and ocean thermal energy* - TERI University Onlineodl.teriuniversity.ac.in/ren_en/pdf/Energy_Sources/rb11.pdf · Geothermal energy, tidal energy, wave energy, and ocean thermal energy

Geothermal energy 593Tidal power 604Wave power 606Ocean thermal energy

conversion 612Appendix 621Nomenclature 621References 622Bibliography 623

11Geothermal energyGeothermal energyGeothermal energyGeothermal energyGeothermal energy, tidal energy, tidal energy, tidal energy, tidal energy, tidal energy, w, w, w, w, wave energyave energyave energyave energyave energy,,,,,and ocean thermal energy*and ocean thermal energy*and ocean thermal energy*and ocean thermal energy*and ocean thermal energy*V V N Kishore, Senior Fellow and Prashant Bhanware, Research AssociateEnergy–Environment Technology Division, T E R I, New Delhi

Stuart L Ridgway#

537, 9th Street, Santa Monica, CA 90402, USA

Geothermal energy

When the earth was formed some 4600 millionyears ago, it was a mass of hot substances. After itcooled, the outer surface became a crust, but theinner mass remained hot and molten. Thetemperature at the centre of the earth is about7000 ºC and because of the large temperature dif-ference between its interior and the surface, heatflows out at a steady rate. Also, the earth containssmall quantities of long-lived radioactive isotopes,principally, thorium-232, uranium-238, and potas-sium-40, which liberate heat as they decay. Theamount of heat flowing from the interior of theearth through the surface is about 1021 J per year,which, though small compared to about 5.4 × 1024 Jper year of solar energy falling on it, is substantialfor warranting exploration as a source of energy.Heat is transferred through the earth mainly bycreep processes in hot solids.

The convective heat transfer process isquite efficient, resulting in small variations intemperature across the depth of the convectinglayer. However, close to the surface (across the

* The sections on geothermal, wave and tidal power are primarily based on The Open University (1994) T521:

Renewable Energy Resource Pack for Tertiary Education, subsequently republished as Boyle G (ed.) 2004

Renewable Energy: Power for a Sustainable Future, published by Oxford University Press, and reproduced

with permission from The Open University.

Copyright © 2004, 2007, The Open University.

# Author of the section on ocean thermal energy conversion

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594 • Renewable energy engineering and technology

outer 100 km of the earth), the material is too hard to convect, so heat istransported by conduction. Because of the low thermal conductivities,temperature increase with depth is much larger. The rigid outer boundarylayer or shell is broken down into a number of fragments called thelithospheric plates that move around the surface at speeds of a fewcentimetres per year, their movement being controlled by the convectivemotions beneath. At the boundaries between the plates where they are inrelative extension or compression, the heat flows are high (~300 mW/m2)compared with a global mean of about 60 mW/m2.

Along the plate margins, there are localized spots of very high heatflows because the rock material reaches the surface in molten form resultingin volcanic activity. The storage of molten or partially molten rock at about1000 ºC, just a few kilometres beneath the surface, strongly augments heatflow around even dominant volcanoes. These high heat flows result in hightemperature gradients. Over the geological periods of time, these flowshave resulted in large quantities of heat being stored in rocks at shallowdepths, and these rocks form the sources of geothermal energy. The regionsof highly concentrated heat flows are termed as ‘high-enthalpy’ regions. Heatis available in the form of steam and hot water at temperatures of 150–200 ºCin the high-enthalpy regions. In areas of lower heat flow, where the convectionof molten rock or water is reduced or absent, the temperature in the shallowrocks is lower (below 100 ºC) and these are termed as the low-enthalpyregions. While high-enthalpy sites are suitable for power generation, low-enthalpysites can be directly used for heating application, drying, etc.

The extraction of heat for useful purposes can aptly be termed as ‘heat-mining’. Geothermal sources are not strictly renewable though at one time itwas thought that many high-enthalpy resources were indeed renewable, in thesense that they could be exploited indefinitely. However, the experience ofdeclining temperatures in steam fields and the simple calculations of heatsupply and demand show that heat is being mined on a non-sustainable basis.The known and potential geothermal energy sources, however, can beexploited on a near-sustainable basis for several years to come.

Potential sites for geothermal energy utilization worldwide include theHimalayan geothermal belt, eastern China, Russia, Japan, the Philippines,Indonesia, New Zealand, Canada, United States, Mexico, Central Americanvolcanic belt, the Caribbean, Ireland, eastern and southern Mediterranean,and the East Africa rift system (Figure 11.1).1

1 Details available at <www.geothermal marin.org>, last accessed on 17 July 2005.

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Geothermal energy, tidal energy, wave energy, and ocean thermal energy • 595

Figure 11.1 Potential sites for geothermal energy utilization worldwideReproduced with permission from Geothermal Education Office, Tiburon, CaliforniaSource http://geothermal.marin.org

Figure 11.2 Geothermal energy potential and current utilization (in megawatts)Source http://www.geo-energy.org/publications/reports/preliminary%20report.pdf

The exploited potential and the ultimate potential are given in Table 11.1and Figure 11.2. (Gawell, Reed, and Wright 1999) (WEC Member Committees2000). It can be seen that the potential is quite large compared to the currentlevel of use.

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Continued...

Table 11.1 Geothermal energy: electricity generation and direct use (end of 1999)

Electricity generation Direct useInstalled Annual Annual Installed Annual Annualcapacity output capacity capacity output capacity

Country (MWe) (GWh) factor (MWth) (GWh) factor

Algeria 100 441 0.50Ethiopia 9 30 0.40 Kenya 45 390 0.99 1 3 0.25Tunisia 20 48 0.28Total Africa 54 420 0.89 121 492 0.46

Canada 378 284 0.09Costa Rica 115 804 0.80 El Salvador 161 552 0.39 Guadeloupe 4 25 0.67 Guatemala 33 216 0.74 3 30 1.00Honduras 1 5 0.76Mexico 750 5 642 0.86 164 1 089 0.76Nicaragua 70 583 0.95 United States of America 2 228 16 813 0.86 5 366 5 640 0.12Venezuela 1 4 0.63Total North America 3 361 24 635 0.84 5 913 7 052 0.14

Argentina 1 0.67 26 125 0.55Chile N 2 0.55Colombia 13 74 0.63Peru 2 14 0.65Total South America 1 0.67 41 215 0.60

China 29 100 0.39 2 814 8 724 0.35Georgia 250 1 752 0.80India 80 699 1.00Indonesia 590 4 575 0.89 7 12 0.19Japan 547 3 451 0.72 258 1 621 0.72Korea (Republic) 51 299 0.67Nepal 1 6 0.66The Philippines 1 863 10 594 0.65 1 7 0.79Thailand 1 0.38 1 4 0.68Turkey 15 81 0.62 820 4 377 0.61Total Asia 3 044 18 802 0.71 4 283 17 501 0.47

Austria 255 447 0.20Belgium 4 30 0.87Bulgaria 107 455 0.48Croatia 114 153 0.15Czech Republic 13 36 0.33Denmark 3 15 0.52Finland 81 167 0.24FYR Macedonia 81 142 0.20France 326 1 365 0.48Germany 397 436 0.13

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Geothermal resources should, in general, have three important char-acteristics, as shown in Figure 11.3, viz an aquifer2 containing water that can beaccessed by drilling, a cap rock to retain the geothermal fluid,3 and a heatsource.

A geothermal aquifer must be able to sustain a flow of geothermal fluidso that even highly porous rocks are suitable only if the pores areinterconnected. The velocity u of a fluid moving through a porous mediumcan be described by Darcy’s law

w

Hu = K

L

...(11.1)

Table 11.1 (Continued)

Electricity generation Direct useInstalled Annual Annual Installed Annual Annualcapacity output capacity capacity output capacity

Country (MWe) (GWh) factor (MWth) (GWh) factor

2 Porous rock that can store water and through which water can flow.3 Geyser or hot springs.

Greece 57 107 0.21Hungary 328 1 400 0.49Iceland 170 1 138 0.76 1 469 5 603 0.44Italy 621 4 403 0.81 680 2 500 0.42Lithuania 21 166 0.90The Netherlands 11 16 0.17Norway 6 9 0.17Poland 69 76 0.13Portugal 20 79 0.45 6 10 0.20Romania 110 120 0.12Russian Federation 23 85 0.42 307 1 703 0.63Serbia, Montenegro 80 660 0.94Slovakia 132 588 0.51Slovenia 103 300 0.33Spain 70 292 0.47Sweden 377 1 147 0.35Switzerland 547 663 0.14United Kingdom 3 10 0.38Total Europe 834 5 705 0.78 5 757 18 616 0.37

Israel 63 476 0.86Jordan 153 428 0.32Total Middle East 216 904 0.48

Australia 1 0.60 10 82 0.90New Zealand 410 2 323 0.65 308 1 967 0.73Total Oceania 410 2 324 0.65 318 2 049 0.74

Total world 7 704 51 886 0.77 16 649 46 829 0.32

Source WEC Member Committees (2000)

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where Kw is the hydraulic conductivity and (H/L) is the hydraulic gradient, or change in the head H per metre of distance L along the direction of flow. The volumetric rate Q is given by

...(11.2)where A is the cross-sectional area. Some values of Kw for different rocks are given in Table 11.2.

Figure 11.3 A schematic cross-section showing essential characteristics of a geothermal sitePermission granted by The Open University©Source Open University (1994)

Table 11.2 Porosities and hydraulic conductivities of selected materials

Material Porosity (%)* Kw (m/day) Gravel 25–35 500–10 000 Sand, volcanic ash 30–40 1–500 Silt 40–50 10–2–1 Clay 45–60 <10–2 Mudrock 5–15 10–8–10–6 Sandstone 5–30 10–4–10 Limestone 0.1–30 10–5–10 Solidified lava 0.001–1 0.0003–3 Granite 0.0001–1 0.003–0.03 Slate 0.001–1 10–8–10–5

*Cavities present in rockSource Open University (1994)


Cap rock consists of material that is relatively impermeable so that itacts like a seal for the geothermal fluid. The importance of cap rocks wasdiscovered by the Italians in the early 1980s while exploring geothermalsources in an obvious place: the flanks of the Vesuvius volcano. Only smallamounts of low-pressure fluid were discovered because the volcanic ashes thatformed its flanks were apparently quite permeable. This proves that over time,gradual alteration in the uppermost deposits by hot water and steam createsclay or salt deposits that block the pores and thus seal the aquifer. This also ex-plains why the youngest volcanic areas, like Vesuvius, are not necessarily themost productive geothermal sources.

The third pre-requisite for exploiting geothermal energy is the presenceof a heat source. There are three different types of sources: volcano-relatedheat sources, sedimentary basins, and hot dry rocks.

Volcano-related heat sources

Several of the world’s most advanced geothermal sites are located in theextinct volcanic areas. As rocks are good insulators, magmatic4 intrusions maytake millions of years to cool, and thus act as the focus for the ‘hot fluid’ inthe permeable strata. The nature of the resource depends upon the localconditions of pressure and temperature in the aquifer, and especially on theP–T (pressure–temperature) depth profiles. Such profiles determine theextraction technology and economic viability of the site (Figure 11.4).

For a typical P–T depth curve shown in Figure 11.4, at shallow depths(~250 m), the temperature is too low for any water present to boil and pressureincreases hydrostatically. However, in the depth interval 250–575 m, thetemperature is high enough for water to vaporize so that the temperaturecurve lies slightly to the right of the boiling point curve. Pressure increaseacross this region is small because the pores are occupied by convecting watervapour rather than liquid water. Due to this reason, this is also an isothermalzone. Between 575 and 700 m, the rate of increase of temperature is propor-tional to the pressure corresponding to the boiling point so this is aliquid-dominated zone. Below 700 m, the pressure increases more rapidly andthe P–T path deviates from the boiling point curve of water. High-enthalpysystems are subdivided into vapour- and liquid-dominated (that is, steam orliquid water) systems in the reservoir. The vapour-dominated systems are bestand most suitable, mainly because steam is dry and has very high enthalpy. In

4 Magma is the partially molten rock.

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the liquid-dominated systems, steam is often wet and of lower enthalpy, which adds to the technical problems of electricity production.

Sedimentary basins

These are heat sources where aquifers carry water to the depths where it becomes warm enough to exploit. Heat flow in rocks is usually governed by the conduction equation

...(11.3)

where q is the heat flux in W/m2, k is the thermal conductivity, and ∆T is the temperature difference over depth z.

The value of k is 2.5–3.5 W/m K for sandstones, limestones, and many crystalline rocks. However, mud rocks (clays and shales) have lower values of the order of 1–2 W/m K. These are also quite impermeable, so mud rocks act as impermeable cap rocks and enhance the geothermal gradient above aquifers in regions with otherwise normal heat flows. This has led to explorations aimed at locating natural warm waters in the areas of thick sedimentary

Figure 11.4 Variation of pressure and temperature with depth for a typical geothermal fieldSource Open University (1994)


rock sequences containing both mud rocks and permeable limestones orsandstones. Large-scale heat applications of geothermal energy worldwidetake place in basins where the background heat flow is above average. Thegeological reasons for the association of high heat flow with sedimentary ba-sins are reasonably well established.

Hot dry rocks

Hot dry rock resources refer to the heat stored within the impermeable orpoorly permeable rock strata and to the process of extraction of heat. Anartificial fracture zone is created within the suitably hot rocks and water iscirculated through such a zone for extracting heat. Drilling is expensive,so only the top 6 km of the earth’s crust is used for calculating the geothermalpotential with the present technology. Given the current technical andeconomic constraints on drilling depths, a minimum geothermal gradient ofabout 0.025 ºC/m is required. With a typical k value of 3 W/m k, this requires aheat flow of 75 MW/m2, which is only a little above the earth’s average. Inpractice, it is customary to look for rocks with much higher heat flows and theideal targets are granite bodies.

Technologies for utilization of geothermal resources

The first stage in prospecting for geothermal resources in the volcanic areasinvolves a range of geographical studies aimed at locating rocks that have beenaltered chemically by hot geothermal brines. Surface thermal manifestations,such as hot springs5 or mud pools, are also examined carefully. Chemicalinvestigations and the release of gases through fractured rocks provide meansof assessing the composition and resource potential of the trapped fluids.Geophysical prospecting, particularly by resistivity surveying and otherelectrical methods for detecting zones with electrically conducting fluids(brines), is probably the most effective technique in locating buriedgeothermal resources. Once a suitable geothermal aquifer has been located,exploration and production wells are drilled using special techniques to copewith the much higher temperatures and, in some cases, harder rock conditionscompared to the oil and water wells. As fluid pressures in the aquifer can go upto 10 MPa, dense drilling muds are required to counter these pressures and toavoid a ‘blow out’ where an uncontrollable column of gas is discharged. Thedimensional and constructional details of the wells to be dug are wellestablished.

5 Hot water only.

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The technologies for utilizing geothermal heat for useful applicationscan be broadly divided into two categories: using the geothermal fluidsdirectly and using a heat exchanger for the extraction of heat. The directmethods of using heat for power generation can further be classified as(1) dry steam plants, (2) single-flash steam power plants, and (3) double-flashpower plants, shown in Figures 11.5 (a), (b), and (c), respectively.

The dry steam power plant is installed in sites where superheated steamat 180–185 ºC and 0.8–0.9 MPa is available. The power plants of 1960srequired about 15 kg of steam per saleable kWh, but modern plants withhigher temperature steam and better turbine designs can achieve6.5 kg steam/kWh. Plant efficiency is strongly affected by the presence ofnon-condensable gases, such as carbon dioxide and hydrogen sulphide.

In the single-flash system (Figure 11.5b), the fluid reaching the surfacemay be wet steam (water that has flashed within the well during ascent) or hot

Figure 11.5 (a) Dry steam power plant Figure 11.5 (b) Single-flash steampower plant

Figure 11.5 (c) Double-flash steam power plantPermission granted by The Open University©Source Open University (1994)

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water at high pressure. It is often better to avoid flashing in the well because itcan lead to scale deposits and plugging of the well. Conventional steamturbines are employed but these operate at lower efficiencies due to lowersteam pressures and temperatures. The bulk of fluid mined (often up to 80%)may be in the form of unflashed brine, which is re-injected, unless there arelocal direct uses.

Double-flash systems (Figure 11.5c) are ideal where geothermal fluidscontain low levels of impurities. The scaling and non-condensable gasproblems are minimal in such cases. Unflashed liquid remaining after theinitial high-pressure flashing flows to a low-pressure tank where anotherpressure drop provides additional steam. This steam is mixed with the exhaustfrom the high-pressure turbine to drive a second turbine, raising poweroutput by 20%–25% for a 15% increase in plant cost.

The indirect method of using geothermal heat for power productionemploys a binary cycle power plant, also called the ORC (Organic RankineCycle) system (Figure 11.6). It uses a secondary working fluid with a boilingpoint lower than that of water, such as pentane or butane, which is vaporizedand used to drive the turbine. The geothermal brine is pumped at reservoir pres-sure through a heat exchanger unit and is then re-injected. Though there areclear advantages, such as the utilization of low-temperature heat and protectionfrom impurities, the capital costs are high. Also, the parasitic power consump-tion is high (~30%) due to the need for using large pumps for injection,secondary fluid circulation, etc. These kinds of power plants had been used insolar ponds and the OTEC (ocean thermal energy conversion) systems.

Figure 11.6 Binary cycle power plantPermission granted by The Open University©Source Open University (1994)

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An example of the indirect method of using geothermal energy for non-electrical application is shown in Figure 11.7. Here, heat pumps are used toenhance the system efficiency. Heat pumps, which work on the same principleas refrigeration, use electricity, and also increase the overall efficacy of athermal application, such as residential heating. In the longer term, heatpumps may allow widespread economic development of even shallower, coolergeothermal aquifers.

Tidal power

The use of tides to provide energy has a long history. The idea of using tidalenergy on a large scale to generate electricity was first considered in 1938 withturbines mounted in large barrages (essentially low dams) built across suitableestuaries.

The upstream ‘tidal flow’ in an estuary (usually called the ‘flood tide’) istrapped behind a barrage. The incoming tide is allowed to pass throughsluices, which are closed at high tide, trapping the water. As the tide ebbs, thewater level on the downstream side of the barrage reduces and a ‘head’ ofwater develops across the barrage. The basic technology of power extractionis then similar to that of low-head hydro (refer to Chapter 10). The maindifference, apart from the salt-water environment, is that the power-generating turbines in the tidal barrages have to deal with regularly varyingheads of water.

The variation in tidal height is primarily due to the gravitationalinteraction between the earth and the moon. As the earth rotates on its axis,gravitational forces produce, at any particular point on the globe, a twice-dailyrise and fall in sea level; this being modified in height by the gravitational pullof the sun, and by the topography of landmasses and oceans.

Figure 11.7 A scheme for utilization of geothermal energy for non-electrical applicationPermission granted by The Open University©Source Open University (1994)

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The actual analysis of the interaction between the earth, the moon, andthe sun is quite complex. In simple terms, the gravitational pull of the moondraws the seas on the earth ‘nearest’ to the moon into a bulge towards themoon while the seas furthest from the moon experience a less than averagelunar pull and bulge away from the moon (Figure 11.8). As the earth rotates onits axis, the lunar pull maintains these high-tide patterns. The two high-tideconfigurations will in effect, be drawn around the globe as the earth rotates,giving, at any particular point, two tides per day occurring approximately12.5 hours apart. As the moon also moves in an orbit around the earth,the timing of these high tides at any particular point will vary, occurringapproximately 50 minutes later each day.

This basic pattern is modified by the pull of the sun, as shown inFigure 11.9. When the sun and the moon pull together (in line), the result isvery high ‘spring tides’. When they are 90º apart, the result is the lower ‘neaptides’. The period between the neap and spring tides is approximately14 days, that is, half the 29.5-day lunar cycle. The ratio between the height ofthe maximum spring and minimum neap tides can be more than two.

The above pattern is modified in reality by the fact that the moon’s orbitis elliptical. There are other longer-term variations, for example, a semi-annualcycle caused by the inclination of the moon’s orbit in relation to that of theearth. There are also forces due to spin of the earth, called the Coriolis forces,

Figure 11.8 Relative rotation of the moon and the earthPermission granted by The Open University©Source Open University (1994)

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(see Chapter 8) which modify tides in some locations. The tidal flow is concen-trated as the tide approaches the shore and can be increased typically up to 3 m. In suitably shaped estuaries, it can be heightened up to 10–15 m, with complex ‘resonance effects’ playing a major role.

Tidal power availability is thus very site-specific (Figure 11.10).The practical tidal potential is estimated at about 120 GW, producing

approximately 190 TWh/year. The potential for India is in the Gulf of Cambay (16.4 TWh/year) and the Gulf of Kachchh (0.48 TWh/year).

Wave power

Ocean waves are generated by the passage of wind over stretches of water. The power P (in kW/m) of an ideal wave is given by

where H is the wave height in metres and T is the wave period in seconds.

Figure 11.9 Influence of the sun on tidesPermission granted by The Open University©Source Open University (1994)


By deploying a wave-rider buoy, it is possible to record the variation inthe surface level during a chosen period of time (Figure 11.11).

For a typical irregular sea, the average total power is given by

Ps (kW/m) = α

sH

s

2T

e

Figure 11.10 Some promising tidal power sites in the worldPermission granted by The Open University©Source Open University (1994)

Figure 11.11 A typical wave recordPermission granted by The Open University©Source Open University (1994)

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where αs is a constant equal to 0.49 kW/s m3, H

s is the significant wave height

given by 4 × rms (root mean square) value of water elevation, and Te is the zero

up-crossing period (second). The values of Hs and T

e change throughout the

year.A statistical picture of the distribution of wave conditions is given by a

scatter diagram (Figure 11.12), which gives the relative occurrence in parts per1000 of the contributions of H

s and T

e.

The estimates of wave power density at various locations in the world areshown in Figure 11.13.

Figure 11.12 Scatter diagram of significant wave heights

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Wave energy technology

In order to capture energy from the sea waves, it is necessary to intercept thewaves with a structure that can respond in an appropriate manner to theforces applied to it by the waves. The structures can be classified as fixed orfloating. Fixed seabed and shore-mounted devices are the only wave energyconverters to have been tested as prototypes at sea. The majority of devicestested and planned are of the OWC (oscillating water column) type. In thesedevices, an air chamber pierces the surface of water and the contained air isforced out of it into the chamber by the approaching wave crests and troughs.On its passage from and to the chamber, air passes through an air turbinegenerator and produces electricity. A novel axial flow air turbine, the Wellsturbine, is proposed for many OWCs (Figures 11.14 and 11.15).

The Wells turbine rotates in the same direction, irrespective of whetherthe airflow is into or out of the chamber, and has aerodynamic characteristics,particularly suitable for wave applications.

Wave energy research has been going on in many places in the world butmore so in Japan. The other countries doing research and development till thelate 1980s were China, Denmark, India, Norway, Portugal, Spain, Sweden,and the UK. The status of various prototypes in the early 1990s is shown inTable 11.3.

Figure 11.13 Annual average wave power in kW/m for various locations in the world

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Figure 11.14 The Wells turbine for conversion of wave energy

Figure 11.15 Principle of operation of an oscillating water column wave energy converter

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Table 11.3 Summary of various prototypes of wave energy systems

InstalledYear Type Location Owner capacity (kW) Comments

1965 Navigation Japan Maritime 0.05 Several hundred deployedbuoy OWC agency around the coastline of Japan

1978–86 Kaimei Japan IEA 375–1000 Vessel motion compromisedthe system performance. Nofurther interest in energytesting but fundamental dataon moorings and materials

1983 OWC Sanze, Mitui and Fuji 40 Low output; decommissionedJapan after one year

1983 Pendulor Muroran, Muroran 5 Still operationalJapan Institute of

Technology1984 Kaiyo Okinowa, Institute Not known Research programme

floating Japan of Ocean completedterminator Environmental

Technology1985 OWC Toftestallen, Kvaerner 600 Good performance. Destroyed

Norway Brug by storms in December 19881985 Tapchan Toftestallen, Norwave 350 Good performance, still

Norway operational1985 Pendulor Mashike, Mashike 5 Supplies hot water, still

Japan Port operational1985 OWC Neya, Taisei 40 Wells turbine driving a heat–

Japan Corporation generating eddy current-typedevice. Tests finished in 1988

1988 OWC array Kujukuri, Takenaka 30 Array of 10 OWCs, with recti-Japan Komuten fying valves feeding a common

Company high-pressure reservoirPlanned to continue until 1995

1989 Hinged flap Wakasa Bay, Kansai Electric 1 Under testJapan Power Company

1989 Tethered Hanstholm, Danish Wave 45 Problems with rubber seals.float Denmark Power APS Further trials are planned.

1989 OWC Sakata, Port and 60 The OWC is an integralJapan Harbour Research part of a new harbour

Institute wall. Now operational1991 OWC Islay, UK Queen’s 75 Still operational. Over

University, 1000 hours of testingBelfast

1991 OWC Trivandrum, IIT Madras 150 Latest available informationIndia in 1992 was that device was

nearing completion1992 FWPV West Coast Sea Power AB 110 Successful operation, not

of Sweden sensitive to tidal rangelike Tapchan

1992 OLAS 1000 West coast Union Fenosa, Array of seven hoseof Spain Madrid pumps/buoys

OWC – oscillating water column; FWPV – floating wave power plant;IEA – International Energy Agency; IIT – Indian Institute of TechnologySource Open University (1994)

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Ocean thermal energy conversion6

The tropical oceans of the world are an enormous energy resource. Theirsurface water is a heat source typically at 25–27 ºC, and the deep water below isavailable for a heat sink at a temperature of about 5 ºC. This temperaturedifference used to make useful energy is called OTEC. Temperaturedifference between surface and depth of 1000 m for different zones is shownin Figure 11.16. India is fairly endowed to take advantage of this resource, andhas a need for a steady supply of low-cost non-polluting energy. In particular,there is no greenhouse carbon dioxide emission.

In 1994, William H Avery, Applied Physics Laboratory, Johns HopkinsUniversity, and Chih Wu, United States Naval Academy, published ‘RenewableEnergy from the Ocean’. From their preface

Figure 11.16 OTEC resource mapReproduced with permission from US Departement of EnergySource http://www.nrel.gov.otec/

6 Contributed by Stuart L Ridgway

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‘The upper layers of the tropical oceans are a vast reservoir of warm waterthat is held at a temperature near 27 oC by a balance between the absorptionof heat from the sun and the loss of heat by evaporation, convection, andlong-wavelength radiation. On an average day, the water near the surfaceabsorbs more heat from the sun in one square mile (2.5 km2) of ocean areathan could be produced by burning 7000 barrels of oil. For the whole tropicalocean area, the solar energy absorbed per day by the surface waters is morethan 10 000 times the heat content of the daily oil consumption of theUnited States. A practical and economical technology for convertingeven 0.01% of the absorbed solar energy into electricity or fuel in a formsuitable for delivery to consumers on land could have a profound impact on

world energy availability and economics.’

OTEC needs supplies of warm and cold water with a temperaturedifference of at least 20 oC, an inexpensive and very efficient heat engine forextracting work from the heat supply heat sink combination, and reasonablemethods for delivering the output to users.

History

The French physicist Jaques D’Arsonval suggested the use of OTEC resourceover a hundred years ago. The Rankine (closed) cycle heat engines he suggestedwould have used a typical refrigerant as working fluid. For example, liquidammonia would have been boiled using warm water heat, the vapour expandedthrough a turbine to do the cycle work, and then condensed back to liquid tobe pumped back into the boiler. For modest yields of output work, largequantity of heat was required for which large boilers and condensers wereneeded.

Georges Claude, who had liquefied air and proposed separating neon byfractional distillation for the ‘neon sign’, attempted to build OTEC plants inthe 1920s and 1930s, which used water vapour flashed from warm surface wa-ter in a vacuum as the working fluid. The warm and cold waters flowingthrough the apparatus were the essential heat exchange surfaces, saving heatexchanger costs. However, very low density of water vapour at OTEC tem-peratures makes it a poor working fluid for a power extraction turbine. Thesize and cost of a turbine adapted to this very low density vapour is a handi-cap to the Claude open cycle. He did achieve 22-kW output from his turbine,but the utilities supplying water and creating vacuum used more power.

Back then, the supply of low-cost fossil fuels was more and theenvironmental consequences of uninhibited combustion did not loomlarge. Forests dying of acid rain were rare. Global warming due to carbondioxide emissions to the atmosphere was not an issue. So economic successeluded him.

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The Arab oil embargo of 1973/74 emptied the Los Angeles freeways; lowspeed limits were made mandatory on the interstate highways; and there werelong queues for fuel at the gas stations. There was a rush to buy locking lids forone’s fuel tanks to frustrate the siphoners.

All this stimulated a substantial programme in the 1970s and early 1980sto develop the technology to exploit the OTEC resource.

Various attempts have been made to develop low-cost heat engines thatcan exploit this small temperature differential to provide useful energy.However, the heat engines seemed too costly, so it was back to inexpensive oil.Now that oil is becoming more expensive again, and the climate experts areinsisting on reducing carbon dioxide emissions, the possibilities of OTECought to be re-examined.

Closed-cycle development

Closed-cycle or Anderson cycle systems (Figure 11.17) use fluid with a lowboiling point, such as ammonia, to rotate a turbine to generate electricity.Heat is transferred in the evaporator from the warm sea water to the workingfluid. The working fluid exits from the evaporator as a gas near its dew point.The high-pressure, high-temperature gas is then expanded in the turbine.From the turbine exit point, the working fluid enters the condenser where it

Figure 11.17 Schematic diagram of a closed cycle system for OTECSource http://www.answers.com/topic/ocean-thermal-energy-conversion

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rejects heat to the cold sea water. Usually, the subcooled liquid leaves the con-denser and finally, this liquid is pumped to the evaporator, completing a cycle.The T–S diagram of the closed system is shown in Figure 11.18.

The heat exchangers (evaporator and condenser) are a large and crucialcomponent of a closed-cycle power plant. Much work has been done onmaterials for OTEC heat exchangers. Inexpensive aluminium alloys are usedfor this purpose.

This process can be modified to produce desalinated water as a by-product; the cold water (warmed only about 10 ºC by the OTEC process) cancondense large volumes of fresh water when it is passed through a heatexchanger in contact with a humid tropical atmosphere.

The world’s first net power producing OTEC plant, ‘Mini-OTEC’, was aclosed cycle plant and was deployed in 1979 on a barge off the Natural EnergyLaboratory of Hawaii by the State of Hawaii, Lockheed Ocean Systems, andother private sector entities. It used ammonia as its working fluid.

This plant operated for three months, generating approximately 50 kWof gross power with net power of 10–17 kW (Ridgway 1984). Though onlyabout 20% of the Mini-OTEC’s gross power was available for export, the net-to-gross ratio was expected to approach 75% for closed-cycle plants largerthan about 10 MW, making the process commercially more viable.

Figure 11.18 T–S diagram of the closed power plantSource http://www.answers.com/topic/ocean-thermal-energy-conversion

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Other considerations associated with a closed-cycle OTEC powerplant are the potential leakage of ammonia and the discharge of smallamounts of chlorine that are added to the ocean water to prevent foulingof heat exchangers. Practices developed over the past 100 years in therefrigeration industry can minimize ammonia leakage. Experiments atthe Natural Energy Laboratory of Hawaii (Ridgway, Hammond, and Lee 1981)have demonstrated that very small, environmentally benign levels of chlorinecan successfully control the micro-fouling that could dramatically diminishthe efficiency of the heat exchangers at the small ∆T available for the OTECoperation.

A plant was built by the Japanese, and was operated next to the islandof Nauru. Power was 100 kW gross. It was studied for a year or two, andproduced power for three months in 1981.

Open-cycle development

In the open-cycle OTEC process (Figures 11.19 and 11.20), also known as theClaude cycle after its inventor Georges Claude (Claude 1930), water vapour isthe working fluid. The boiling temperature of water is a function of pressure.The warm surface sea water evaporates (boils) inside a vacuum chamber that ismaintained at a low pressure of approximately 0.34 psi (the pressure at80 000 feet [24.4 km], about 1/40 atmospheric pressure at sea level). Theresulting low temperature vapour (steam) flow is then directed through aturbine generator. Afterwards, steam is condensed back into liquid by the flow

Figure 11.19 Schematic diagram of open-cycle OTECSource http://www.answers.com/topic/ocean-thermal-energy-conversion

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of cold deep sea water. The most efficient condensation and hence, the high-est electricity output, can be achieved if this steam is brought into directcontact with the cold sea water.

However, if steam flows through a surface condenser, in which it doesnot directly come in contact with the cold sea water, the resulting condensateis desalinated water. This pure fresh water ‘by-product’ is valuable for humanconsumption and agricultural purposes, especially in regions where naturalfreshwater supplies are limited. The reduced efficiency of the surfacecondenser, however, significantly reduces the production of electrical energyfrom the turbine. Designs that condense half the steam with a surfacecondenser, and then finish the steam condensation with the partially warmedsea water exiting the surface condenser can greatly improve electrical effi-ciency but reduce the freshwater output.

As the pressure drop across the turbine is the difference between the lowpressure at which the water vaporizes and the lower pressure remaining aftercondensation, open-cycle systems require very large turbines to capturerelatively small amounts of energy. Claude calculated that a 6-MW turbinewould need to be about 10 m in diameter, and he could not design a realisticturbine larger than this. Recent re-evaluation of Claude’s work (Parson,Bharathan, and Althof 1985) indicates that modern technology cannot

Figure 11.20 T–s diagram of open cycle power plantSource http://www.answers.com/topic/ocean-thermal-energy-conversion

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improve significantly on his design, so it appears that the open-cycle turbinesare limited to about 6 MW.

Less than half of the one per cent of the incoming warm ocean waterbecomes steam, so large amounts of water must be pumped through the plantto create enough steam to run the large, low-pressure turbine. However, thisdoes not substantially reduce the surplus or net electrical power as pumpingsurface sea water requires little energy. In an ideal open-cycle plant, thevacuum pumps could be shut down after start-up, as all the water vaporized inthe evaporator would be condensed in the condenser, leaving behind avacuum. In the real world, however, both inevitable vacuum leaks andnon-condensable gases dissolved in the surface and deep sea waternecessitate continuous operation of vacuum pumps. The overall thermal-to-electrical efficiency of these traditional open- and closed-cycle OTECplants is very similar, approaching 2.5%. Though this is low compared to thetraditional power generation systems, the extent of the ocean thermalresource is sufficient to offer tremendous power outputs.

In 1993, the PICHTR (Pacific International Center for High TechnologyResearch) designed, constructed, and operated a 210 kW open-cycleOTEC plant at Keahole Point, Hawaii. When this demonstration plant wasoperational, it set the world record for OTEC power production at 255 kW(gross) (Vega and Evans 1994). The sea water pumps and vacuum systemsconsumed about 170 kW, so the nominal net output of this experimental plantwas about 40 kW.

The PICHTR was tasked by the MHI (Mitsubishi Heavy Industries)to maintain the open-cycle OTEC Experimental Facility open through De-cember 1998, and update their conceptual design of a small land-basedopen-cycle OTEC plant for production of electricity and fresh water forPacific Islands. In 1991, PICHTR, under the sponsorship of MoFA (Ministryof Foreign Affairs), documented the conceptual design for a 1.8-MW (gross)land-based open-cycle-OTEC plant for production of electricity and freshwater. The MHI task was to update the design using information gathered atthe open-cycle OTEC Experimental Facility since 1992. The conceptualdesign was to include technical specifications for the major Open CycleOTEC components that the MHI was able to manufacture. These are thelow-density steam turbines and the pumps required to maintain theprocess vacuum by removing non-condensables (air and steam).

Following the successful completion of experiments, the 210-kW OTECplant was shut down and demolished in January 1999.

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Mist Lift

The mist lift process, a new open-cycle concept introduced in 1977, offers away around the high cost difficulties of the previous OTEC engines. It avoidsthe giant heat exchangers of the ‘closed cycle’ originally proposed byD’Arsonval, and the enormous water vapour turbine required by Claude’s‘open cycle’.

In the mist lift process, warm ocean water is sprayed upward from thebottom into an evacuated vertical duct. The ambient pressure is of the orderof 2400 Pascals (0.348 psi). Vapour evaporates from warm water. Mist, amixture of water droplets and water vapour, is formed. At a distance of10–20 m above the bottom, cold water is sprayed upward into the duct. Itcondenses the vapour, and establishes a pressure of 1200 Pascals, which islower than the bottom pressure. Driven by the pressure difference, the vapourflows upward from the bottom to the cold water spray-condensing region,dragging warm water droplets with it. The mist is thus accelerated to asubstantial velocity. As the vapour condenses, the mist and cold water merge,forming a single-phase fluid, which goes to the top of the duct. The liftedwater is then collected and passed through a hydraulic turbine to provide theoutput power of the plant.

The process uses the vapour flashed from a spray of very fine warm waterdroplets to lift these droplets to the height of Niagara Falls (140 feet [42.6 m]).Alternatively, water can be first dropped through a hydraulic turbine toprovide the desired power output from the cycle, and then the mist is liftedand merged with the condensing cold water, and returned to the ocean.The mechanical coupling between the droplets and the lifting vapour dependsupon the viscosity of the vapour, which does not diminish with loweringpressure, whereas the coupling between vapour and the turbine blades of theClaude-cycle depends on very low vapour density, which requires largeturbines.

By placing the warm water and cold water injectors sufficiently belowthe sea level, one may dispense with cold and warm water supply pumps,which gives the concept an additional cost advantage over closed orClaude cycle OTEC.

The concept was initially tested on a 3.7-m high column erected in alaboratory at Dynamics Technology. The experiments demonstrated sufficientcoupling to lift the mist to a height of 50 m with temperature differences typi-cal of the tropical seas. These freshwater results were verified in experimentsat the Natural Energy Laboratory of Hawaii in the early 1980s for droplet tovapour coupling with sea water.

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Table 11.4 Characteristics of 1.6-MW two-stage mist lift OTEC

Cold water flow 2.0 T/s (tonnes per second)Head loss cold water pipe 5.0 mWarm water flow stage 1 1.8 T/sWarm water flow stage 2 1.8 T/sWarm temperature 25 oCCold temperature 5 oCTransfer temperature 14 oCOutput water temperature 17.8 oCStage 1 power 900 kWStage 2 power 720 kWInput turbine 3.6 m3/s at 10 m head

Output turbine 5.6 m3/s at 18 m head

A cost estimate of a conceptual design of a 4-MW mist lift OTEC powerplant was prepared and published in 1984. This design was based on a modestextrapolation of the mist transport data acquired in the freshwater experi-ments in 1980/81 and ocean water experiments in 1983. It was optimized forminimum cold water use with a condenser effectiveness of 0.9, yielding anoutput of 450 kJ/m3 of cold water. Allowances for cold water pumping power,mist generator loss, filter loss, hydraulic turbine efficiency, exit loss, and non-condensable removal power reduced this yield to a net value of 300 kJ/m3 ofcold water. The projected cost was 10 million dollars.

The two-stage mist lift

The cold and warm waters that emerge from the mist lift plant are mixed.The process uses a larger flow of cold water than warm water. Theemerging water is cool and could accept more heat. Improved performanceand lower costs could be achieved by adding a second stage that uses thecool water output from the first stage, and thus reduces the total cost(Tables 11.4 and 11.5).

A recent design analysis of this concept predicts that a two-stage mistlift plant can provide net power of 600 kW/m3/s of cold water.

Thus, the two-stage mist lift can yield twice the output per unit coldwater supply of the present OTEC versions, and has many other potentialeconomies. Further research and development in this direction promise largereturns.

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The theoretical maximum power, given Twarm = 298 °, Tcold = 278 °, and warm/cold flow ratio = 1.0,is 1.4 MW/T/s, and for warm/cold = 1.8 is 1.9 MW/T/s. Therefore, there is much possibility forsubstantial increase in the OTEC performance.*The maximum output was 200 kW; the water was supplied by NELH pumps that are not optimizedfor the power plant; a 100-kW charge was rather arbitrarily taken for the pumping. Zero chargewould make its power/cold = 0.5.

Table 11.5 Performance comparisons of recent plants and designs

Power/cold flowItem Warm/cold ratio (MW/T/s)

Closed cyclesNauru 1.03 0.257GE 40 MW 1.15 0.217John Hopkins 0.97 0.292

Open cyclesNELH net power 1.5 0.246*Mist Lift 1984 0.6 0.308Two-stage Mist Lift 1.80 0.605

Source Avery and Wu (1994)

Appendix

The maximum thermodynamically possible workout from a flow Wh of warm

water at temperature T1 and flow W

c of cold water at temperature T

0 is given

by

(1) Work/cp = W

h(T

1 – T

2) – W

c (T

2 – T

0) from conservation of energy, where

cp is the fluid-specific heat and T

2 is the common temperature of the exit

water. If the heat engine is ideal and no entropy is created, T2 can be

found as follows(2) (W

h + W

c )*ln(T

2) = W

h*ln(T

1) + W

c *ln(T

0) and substituted back into (1) to

obtain the possible work.

Nomenclature

Geothermal energyH HeadH/L Hydraulic gradientk Thermal conductivity (W/mK)K

wHydraulic conductivity

L DistanceP Pressure

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q Heat flux (W/m2)T Temperatureu Velocity (m/s)z Depth (m)

Wave powerH Wave height (m)H

sSignificant wave height (m)

P Power (kW/m)T Wave period (s)T

cZero-up-crossing period (s)

Appendixc

pSpecific heat

T TemperatureW Flow rate

References

Avery J H and Wu C. 1994Renewable Energy from the Ocean: a guide to OTEC, 446 pp.New York: Oxford University Press

Claude G. 1930Power from the tropical seasMechanical Engineering 52: 1039 pp.

Gawell K, Reed M, and Wright P M. 1999Geothermal Energy: the potential for clean power from the earthGeothermal Energy AssociationDetails available at http://www.geo-energy.org/PotentialReport.htm,last accessed on 17 July 2005

Harrison R, Mortimer N D, and Smarason O B. 1990Geothermal heating: a handbook of engineering economics, 572 pp.New York: Pergamon Press

Open University. 1994.T521 Renewable Energy: a resource pack for tertiary educationMilton Keynes: The Open University

Parson B K, Bharathan D, and Althof J A. 1985Thermodynamic Systems Analysis of Open-cycle Ocean Thermal EnergyConversion (OTEC)[SERI TR-252-2234]Golden, CO: Solar Energy Research Institute

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Ridgway S L. 1984Projected capital costs of a mist lift OTEC power plant 84-WA/Sol-33[Winter meeting ASME, December 1984, New Orleans]

Ridgway S L, Hammond R P, and Lee C K B. 1981Experimental demonstration of the feasibility of the mist flow ocean thermalenergy process[American Institute of Aeronautics and Astronautics, Terrestrial Energy Systems Conference,2nd, Colorado Springs, CO, 1–3 December, 8 p, Organized by American Institute ofAeronautics and Astronautics]

Vega L and Evans D E. 1994Operation of small open cycle OTEC experimental facility[Proceedings of Oceanology International 1994, Vol. 5]Beighton, UK

WEC Member Committees. 2000Geothermal energy: electricity generation and direct use at end-1999Details available at http://www.worldenergy.org/wec-geis/publications/reports/ser/geo/excel_files/geothermal_12_1.xls, last accessed on 4 August 2005

Bibliography

Baker C. 1991Tidal powerEnergy Policy 19(18): 792–797

Charlier R H. 1982Tidal Energy, 351 pp.New York: Van Nostrand Reinhold

Deffeyes K. 2001Hubbert’s Peak: the impending oil shortage, 208 pp.Princeton, NJ: Princeton University

Goodstein D. 2004Out of Gas: the end of the age of oil, 128 pp.New York: W W Norton & Company

Heinberg R. 2003The Party is Over: oil, war and the fate of industrial societies, 288 pp.East Sussex: Clairview Books

Salter S. 1992Wave energy: some questions and answersInternational Journal of Ambient Energy 14(1): 17–23

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