Tidal Power is the ability to harness the potential energy created between the moon the Earth as they rotate around the Sun. Most of the energies we’ve talked about in class are dependent upon the Sun. Fossil Fuels, Wind power, Thermal Energy, Solar Power, and Bio-Fuels are all bi-products of the sun or some solar process. Nuclear power is of course a chemical reaction. Geo-Thermal is taps into the convection occurrence at the Earth’s core. Tidal power on the other hand is “the only technology that draws on potential energy inherent in the orbital characteristics of the Earth–Moon system.”
The moon is majorly responsible for the changing of the tides. When the planet rotates, the portion of the Earth that is facing the moon changes, which in turn creates a change in the distribution of water on the earth. Because water is a liquid, it is susceptible to the gravitational pull of the moon and a change in the Tidal level occurs. In other words, the level of the ocean at a specific place on the Earth changes height twice a day.
Tidal power takes advantage of this height differential and uses it to generate power. To be clear, Tidal power is categorically different than “Wave Power.” “Wave power devices extract energy directly from surface waves or from pressure fluctuations below the surface.”  Basically, wave power uses the motion at the surface of the ocean to create energy.
Like any other type of energy generation, the flow of water pushes a magnet back and forth past a large coil to excite electrons. Tidal Power is defined by the ebb and flow of the currents in the ocean in combination with the height of the water level. Since the ocean amounts for 70% of the Earth’s surface, there is amazing potential to use this resource to generate power for a variety of reasons. Tides are extremely consistent and therefore more reliable than any wind or solar options. Because you’re using the existing movement of the tides, Tidal power is a source of energy that’s available indefinitely. Certain coastal cities are prime candidates for tidal power, providing options for areas that aren’t able to take advantage of wind or solar power farms.
Tidal Energy cannot be harvested just anywhere, however. In fact, there are only about 40 sites suitable for tidal power plants since there needs to be height difference of at least 16 feet (5 meters) between low and high tides. 
There are very specific conditions for collecting the ocean’s potential energy and those conditions have engineered very different mechanisms for harvesting the energy.
1. A “Barrage System”
A barrage is a method of harvesting the difference in wave height between low and high tide. Essentially a dam along an estuary, this system has many environmental impacts not to mention extremely limited suitable sites worldwide. As the water level is lower on the ocean side of the barrage, the water wants to return to equilibrium. By forcing the water to flow past a turbine, energy is generated when the tide is low and water moves from the “basin” or estuary side to towards the ocean.
The first Tidal Power Plant began operation in 1966 in Brittany, France as a Barrage system. Today it is still in operation as the largest Tidal Power Station in the world. With a peak rating of 240 Megawatts, generated by its 24 turbines, it supplies 0.012% of the power demand of France. Its capacity factor of approximately 40% allows it to supply an average 96 Megawatts, giving an annual output of approximately 600 GWh.
As mentioned, the sites which are capable of supporting a Tidal Barrage are limited. Some prospective sites for Barrages can be found here.
2. Tidal Stream Generator
The second type of Tidal Power mechanism is a tidal stream generator and requires the least amount of infrastructure among the three main forms of tidal power generation. Like a wind turbine, the tidal stream generator (often called a Tidal Turbine) makes use of water flowing across an underwater blade. Unlike wind turbines, though, Tidal Turbines can be turned horizontally to take advantage of of the currents that flow laterally. Because water has a much higher density than wind (about 800 times more dense), it can generate a large amount of kinetic power. In fact, “water speeds of nearly one-tenth of the speed of wind provide the same power for the same size of turbine system.” These devices are best installed in channels, between islands or in estuaries, where the current is forced to move past the turbines.
Sites that have been selected as optimal for Tidal Stream Generation:
Pembrokeshire in Wales
River Severn between Wales and England
Cook Strait in New Zealand
Kaipara Harbour in New Zealand
Bay of Fundy in Canada.
East River in the USA
Golden Gate in the San Francisco Bay
Piscataqua River in New Hampshire
The Race of Alderney and The Swinge in the Channel Islands
The Sound of Islay, between Islay and Jura in Scotland
Pentland Firth between Caithness and the Orkney Islands, Scotland
Humboldt County, California in the United States
Columbia River, Oregon in the United States
There are a variety of fans and oscillators used for tidal stream generation systems. The outer two turbines shown here are known as VAWT or vertical axis wind turbines, whereas the centered turbine is operates on the horizontal axis.
Here you can also see a rendering of an underwater kite, which is tethered to the ocean floor and has turbines on its wings. By tethering the turbines and moving them around, it increases the flow of current flowing through, keeping the turbines active at all times. There is an increased payback for a smaller package. This device generates the same amount of power in 3 weeks as an onshore wind turbine does in 8 months.
In 2007 Verdant Power began operating a prototype in the East River between Roosevelt Island and Queens, which is the first tidal project in an major US urban area. The prototype has seen some challenges though. The blades of an earlier prototype broke off and were replaced with more robust turbines in 2008. The company holds a patent for devices that generate power in slower moving canals and waterways.
Current Tidal Harvesting Projects:
In March a project was approved to create the “first of its kind” wind farm off the coast of Scotland. The size of the project is what is monumental here, not necessarily the technology as they will be using devices that have been in operation for over six years.
3. Dynamic Tidal Power
The third type of Tidal Power harvesting is still mostly a theoretical proposal. As of yet, this system has not been implemented. The proposal is to create a T-shaped structure which intersects the coast perpendicularly, as opposed to a regular barrage dam which is parallel to the coast. The T-shape part extends into the ocean, and does not fully enclose the water but instead, creates a phase difference of tides on each side of the structure. In this way you are able to create a barrier that does not completely contain the water, but rather creates an artificial difference in the water level. The water is then moved in a similar way as the barrage, past a turbine to generate power. This concept is enticing because it allows for Tidal Power in areas where there is a large coastline, but not a estuary. Each dam would generate between “6-15 GW.” 
Tides move parallel to the shoreline in most places, not perpendicularly. If you are able to impeded the movement of the tide as it’s moving outward you’re able to create a height differential and direct the flow through turbines in order to reach equilibrium on either side of the DBT.
“Wave power devices extract energy directly from surface waves or from pressure fluctuations below the surface. Renewable energy analysts believe there is enough energy in the ocean waves to provide up to 2 terawatts of electricity. (A terawatt is equal to a trillion watts.)
Wave power can’t be harnessed everywhere. Wave-power rich areas of the world include the western coasts of Scotland, northern Canada, southern Africa, Australia, and the northeastern and northwestern coasts of the United States. In the Pacific Northwest alone, it’s feasible that wave energy could produce 40–70 kilowatts (kW) per meter (3.3 feet) of western coastline. The West Coast of the United States is more than a 1,000 miles long.” 
There are two main types of wave power generation, offshore systems and onshore systems.
1. Offshore Systems
Offshore Systems generate power by harnessing the movement of the water’s surface to turn generators. One example of this is the Salter Duck, a device which sits on top of the waves, and bobs up and down, the motion of which turns a turbine inside of it attached to a generator.
Another example is the Scottish developed Pelamis, which is currently part of the first large-scale commercial wave power farm off the coast of Portugal, which “will generate clean electricity for more than 1,000 family homes in its first phase.” 
2. Onshore Systems
Onshore systems generate power by harnessing the energy in breaking waves. There are various types of mechanisms that do this, among them are:
- Oscillating Water Column: “The oscillating water column consists of a partially submerged concrete or steel structure that has an opening to the sea below the waterline. It encloses a column of air above a column of water. As waves enter the air column, they cause the water column to rise and fall. This alternately compresses and depressurizes the air column. As the wave retreats, the air is drawn back through the turbine as a result of the reduced air pressure on the ocean side of the turbine.” 
- Tapchan: A tapered channel system where water is fed through a channel into a reservoir, usually on top of cliffs. The height difference causes water
- Pendulor Device: A an open box with a lid facing the sea. Waves traveling back and forth cause the lid to move on hinges, which powers a generator.
Also, there is the potential for partnership between offshore oil and gas companies, since they have already researched deep sea conditions. 
World Energy Council Report on Tidal Energy: http://www.worldenergy.org/publications/survey_of_energy_resources_2007/tidal_energy/default.asp
Wikipedia page on Tidal Power: http://en.wikipedia.org/wiki/Tidal_power
List of Operating Tidal Power Plants: http://en.wikipedia.org/wiki/List_of_tidal_power_stations