Wave and tidal power

last updated 07/00

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Originally appeared in FRR issue:September 1998, web version updated July 2000

Keywords: wave power, tidal power, oceans, currents, waves, electricity generation
Sources: The New Scientist, 16 May and 20 June 1998, Professional Engineering 10 June 1998, Renewable Energy, Sep-Dec 1996, vol. 9 No. 1-4, pp.870-874, Blue Energy Canada Website at http://www.bluenergy.com.

[Additional material from The New Scientist, 3 October 1998, Renewable Energy 2000 (held at Brighton, England July 2-4 2000), and the wavegen website (http://www.wavegen.co.uk) have been added to the original printed report.]

Abstract

Generating electricity from the oceans has been widely discussed for 40 years. Time and again attempts have been made to harness the both wave and tidal power, usually with little success. However, with advances in engineering in the past few years, the oceans have become an economically feasible source of energy.

Introduction

The World electrical energy market is at $800-billion-a-year(US) and rising. It has been estimated that " there are 2 billion people who still lack electricity today, and the world demand in developing countries is doubling every eight years" (World Watch Institute, May 1997). In order to meet that demand, while limiting production of green house gases, renewable energy sources must be developed.

The sea has long been seen as a source of energy. In the middle ages (1200-1500 AD) farmers used to trap sea water in mill ponds and use it to power water mills as the tide dropped. Over the last fifty years, engineers have begun to look at tidal and wave power on a larger, industrial scale. However, until the last few years, particularly in Europe, wave power and tidal power were both seen as uneconomic. Although some pilot projects showed that energy could be generated, they also showed that, even if cost of the energy generated was not considered, there was a real problem making equipment which could withstand the extremely harsh marine environment.

In the late 1990s, it has become clear that technology has advanced to the point where reliable and cheap electricity from the oceans is becoming a real possibility. The UK will have its first electricity supplied to the national supply by the year 2000, and other countries are seriously considering doing likewise.

Wave Power I - sea-based devices

Europe, and in particular the United Kingdom, are looking again at wave power. A recent review by the government has shown that there are now types of wave power devices which can produce electricity at a cost of under $US0.10/kWh, the point at which production of electricity becomes economically viable. The most efficient of the devices, the “Salter ”Duck can produce electricity for less than $US0.05/kWh.

The “Salter ”Duck was developed in the 1970s by Professor Stephen Salter at the University of Edinburgh in Scotland (email Shs@srv1.mech.ed.ac.uk) and generates electricity by bobbing up and down with the waves. Although it can produce energy extremely efficiently it was effectively killed off in the mid 1980s when a European Union report miscalculated the cost of the electricity it produced by a factor of 10. In the last few years, the error has been realised, and interest in the Duck is becoming intense.

Operation of the Salter Duck

salter duck

©1996 Ramage

The “Clam” is another device which, Like the “Salter ”Duck can make energy from sea swell. The Clam is an arrangement of six airbags mounted around a hollow circular spine. As waves impact on the structure air is forced between the six bags via the hollow spine which is equipped with self-rectifying turbines. Even allowing for cabling to shore, it is calculated that the Clam can produce energy for around $US0.06kW/hr.

Wave Power II- Shore based systems

Where the shoreline has suitable topography, cliff-mounted oscillating water column (OWC) generators can be installed. OWC systems have a number of advantages over the Clam and the Duck, not the least of which is the fact that generators and all cabling are shore-based, making maintenance much cheaper.

The OWC works on a simple principle. As an incoming wave causes the water level in the unit's main chamber to rise (see diagram), air is forced up a funnel which houses a Well's counter-rotating turbine. As the wave retreats, air is sucked down into the main chamber again. The Well's turbine has been developed to spin in the same direction, whichever way air is flowing, in order to maximise efficiency. Although most previous OWC systems have had vertical water columns, that in LIMPET is angled at 45° - which wave tank test show to be more efficient.

OWC schematic

limpet

OWC machines have already been tested at a number of sites, including Japan and Norway. A commercial-scale (500 kW) installation is due to be commissioned on the Scottish Island of Islay in September 2000. The Islay OWC (known as LIMPET) is a joint venture between Queens University, WAVEGEN, Instituto Superior Técnico (Portugal), the European Union and Charles Brand Engineering. It is the direct successor of an experimental 75 kW turbine (built by researchers from the Queen's University of Belfast) which operated on the island between 1991 and 1999. Another LIMPET is currently being developed (at pilot-plant scale) on the Azores.

Construction of OWCs

One of the great problems with shoreline-based OWCs is their construction, which must necessarily take place on rocky shores exposed to wind and waves. In the case of the prototype Islay OWC system it was relatively easy to build a temporary dam on the shoreline to protect the unit. However, LIMPET is a much larger system, with a lip 20m wide. It was therefore ultimately decided to build the unit back from the coastline and remove a bund to make the system fully operational (see figure, below).

Installing an OWC unit

limpet

However, both OWC-systems and ocean-wave systems suffer from trying to harness violent forces. The first Norwegian OWC was ripped off a cliff-face during a storm, the Islay station is completely submerged under storm conditions. Thus, researchers are looking at other ways of generating electricity from the ocean, and are increasingly turning to tidally-generated coastal currents.

Power from tidally-generated coastal currents

Over the past forty years, there has been constant interest in harnessing tidal power. Initially, this interest focused on estuaries, where large volumes of water pass through narrow channels generating high current velocities. Engineers felt that blocking estuaries with a barrage and forcing water through turbines would be an effective way to generate electricity. This was proved by construction of a tidal barrage at St. Malo in France in the mid 1960s. La Rance tidal power plant still provides 90% of Brittany's, and a major refurbishment program (due for completion in 2007) means it will continue in operation well into the new millenium.

Despite the success of La Rance, no other major tidal barrages have been built since, due in some part to environmental concerns. Barages present a barrier to navigation by boats and fish alike; reduced tidal range (difference between high and low water levels) can destroy much of the inter-tidal habitat used by wading birds; and sediment trapped behind the barrage could also reduce the volume of the estuary over time. By the early 1990s, interest in estuarine-derived tidal power had largely ceased, and scientists and engineers began to look at the potential of tidally-generated coastal currents instead.

As tides ebb and flow, currents are often generated in coastal waters (quite often in areas far-removed from bays and estuaries). In many places the shape of the seabed forces water to flow through narrow channels, or around headlands (much like the wind howls through narrow valleys and around hills). However, sea water has a much higher density than air, meaning that currents of 5-8 knots generate as much energy as winds of much higher velocity. In addition, unlike the wind rushing through a valley or over hilltops, tidally-generated coastal currents are predictable. The tide comes in and out every twelve hours, resulting in currents which reach peak velocity four times every day. Two rival technologies -- tidal fences and tidal turbines -- are now being developed to catch the energy of these currents.

Coastal currents are strongest at the margins of the world's larger oceans. A review of likely tidal power sites in the late 1980s estimated the energy resource was in excess of 330,000 MW. South East Asia is one area where it is likely such currents could be exploited for energy. In particular, the Chinese and Japanese coasts, and the large number of straits between the islands of the Philippines are suitable for development of power generation from coastal currents.

Tidal Fences

Tidal fences (see figure 1) are effectively barrages which completely block a channel. As discussed above, if deployed across the mouth of an estuary they can be very environmentally destructive. However, in the 1990s their deployment in channels between small islands or in straights between the mainland and island has increasingly been considered as a viable option for generation of large amounts of electricity.

A Tidal Fence

tidal fence

The advantage of a tidal fence is that all the electrical equipment (generators and transformers) can be kept high above the water. Also, by decreasing the cross-section of the channel, current velocity through the turbines is significantly increased.

The first large-scale commercial fences are likely to be built in South East Asia. The most advanced plan is for a scheme for a fence across the Dalupiri Passage between the islands of Dalpiri and Samar in the Philippines, agreed between the Philippines Government and Blue Energy Engineering Company of Vancouver, Canada in late 1997. The site, on the south side of the San Bernardino Strait, is approx. 41 m deep (with a relatively flat bottom) and has a peak tidal current of about 8 knots. As a result, the fence is expected to generate up to 2200 MW of peak power (with a base daily average of 1100 MW).

Once given final government approval (expected before the end of 2000), work will begin on a 4km-long structure designed to withstand typhoon winds of 150 mph and tsunami waves of 7 meters. The Dalupiri Ocean Power Plant will utilize 274 ocean-class Davis Turbines, each generating from 7MW to 14 MW. However, the $US2.8 billion project is just the first phase one of a much-larger proposed Build Own Operate Transfer (BOOT) project that will be transferred to the Philippines after 25 years. Used to generate large scale renewable energy, the San Bernardino passage could help the Philippines to become a net exporter of electrical power.

The modular nature of the Blue Energy Power System allows for power to be generated in the fourth year of the project, with the installation of the first module in the chain, which gradually increases to full capacity by project completion in year six. Once begun, this project will be one of the largest renewable energy developments in the world.

Tidal Turbines

Tidal turbines are the chief competition to the tidal fence. Looking like an underwater wind turbine they offer a number of advantages over the tidal fence. They are less disruptive to wildlife, allow small boats to continue to use the area, and have much lower material requirements than the fence.

Tidal turbines function well where coastal currents run at 2-2.5 m/s (slower currents tend to be uneconomic while larger ones put a lot of stress on the equipment). Such currents provide an energy density four times greater than air, meaning that a 15m diameter turbine will generate as much energy as a 60m diameter windmill. In addition, tidal currents are both predictable and reliable, a feature which gives them an advantage over both wind and solar systems. The tidal turbine also offers significant environmental advantages over wind and solar systems; the majority of the assembly is hidden below the waterline, and all cabling is along the seabed.

There are many sites around the world where tidal turbines could be effectively installed. The ideal site is close to shore (within 1 km) in water depths of about 20-30m. Peter Fraenkel, director of UK-based Marine Current Turbines, believes the best sites could generate more than 10 megawatts of energy per square kilometer. The European Union has already identified 106 sites which would be suitable for the turbines, 42 of them around the UK. Further afield, Fraenkel believes the Philippines, Indonesia, China and Japan could all develop underwater turbine farms.

Fraenkel intends to deploy a commercial-scale prototype turbine off the southwest coast of England in the summer of 2001. It will generate 300 kW (enough to power a small village). Although the cost of energy from the prototype turbine will be $US0.10/kW, costs will drop as the technology matures. Fraenkel hopes that the first "turbine farm" (£MW) will operational by 2004 and aims to have 300MW capacity being installed every year by 2010.

Artist's impression of turbine farm

MCT tidal turbines

image © marine current turbines ltd.

Conclusions

Wave power (and tidal power) are beginning to come into their own. They have many benefits, including:

Clearly there are still technical difficulties to overcome, but in the next few years, countries will begin to see wave power connected to national supplies. It will be a big market.

 

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