Re: So what exactly do scientists know about global warming?

Thus spake AJH <sylva@xxxxxxxxxxxxx>
>On Thu, 26 Jan 2006 08:27:51 +0000, Derek Moody
><derek@xxxxxxxxxxxxxxxxx> wrote:
>
>>In article <ks2gt19b5876esieeuk5de47f8udj1ges2@xxxxxxx>, AJH
>><URL:mailto:sylva@xxxxxxxxxxxxx> wrote:
>>> On Wed, 25 Jan 2006 14:29:58 +0000, Derek Moody
>>> <derek@xxxxxxxxxxxxxxxxx> wrote:
>>>
>>> >Offshore tidal stream is greatest at high/low tide, typically 1 - 2
>>> >knots at
>>> >springs and 1/2 - 1 knot at neaps. At half tide the tidal stream stops and
>>> >then reverses direction.
>>>
>>> Are these speeds obtained in deep ocean, i.e those unaffected by shore
>>> and channel artifacts?
>>
>>Deep waters run still ;-)
>>
>>Negligable tidal flow in the deep oceans. Even on the continental shelf
>>it's not worth factoring if deeper than 200m.
>>
>>> I had assumed tidal stream would need to be in some sort of narrows to
>>> get the mean speed up in the order of 5km/hr.
>
>So I was right in my initial thoughts, we are talking of a narrows
>with less than 60m of water to get at best tidal current variation
>with a maximum of 10km/hr.

Not necessarily narrows. There can be substantial currents around any
headland. Portland Bill can be quite scary in a small boat. It takes an
awful lot of water to fill the north sea, and this has to flow right
round Scotland. Hence the high current around the islands.

>What tidal height range would be typical in
>such a place? Is the 60m an average, max or min depth?

60m is the highest tide recorded in an estuary world wide. Estuaries
amplify the height of a tide, so make the best sites for tidal lagoons.
Probably less good for tidal stream generators. The channels are
constantly shifting and needed for boats, sometimes ships.
>
>What's the depth of water needed for the biggest ships?

I don't think tidal stream generators really have to vie for space in
the sea with ships. No one would take an oil tanker between the islands
of the Hebrides, even if there were reason to do so. If you got slightly
cross channel the current would spin the ship sideways on in no time,
and you would completely lose control of the ship, terminating when you
hit a rock or an island.
>>
>>> Contrary to what Oz posted earlier the power will be proportional to
>>> the cube of the velocity and directly proportional to the conversion
>>> of the turbine.
>>
>>There's plenty of power there but it's slow. Way below conventional turbine
>>speeds.

More than 4knots is too much for the most conventional submerged
windmill type design. Vertical axis designs can cope with more current.
The Canadian company doing these shows a prototype tethered to the
ground. Tethering may be a cost effective way of ensuring that the
generators are always directed with the flow of the current, and easier
than building or fixing solid structures underwater.
>
>Again I was assuming there is some limiting average below which it is
>not worth trying to capture. At current stage of the art the perceived
>wisdom for wind is that the average speed should reach 7metres/sec.

Water is much more dense than air, so speeds are correspondingly much
lower. Different designs are appropriate for different levels of
current, but this is early days and only a few prototypes have been
built.



Regards

--
Charles Francis
Please reply by name
.

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