Week 3 and 4

Question 1

What is tidal range?

Answer 1

The difference in height between the high tide and the low tide is called the tidal range

Question 2

What is the simplistic difference between tides and currents?

Answer 2

Tides go up and down; currents move left and right

Question 3

What are the two most major causes of tides? Which one has more of an effect and why?

Answer 3

The Sun and the moon.

The moon has more of an effect on the tides. Whilst the mass of the sun is significantly larger than that of the moon, the distance to the moon from Earth is significantly less than the Sun and distance has more of an impact on the effect on gravitational attractive force (refer to equation)

Question 4

How often do coastal areas experience tides?

Answer 4

There are two high tides and two low tides every lunar day.

A lunar day is 50 minutes longer than a solar day (24hr and 50 minutes)

Question 5

Describe the concepts of spring and neap tides.

Answer 5

As indicated, the Sun has less of an effect on the tides, however it still does have an effect.
At the time of a new or full moon this effect is at its greatest where the sun, moon and Earth are all aligned meaning the effect of the moon and Sun on the tides is additive creating extra high tides and very low tides (spring tides).
Neap Tides occur a week after the sun and moon are t right angles to each other, so the Sun partially cancels out the effect of the moon creating moderate tides (neap tides)

Question 6

Other than gravitational attractive forces what are some factors that affect on the size of tides?

Answer 6

Land mass
Resonant Frequency of the ocean
Location

Question 7

What is the difference between Hydro power and Tidal Energy?

Answer 7

Hydro Power relies on the hydrological climate cycle which is powered by solar energy whilst tidal energy comes from the result of the interaction of the gravitational pull of the moon and the sun on the seas.

Question 8

Roughly what percentage and approximate value of tidal energy resources could be recovered for electricity generation?

Answer 8

2% - 60GW

Because of economic and financial viabililty

Question 9

What are the three primary tidal stream technologies?

Answer 9

Tidal Barrage
Tidal Lagoon
Tidal Fence

Question 10

What is the difference between a tidal barrage and a tidal lagoon?

Answer 10

A tidal barrage spans an
entire river estuary in a straight line, a tidal lagoon encloses an area of coastline with a high tidal range behind a breakwater, with a footprint carefully designed for the local environment.

Question 11

What are the advantages and disadvantages of a tidal lagoon?

Answer 11

Advantages - Offshore construction has much less impact on the environment and they are simpler to build.

Disadvantages - long construction time for the power output.

Question 12

Discuss the basic process of tidal lagoon power generation cycle.

Answer 12

1. High Tide - Enclosure Full
2. Tide goes down, creating ‘head’
3. Power generation
4. Tide goes up, creating ‘head’
5. Power generation
6. Return to 1.

Question 13

What is a tidal fence?

Answer 13

A tidal fence installation is a cross between a tidal barrage and a tidal turbine stream system. Tidal fences are composed of individual vertical-axis turbines that are mounted together with a single fence-like structure.

Question 14

What are the advantages of the use of a tidal fence?

Answer 14

They can be used in unconfined basins
They have a limited impact on the environment
They are significantly cheaper to install than barrages and lagoons
They are able to generate electricity once the initial modules have been installed

Question 15

What is the general tidal energy and power generation process

Answer 15

• Extract energy from the rise and fall of the tides
• Turbines are located in water passages
• The potential energy, due to the difference in water levels
across the barrage, is converted into kinetic energy in the form
of fast-moving water passing through the turbines.
• This is converted into rotational kinetic energy by the blades
of the turbine, the spinning turbine then driving a generator to
produce electricity.
• The average power output from a tidal barrage is roughly
proportional to the square of the tidal range

Question 16

What are the three major types of tidal turbines?

Answer 16

Bulb turbines
Rim turbines
Tubular turbines

Question 17

Describe a bulb turbine and give some advantages and disadvantages of them

Answer 17

a turbine that has the generator inside it. This means that it is very efficient in terms of space and size–by putting the generator inside the turbine, the need for a separate housing unit is
eliminated, which means the turbine is less of an eyesore.
On the other hand, these turbines cannot be maintained without stopping the flow of water to them. This means that when the
inevitable maintenance needs to be carried out the turbine will have to
stop producing power for the duration of the maintenance.

Question 18

Describe a rim turbine and give some advantages and disadvantages of them

Answer 18

A rim turbine’s generator is separate from the turbine itself, and is connected through a shaft that moves with the turbine. This means that the generator can be maintained easier. However, the more-delicate nature of the rim turbine means that it
is harder to pump water through it and regulate the amount of power it produces. Where the bulb turbine is hardy, yet difficult to maintain, the rim turbine is precisely the opposite.

Question 19

Describe a tubular turbine and give some advantages and disadvantages of them

Answer 19

Rather than have the generator mounted directly on top of the turbine, it is
mounted at a 45-degree angle from it.
The real advantage of a tubular turbine is that the blades can be adjusted. This means that they can be changed to meet electricity demand; smaller blades will generate less power while
larger blades will generate more power. This allows the turbine to run more efficiently, generating only the amount of power it needs to without excessive wear and tear

Question 20

Why is duct design important?

Answer 20

Pressure losses in the duct and at the entry and exit and at turbine mounting structure and vanes waste energy. Large diameter ducts will lose less energy than small diameter ducts but the turbine may need
to be smaller for cost and efficiency reasons.

Question 21

Discuss the process of environmental concerns raised by tidal barrages starting with the changes, followed by the affects and finally those who would suffer these consequences.

Answer 21

Changes: Water levels and salinity
Surges and currents
Sedimentation and erosion (water turbidity)

Affects: Vegetation around the coast
Quality of the water in the basin or estuary

Sufferers: birds and fish

Question 22

Where do tidal streams tend to be concentrated?

Answer 22

Through narrow channels around headlands

Question 23

Large diameter rotors can be fixed to the bottom and/or tethered to the sea bed and the energy can be harnessed. Discuss the advantages and disadvantages of this.

Answer 23

Advantages
Constructed on a modular basis
Installed incrementally
Expensive and environmentally intrusive barrages not
required
Little visual impact

Disadvantages
Fouling
Tethering
Power take-off

Question 24

Power output from a tidal stream machine is much larger than from a wind turbine of equivalent blade size. Why?

Answer 24

Because the density of the working fluid is much higher

Question 25

Compare the output between tidal current turbine and an equivalent sized conventional turbine in a barrage. Explain this.

Answer 25

The power created by tidal current is less than that of a tidal barrage.
This is because of funnelling effect of estuaries and the creation of enhanced head of water.

To compensate for this it should be possible to use larger rotors in free streams than is possible in barrage-mounted turbines.

Question 26

Where are the strongest marine currents?

Answer 26

In shallow waters

Through narrow channels that connect large areas of water

Question 27

What factors are current dependant on?

Answer 27

• In open water, the flow depends on the direction of the tidal wave
• In channels and straits, current is constrained to flow up and down the channel
• The currents change direction as we move from flood (tide in) to ebb (tide out)

Question 28

What two factors can modify predicted tidal patterns?

Answer 28

1) Wind acting on the sea surface

2) Atmospheric pressure causing storm surges

Question 29

What is slack water?

Answer 29

A body of tidal water when the water is completely unstressed, and therefore no movement either way in the tidal stream, and which occurs before the direction of the tidal stream reverses

Question 30

What is SeaGen technology? Where are the rotors positioned on this system and why?

Answer 30

The SeaGen S system has the largest swept area of all the commercial scale tidal products currently available. The turbines have a patented feature whereby the rotor blades can be
pitched through 180 degrees, allowing them to optimise energy capture and operate in bidirectional
flows.

The rotors are positioned in top third of the water column where tidal currents are the
strongest, therefore maximising the energy capture.