1987 on this proposal to dam the Severn Estuary between Wales and England. The tidal range in the Severn is upwards to 40 feet in places and the potential power from a barrage could provide 12% of the United Kingdom’s requirements. Major engineering consultancies, large construction companies, several universities, and the U.K. Government’s Department of Trade and Industry combined to fund and conduct the 13 years of studies costing almost $100 million.
The STB proposal was shelved in 1987 due to “economic problems,” but the proposal likely would have met with fierce opposition from a broad array of environmental groups and local inhabitants. The STB and other large-scale tidal barrages suffer from four types of environmental problems:
· Barrages block navigation
A barrage is a dam across a tidally-affected inlet or estuary and blocks the egress to the ocean. Locks can be installed, as they are in France, or not, as in Canada. The lock allows some traffic, but it is a slow and costly alternative to free access to the ocean.
· Barrages impede fish migration
Anadromous fish spawn in fresh water and outmigrate to salt water, then return after three or four years to spawn and die, ineffably drawn to the exact location of their birth. Fish are, therefore, instinctively obliged to pass through the turbines of an intervening barrage at least twice. Some fish actually pass through the turbines multiple times during one outmigration or one return. The mortality rate for fish passing through the low-head turbine is about 6%. Fish ladders are sometimes provided as an alternative means of bypassing the dam, but the mortality rate of fish ladders is slightly higher than that of passing through the turbines and most fish avoid them.
· Barrages change the size and location of the intertidal zone
The intertidal zone is the area that is alternatively wet and dry during the tidal cycles. The wet/dry habitat is unique and only certain types of plants and creatures thrive there. A barrage re-times the tidal cycle and changes the water levels, thereby “moving” the wet/dry intertidal zone, obliging the plant and animal life to adapt or “move” to the new location. The humans living around the headpond of the tidal barrage in Annapolis Royale, Canada, have limited the functioning of the barrage so as to maintain water levels that are nearly normal, but at a cost of about 50% of the potential output of the 16-megawatt unit.
· Barrages change the tidal regime downstream
Canada’s Bay of Fundy has the largest tidal ranges in the world and has been the subject of numerous studies of proposed tidal power plant installations. Huge barrages have been proposed and one of the major concerns was the fact that coastal process modeling conjectured that the highest tides downstream of the barrage might be raised as much as 9 inches as far away as Boston, more than 800 miles. This finding was controversial, but, even the possibility of such an impact was seen as sufficient to draw lawsuits from every property owner with a flooded basement from Nova Scotia to Cape Cod. Similarly, the Severn Estuary is the outbound pathway for much of the waste created in central England and southern Wales and the proposed barrage would make a 1300 square mile head pond and impede that flushing action, thereby forming the world’s most offensive body of water.
Economic Problems of Barrages
The aforementioned environmental problems of tidal barrages have created opposition from environmental groups and local inhabitants, requiring either (1) costly efforts to overcome the objections through further studies or (2) abandonment of the proposals. The barrage also suffers from high capital costs and a relatively low load factor (environmental considerations limit generation to single-effect ebb tide-only generation) of about 28%.
[i] Tidal cycles last 12 hours and 25 minutes in most places.
[ii] The tidal dredging kept a shipping channel open for the delivery of coal to a coal-fired power station. When the coal-fired power plant was closed, the tidal basin was donated to the Royal Society for the Protection of Birds (“RSPB”). Subsequently, the shipping channel has silted up, but the RSPB refuses to give back the tidal basin or to resume using it for the dredging activity, as this would disturb the habitats that have developed due to the inactivity.
[iii] Clare, R., 1992. Tidal power: trends and developments. Thomas Telford, London.