Plugging Into The Ocean

Plugging into the ocean

Senior Editor

The Sun and Moon's gravitational pull on the Earth generate tides, which could provide anywhere from 10,000 to 30,000 MW of electricity if we tapped areas suitable for power generation. Toss in the electricity that might be squeezed from undersea currents and surface waves — maybe another 300,000 MW or more — and we're talking serious generating capability. This untapped hydropower promises reliable and predictable generation with no fuel costs, wastes, hazardous byproducts, or emissions of any kind. But it still costs too much to be competitive with coal, oil, and gasfired plants. Engineers and scientists worldwide, however, are developing new technologies and approaches that should bring the price down and expand hydropower's role in generating 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 Rance, France, since 1966 to generate 240 MW of electricity. Experimental tidalplants 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.

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 $1,200/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-fiberreinforced 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.

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.

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 turbogeneration 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 drinking.

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 turbogenerating 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 presents no danger to aquatic life or boaters.

"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 breakwater, 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 offlimits 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 atm, and push it into a compression manifold and a one-way valve feeding a reservoir," explains John Kemp a scientist at OWEL. 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 (O) 239-281-8745, www.owel.co.uk
Wavegen, 44 (0) 146-323-8094, www.wavegen.com