Solar Energy, Wind Energy, Hydro and Geothermal Energy: A Review of Ocean Energy Systems

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1 Solar Energy, Wind Energy, Hydro and Geothermal Energy: A Review of Ocean Energy Systems Les Duckers and Wirongrong Mongkonthum * School of Science and the Environment Coventry University, Coventry, United Kingdom Abstract: The Kyoto protocol recognised the need to reduce carbon dioxide emissions and many countries have considered the potential contribution from renewables. The total world ocean energy resource is extensive and this paper will address how exploitation of this resource is progressing. In the UK wave energy research and development began as a response to the first oil crisis in 1973, and subsequent work in many countries has resulted in a wide range of wave energy concepts. France installed the World s largest fixed tidal scheme in 1969 and has considerable experience of operation of a two-way system. Modern concepts are being developed to harvest tidal currents. Ocean Thermal Energy Conversion (OTEC) has applications where the surface temperature of oceans is over 25C. This paper will examine the nature of ocean energy resources and report some of the recent work in each of the technologies. Keywords: Wave, Tidal, OTEC, Environment. 1. INTRODUCTION Tidal power has a long history. Records indicate that tide mills were being worked on the coasts of France, Spain and Britain prior to 1100 AD. These consisted simply of a storage pond, which was filled during the flood tide through a sluice and emptied during the ebb tide through a waterwheel. See Fig. 1. The technology used in these mills and the applications of the mechanical power were much the same as in hydropower water wheels. The tidal mills captured seawater at high tide and released it to provide low speed high torque output to grind corn etc. These mills remained in common use for many centuries, they were gradually displaced by the cheaper and more convenient fuels and machines made available by the Industrial Revolution. A 240MW tidal scheme was installed at the mouth of the river Rance in northern France in 1969,but apart from that and small scale OTEC schemes and pilot wave schemes the ocean energy resource remain largely untapped. The international acknowledgement of carbon dioxide induced climate change has reopened the quest for obtaining energy from the oceans. Fig. 1 An example of an ancient tidal mill in Saint Suliac in La Rance estuary in France. The Oceans can provide huge amounts of renewable energy in the form of waves, tides and thermal stratification. Table 1 shows approximate values of the resource for water Corresponding author: mongkonw@coventry.ac.uk technologies in terms of practical power available, and it is clear that they collectively offer large rewards if we can find economic and environmentally sensitive means of harvesting them. The hydro resource is shown for comparison; about one quarter of the World s large-scale hydro resource is already exploited, often in controversial circumstances which inhibits further exploitation. Small-scale hydro, on the other hand is still open for further development, but the oceans may provide great opportunities for energy farms. The global tidal resource may actually be 3000GW (DTI, 2002) but much of it is not located near to centres of energy demand. Similarly the World wave and OTEC resource may be tens of times greater than shown here it is not practical to envisage extracting all of the physical resource of these resources and so we report reasonably realistic figures in this table. Table 1 The authors estimates of realistically accessible resources GW World Potential World Exploited Hydro large scale Hydro small scale Tidal Barrage Tidal Stream Wave on-shore Wave off-shore OTEC OCEAN THERMAL ENERGY CONVERSION The oceans cover a little more than 70 percent of the Earth's surface. This makes them the world's largest solar energy collector and energy storage system. On an average day, 60 million square kilometres of tropical seas absorb solar radiation. OTEC, or ocean thermal energy conversion, is an energy technology that converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradient the fact that the ocean's layers of water have different temperatures to drive a power-producing thermodynamic cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20 C, an OTEC system can produce a significant amount of energy. The oceans are thus a vast renewable resource, with the potential for OTEC plants to produce 3000 GW of electric 961

2 power. This potential is estimated to exceed 1000 GW, although we have been more modest in the table above. The cold, deep seawater used in the OTEC process is also rich in nutrients, and it can be used to culture both marine organisms and plant life near the shore or on land. Moon are out of phase. Spring tides can be twice as large as neap tides. Over much of the surface of the oceans, the tidal range (the vertical rise and fall) is rather small, less than one metre, but in certain places, there is an enhancement of the range. Enhancement may be due to: 1. Funnelling (as is the case of Fig. 3, Severn Estuary) - The tide is gradually constrained from the sides and so increases in height - or the reverse, later in the cycle. 2. Resonance (as is also the case in the Severn) - The estuary has a resonant period equivalent to the tidal period. The length and depth of the estuary are very important for Resonance: they should approximately satisfy the expression: (L/h) 0.5 = m 0.5 Fig. 2 World ocean surface temperature (NREL, 2004). The economics of energy production today have delayed the financing of a permanent, continuously operating OTEC plant. The cost of constructing and operating equipment to move cold water through several hundred meters to the surface makes the economics rather marginal. However, OTEC is very promising as an alternative energy resource for tropical island communities that rely heavily on imported fuel. OTEC plants in these markets could provide islanders with much-needed power, as well as desalinated water and a variety of mariculture products. The United States National Renewable Energy Laboratory (NREL, 2004) has withdrawn from supporting OTEC activities. 3. TIDAL ENERGY The Resource Tidal schemes divide into two types. Tidal range schemes capture potential energy, usually by means of a tidal barrage, while tidal stream or flow schemes harness only the kinetic energy of a tidal current. Much work has been done on feasibility studies for range schemes at various sites and of course a considerable amount of civil engineering is often involved. Flow schemes are obviously most attractive where local topography causes strong currents, using the same technology as `run of the river' flow schemes. Not many of either type of tidal scheme have actually been constructed. The tides are produced by the gravitational attraction of both the Sun and the Moon. The Moon dominates this attraction, being much nearer to the Earth and is about 2.2 times more influential. The Moon and the Sun s gravitational fields cause the natural rise and fall of coastal tidal waters. Since the Moon is closer to Earth, albeit much less massive, it has a dominant effect upon tides. The Moon is 2.2 times more influential than the Sun. We could consider tidal energy to be mostly a form of lunar energy! The Earth rotates on its axis once every 24 hours. In the Earth s frame of reference the Sun appears to orbit the Earth once every 24 hours. The Moon orbits the Earth once every 29 days approximately. In the Earth s frame of reference, the Moon appears to orbit the Earth once every 24 hours and 50 minutes. This difference in periods between the apparent orbits of the Sun and Moon leads to phase changes with larger spring tides during in phase behaviour and smaller neap tides when the Sun and Fig. 3 Severn estuary. 3. Coriolis Effect (as is the case at La Rance, France) The spinning of the Earth leads to the Coriolis effect (the effect that causes large scale weather systems to form). The tide is influenced by this and so tends to increase in height at high tide on the northern French coast and be drawn away from this coast at low tide, with the net effect of enhancing the tidal range. For the equivalent coast in Southern England, the effects are in the opposite sense creating a lower tidal range. Fig. 4 is a photograph of the tidal barrage at La Rance in France. The tidal range sometimes exceeds 10m. Fig. 4 La Rance (the tide is flowing into the estuary). 4. Atmospheric pressure Changes in atmospheric pressure can depress or elevate the sea level by tens of centimetres. 5. Storms 962

3 Storms can push water onto the coast and thereby increase the tidal height. Both 4 and 5 are weather related and cannot be predicted on a long term basis, whereas 1, 2 and 3 can be accurately predicted over a long time scale. The barrage at La Rance in France, with an installed capacity of 240MW, is the only significant tidal barrage power scheme in Europe and is - by a considerable margin the largest in the world. A 1991 study commissioned by the EU estimated that the technically feasible energy resources from tidal barrages across the EU could be as much as 105TWh/year (from 64GW of installed capacity). This resource is unevenly distributed across Europe with the UK (47.7%) and France (42.1%) sharing the bulk of the resource and Eire (7.6%) accounting for most of the rest. Worldwide, other examples of barrage schemes include: Canada: a 17.8MW plant at Annapolis. Tidal range 6.4m, basin area 6 km 2 Russia: a 0.4MW experimental plant at Kislaya Guba. Tidal range 2.4m basin area 2 km 2 China: the 3.2MW Jiangxia station. Tidal range 7.1m, basin area 2 km 2 4. TIDAL STEAM OR FLOWS The physics of the tidal flow (or current or flow) resource is similar to wind power. For a current speed u m/s the kinetic energy intercepted by unit area perpendicular to the flow, per second is Tidal Power unit area = ½ Mass flow rate X u 2 = 3 ρu 2 Watts Thus for a current velocity of 3 m/s, the power is 13.8kWm/s 2. Of course the average over a tidal cycle is less than this peak. One of the developers involved in harnessing tidal currents is Marine Current Turbines Ltd, which is starting a programme of tidal turbine development through research and development and demonstration phases, to commercial manufacture. An initial grant of 1 million Euro has been received from the European Commission towards R&D costs and this has been followed by a grant towards the cost of the first Phase of work from the UK government DTI worth 960,000. The German partners also received a grant worth approximately Euro 150,000 from the German government. The company's plan is to complete the initial R&D phase by 2006, and to start commercial installations at that time. It is planned that some 300MW of installations will be completed by 2010 and after that, there is far larger growth potential from a market literally oceanic in size. MCT's main R&D programme is as follows: Installation of the first large monopile-mounted experimental 300kW single 11m diameter rotor system off Lynmouth, Devon, UK. This uses a dump load in lieu of a grid-connection (to save cost) and will only generally operate with the tide in one direction; phase cost 3.3 million Design, manufacture, installation and testing of the first "full size" twin rotor system to be rated at 750 to 1200kW (each rotor being slightly larger than for the Phase 1 System - the variation depends on the rated velocity for the site chosen, see Fig. 5). This will be grid-connected and will function with the flow in both directions - it will in fact be the prototype and test-bed for the commercial technology. This phase is expected to cost approximately 4.5m including grid connection. Fig. 6 Stingray tidal stream device Other options include the artist's impression, shown in Fig. 6, of Stringray, which reacts to the flow of water passing over its aerofoil shaped wings (Trapp, 2002). 5. WAVE ENERGY Fig 5 Marine current turbines Ltd twin rotor concept Electricity, heat or mechanical energy can be obtained by converting some of the energy contained in ocean waves. Although electricity is the most likely form of delivered energy, some applications such as desalination or the heating and pumping of sea water for marine culture are potentially viable. The cost effectiveness of wave energy schemes is being tested by recent pilot projects, including three contracted to deliver electricity under the Scottish Renewables Obligation (SRO, 1999). Wave energy technologies have been investigated over the past 25 years, initially in Europe, but also in the USA, Asia and Australia. Some early concepts and prototypes were very successful, but political support was not always strong and progress was slow. The present realisation that climate change must be addressed has prompted new commitment to exploiting wave energy because of its low environmental impact. Wave Resources Wind passing over the surface of water gradually passes some of its energy into the water to create waves. If the wind has a reasonable velocity and persists for a long time across a long stretch of water then the resulting waves will be large and powerful. Fig. 7 shows the World wave power resource, as kw per metre of wave crest. The total exploitable World wave power resource is estimated at 2-5 TW, largely to be found in offshore locations where the water is deeper than 40m, and the power density can be 50 to 70 kw per metre of wave crest. The shoreline resource, although easier to exploit, has lower power density (around 20 kw/m) since the energy content of waves is partially dissipated as they run through shallow water on their approach the shore. The high power density of deep water waves off the coast of the UK and Eire are due to the persistent south westerly winds crossing the Atlantic. That ocean waves represent a huge potential source of energy drew optimistic UK Government support in 1974 as a response to the oil crises. A number of UK research teams investigated wave energy conversion systems during the late 1970s and early 1980s, and the longer lasting concepts were reviewed by Thorpe (1992). Successive UK Governments failed to support wave energy R&D, Ross(2002), and while the UK effort receded, with only a few research teams able to continue, elsewhere Norway, Sweden, Denmark, Portugal, Japan, China and India were 963

4 continuing to carry out R&D. Several prototype wave energy converters were commissioned around the world. The most significant in size being the two demonstration systems built in Norway, a 600kW Oscillating Water Column and a 350kW Tapchan. Neither of these is operating today but they successively demonstrated the principles and subsequent developments have benefited from the experience gained. Fig. 8 LIMPET, in Scotland, this rear view shows the turbine ducting which allows the air to flow into and out of the chambers. Fig. 7 World wave power density, annual average values in kw per metre of crest length. Wave Converter Technology The first generation of devices has been those attached to the shore. As the energy content of the waves is much lower (typically 20kW/m) the devices are less productive but more likely to survive storm loading as well as being easier to maintain and to transmit electricity from. The essence of most of the shoreline technologies is that they can be firmly mounted onto the rocky shore or to the seabed, ensuring a good frame of reference for the active part of the device to operate against. Oscillating Water Columns (OWC) OWC are probably the most common type of wave energy prototype. As waves approach the OWC they cause the water within the base of the column to oscillate backwards and forwards. The upper part of the column contains air, which is successively driven out of and drawn into the column by these oscillations. The air then flows to the outside/back to the inside via an air turbine lined to a generator. A self rectifying air turbine named after its inventor, Professor Wells, obviates the need for rectifying valves and thus the only moving part of the device is the rotating shaft attached to the turbine and generator. An interesting development is the 500kW LIMPET (Fig. 8) which has been built on Islay, Scotland under the SRO1999. A carefully orchestrated construction operation was conducted to install LIMPET, an OWC type device, into position in a "designer gully" before blasting the rock sea wall away. The floor geometry of LIMPET is designed to maximise the energy capture instead of reflecting much of it as was the case in the earlier Islay OWC prototype (Heath etal, 2000). Energetech Australia Pty Ltd is developing a new wave energy system. This is an advanced shoreline OWC, which uses a novel, variable pitch turbine to improve efficiency and a parabolic wall to focus the wave energy on the OWC collector. It is designed for use in harbours or rocky outcrops where the water at the coastline is deep. As wave fronts approach they are amplified up to three times at their focal point by the parabola shaped collector before entering the 10m by 8m OWC structure. A 500kW scheme is being constructed at Port Kembla, New South Wales, Australia, with a power purchase agreement with the local utility. The Mighty Whale, a floating OWC, 30m wide, 50m long and 8m deep and rated at 110kW, was launched for trials by the Japanese Marine Science and Technology Centre in September 1998 (Washio etal, 2001). Another floating OWC, the Backward Bent Duct Buoy, (BBDB) is the subject of collaboration between Japan and China (Masuda etal, 2000). Tapchan TAPCHAN stands for Tapered Channel, and it is the design of this tapered channel which enables the scheme to harvest energy from the ocean waves which arrive at the mouth of the channel. The mouth of the 350kW prototype built in 1985 in Norway was 40m wide. Waves entering the collector are fed into the wide end of the tapered channel where they propagate towards the narrow end with increasing wave height. Because the waves are forced into an ever narrowing channel their height is amplified until the crests spill over the walls into the reservoir at a level of 3.5m above the mean sea level. The wave energy has been converted into potential energy and is subsequently converted into electricity by allowing the water in the reservoir to return to the sea via a low head hydroelectric Kaplan turbine attached to a 350kW generator. Construction of a 1.1MW Tapchan was started on the Indonesian island of Java in 1998 by Indonor AS, a joint Norwegian and Indonesian company. A Floating Wave Power Vessel (FWPV, Fig. 9), based on ship construction, will be anchored in depth of water 500m from the Shetland coast in It will be designed to have a maximum power output of 1.5MW, producing about 5.2 million kwh per year at a cost of 0.07/kWh under a SRO1999 contract (Lagström, 2000). This design functions by capturing the water from waves that run up its sloping front face. The captured water is returned to the sea via a standard Kaplan hydro-electric turbine. In many respects this is a floating Tapchan. 964

5 Fig. 9 The floating wave power vessel. Tethered buoys An interesting area of development is that of the buoys which are tethered to the sea bed and which extract energy by the relative motion of the surface following part of the device. Examples are the IPS and Hose pump from Sweden. In the former a float is connected to a submerged, weighted vertical tube and causes it to move up and down with respect to a piston located within the tube. The relative motion of piston and tube generates electricity. The latter pumps sea water to a reservoir on land and some hundred metres above sea level. Electrical energy is obtained by releasing this stored water back to sea via a hydro turbine. Apart from developing the floating buoy concept the Danes have also considered other devices including the Waveplane which collects waves at the surface and effectively rectifies the captured water into high/low pressure chambers. By allowing water to flow between them energy can be extracted by a water turbine. Apart from developing the floating buoy concept the Danes have also considered other devices including the Waveplane which collects waves at the surface and effectively rectifies the captured water into high/low pressure chambers. By allowing water to flow between them energy can be extracted by a water turbine. The Dutch Archimedes Wave Swing consists of a number of interconnected mushroom shaped air filled chambers located just below the sea surface. These chambers are topped with moveable floats or hoods. This movement is generated by the changes in buoyancy of the air within the floater as waves pass over the top. The resulting changes apply forces to a tether which actuates a hydraulic system and generator. A 2 MW Pilot scheme is currently being constructed in Romania for deployment in Portugal. The Wave Energy Converter developed by Ocean Power Technology, USA consists of a modular buoy-based system to drive the generators using mechanical force developed by the vertical movement of a wave energy converter. Each module is relatively small permitting low cost regular maintenance, leading to lifetimes of at least 30 years and hence delivered energy at 2 to 7 p/kwh (Taylor G W 2000). A unit of the system has been extensively tested at a large scale off the coast of New Jersey, USA. The first commercial schemes are being built for an electrical utility in Australia, the US Navy and for the State of New Jersey. Floating offshore devices It is clear that the major resource lies in the deep water offshore wave climates. To harness this would require the development and exploitation of large numbers of floating offshore devices like the Duck and Clam (Duckers, 1998). The Pelamis is a device being developed by the UK firm Ocean Power Delivery Ltd for deployment offshore. Pelamis (Fig. 10) has its ancestry in the Edinburgh Duck, since it consists of a number of cylindrical sections hinged together but these are arranged as an attenuator and are the active components of the device. Survival has been a key feature of the development programme; it is capable of inherent load shedding and as an attenuator it sits down the waves rather than across them and so becomes detuned in the long storm waves when the waves are much longer than the device. The wave-induced motion of the cylinders is resisted at the joints by hydraulic rams that pump high pressure oil through hydraulic motors via smoothing accumulators. The hydraulic motors drive electrical generators to produce electricity. A 750kW device will be 150m long and 3.5m in diameter and composed of five modular sections. OCP have signed a 15 year Power Purchase agreement under the Scottish Renewable Order (1999) to deliver electricity at less than 7 p/kwh. Pelamis began delivering electricity into the UK grid in 2004 (Ocean Power Delivery Ltd, 2004). Fig. 10 Pelamis. 6. OCEAN ENERGY COSTS Ocean energy technologies are in their infancy and electrical generating costs from the pilot schemes are generally more expensive than the fossil fuel competitors. This is not necessarily a dramatic difference: the La Rance tidal scheme has relatively low costs, and since it amortised over a long lifetime like classical hydro schemes, this is to be expected. The pilot Norwegian wave schemes were also reasonably cost effective. The new Scottish wave schemes are contracted at 7p/kWh (about 12 US cents/kwh). We can expect production costs to full as the technologies mature, but the World fossil fuel market is becoming more volatile and expensive, making the renewable more attractive. Assigning environmental costs to the generation of electricity is increasingly likely especially within the European Union, where externality calculations and hypothecated taxes are weapons in the fight to reduce carbon emissions. Ocean energy schemes should compete economically in both remote offshore islands and mainland grid connections in the next decade. 7. THE FUTURE Some of the ocean technologies have restricted geographical applications because they rely upon high temperature surface water, large tidal range or current or a strong wave regime. They are not universally deployable, but they do offer enormous energy resources. The first string of technologies to harness these resources are reaching maturity and already showing that their environmental benefit of low carbon emissions can be matched by economic competitiveness. 965

6 Investment in offshore renewable is set to increase quickly as can be seen in Fig. 11, although at this time much of this growth will be attributable to offshore wind developments. In the longer term, though, the ocean technologies outlined here may well play a dominant role [10] Taylor, G. W. (2000) The history, current status, and future prospects for the modular OPT wave power system. Wave Power: Moving towards commercial viability, IMechE Seminar Publication, pp [11] Thorpe, T. W. (1992) A Review of Wave Energy, ETSU Report R72, December. [12] Thorpe, T. W. (1998) Overview of Wave Energy Technologies, AEAT-3615 for the marine foresight panel, May [13] Trapp, A. (2002) Developing and building the Stingray tidal stream generator. [14] Washio, Y., Osawa, H. and Ogata, T. (2001) The open sea tests of the offshore floating type wave power device mighty whale characteristics of wave energy absorption and power generation, Ocean 2001 Symposium. [15] Yemm, R W. (2000) The history and status of the Pelamis wave energy converter. Wave Power: Moving towards commercial viability, IMechE Seminar Publication, pp Other information can be found on the CADDET web page: Fig. 11 Capital expenditure in world offshore renewable energy technologies (DTI, 2002). ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of Coventry University and Khon Kaen University. REFERENCES [1] DTI (2002) The World Offshore Renewable Energy Report : Summary of a report by Douglas- Westwood Limited for Renewables, UK report number [2] Duckers, L.J. (1998) Wave Power Developments Renewable Energy World, James and James, 1, (X), pp [3] Heath T., Whittaker, T.J.T. and Boake C.B. (2000) The design, construction and operation of the LIMPET wave energy converter (Islay, Scotland), Fourth European Wave Energy Conference, pp , ISBN [4] Lagström, G. (2000) Sea Power International- Floating Wave Power Vessel (FWPV) Fourth European Wave Energy Conference, pp , ISBN [5] Vega, L.A. (2001) Ocean Thermal Energy Conversion (OTEC). [6] Masuda, Y., Kuboki, T., Xianguang, L. and Peiya, S. (2000) Development of Terminator type BBDB, Fourth European Wave Energy Conference, pp , ISBN [7] National Renewable Energy Laboratory (2004) Ocean Thermal Energy Conversion. [8] Ocean Power Delivery Ltd (2004). [9] Ross, D. (2002) Scuppering the Waves: how they tried to repel clean energy, Science and Public Policy, 29, (1), pp. 966