Tidal Energy

• Most of the energy sources we have been discussing derived their energy from the sun.

• Tides are driven by gravity• Gravity is a force that exists between any two

objects based upon their mass and the distance between them

• Fg = GmM/R2 where M and m are the masses of the two objects, R is the distance between them and G is the gravitational constant = 6.67300 × 10-11 m3 kg-1 s-2

Tides

• So the moon and Earth exert a force of gravity on each other. The motion of the moon around the Earth counteracts the Earth’s pull, so the moon does not fall into the Earth.

• The moon’s pull on the Earth causes any material that can flow on the Earth’s surface, like large bodies of water, to pile up underneath the moon.

• See we do understand the tides!!!

Tides

• The sun also causes tides the Earth, though the effect is small, unless the sun and moon line up and work together (spring tide) or are at right angles to each other and work against each other (neap tides).

• In areas where there are natural basins on the coastline, water flows in and out of these basins.

• So there are regular, predictable motions in the oceans which could be used as an energy source.

Capturing Tidal Power

• Dams or barrage with gates are usually built across the mouth of basins

• This allows the current to be directed into the turbines and enhances the effect.

Rance River Tidal Power station in France

Current and Future tidal power stations

• Rance River, France 240Mw• White sea, Russia 1 MW• Annapolis River, Nova Scotia, Canada, 18mW• Two most favorable sites in the US: Cook Inlet

and Bristol Bay in Alaska and Bay of Funday which covers the Northeastern US and southeastern Canada.

• Development of the Bay of Funday would provide 15,000mW to the northeastern US and 15,000mW to Canada

Bay of Fundy

Power

• Power: * P = Cp x 0.5 x ρ x A x V³• Cp is the turbine coefficient of performance• P = the power generated (in watts)• ρ = the density of the water (seawater is

1025 kg/m³)• A = the sweep area of the turbine (in m²)• V³ = the velocity of the flow cubed (i.e. V x V

x V)

Environmental Issues• alters 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 • Some species lost their habitat due to La Rance’s construction, but other species colonized the

abandoned space, which caused a shift in diversity.• Turbidity (the amount of matter in suspension in the water) decreases as a result of smaller volume

of water being exchanged between the basin and the sea. This lets light from the Sun to penetrate the water further, improving conditions for the phytoplankton. The changes propagate up the food chain, causing a general change in the ecosystem.

• If the turbines are moving slowly enough, such as low velocities of 25-50 rpm, fish kill is minimalized and silt and other nutrients are able to flow through the structures . Tidal fences block off channels, which makes it difficult for fish and wildlife to migrate through those channels. Larger marine mammals such as seals or dolphins can be protected from the turbines by fences or a sonar sensor auto-breaking system that automatically shuts the turbines down when marine mammals are detected

• As a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem

• Estuaries often have high volume of sediments moving through them, from the rivers to the sea. The introduction of a barrage into an estuary may result in sediment accumulation within the barrage, affecting the ecosystem and also the operation of the barrage.

Innovative strategies• East River in New York-tidal river

– Plans for 30 underwater turbines to tap the rivers 4 knot tidal flow and produce 1mW

– Already tested with a prototype, took 3 attempts. First two the blades were torn off by the currents.- If permits are approved, on schedule

for a fall installation • Tidal Lagoons

– Artificial lagoons with high walls. – Lagoon fills and empties through apertures, turbines are spun and

generate electricity– doesn’t disturb current environmental conditions as much and

expands locations by only requiring large tidal variations

Wave Energy

• It is estimated that there is 2-3 million mW of energy in the waves breaking on the world coastlines, with energies derived ultimately from the wind

• In Great Britain alone, almost twice the current electricity demand breaks on the countries coastlines every day.

• A vast untapped resource, but how to harness it?

How are waves formed• As wind blows along the surface of a

body of water, a surface wave develops.

• As the wind blows, pressure and friction forces perturb the equilibrium of the water surface

• These forces transfer energy from the air to the water, forming waves.

• The water molecules actually move in circular motion

• When a wave can no longer support its top, it collapses or breaks.

• Usually happens when a wave reaches shallow water, such as near a coastline.

Harnessing the energy

• LIMPET (Land Installed Marine Powered Energy Transformer)

• Breakwater Design• PowerBuoys• Pelamis

LIMPET

• Takes the wave into a funnel and drives air pressure past two turbines, each of which turns a 250 kW generator.

• Installed on the island of Islay, off Scotland’s west coast.

Breakwater• Installed where there would

normally be a breakwater• a series of layered

‘reservoirs’ up a carefully calculated slope.

• This is then converted to kinetic energy (by falling down), and this turns the turbine/generator.

• A 500m breakwater can produce respectable 150 kW generator capacity

• Only in design phase, non of these up and running yet

PowerBuoys• In a permanent magnet linear

generator buoy, an electric coil surrounds a magnetic shaft inside the buoy

• the magnetic shaft is anchored to the sea floor. When waves cause the coil to move up and down relative to the fixed magnetic shaft, voltage is induced and electricity is generated.

• Each buoy could potentially produce 250 kilowatts of power.

• A fleet of about 200 such buoys could power the business district of downtown Portland.

PELAMIS• made of large semi-

submerged sections, like a submarine cut into pieces

• the wave action makes the Pelamis bend between the sections.

• This bending action forces hydraulic pistons to move in the device and push fluid around generating a linear flow, which produces energy.

• A 1 sq kilometer farm could produce 30 Megawatts.

Geothermal Energy• Based on the heat that naturally occurs in the

Earth’s interior• Sine the Earth’s interior is hotter than the

surface, this heat flows from the interior towards the surface.

• Heat is the result of decay of radioactive nuclei inside the Earth, heat left over from the formation of the Earth and heat from the friction of heavier material sinking towards the center.

• Earth’s core temperature is 7200 F, which is close to the temperature on the Sun’s surface.

Radioactive Decay• the process in which an unstable atomic nucleus loses energy by

emitting ionizing particles and radiation.• Most important source of heat inside the earth• loss of energy results in an atom of one type (parent nuclide)

transforming to an atom of a different type, (daughter nuclide). • Eg: a carbon-14 atom (the "parent") emits radiation and transforms

to a nitrogen-14 atom (the "daughter").• Inside the Earth, the main elements that decay are 235U, 238U 232Th,

40K .• This is a random process on the atomic level, in that it is impossible

to predict when a given atom will decay, but given a large number of similar atoms the decay rate, on average, is predictable.

Radioactive decay

Thorium Decay

Uranium 238 decay

Geothermal• Not a new idea• Used the heat from inside the Earth for thousands of years

for bathing (hot springs) and space heating.• First electric power use was by Prince Piero Ginori Conti

tested the first geothermal power generator on 4 July 1904 in Larderello, Italy.

• In 1958 New Zealand built a plant of its own.• First geothermal power plant in the United States was

made in 1922 by John D. Grant at The Geysers Resort Hotel.• In 1960, Pacific Gas and Electric began operation of the first

successful geothermal power plant in the United States at The Geysers. The original turbine installed lasted for more than 30 years and produced 11 MW net power.

The Geysers• The Geysers is a geothermal

power field located 72 miles (116 km) north of San Francisco, California.

• It is the largest geothermal development in the world.

• It is currently outputting over 750 MW. It consists of 22 separate power plants that utilize steam from more than 350 producing wells.