What are the advantages and disadvantages of renewable tidal power as an energy source?

Tidal power stations take in water from high tides, usually at a river mouth, and then release it using dams or barrages where it drives turbines that produce electricity. Sometimes dams and barrages are not used but turbines are laid in a moving current.

Advantages:

• Tidal energy is renewable.
  
• The energy produced is clean and non polluting.
  
• There is no carbon dioxide or any other by-products released. It produces no greenhouse gases or other waste.
• It is a renewable energy that will help reduce our reliance on the burning of fossil fuels.
  
• There are two tides every day and they can be relied on. The energy is there for the taking.
• So the electricity supply is constant and efficient.
• Once you've built it, the energy is free because it comes from the ocean's power
• It needs no fuel.
• It produces electricity reliably.
• Not expensive to maintain.
• Tides are definitely predictable.
• Offshore turbines and vertical-axis turbines are not ruinously expensive to build and do not have a large environmental impact.
• A plant is expected to be in production for 75 to 100 years
• Uses an abundant, inexpensive fuel source (water) to generate power
• May protect coastline against damage from high storm tides and provide a ready-made road bridge

In most cases, nothing can stop tidal flows as they are huge masses of water with lots of energy behind them. As long as the moon exists, and as long as the earth spins, the tides will come in and go out and rotate a turbine - all for an initial upfront one-off cost with a whole pile of maintenance considerations.

Disadvantages:

• Holding back the tide allows silt to build up on the river bed.
• The dams and barrages sometimes interfere with shipping.
  
• You will need to find a way to connect the electricity to the grid.
• Pose same threats as large dams, altering the flow of saltwater in and out of estuaries, which changes the hydrology and salinity and possibly negatively affects the marine mammals that use the estuaries as their habitat
• Turbidity decreases as a result of smaller volume of water being exchanged between the basin and the sea.
• The average salinity inside the basin decreases, also affecting the ecosystem
• A barrage across an estuary is very expensive to build, and affects a very wide area - the environment is changed for many miles upstream and downstream. Many birds rely on the tide uncovering the mud flats so that they can feed.
• There are few suitable sites for tidal barrages.
• Only provides power for around 10 hours each day, when the tide is actually moving in or out.
• It only provides about 7% of the power needed for England and Wales that means that some people get their energy close to free and some pay a lot of money
• Water is not replenished, it cannot flow away so any dirt or pollution lingers around the coast much longer
• Needs a very big piece of sea to be cost effective
• Cannot be used inland
• Barrage systems require salt resistant parts and lots of maintenance
• Affects the lives of the people who rely on fishing for a means of living
• Limited because the tide never speeds up or slows down, and occurs on 6 hour cycles. It is also dependent on the fetch distance. The fetch is the distance the tide rises and falls, so some beaches have a very small fetch, and others have a big fetch but hardly any have a large enough fetch to support tidal energy
• Tidal energy is currently more expensive to generate than conventional energy or that from many other renewable sources.
• Effects on marine life during construction phases.
• Operation and control must be provided remotely and maintenance is complicated due to sea-basing of the generation facilities.
• Sea-based moorings and towers to hold the generators must be placed on the sea bottom.
• The generating facilities and mooring infrastructure are potential navigational hazards.
• Is only available in a small number of regions - it requires a basin or gulf that has a mean tidal amplitude of 7 meters or more. Also need semi-diurnal tides where there are two high and low tides everyday.
• Even with the best barrage designs, fish mortality rate per pass through the barrage is about 15%. Solutions to this problem have either failed or are too impractical and too expensive.
• Dams used in the production of tidal power can raise tide levels.
• Damages like reduced flushing, winter icing and erosion can change the vegetation of the area and disrupt the balance.
• Expensive to construct
• Power is often generated when there is little demand for electricity
• Barrages may block outlets to open water. Although locks can be installed, this is often a slow and expensive process.
• Barrages affect fish migration and other wildlife- many fish like salmon swim up to the barrages and are killed by the spinning turbines. Fish ladders may be used to allow passage for the fish, but these are never 100% effective. Barrages may also destroy the habitat of the wildlife living near it
• Barrages may affect the tidal level - the change in tidal level may affect navigation, recreation, cause flooding of the shoreline and affect local marine life
• Causes a continual loss of mechanical energy in the Earth-Moon system (Due to pumping of water through the natural restrictions around coastlines and viscous dissipation at the seabed and in turbulence.
• Loss of energy has caused the rotation of the Earth to slow in the 4.5 billion years since formation losing 17% of its rotational energy.
• May take additional energy from the system, increasing the rate of slowing over the next millions of years.

There are a lot of links on this topic on the internet. I have included links. I would suggest you review them carefully. In comparing any form of energy, you must be clear what the alternative exists. In comparison to non-renewable fossil fuels, tidal power is a renewable energy source and it does not pollute the atmosphere. Tidal power cost of generation should stay constant while oil prices will go up as the supply diminishes. It requires the right conditions, and only a small number of locations seem adoptable to this technology. (See list of 9 sites in the world). It requires large investments. Much of the technology is unproven. There are environmental issues with generating energy by placing a barrage in an estuary (see barrage tidal power under wikipedia).
There are a lot of links on this topic on the internet. I have included links. I would suggest you review them carefully. In comparing any form of energy, you must be clear what the alternative exists. In comparison to non-renewable fossil fuels, tidal power is a renewable energy source and it does not pollute the atmosphere. Tidal power cost of generation should stay constant while oil prices will go up as the supply diminishes. It requires the right conditions, and only a small number of locations seem adoptable to this technology. (See list of 9 sites in the world). It requires large investments. Much of the technology is unproven. There are environmental issues with generating energy by placing a barrage in an estuary (see barrage tidal power under wikipedia).