Plugging into the Ocean
Sep 26 - Machine Design
Engineers are developing clean, renewable sources of electricity that rely on the constantly moving ocean and its tides.
An onshore installation collects wave energy, converting it to pressurized
air and then to electricity.
TIDAL POWER
Tidal mills date back to 800 A.D. on the Atlantic cost of Europe. Innovative
millers built storage ponds that would fill at high tide, then emptied them at
low tide to turn a wheel as water rushed back to the ocean. A similar concept
has been used commercially in La Ranee, France, since 1966 to generate 240 MW of
electricity. Experimental tidal-plants built on the same idea are also operating
in Nova Scotia's Bay of Fundy (20 MW) and near Murmansk, Russia (0.4 MW). But
these plants are prohibitively expensive to build in today's economy, disrupt
the local marine ecology, and need to be continually dredged to remove silt.
An alternative, the vertical-axis Davis hydro turbine, is being developed by
Blue Energy. The company has six operating prototypes and claims they can be
mass produced using current construction methods and materials. The company
proposes several different modular units, some in the 5 to 500-kW range for
rivers, and others for 200 to 8,000-MW sites.
Davis underwater hydro turbine
Four fixed vertical blades connect to a rotor shaft that drives an integrated
gearbox/generator. The turbine sits in a concrete marine caisson, anchoring it
to the ocean floor. The caisson also directs water through the turbine and
supports the generator and other machiner/ above it. Blades are shaped to take
advantage of hydrodynamic lift, letting them move faster than the water
surrounding them. The blades are also shaped so that water flowing either way
spins the turbine, letting the unit generate power during a tide's ebb and flow.
Blue Energy is currently developing a 250-kW system that will work in waters
at least 10-m deep with currents of at least 1.75 m/ sec (3.5 knots). It cannot
produce electricity during slack tide, so the first 24/7 commercial systems for
off-grid applications will have diesel generators. An electronic controller
activates the diesel when it detects demand about to exceed 250 kW. A planned
upgrade adds a hydrogen fuel cell. Unused power from the 250-kW unit will
electrolyze water into hydrogen, which the fuel cell later converts to
electricity when there is not enough tidal movement. Hydrogen could also be
stored and used for cooking and transportation.
For larger applications, Blue Energy envisions a tidal fence or bridge
spanning a river, tidal estuary, or ocean channel. It would connect several
turbines, each 11 meters in diameter and rated at 12 MW. The site would need a
difference between high and low tide of at least 1.75 meters and currents of at
least 3.5 knots. The structure could easily be engineered to withstand typhoons
and earthquake- generated waves and tsunamis, says the company. Blue Energy
researchers suggest the finished structure could be used as the foundation for a
bridge or an offshore wind farm. The company also says most countries can
optimize load capacity to meet tidal power's inherently cyclic nature. It gives
three strategies for doing so: store energy until it's needed, only use tidal
power to meet peak loads, or convert unused tidal-generated electricity to
hydrogen, which could be considered storage.
Blue Energy's large-scale projects average about $l,200/kW (capital cost) and
they believe continued technical improvements will bring this figure down
considerably.
Like all hydropower concepts, Blue Energy exploits the fact that water is.,
over 800 times more dense than air, giving moving water considerable energy,
especially when compared to wind power. For example, seawater flowing at 8 knots
has the same energy as wind blowing at 217 knots. And while winds are
intermittent at best, tides and sea currents are completely predictable.
A concept much like that of Blue Energy's comes from Hammerfest Strom AS, a
Norwegian company. Norway already generates over 99% of its power in
hydroelectric dams, but Norwegians are now more concerned with preserving
pristine rivers and streams than building more dams for power. Hammerfest
proposes building verticalaxis turbines with two or three 10-meter blades made
of glass-fiber- reinforced composites to take advantage of tidal streams. Pitch
control will change the blades' angle of attack, letting the turbine spin
regardless of the tide's direction and without having to turn the nacelle.
Tidal currents are highly predictable and will put accurately known loads on
the tidal-stream turbines, making design of the mills fairly straightforward.
The challenge is the fact they sit on the bottom of the ocean, making access
difficult. To simplify installation and maintenance, Hammerfest will design
modular turbines with all critical components in one module (the nacelle), so it
can be lifted out of the water in one operation.
Hammerfest plans to install several mills at the same location, with the
first site being Kvalsundet, a narrow strait off Norway's coast about 50-m deep
with average tidal flows of 1.8 m/sec. The entire site is scheduled to have 20
mills and produce 32 GW-hr per year. Power will be sent ashore via a cable on
the seafloor and plugged into the national grid.
Mills similar to Blue Energy and Hammerfest Storm's could one day be deployed
to deep-sea sites and convert sea currents to electric power. Some developers
want to use the energy on site to convert water to hydrogen, fill a tanker with
the hydrogen, and periodically replace the full tanker with an empty one.
A shore-based Limpet
A land-based version of the oscillating water-column turbine from Wavegen
(below) and an inside look (left) at how wave action turns a turbine.
WAVES OF POWER
It seems natural for engineers at Wavegen to develop machines that convert
wave action into electricity, considering they are based in wave-battered
Scotland. Their Limpet (land-installed marine- powered energy transformer)
relies on an oscillating water column (OWC) shaped to take advantage of local
waves and hooked to a Wells turbine. Waves push water inside a collector (i.e.,
the water column) up and down, which pushes air in and out of a turbine invented
by Professor Alan Wells, a founding director of Wavegen. The turbine uses
symmetrical airfoils and spins in one direction no matter which direction air
flows through it. The turbine sits directly on the shaft of an induction
generator and uses an inverter drive, so the running speed can vary according to
wave conditions. There is no gearbox, making the turbine efficient and easy to
maintain.
Vertical-axis turbine
Hammerfest's underwater plant will not be a lightweight. The nacelle weighs
54 tons. The entire device weighs 120 tons, and an additional 200 tons of
weights will help secure the mill to the ocean floor
One problem is that with airflow reversing every wave cycle, the power
driving the turbine peaks and falls to zero every half cycle, says Jimmy
Ferguson, managing director of Wavegen. Unless steps were taken, electrical
generation would also vary in the same way. Instead, the system was designed to
have significant mechanical inertia, so it stores excess energy in flywheels
when input power is above average, When input energy falls below average, the
system draws additional energy from the flywheels to make up the difference.
This smooths operation during normal seas, but the system cannot cope if the sea
is too calm. "The units have detectors that determine when it is economical
to generate power, and shut down and start up the turbo-generation plant
accordingly," says Ferguson." "There is no need for a crew."
Such plants' output varies with the sea state, so average output will be less
than the maximum. But the unit's maximum output, which is limited by the
generator, determines the units output rating. And, according to Wavegen, it is
easy to integrate power from an OWC/ Wells turbine into a national power grid as
long as that power is a small percentage of the overall grid capacity. This
means it would be easy in industrialized or urban areas but it might be a
problem in more remote areas where such units would be most useful. Limpets
could also be used in arid countries near the sea to power desalination plants
that produce fresh water for farming and drin\king.
A Limpet in Islay, an island off the Scottish coast, has been feeding
electricity into the local grid since 2000. The 500-kW plant cost about $2
million, but that includes development and research funds. And they spend
$100,000 annually maintaining and monitoring the plant, but much of that is also
considered research. (They sell power to the grid at 12 cents/kW-hr.) "We
hope to get costs down to $800/kW for the turbo-generating equipment and another
$800/kW for the collection chambers," notes Ferguson.
In terms of life cycle, Limpet's concrete structure should last 100 years and
survive any high waves or rough weather, according to the company. Regular
maintenance and replacement should let them keep Limpets operational for those
100 years. The company also says Limpet 1 presents no danger to aquatic life or
boaters.
Wave-energy converter
The WEC from the Offshore Wave Energy Co. uses the shape of its duct to
capture and compress air for a turbine. The company has plans to connect several
together into an offshore generating platform, complete with windmills to take
advantage of the sea breezes.
John Kemp watches a scale prototype of the company's Wave Energy Converters
tested in a lab at Offshore Wave Energy Ltd.
"Although there is no limit to the size of an onshore Limpet site that
could be built, we believe a better course is to develop a linked system of
smaller units," explains Ferguson. "We are already doing this on a
deepwater break-water, installing 250 20-kW Limpets on a 1-km site. It will be
rated at 500 MW when finished."
The company is also developing a near-shore (water depths to 15 m) Limpet. It
takes advantage of the stronger waves in deeper waters. (Friction with the sea
floor robs waves of their power.) And there are more suitable sites for
near-shore rather than onshore installations. Shoreline real estate is usually
expensive and already developed, or off-limits to development.
Wavegen has plans for offshore (sea depths to 200 m) floating OWC buoys. But
right now they are concentrating on making the devices robust, reliable, and
capable of generating electricity and profits.
At Offshore Wave Energy Ltd. (OWEL), a company based in Portsmouth, U.K.,
engineers have an idea similar to the Limpet but with some engineering twists.
Their Wave Energy Converter (WEC) will float offshore in water at least 40-m
deep to get waves at their strongest and keep performance independent of high
and low tides. The goal is to make WECs simple to construct, with few moving
parts, none in contact with seawater, and able to survive major storm waves by
absorbing only a fraction of their power.
WECs mainly consist of a duct, its open end pointed toward incoming waves,
and sized to match waves at the proposed site. The distance between the floating
deck and the water line, for example, would be the same height as that of local
waves. And it should be as long as the longest anticipated waves. That way, air
in troughs between waves is trapped and carried into the duct. "The duct
will have angled sides and floor, which will compress air, ideally to about 10
aim, and push it into a compression manifold and a one-way valve feeding a
reservoir," explains John Kemp a scientist at OWBL. The reservoir will hold
about 1,000 gallons of air at a pressure slightly less than the manifold
pressure. Aft of the manifold, a series of baffles disperses any remaining
energy in the waves so it doesn't reflect back into the duct and interfere with
following waves. Air in the reservoir drives an air turbine to generate
electricity.
At start up, a few waves would first charge the reservoir, but then air will
be taken out constantly to drive a turbine and generator. How much air is
compressed depends on the size of the waves. But for a 2-m wave, a WEC would
take in 10,000 cubic meters of air, and the compression ratio would be about
10:1. Wave action will feed new pulses of compressed air into the reservoir
every few seconds to maintain pressure.
Practical dimensions for the WEC are on the order of 200-m long, 33-m wide
narrowing to 7, and 30-m deep. OWEL envisions six WECs rigidly connected side by
side, forming a triangular platform about 200-m long, 200-m wide and narrowing
to 40 m. It could be moored to the ocean floor, unmanned, and produce an
estimated 6 MW of electricity if optimized for wave conditions.
OWEL engineers believe 30,000-ton platforms could be made of environmentally
benign concrete and possibly serve as sites for windmills which would share
transmission lines and maintenance resources to bring down costs. They estimate
it would cost $21.5 million to build, $4.5 million to maintain, and could
produce 5 MW for 25 days a month. This yields 36 million kW-hr at 12.7 cents per
kW-hr. The current wholesale price of electricity in the U.K. is 5.4 cents per
kW-hr, but the government fines supply companies 5.4 cents per kW-hr if they do
not get some power from renewable sources. "This means we can be
competitive if we get the costs down below 10.8 cents per kW-hr," says
Kemp. "And that should not be difficult when significant numbers of
platforms are constructed so that economies of scale kick in."
OWEL is tank-testing models and have gotten encouraging results. For example,
they've shown that air pressure, as measured in millimeters of water, increased
in the duct by as much as twice the height of incoming waves and in some cases
three times the height. The team is also computer modeling the WEC to test
different sizes and configurations in a variety of sea states, as well as to
refine the computer model.
MAKE CONTACT:
Blue Energy Canada Ltd., (604) 682-2583, www.bluenergy.com.
Hammerfest Strom AS, 47 (7) 841-7103, www.e-tidevannsenergi.com.
Offshore Wave Energy Ltd., 44 (0) 239-281-8745, www.owel.co.uk
Wavegen, 44 (O) 146-323-8094, www.wavegen.com
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Copyright Penton Media, Inc. Sep 16, 2004