Advocates for all energy technologies must answer the following questions: Is there enough?  Is it safe? Is it secure? Is it environmentally acceptable? What are the costs? This paper attempts to answer the questions for wave power 

Salter, S. H.

English

Edinburgh Research Archive

I I S. H. Salter, Department of Mechanical Engineering , Univers ity of Edinburgh, Mayfield Roa d, Edinburgh EH9 3JL. THE ENVIRONMENTAL ASPECTS OF WAVE POWER Mr. Sa lter' s tel ephone number is 031-667-1081 ext ension 3276 ABSTRACT Advocates for all energy technologies must answer the following questions: Is there enough? / Is it safe? Is it secure? I s it environmentally acceptable? What are the costs? This paper attempts to answer the questions for wave power . - 1 -Is there enough? Waves cannot approach solar radiation in total amounts of energy but t hey provide greater power density than is available to wind machines . A wave installation is the second stage of a windmill of which the first stage is the open sea. The size of waves depends on the fetch of sea as well as the strength and duration of the wind . Instrum.ent observations of waves in British waters have been made by Draper of the British Oc ean.ographic Data Service( 1 ). I based my first estimate of power levels on his findings. I concluded that the average power density in the North Atlantic was about 80 kil owatts per metre( 2). Work in progress by Mollison and Buneman using more refined techniques suggests that it is actually more than 90 kil owatts per metre . The peak of supply is in winter. Waves are directly complementary to sun. Visual observations from around the world have been collected by Lewis(:3) and by Hogben and Lumb( 4)_ They show that power densities in open oceans are nearly always greater than 10 kil owatts per metre . Drape) 5) presents a tabl e which shows that a fetch of 100 kilometres is sufficient to produce large enough waves to be worth harvesting . In-shore waves on a 500 kil ometre front could produce all the electricity now used in the U.K. If ways can be found of transporting energy from offshore stations t hen the world wave potential is several times the present world demand for all forms of energy. - 2 -Is it safe? Our designs for wave power installations are unmanned but from time to time the plant will need to be brought in for servicing. This activity will be like fishing. Men's lives will be part of the price of wave power just as we pay about one life per week for coal and twenty lives a day for road transport. With money and common-sense and sound legislation we can reduce this price. Most of the accidents will happen to yachtsmen attracted by calm water and good winds . Most shipping keeps to the economical line between two points . This leaves very large infrequently visited polygons inside the great circle routes . Wave power installations will be more or less stationary in marked chart positions and well equipped with navigation warnings . They nhould be less of a hazard to ships than other ships or the land itself. However, no system of human devising is perfect and there wj_ll be many small accidents, some medium ones , and a fe,., large ones . - 3 -Is it secure? Security is affected by the interruption or exhaustion of the flow of some ingredient . These da ys we have to consider interruption by political or terrorist activity. A widely dispersed target with parallel redundant connections and controls is not attractive to terrorists . Indeed, it would take a hard- wor king group to make much impression on 500 kilometres of wave plant . There are no secondary hazards. In the very long term ws.ve power is as secure as we could wish. We know tha t the winds will blow for ever . In the very short term wave power is at least predictable. We know enough to prepare reliable forecasts for twenty- four hours ahead so that stand- by plant should have plenty of warning . In the medium term wave power security can be expressed in terms of a statistical probability. Figures 1 and 2 show summer and winter scatter di agrams with wave power density cont ours for the !forth Atlantic . In Briti sh waters wave power is worth having for Bo% of the time , and in the winter this figure moves to 9C/fo. The probability of zero power i s n ot zero . The Engl ish Central Electricity Generating Board have found a week in May 1961 in which there was no wave power at a station in the Atlantic . A secondary source must be provided . T'nere is no diurnal variation and so no match wi th dail y patterns of demand . Methods of short and medium term storage will become important when the amount of wave power exceeds the base load . - 4 -Is it environmentally acceptable? Wave power introduces no new chemicals or heat into the biosphere. But it does introduce a temporary diversion of heat . There will be a cooling of water on the beaches. We could measure the effects by putting a sufficient number of electric fir e elements into the surf zone and noting the rise in temperature . This rather extravagant-experiment is now done continuously by those generating boards who draw cooli..Tlg water from the sea and put into it twice the energy that they deliver to consumers. We may als o attempt calculations based on the rates of replacement of water . Stomme1 (6 ) reports a flow of 109 cubic metres a second in theUorth Atlantic drift . If this were t o be evenly distributed across the western approaches then full deployment of wave power installations would not reduce the beach temperatures by more than one hundredth of~ centigrade degree . I n modera te weather the size of waves to leeward of a wave power i nstallation will be reduced to between one-tenth and one- half t ha t of the incident ones . People using the sea lanes inshore will find l i fe l ess exciting and the required levels of hardihood and seamanship will be reduced . No wave installation can absorb or reflect the extreme c onditions and the very large waves wit:.1 power levels above one megawatt per metre will pass unattenuated . I do not believe that the present causes of . beach formation and erosion which make such a large difference between the east and west coasts of the Hebrides should be much affected . But if they were then we have many examples of beaches in sheltered seas t o help us predict the outc ome . The wave power engineer will , if he can, avoid sites wit:i hi gh current flows . I f mistakes are made and silting of harbours results then the machinery may be re- sited at little expense . - 5 -Among the many requirements of modern industry are power, cooling water , deep harbours and easy disposal of waste. Some indus trialists may feel that threats of nationalisation and factory regulations restrict their freedom and they might be tempted to set up at sea . I believe that waste disposal from an offshore installation in international waters could be dangerous and difficult to regulate in the present state of international law, and that this is the only serious environ-mental risk inherent in wave power . Perhaps this risk is really i nherent in having industrialists . ~ What are the costs? To calculate the cost one should add up cost of research and development, land , factories , processing plant , fuel, labour and interest charges that can fairly be carried by the project , and divide this by the output produced over some period of time . This tedious exerc i se is not always done amid the exci tements of technological advance. T'ne answer is most needed at the start to help in deciding between competing proposals , but cannot be lmown with any certainty until the end. It is particularly difficult to decide whether or not some piece of research done many years before has to be paid for by this account or an other . We set out to build Atl antic plant rated for an average of 50 kilowatts costing £20 OOO ( 1974) (October ) per metre giving a target capital cost of £400 per kil owatt . After considering a wide number of possible mechanisms and conducting model tests of several , we settled on the one shown in Figures 3 and 4 . It consists of a number of 'duck '-shaped segments ·rotating about a common backbone . E:tch duck i s designed to be slightly heavier than the water it dis places so that i f i t should Mf' le,j Hl~H f,f'f/(/ 1Shl(Vf'6 JEE 5AiT F R'5 N ,J~t- PI( IC l - 6 -break it will sink. The whole structure has a very low freeboard so that it could be easily submerged. The rear surface of each duck is a cylinder coaxi al with the centre of rotation so that no water is displaced behind and no rear wave created . The front curve is designed to match the displacements of water in approaching waves . The natural 'nodding' period i s designed to coincide with that wave period where maximum efficiency is required and attempts are made to broaden the frequency response. Laboratory tests on single units show an extra-ordinary efficiency for monochromatic and mixed spectrum waves. In full scale designs prepared by my colleague Eric Wood each duck runs on rollers which are the bodies of commercially available rotary hydraulic pumps ( ? )_ High pressure oil drives hydraulic motors One unit_ will be about 500 metres long . The concrete , the electrical generating plant , th e hydraulic parts and the l abour can all be costed fairl y accurately. The result is wit~in the target . The only problem lies in the strength requirement s of the common backbone. The laboratory models are mounted on fixed bearings . At sea this reference must somehow be synthesised. I t is clear that a sufficiently long backbone would span a large enough sample of wave pr..ases so that it would average the alternating components of wave force . But the resulting structure would suffer a dangerous bending moment in the centre . We calculated that the really extreme 1 50 year wave'(S) would require steel cos ting ten times more than we could afford . The crucial question was w~ether we could find ways of evading those bending moments . The key to the problem has been found by Eric Wood. His design gives a rigid backbone f or l ow bending moments which turns into a flexible one for high bending moments . A model tested in a multidirectional sea behaved as we had hoped . - 7 -Our approach is by no means the only one, and efficiency itself is of no concern when the gods pay for the waves . But in structures of this size the wave forces depend on the displacement, and the cost depends on strength, so that there are powerful economic incentives to get the most power out of the lowest displacement. We are certainly interested i n the highest possible efficiencies for those times when wave power levels are low . Wave power plant can be added in amounts of £10 OOO OOO at a time . All engineers make mistakes . If we are wrong we can be stopped early. Wave power plant consis ts of multiple small modules which will have the advantage of repetitive production. Each will take only a few months to build so that interest during construction is low. The makers of the hydraulic parts advise us that we will need to replace bearings and seals after six years . Ships need antifouling treatement after two years . This work will have to be done in p:::-otected water and will be the major r unning expense. Ships can be made t o last for forty years and i ndeed , t he first ferro-cement boat made in 1855 is still in perfect working condition • . It is an obvious disadvantage of wave power that almost a l l t he costs come at the beginning but that benefits may accrue t o future generations. Chapman(g ) gives figures for the energy content of raw materials . Structural steel consumes~ ilowatt hours per ton while cement needs 2 200 kilowatt hours . If we use a five to one aggregate ratio , we will need to run our plant for 2 OOO hours to earn the energy t o build it . 1 vr -~ f"F R- l..-t C. H . f!__ rrf ~ra DI vile cJ ~'""'= "l-nt = CIJ~ T Ftf< G-tr f l-Og/k w '::}tl O OOO/~ I 00 TO'!'IJ ~1.000 ~ 50 .,_ 1 ,OOO TOT AL L f rvi:;, T l--1 500,yy, 1 GO ,OOO )1wlJ/~.rt l,ooo kwLi/~ - 8 -Conclusions I claim that the answers to the questions at the beginning of this paper are as folloivs : There is enough wave energy . I ts safety is acceptable . The dangers are clear and well understood . I ts security is as good or better than other technologies . Wave power is clean and cool . As model work continues , the cost estimates c ontinue t o fall . I f all costs are considered i t is already competitive for good sites . I rest my case . Let the ultimate judges be our children and , of course, the sea . - 9 -References (1) Draper, L., & Squire, E.M. Waves at Ocean Weather Ship Station 'India' (50°N, 19°w) Trans. Inst. Nav. Arch • .1Q2. 85 (1967) (2) Salter, S.H. Wave Power Nature Vol 249 No 5459 720-724 (3) Lewis, E.V. Motion of Ships in Waves Principles of Naval Architecture (Ed. John P. Comstock) Chap IX 624-626 Society of Naval Architects and Marine Engineers New York ( 1967) (4) Hogben, N. & Lumb, F. E. Oc ean Wave Statistics H.N. Stationery Office London (1967) (5) Draper, L. Environmental Conditions Paper No 1 Symposium of Offshore Drilling Rigs 1970 Roy. Inst. Nav. Arch. (6) Stommel, H. The Gulf Stream - A Physical and Dynamical Description Berkeley University of Calilornia Press (1958) (7) Korn, J. (Ed) Hydrostatic Transmission Systems Intertext Books London ( 1 972) (8) Draper, L. Extreme wave conditions in British and adjacent wat ers Proc. 13th Coas tal Engineering Conference 1972 Vancouver Canada (reprinted by Am. Soc. Civ. Engrs.) (9) Chapman, P.F. Energy Policy March 1975 Vol 3 No 3 231-243 Acknowledgment This work was supported by the Department of Industry. .µ a, (1) f+-1 ~ •rl D) ::r:l .µ fo •rl (1) :r: (1) > ~ .µ s:: . ~ •rl f+-1 •rl ~ Z1 50 48 46 44 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 6 2 4 3 2 11 17 2 ~25 6 5 ~ 50 20 9 ,1-5 23 4 1 18 6 26 30 32 22 8 2 2 6 1 4 4 2 7 8 9 10 11 12 13 Zero-Cr ossing Peri od T ~n seconds z ,-o. w. s. ' INDIA ' SUMMER FI GURES 1 and 2 1 14 Dat a is based on observations made by The British Oceanographic Data Service between 1955 and 1965 , The isodynes show power density levels i n kilowatts per metre . Each entry represents the number in parts per thousand of a particular c ombination of significant wave height and zero crossing period . ( Power Densi ty lili M 1280 640 320 160 80 40 20 10 Po.·rer Density KW 50 48 M 46 44 1280 42 40 2 38 1 36 2 2 .µ 34 a, a, 3 1 f+..t 32 640 s:: 3 3 . 3 2 •.-f 30 Ill 1 3 2 4 2 1 tr! 28 .µ t 26 2 3 1 •,-f 2 3 a, ·24 tr! 2 5 6 7 320 a, 22 > 3 8 9 9 2 1 ~ 20 .µ ~ ra 2 3 1 2 4 17 3 2 ,0 160 •,-f 16 f+..t 14 6 5 2 •.-f t1 14 ~ 12 5 8 23 18 8 4 3 10 9 28 26 29 18 13 6 2 80 16 42 30 1 1 40 8 1 Hl 2Q ~ 5 6 - 20 3 12 ~ 9 ~ 1 a 4 10 3 5 2 (' 6 7 8 9 10 11 12 13 14 Zero-Crossing Period~~ in -seconds o.w.s. I INDIA' WINTER ( + - -· ·- ·-- - ---Figure 3 A cross section of duck and backbone and hydraulic rollers . The diameter of the rear portion will be about 10 metres. ~ -.. :·: : / /:. .. :.; : ;: t • .• • I . I . : · .. . . ··. ·1 ... ·;. : .. . ·' ··. .. . . . · : · . . : ~ ' . .• : · ' , .. -----.... -.... , -- .. ........... ~. '. .. '' ,,. ' ,, . ' i ~ f I . ,. '' .,, • • • • • ,; .. .. • • --. __ 41' ,,., , 7 i · ., .. . , . . ~: J ; '1· • . !. .. ,' • ' :- . I• ··-1 , . -~ . • :: r··_. : 11, .. . , '. • ' ·,'. ,i. c'· . ·, · , ·;· '·': ·: . - , . . .... · .

The Environmental Aspects Of Wave Power

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The Environmental Aspects Of Wave Power

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