Offshore Alternative Energy Generation

Transcription

1 Offshore Alternative Energy Generation Rob Bioletti and Ian Potter Carbon & Energy Management Alberta Research Council Edmonton, Alberta, T6N 1E4, Canada Abstract Alternative energy sources have garnered attention in recent years in an attempt to decrease mankind s dependence on fossil fuels. Of these, offshore alternative energy generation has been in the development stage for a number of years due to the great amount of energy available within the Earth s oceans. Wave energy, tidal energy, marine current energy and ocean thermal energy conversion are four groups of technologies that have been studied and developed in recent years. This paper presents a brief review of the technologies, their current operational status and their advancement towards commercialization. Theoretically, all four can function to create electricity, and many technologies have been proven to be effective. However, only tidal energy has a few commercial installations and future facilities have yet to be planned. None of the other technologies are in commercial operation due to prohibitive capital costs. Projects are ongoing that may make the technologies more feasible in the years to come. Introduction The increasing global requirement for energy is resulting in greater fossil fuel consumption. In recent years, heightened public awareness about environmental issues has created a greater interest in alternative energy sources, such as wind, hydro and solar power, to name a few. This paper focuses on alternative energy sources from offshore sites and their current technologies. The offshore energy sources have been categorized into four groups: wave energy, tidal energy, marine current energy, and ocean thermal energy. Wave Energy Oceanic waves are generally considered to be a concentrated form of solar energy. Waves are produced by winds that are created by pressure differences in the Earth s atmosphere, which are created by differential solar heating. The energy transferred from wind to water is in the form of potential energy (mass of water in wave above sea level) and kinetic energy (movement of water molecules). The amount of energy transferred is dependent upon the wind speed, the amount of time that the wind is blowing and the distance over which it travels, also known as the fetch (Thorpe, 1998). The power potential for waves can be described as units of power per metre of wave crest length. The potential wave power can be converted to electricity via mechanical means. Harnessing the energy provided by oceanic waves has been developed for over the past thirty years via numerous technologies, with research being conducted primarily in Western Europe. This paper focuses only on the most well-known and promising technologies. Wave Energy Potential Worldwide, the highest concentrations of wave energy (that is, the strongest waves) are found on western coasts between 40 o - 60 o latitude in the northern and southern hemispheres. The strongest waves that provide the most concentrated energy are found in the Atlantic Ocean off the coast of Ireland, the Southern Ocean off the southern coast of New Zealand and off of Cape Horn of South America, as shown in Figure 1.

2 Figure 1: Global Distribution of Offshore Wave Power (Source: Thorpe, 1998) The power available from each wave is proportional to the square of the wave height and the wave period. This is shown in Equation 1: 2 2 ρ g T H P = (1) 32π Where P equals power per unit crest length of wave (e.g. kw/m), ρ equals the density of water, g equals acceleration due to gravity, T equals time between successive wave crests (period), and H equals the height of the wave. As oceanic waves approach shorelines, their power output generally decrease due to frictional losses to the seabed. However, operating power generating units would be easier and waves can be concentrated as they approach shorelines with adequate seabed formations. Five ways this can occur are: Refraction wave fronts bend to become more parallel with the shoreline due to shallow waters. Diffraction wave fronts bending around and behind obstacles. Shoaling wave height varies with water depth (e.g. higher waves in shallower water). Breaking waves break as they become steeper, resulting in loss of potential energy. Friction waves lose energy due to friction between seabed and water molecules. With the correct seabed formation close to shore, powerful wave hotspots can occur, which are ideal locations for near-shore applications. However, even near-shore hotspots are only about one-third as powerful as the average deep-water (>40m) location (Thorpe, 1992). Seasonal variation is also a factor when determining where the greatest wave power potential is situated, but the greatest power potential usually coincides with power demand. That is, the strongest waves coincide with winter months and greatest energy demand. The worldwide power potential for deep-sea waves has been reported to be 1 10 TW ( W) (Panicker, 1976).

3 Offshore Wave Energy Technologies The means by which wave energy is extracted to a useful form is generally accomplished via an interface that transfers the force of a wave to fluid pressure or mechanical motion. There are more than 1000 patents for wave energy converters. The most common offshore devices are float-based devices: objects that float at or near the surface to extract the maximum power from incident waves. Thus, they need to be constructed with flexible moorings and electrical transmission cables. The four most developed and promising offshore technologies are described below. Hosepump The hosepump consists of an elastomeric hose that decreases in internal volume as it stretches. The hose is connected to a float that oscillates with the surface waves. The resultant pressurizing of the water in the hose forces water out a non-return valve at the bottom of the hose and to a turbine and generator unit. A series of hosepumps can be connected to a central turbine for larger installations. A schematic of the hosepump is shown in Figure 2. McCabe Wave Pump Figure 2: Hosepump Schematic (Source: WEC, 2001) McCabe wave pumps have been in the development stage since 1980, and were originally designed for seawater desalination using reverse osmosis, but have recently been designed for power generation. Wave pumps are devices that extract energy from waves by the rotation of pontoons around hinges via linear hydraulic pumps. The hydraulic pumps may be constructed either as a closed-loop oil system or an open-loop seawater system, which drive a turbine and generator to create ~400 kw). A schematic of the wave pump is presented in Figure 3. A 40m-long prototype has been tested offshore of Ireland in 1996, and further demonstrations were scheduled for Figure 3: McCabe Wave Pump Schematic (Source: WEC, 2001)

4 Pelamis The Pelamis water snake device is composed of hollow, cylindrical sections linked by hinged joints. The sections point into the oncoming waves and move with respect to each other as the waves pass down their length. Energy is extracted by hydraulic rams at the joints, which drive electrical generators. A Pelamis is being developed for deployment off a Scottish island in 2003, which is rated at 375kW and is 130 m long and 3.5 m in diameter. A pelamis device is shown in Figure 4. Figure 4: Pelamis Drawing (Source: Archimedes Wave Swing (AWS) The Archimedes Wave Swing consists of a cylindrical, air-filled chamber that can move vertically with respect to an anchored smaller-diameter cylinder. The air within the 10m - 20m diameter floater cylinder ensures buoyancy. When a wave passes over the floater, the buoyancy is changed due to pressure changes, causing the floater to move up and down. This relative motion between the floater and the anchored basement is used to produce energy. To date, the AWS is the most powerful device built: a 2 MW Pilot project was scheduled for launch off the coast of Portugal in April A photo of the an AWS above-surface and the principle behind the AWS is shown in Figure 5. Figure 5: AWS Above-surface and Technical Principle (Source: Wave Energy Summary Determining the unit cost for offshore wave power technologies is prohibitive at this time since the technologies are still in the development stage and no commercial applications have been proven.

5 Nevertheless, the capital costs for wave energy devices will be relatively high in the near future when compared to more conventional methods of energy production. The environmental impact of offshore wave energy devices is generally considered to be minimal, save for potential noise and obstacles for shipping routes and fishing. Anchoring the devices to the seabed would cause some environmental damage, especially if they are deployed in large numbers. Tidal Energy Tides are generated by the Earth s rotation within the gravitational fields of the sun and moon. The varying gravitational forces due to the relative positions of the Earth, sun and moon cause the ocean s surface to raise and lower periodically. There are a number of interacting cycles that affect the level of the ocean surface: - A half-day cycle caused by the Earth rotating within the moon s gravitational field - A fourteen-day cycle caused by constructive and destructive interference of the gravitational fields of the sun and moon. During new moon and full moon, there is constructive interference between the gravitational forces of the sun and moon, which results in maximum (spring) tides. During quarter phases of the moon, there is destructive interference that results in minimum (neap) tides. Neap tides generally have about half the tidal range as spring tides - A half-year cycle caused by the inclination of the moon s orbit relative to that of the Earth, which results in maximum spring tides in March and September In the absence of land, the maximum amplitude of tides is approximately one metre. Closer to land, this range can be dramatically increased due to shelving of the seabed and funneling of the seawater in estuaries. Additional effects can be created by tidal resonance and reflection by the coastline. This can occur in trumpet-shaped estuaries where the length of the estuary is nearly one-fourth of the tidal wave length. Tidal Energy Potential Due to the physical concept of tides, their potential power output varies with time and location. Tidal energy output varies approximately with the square of the tidal range. As an example, spring-neap cycles tend to range by a factor of two, and the power output can vary by a factor of four. A benefit of tidal energy is that tides are very predictable in terms of timing and power output. A problem with tidal energy development is the relatively low number of locations that are close to centres of demand where tides are great enough to warrant energy extraction. However, places such as the Bay of Fundy in Eastern Canada, the La Rance estuary in North-western France, and the Severn estuary in South-western United Kingdom have tidal ranges that can exceed 14 metres and could potentially harbor tidal energy power plants. Some locations where tidal energy could be exploited are presented in Table 1.

6 Table 1: Potential Tidal Energy Power Plant Sites (Source: WEC, 2001) Country Location Mean tidal range (m) Basin area (km 2 ) Installed capacity (MW) Approximate annual output (TWh/year) Annual plant load factor (%) Argentina San José Golfo Nuevo Rio Deseado Santa Cruz Rio Gallegos Australia Secure Bay Walcott Inlet Canada Cobequid Cumberland Shepody India Gulf of Kutch Gulf of Khambat Korea (Rep.) Garolim Cheonsu Mexico Rio Colorado UK Severn Mersey Duddon Wyre Conwy USA Pasamaquoddy 5.5 Knik Arm Turnagain Arm Russian Fed. Mezen Tugur Penzhinsk Tidal Energy Technologies Tidal energy differs somewhat from other offshore technologies in that it is most feasible when a barrage or dam is built across an estuary entrance, essentially linking the power plant to land. It has been under development for a number of years due to its technological similarities with conventional hydropower plants. The barrage or dam consists of a series of gated sluices and banks of low-head axial flow turbines. During ebb tides, the gates are opened and water is allowed to flow into the estuary. During the tide shift, the gates are closed and the retreating water flows through the turbines to drive a generator that produces electricity. This is facilitated by the head that is created from the difference in water levels on either side of the dam. The most suitable turbines for use with tidal

7 energy are Kaplan or propeller-type turbines because they can handle large volumes of water and variation of head. They are slow turning, usually in the range of 50 to 100 rpm, and the runner diameter can be up to 9 metres long. They are uni-directional, which is suitable for ebb tide generation. Studies have shown that two-way generation (turbine operation during ebb and flood tides) requires more expensive machinery (namely two-way turbines) and more sophisticated controls that are not warranted for the gain in energy production (Pontes, 2001). The sluice gates must be large to allow large volumes of water to flow through. The best type of gates has been reported to be the vertical lift gates with a motor operated hoist. In some cases ship locks may need to be installed, which would increase the cost of the project substantially. A cross section of a potential tidal energy installation is shown in Figure 6. Figure 6.: Cross Section of Potential Tidal Energy Installation (Source: europa.eu.int/comm/energy_transport/atlas/htmlu/tidal.html) There have been a few installations of tidal energy power plants in the past 50 years. In 1967 a 240 MW plant was completed across the La Rance estuary on North-western France, which has been operating successfully since. It entails a 750 metre-long dam with 24 bulb-type Kaplan turbines rated at 10 MW each. In 1984 an 18 MW plant was operated at Annapolis Royal on the Nova Scotia coast of the Bay of Fundy. It was constructed for demonstration of a large Straflo turbine but has not been expanded upon to date. Other power plants have been constructed, such as the 400 kw experimental plant Kislaya Guna on the Barents Sea in Russia built in 1968, and the 3.4 MW plant built at Jianxia in China between 1980 and Considerable work has been performed in the United Kingdom to investigate the feasibility of tidal energy installations. No projects progressed past the technical appraisals due to the less favourable economics of tidal energy compared to more conventional means, such as hydropower. Other projects are underway to investigate interconnected channels between estuaries to provide continuous power. The projects are in the feasibility study stages in Derby, Australia and in Alaska and Chile. Tidal Energy Summary Tidal energy can be perceived as being more environmentally friendly compared to fossil fuels, but the alterations to the estuary ecosystem resultant from different tides must be considered. Once completed, marine energy systems can benefit the local economy by providing storm surge and coastal flooding protection and potential roads across the dams on large installations. Tidal energy is a fairly advanced alternative energy technology. Currently, there are operational power plants, such as the 240 MW La Rance facility in France, but construction of new plants in the near future was not identified. Major obstacles for large tidal energy plants using the aforementioned technologies are the high capital cost and long construction times necessary for tidal energy power plants.

8 Marine Current Energy Marine currents are mostly driven by tidal forces, which force seawater to flow towards shore twice a day (flood tide) and back out to sea twice a day (ebb tide) due to the gravitational forces from the relative positions of the Earth, its moon and the sun. These currents are called tidal currents. Oceanic currents are mostly affected by the Earth s overall thermal flux, but they are also affected by the shape of coastline and the shape of the seabed (bathymetry), as well as local temperature and salinity. For example, near a jagged coastline with a high tidal range the marine current would typically be strong. Along a straight coastline or in the middle of a very deep ocean the current would typically be low. There are locations where the current flows continuously in one direction, such as the Gulf Stream, which moves approximately 80 million cubic metres of water per second. These currents are generated mostly from large thermal movements within the Earth s hydrosphere. Marine currents are an attractive energy source due to their predictability and potential high load factor (up to 80% for non-tidal flows). However, there has been little research in the field and there are no gridconnected installations put in place to date. Marine Current Energy Potential The strongest marine currents are generally found in narrow straits, around headlands, between islands and entrances to bays and lochs. In these locations, the areas that have the greatest tidal variation and shallow water will have the strongest marine currents. In Western Europe studies have conducted which pegged the present-day potential marine current energy to be 48 TWh per year (CEC, 1996). It was estimated in 1973 that the world s total marine current energy is approximately 5 TW (Isaacs, 1973). However, only a fraction of this could be used practically, and in 2000 it was reported that 450 GW could potentially be harnessed (Blue Energy, 2000). A map of the Earth s major ocean surface currents is presented in Figure 7. Figure 7: Map of Major Oceanic Currents (Source: Marine Current Energy Technologies The technique that has been investigated the most is the use of a submerged turbine rotor set normal to the current flow and connected to a generator to generate electricity, much like a wind turbine. This transfers the kinetic energy of the marine current to useful rotational and electrical energy. The

9 potential power output from a marine current is a function of the density and velocity of the water, the area of the rotor blades and the efficiency of the turbine, as shown in Equation 2: 1 3 P = ρ Aν η (2) 2 Where P equals the power output, ρ equals the density of the water, A equals the area of the rotor blades, ν equals the velocity of the current and η equals the turbine efficiency. The turbine rotors can be anchored to the sea floor in shallow water or moored with a floating platform in deeper water. There are many types of turbines that have been tested, grouped into horizontal and vertical axis categories. Figure 8 shows the four most common types of marine current energy technologies. Horizontal Axis Turbines Figure 8: Four Common Types of Marine Current Energy Technologies (Source: WEC, 2001) Horizontal axis turbines (axial flow turbines) are very similar to the common wind turbines, although the rotor blades can be much shorter due to the much greater density of water relative to air. There have been prototypes built that produced up to 10 kw of electricity, and there are plans to install a 300 kw unit off the south coast of the United Kingdom. Horizontal axis shrouded turbines can be used to concentrate the flow of water around the blades to increase the power output. Vertical Axis Turbines Vertical axis turbines (cross flow turbines) have been developed and tested using many different designs. The most promising may be the Darrieus lift turbine that is composed of three or four thin blades of aerofoil. A Darrieus turbine was proven to generate 5 kw in the Kurushima Straits of Japan. A 130 kw vertical axis turbine with a floating platform has been deployed in the Strait of Messina between Sicily and Italy. Tidal fences have also been proposed, which incorporate a large number of small vertical axis turbines to make a fence. A 30 MW vertical axis tidal fence is being planned for deployment near the Philippines.

10 Marine Current Energy Summary There are a number of problems associated with utilizing marine current energy. First, since there are bearings and other moving parts submerged in seawater there will be maintenance issues that will need to be addressed. Second, cavitation at the blade tip will need to be avoided, which will limit tip speeds to approximately 8 m/s. Third, build-up of marine organisms and collection of debris will need to be prevented. The lack of operating knowledge and high capital costs will ensure this technology to be expensive in the near future. These obstacles aside, marine current energy has large potential for future applications if more research is conducted, mostly due to its theoretical energy density. The units can be modularized for easy installation, and environmental impact can be kept relatively low compared to conventional energy sources. Ocean Thermal Energy Conversion (OTEC) Ocean Thermal Energy Conversion is a method by which temperature differences between the sea surface and depths are utilized to create electricity. It is a simple heat exchange that operates using the Rankine thermodynamic cycle. It has been investigated by French, American and Japanese teams in Brazil, Cuba, Hawaii and Japan. Ocean Thermal Energy Conversion Potential OTEC uses the difference between the sea surface temperature of tropical areas (typically between 24 o C and 33 o C) and the water temperature at depths of 500 m to 1000 m (typically between 3 o C and 9 o C). Since OTEC can operate with temperature differences of only 20 o C, seas between 20 o N and 20 o S latitudes can be potential sources. Ocean Thermal Energy Conversion Technologies OTEC operates using the Rankine thermodynamic cycle. It is the same cycle used in conventional thermal power plants where a working fluid is evaporated, expanded through a turbine and then condensed. The working fluid can be seawater (open cycle) or another fluid, such as ammonia, propane or a refrigerant (closed cycle). In the open cycle scheme, warm seawater at the surface is flash-evaporated via a partial vacuum. The resultant steam propels a turbine and is then cooled in a condenser that uses cold water from the ocean depths. The turbine drives a generator to produce electricity. The thermal efficiency of an OTEC plant is approximately 2-3% after considering pumping power and frictional losses. A schematic of a typical open cycle OTEC is presented in Figure 9. Figure 9: OTEC Open Cycle Schematic (Source: Petroleum Corporation of Jamaica)

11 OTEC installations can be lad-based, floating or roving, which adds to their versatility as a stable energy provider. As well, if the device is far out at sea and power transmission lines are not feasible, the power can be used to electrolyze water to create oxygen and hydrogen. These valuable gases can be shipped out for sale. Considerable work has been performed developing OTEC in Hawaii. In 1979, a 50kW closed cycle plant was demonstrated. In 1993, 50 kw of electricity was realized using a 210 kw open cycle OTEC unit. Ocean Thermal Energy Conversion Summary A crucial problem associated with OTEC is the size and quality of equipment necessary for adequate power production. The heat exchangers (evaporator and condenser) must be made of highly conductive materials, which raise their capital cost accordingly. In open cycles, the turbine must be large in order to transfer the small amount of available enthalpy from the steam. In addition, the size of the cold water pipes will need to be large, and the pipe would need to be flexible for floating OTEC applications. For instance, the 40 MW OTEC land-based project at Kahe Point in Hawaii requires a pipe 3670 m long, and this cost would represent one quarter to one third of the total project cost. It is evident that OTEC also has problems in terms of environmental feasibility. OTEC installations may disturb oceanic balances by pumping and land-based facilities may require large areas of developed land. Closed cycle operations can pose serious environmental and health risks if a toxic working fluid leaks from the system. However, the theoretical foundation exists for successful OTEC operation, and advances in heat exchange materials and pumps may make the OTEC technologies more attractive. Conclusions The potential for wave energy, tidal energy, marine current energy and ocean thermal energy conversion to be an alternative source of energy has been proven at the experimental level. Wave energy has had many recent developments with a myriad of different technologies being created, and successful tests have been conducted in Western Europe. The greatest power output reported was for a planned 2 MW Archimedes Wave Swing unit off the coast of Portugal. Tidal energy is the most proven technology to date with the largest power plant being the 240 MW La Rance facility in France. However, high capital costs and the lack of suitable locations have kept tidal energy from being a substantial global power producer. Marine current energy has also had success in proving its theoretical base, with promising results attained in Western Europe and substantial plans to build a 30 MW vertical axis tidal fence in the Philippines. Ocean thermal energy conversion (OTEC) has also been proven to create electricity, such as the 50 kw test unit in Hawaii, although no substantial future projects were identified for the OTEC technology. No published figures were identified that reported the unit cost of electricity generation since commercial applications are non-existent. Offshore energy generation has been perceived as being environmentally benign, but issues surrounding the local marine ecosystem need to be addressed before such a conclusion is made. Although each technology is theoretically feasible, economics has been the obstacle towards commercialization. As with many other alternative energy technologies, the capital cost of the equipment is a major deterrent for commercial application. Wave energy, tidal energy, marine current energy and ocean thermal energy conversion may not be economically feasible now, but ongoing research could lead to improvements that may make these technologies more economically attractive in the future.

12 References Blue Energy Canada Inc., (2000), Canada. CEC (Commission of the European Communities), DGXII (1996), Wave Energy Project Results: The Exploitation of Tidal Marine Currents, Report EUR16683EN. Isaacs, J.D., and Seymour, R.J., (1973), The Ocean as a Power Resource, Int. Journal of Environmental Studies, vol. 4(3), pp Panicker, N.N., (1976), Power Resource Potential of Ocean Surface Waves, pp J1-J48, Proceedings of the Wave and Salinity Gradient Workshop, Newark, Delaware, USA. Pontes, M.T., and Falcão, A., (2001), Ocean Energies: Resources and Utilisation, 18 th World Energy Council Congress, Buenos Aires, October Thorpe, T.W., (1992), A Review of Wave Energy, ETSU Report R-72, Harwell, Oxfordshire, UK. Thorpe, T.W., (1998), An Overview of Wave Energy Technologies, ETSU. WEC (World Energy Council), (2001), Survey of Energy Resources, 19 th Edition, London, England. Copyright 2002 The copyright of this document or product, whether in print or electronically stored on a CD or diskette or otherwise (the "Protected Work") is held by the Alberta Research Council Inc. (ARC). The Inter-American Association of Sanitary and Environmental Engineering (AIDIS) and its Chapters have been granted a license to copy, distribute and reproduce this Protected Work on a non-commercial and cost-recovery basis in Latin America and the Caribbean. This Protected Work shall not, in whole or in part, be copied, photocopied, reproduced, translated or reproduced to any electronic means or machinereadable form without prior consent in writing from ARC. Any copy of this Protected Work made under such consent must include this copyright notice. Funding This document has been exclusively prepared for the AIDIS-CANADA Environmental Project. The Project was funded by the Canadian International Development Agency (CIDA), managed by the Alberta Research Council Inc. (ARC) and AIDIS as the Latin American partner. Alberta Research Council Inc. (ARC) 250 Karl Clark Road Edmonton, Alberta T6N 1E4 Tel: (780) Fax: (780) Website: Inter-American Association of Sanitary and Environmental Engineering Permanent Headquarters Abel Wolman Rua Nicolau Galiardi, Sao Paulo, SP, Brazil Tel: (55-11) Fax: (55-11) Website: aidis@unisys.com.br