Tidal & Economics - Tutorial Flashcards
Explain the main mechanisms that lead to the tides
Gravitational attraction of Moon and Sun, drawing
water to sides nearest/furthest to them. Rotation of Earth causes bulges to travel around Earth. A diagram showing Earth-Moon system valuable.
What are the main tidal cycles and what are they governed by? What other factors
manifest themselves as a tidal cycle?
Main cycle is the semi-diurnal cycle arising from attraction of Moon and the
rotation of the Earth which results in 12 h 25 min cycle. Next is spring-neap cycle
arising from relative position of Sun and Moon: this is approximately 14 day cycle.
Other cycles arise from Moon’s elliptical and out of plane orbit as well as Sun’s
elliptical orbit.
Real estuaries do not conform to the simple model suggested. How is their resonant
behaviour examined in practice?
Originally this would have been conducted using scale models of the estuary in
large hydraulics laboratories. The model would be filled with water and the rise
and fall of the tide at the open sea end of the estuary would allow observation of
effect further up the estuary. With availability of cheap computing power, this is
now largely carried out with software able to simulate the shallow water equations
and using detailed bathymetric data, e.g. Telemac, Mike, POL-COMS.
Outline the main components of and the methods for constructing tidal barrages
Either build cofferdam as for hydro scheme (see Boyle (2012), p254) or use
caissons which requires them to be towed into place (discussed in lecture). The 2nd
method does not fully block estuary.
Outline the differences in operating a tidal barrage in ebb-only, flood-only and 2-
way modes. Why is pumping considered valuable with barrages?
See Boyle (2012), pp-251-253.
Turbines are able to be powered in reverse by excess energy in the grid to increase the water level in the basin at high tide
What are the main environmental impacts of tidal barrages?
Impacts in
particular on estuarine ecosystems.
impacts in
particular on estuarine ecosystems.
It is uncertain how changes in water quality
will impact biodiversity, but there is likely to
be a change in the type of animals and
plants that colonise estuaries.
In addition, a barrage may increase levels of
fish mortality due to predation, disease,
habitat loss and disruption to movement.
What are the relative merits of tidal lagoons over barrages?
Lagoons get around impacts on estuarine ecosystems by enclosing part of the
estuary using impoundments.
Is however more expensive.
What are the principal mechanisms for the existence of tidal currents?
Current flows tend to occur due to differences in the water level between points as the tidal wave advances.
The difference in water level forces the water at a higher level to flow down the water gradient.
Tidal current devices can be typically classified in several ways. What are these
and what are the key criteria for classification?
Type: Horizontal axis, vertical axis, linear lift, venturi; Support: Fixed or floating
with subdivisions of fixed by structure type; ‘Generation’: 1st generation is for
shallower water – typically fixed installation; 2nd generation for deeper water where
floating looks more credible: Deployment method: tow and sink; barge/boat and
lift or combination of these; Rotors: number of blades per rotor, number of rotors
per device, contra-rotating or not. Etc, etc.
Discuss and describe 4 tidal current energy conversion concepts, including what
you believe to be the advantages and disadvantages of each.
Horizontal axis TT: (+) Simple and proven; mechanically efficient; low solidity
high speed rotors can simplify generator choice; (-) need to be accurately
positioned to ensure flow in line with rotor axis; size limited by channel depth
(typical max diameter is half depth;
Vertical axis TT: (+) Generator/gearing in base for stability; size not limited by
depth; multi-directional; (-) generally less efficient than HATT; may require
starting assistance; possible higher impact risk with marine life?
Linear lift TT: (+) Not depth limited; (-) complex power conversion technology;
original device overly slow – up to date approach uses multiple foils; sensitive to
flow direction.
Venturi TT: (+) Compact and robust; (-) added complexity – do they add anything?
