Tidal Power Presentation - Notes from Renewable Energy: Power for a Sustainable Future
- It is important to distinguish tidal energy from hydropower and wave energy
- Tidal energy is the result of the gravitational pull by the moon and sun on the seas
- Hydropower is derived from the hydrological climate cycle, which is usually harnessed via hydroelectric dams
- Wave energy is derived from the action of wind over water
Basic Physics of Tides
- As the Earth rotates, gravitational forces create, at any particular point on the globe, a twice-daily rise and fall in sea level, this being modified in height by the gravitational pull of the sun and by the topography of land masses
- Starting first with just the Earth and the moon
- The gravitational pull of the moon draws the seas on the side of Earth nearest the moon into a bulge towards the moon, while the seas furthest from the moon experience a less than average lunar pull and bulge away from the moon
- As the Earth rotates, there are 2 tides per day occurring approximately 12.5 hours apart
- This basic pattern is modified by the pull of the sun – although the sun is much larger than the moon, it is much farther away from the Earth, and the moon’s influence on the seas is about twice that of the sun
- Tidal variation depends on relative orientation…
- When the sun and moon pull together, the result is the very high ‘spring tides’, and when they are at 90 degrees to each other, the result is the lower ‘neap tides’ – period between the spring and neap tides is about 14 days (half the 29.5 lunar cycle)
- The tides are also modified in some locations by Coriolis forces – which are due to the spin of the Earth and deflect tidal currents from the paths that they would otherwise have taken
- The overall effect of the basic sun-moon-Earth interaction is that, in mid-ocean, the typical tidal variation or tidal range is about 0.5 meters – however tidal ranges experienced in coastal sites are usually amplified and reach about 3 meters (as water depth decreases, the tidal flow is concentrated)
- If the tide then enters a suitably shaped estuary (a semi-enclosed body of water where fresh water and salt water meet), it can be funneled and therefore heightened even further (up to 10-15 meters at some sites)
- Tidal patterns, although very much site-specific, are predictable and reliable
- Tidal barrages, built across suitable estuaries, are designed to extract energy from the rise and fall of the tides, using turbines located in water passages in the barrages
- The potential energy due to the difference in water levels across the barrages is converted into kinetic energy in the form of fast moving water passing through the turbines
- This, in turn, is converted into rotational kinetic energy by the blades of the turbine, which drives a generator to produce electricity
- Power can be generated from a barrage either by passing the incoming tide through the turbines mounted in the barrage (Flood Generation); or by allowing the flow to pass through sluices without generating power and then trapping the high tide behind the barrage by closing the sluices. The head of the water is then passed back through the turbines on the outgoing ebb tide (Ebb Generation)
- In simple Ebb or Flood Generation, large installed capacity is only used for a short period of time (3-6 hours) in each tidal cycle, producing short bursts of power which may not match demand
- Thus, the key problem is that tidal energy inputs come in relatively short bursts and it is very difficult integrating them into the national grid
- Two-way operation is also possible - although output will be more evenly distributed in time, there will be a net decrease in power because:
- In order to be ready for the next cycle, neither the ebb nor flood generation can be taken to completion
- Blade design cannot be optimized
- A mean tidal range of at least 5 meters is usually considered to be the minimum for viable power generation, depending on the economic criteria used and energy output is also roughly proportional to the area of the water trapped behind the barrage
- Thus…location of barrages is crucial
- One medium scale scheme has been built at the Rance estuary in France and a small number of small schemes have been built around the world
- La Rance was constructed between 1961 and 1967 – it has a road crossing and ship lock
- Although some mechanical problems were encountered in 1975, typical plant availability has been more than 90%
- La Rance officials report that the operation results in one of the least expensive means in France for generating power – the cost is about 3.7 cents per kW-hr, which compares favorably with nuclear plants at about 3.8 cents per kW-hr – only hydroelectric plants at 3.2 cents per kW-hr produce at less cost
- Construction involved complete closure of the estuary, which resulted in effective stagnation and subsequent collapse of the ecosystem within it
- Since construction, exchange of water from the sea and estuary has “restored” the ecosystem
- The most obvious potential impact of a tidal barrage would be on local wildlife – many fish and birds rely on the estuaries for food, and access to that supply might be affected by a tidal barrage
- BUT, with the barrage in place water would become clearer as silt would drop out due to reduced tidal flows – increasing the biological productivity and thus potential food
- Royal Society for the Protection of Birds sees barrages as reducing habitats for key species
- Changes to the tides and currents during construction and then later during the operation of a barrage will cause changes in sediment characteristics, salinity, and quality of water – which govern to a large extent the ecosystem
- Construction of a barrage would impede shipping
- Barrages would play a useful role in providing protection against floods and storm damage as they would limit local wave generation
- The construction of a barrage would drive employment and the local economy
- Increased tourism due to enhanced opportunities for water sports
- There could also be an option of providing a new road or rail crossing (as installed on La Rance)
- Overall economics of a tidal barrage depends not only on operational performance, but also on initial capital cost
- Estimated total capital cost of the proposed Severn Barrage in the UK is $8 billion
- Once capital and interest costs have been paid off, a tidal barrage would be generating profit for the rest of its life (apart from relatively small operation and maintenance costs = about 1% of its initial cost per year)
- Construction of a tidal barrage creates security and sustainability of supply, local employment gains, and adds possible new road or rail transport crossings
- What are your feelings on the articles and tidal power in general? Did you enjoy the articles?
- Should we sacrifice local ecosystems for tidal power?
- What are the advantages and disadvantages of tidal power versus other alternative energy sources (such as thermal conversion process)?
- What are your feelings on the opportunity cost of a barrage?
- Can integration of other forms of alternative energy into a tidal system mitigate its problem of short bursts of energy?
Class Discussion Summary
There were serious concerns regarding the environmental impact of tidal schemes from several members of the class. One issue of conflict was the precise degree of water and sediment movement following the installation of a barrage. There is a risk that sediments would enter and remain in the barrage due to a weak ebb current. Although this may be mitigated by two-way flow, accumulation of sediment in the tidal basin will take place over decades.
La Rance is the only medium scale tidal project currently operating in the world and its construction prompted the collapse of the local ecosystem. The text claims that the exchange of water from the sea and estuary in La Rance has restored its ecosystem. Because the study on La Rance did not assess the ecosystem before construction, the class emphasized the importance of further research on the effects of a barrage on a local ecosystem. Further research would include ecosystem observations and computer models. The class noted that the use of the computer to model water movement is integral to the successful implementation of tidal schemes.
The amount of time in which little energy can be generated between the tides was a major concern for the class. I suggested that the integration of other forms of alternative energy into a tidal system would mitigate the short bursts of energy and make the projects more economically viable. The class argued that the feasibility of other forms of alternative energy, such as installing wind turbines along the barrage, is very site-specific. Furthermore, integration problems would be prevalent.
The class reiterated the fact that construction costs remain extremely high. They asserted that we must examine the opportunity cost of tidal power. One class member in particular stated that money may be better spent on energy conservation. It was agreed that individuals funding a tidal scheme must have a long-term view in terms of return on investment, which is unusual for many energy companies. Tidal power will need at least some state support.
The class debated the prospects of harnessing tidal power using large submerged turbines supported by floats or fastened to the sea bed. The class envisioned a tidal turbine farm in the sea – the units could be installed piece by piece, and an expensive barrage would not be necessary. But serious problems exist in terms of boat navigation and fishing. Boat motors and fishing nets may interfere with the tidal turbines.
The class noted that the primary potential for tidal power is from a small number of massive barrages. They reiterated that the viability of tidal power is a function of the tidal range. Both a mean tidal range of 5 meters and an indented coastline such that a barrage could enclose an estuary are necessary for a successful tidal scheme. The class asserted that there simply aren’t many suitable sites. This notion is consistent with the text, which outlines only about 15 suitable sites in the world.
The class consensus was thus, tidal power cannot solve the energy crisis. Tidal power, however, can decrease the world’s dependence on fossil fuels. Tidal systems can diversify our energy sources and meet stricter greenhouse gas emission targets. Further research is essential with respect to sediment accumulation in a tidal basin and a barrage’s effect on the local ecosystem.
Boyle, Godfrey. Renewable Energy: Power for a Sustainable Future. Chapter 6 – Tidal Power. Oxford University Press. 1996.
Baker, Clive. Renewable Series - Tidal power. Energy Policy. October 1991. p. 792-797.