4 TIDAL RANGE

Question 1

What is the largest tidal range?

Answer 1

Bay of Fundy, Canada

av tidal range = 16.3m
Results from the narrow bays that create a natural bottleneck, resulting in a high range.

Question 2

What is the second largest tidal range?

Answer 2

Severn Estuary, UK

av tidal range = 15m
Results from the tidal funnelling by the English and Welsh landmasses.

Question 3

How much energy worldwide is produced by marine energy?

Answer 3

<1GW

Question 4

Why marine energy isn’t really being implemented.

Answer 4

• Marine environment can be hostile. Any marine energy conversion device will experience a wide range of forces.
• Need robust infrastructure to survive extreme weather (storms)
• Difficult to install and gain access to an offshore marine energy device, making it difficult to maintain Also transmitting electrical power to the shore using high voltage DC cables is expensive.
• Materials would need to be protective as seawater is corrosive.
• much lower rate of energy generation for water than for wind

∴ Cost of marine projects are much higher than onshore.

Question 5

Why is the energy generation lower for water than for wind?

Answer 5

as 𝜌 of water is 800x that of air

Question 6

How does tidal generation work?

Answer 6

The Ep of water is converted into mech energy of the turbine. This rotates a generator which generates electricity.

1. Tidal range generation exploits the Ep of the difference in the height of the water level at the edge of the oceans that is caused by the tides.
2. A barrage across the mouth of an esturaru creates a difference in water height within and outside of the impounded area, as the tidal height varies.
3. The hydro turbines generate electricity from the Ep of the height difference of the water across the barrage
4. The difference in height as the tide rises and falls results in flows through the turbines into the basin (flood generation) or out of the basin (ebb generation)

Question 7

How does sea level change throughout the day?

Answer 7

~twice a tidal day.

The water level increases to its highest during Flood Tide and drops to low water during Ebb tide.

Question 8

What is the Flood Current?

Answer 8

Water flowing towards shore as water level rises. (this can generate electricity via turbines)

Question 9

What is the EBB Current?

Answer 9

Water moving away from shore as water level decreases

Question 10

How does water level change with location of the moon?

Answer 10

The mass of the moon creates a gravitational force which attracts the water of the ocean towards the moon, with a balancing inertial force acting on the opposite side of the earth.

The tides are caused by a very low frequency water wave that is created by the gravitational attraction of the moon and the sun and the rotation of the earth. This wave has a period of 12.5 hours and leads to variations in the height of the water and in the creation of marine currents as its approaches the shoreline.

Question 11

When do water levels rise in relation to the moon?

Answer 11

When the location is in same plane with the moon. On side of the moon, the tides ‘bulge’ due to gravity and on the opposite side they simultaneously bulge due to inertia.

Question 12

Why do some locations only experience one tide a day?

Answer 12

As the angle of declination of the moon wrt the earth’s equator resultslts

Question 13

Why do some locations only experience one tide a day?

Answer 13

The angle of declination of the moon wrt earth’s surface means the plane of moon isn’t in line with earth’s equatorial plane. ∴ different locations experience tides at different times.

Question 14

What is Perigee?

Answer 14

The minimum distance between the earth and moon

Question 15

What is Apogee?

Answer 15

The max distance between earth and moon.

Question 16

At what distance between moon and earth does the largest tidal range occur?

Answer 16

Perigee. As the min distance means the gravitational forces exerted on earth are at a maximum ∴ larger tides.

Question 17

What is Spring tide?

Answer 17

When all sun, earth and moon are in same plane.

At which point the gravitational forces of sun and moon act together => max impact on sea water levels => largest tides.

Question 18

What is Neap tide?

Answer 18

When there’s a right angle between the moon, earth and sun.

The moon and sun’s gravitational forces don’t leverage each other ∴ smallest tidal range.

Question 19

What are tides affected by?

Answer 19

• angle of declination
• moon’s orbit around the sun
• sun and moon acting together

Question 20

Why are spring and neap tide cycles different for different locations?

Answer 20

Due to position from equator etc

Question 21

Advantage of tidal power systems

Answer 21

• highly predictable for years in advance
• long lifetime (eg La Rance in France was commissioned in 1966 and is still running strong) - estimated lifetime of 100-120 years with servicing.

Question 22

What is ebb generation?

Answer 22

Generation of elec energy as water flows OUT of the impoundment through the turbine.

Ebb flow = when sea level goes down/moves away from shoreline.

Question 23

How does ebb generation work?

Answer 23

1. Impoundment filled with water when sea level at max and sluice gates opened.
2. Close sluice gates then wait for sea level to be at a minimum
3. Once sea at min level, open sluice gates for water to flow from impoundment to ocean through the turbine, which generates electricity.

Question 24

What is two-way generation?

Answer 24

The tidal generation of energy when water flows EITHER way through the turbine. Requires ANY head difference ie doesn’t matter if sea water is higher or lower than impoundment’s.

Question 25

What is two-way generation with pumping?

Answer 25

Use pumping to maximise the amount of energy produced.
Uses energy to pump water into impoundment to increase its water level. This increases the available Ep which increases the amount of electricity that can be produced during a generation cycle.

BUT: depends on current energy prices as needs to be justifiable.

Question 26

Disadvantages of tidal schemes

Answer 26

• anticipated environmental impacts; fish/birds/ecosystems/etc
• interference with shipping and other users of the estuary
• high capital cost and long construction period

Question 27

What are the benefits of tides being completely different in different locations of the UK?

Answer 27

Will help smooth the level of energy produced in a country. => more consistent and reliable.

Question 28

What is the benefit of using a flexible elec generation tidal lagoon?

Answer 28

Means electricity is only produced when it is needed the most/actually demanded. => lowers energy generation but also saves on cost which is desirable.