The gravitiational effects of the sun and moon create tidal
action all over the world but two factors unique to the Bay of
Fundy contribute to the high tides: the gradual tapering and
shallowing of the bay and resonance.
The shape of the bay has been described as funnel shaped. This
tapering, as well as shallowing, constricts the tidal flow and
causes the waters to rise an average of 16m (52ft) at the head of
the Bay of Fundy. Resonance (or rocking) can be likened to water
sloshing around in a bathtub from end to end. It takes about 13
hours for water to travel from the mouth to the head of the bay and
back again. Since the oceans tides rise and fall every 12 hours and
25 minutes, this reinforces the rocking motion of the tide.
To produce practical amounts of tidal power, a difference of a
least five metres between high and low tide is needed. There are
only about 40 sites around the world with this kind of tidal range.
Currently in Canada, the only practical site is the Bay of Fundy.
So when in the 1980s the Canadian government wanted to explore the
feasibility of tidal power, this was a natural site for an
experimental station. The site chosen at Annapolis Royal had
already been closed off by a causeway, built to control tidal flow
further up the Annapolis River. Annapolis Royal became home to the
first (and only) modern tidal power plant in North America.
So how is tidal power generated?
Tracing the plant’s generating cycle, starting with the
incoming tide, when the sea level reaches the level of the head
pond, the causeway’s sluice gates are opened to fill the head
pond. When the pond reaches its maximum level (less than high tide
in the case of Annapolis), the sluice gates close to trap the
seawater upstream from the turbine. As the tide recedes, a head
develops between the head pond and the seaside. When a head of 1.6
metres or more is produced, the 18 wicket gates of the distributor
assembly open. These wicket gates, which resemble venetian blinds
arranged in a circle, control the flow of water through the
turbine. When they open, water rushes through at the rate of 400
cubic metres per second and turn the turbine’s massive
four-blade runner.
This power generating phase of the cycle continues until the
level of the head pond has dropped to within 1.6 metres of the
incoming tide, which it does in just over five hours. Then, the
wicket gates close, awaiting a repeat of this twice daily tidal
cycle. The Annapolis system uses a single-effect turbine that
generates electricity only in one direction - when the flow of
water is towards the sea. With clockwork regularity, in harmony
with the high-tide rhythm of the sea, the Annapolis station feeds
the grid every 12 hours and 25 minutes.
Why doesn’t the Annapolis station use a double-effect
turbine that could generate power in both directions? There are a
couple of reasons. While double-effect turbines generate more than
single-effect turbines, their output is considerably less than
twice as much. More importantly, as far as the Annapolis is
concerned, they also require higher head. Double-effect turbines as
large as the one at the Annapolis plant would be better suited for
use farther up the Bay where the average tides are higher. The
tides at Annapolis average 7 metres, which is about 4.5 metres
short of what’s needed for a double effect turbine.
The Annapolis station attracts more than 35,000 visitors a
year.
In order to claim this earthcache your log
must meet the following requirements:
1. E-mail me, indicating one advantage and one disadvantage of
tidal power. Also indicate "who" stayed in the Annapolis river for
8 days in August 2004. Please do not post answers online.
2. Take a picture of yourself (or group or GPS) and post online
with the time and what phase of the generating cycle was occuring
at the time of your visit (incoming tide, slack tide, power
generation, etc)
YOU DO NOT HAVE TO WAIT FOR
CONFIRMATION FROM ME BEFORE LOGGING YOUR FIND. HOWEVER, LOGS NOT
MEETING ALL REQUIREMENTS WILL BE IMMEDIATELY DELETED!
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