page 413, figure 11.2 (b): The ellipses are drawn incorrectly. The (vertical) minor axis should tend to zero for ellipses approaching the bottom. The diminishing of the (horizontal) major axis should be nearly negligible.

page 415, line 3 below the figures: ‘the particle’ could perhaps be replaced by ‘another particle’?

page 417, line 1 above equation (11.15): The quantity symbol r should presumably be Greek letter rho.

page 418, line 1 below equation (11.23): ‘harmonic motions’ should be ‘a purely propagating wave’. [Comment: E.g., for a standing wave, with nodes and antinodes, there is no potential energy at the nodes, and at the antinodes there is more potential energy than kinetic energy, even if the time variation is harmonic, that is, sinusoidal. However, averaged over half a wavelength the two types of energy are equally large. More general information on wave-related difference between kinetic energy and potential energy may be found in Falnes & Kurniawan (2015), Fundamental formulae for wave-energy conversion. R. Soc. open sci.2: 140305, http://dx.doi.org/10.1098/rsos.140305.]

page 422, line 10 above Fig. 11.9: Comment: If the maximum wave height over 50 years is, say 30 m, then the statement given on line 10 above Fig. 11.9, is correct provided the particular significant wave height, to which it is referred, is as low as 30/50 = 0.6 m.(!)

page 427, line 9 above Fig. 11.13: Replace ‘Attenuators (line absorbers) are several wavelengths long’ by ‘Attenuators may be about one wavelength long, or longer,’.

page 428, lines 2-3: Comment: On lines 2-3, the wavelength lambda should be replaced by lambda/2 (or perhaps lambda/3) in order to be in agreement with information given on lines 7 to 3 above Fig. 11.2 on page 413.

page 428-429, all lines below equation (11.54) including the three first lines on page 429: A comment to this matter may be provided by the following citation:

"Quite often, results have, traditionally, been expressed by quantities as ‘absorption width’ and ‘capture width’, which have dimension length (SI unit metre), or by dimensionless quantities, such as ‘capture width ratio’ and ‘(hydrodynamic) efficiency’. We consider it to be very unfortunate that ‘efficiency’ is applied to a quantity that may, in some cases, be larger than 1. Moreover, it is also misleading to refer to an ‘efficiency’ that may be less than 1 for cases where no energy is being dissipated into heat! Could the frequent use of these terms be partially responsible for the situation that, after decades of research, wave-energy technology is still only a ‘dream’, and not ‘reality’ yet? We should be aware that our language influences our thinking! These technical terms focus on one horizontal dimension (in the direction of the incident wavefront). However, a WEC has two horizontal dimensions, in addition to a vertical one. A most important and urgent challenge is to develop a feasible single unit of a WEC, a unit that maximizes the power output, not with respect to the free wave power that is available in the ocean, but with respect to parameters related more directly to the WEC itself (size, cost of investment and maintenance, etc.)." [Cited from pp. 247-248 of Falnes, J. and Hals, J. ‘Heaving buoys, point absorbers and arrays’. Philosophical Transactions of the Royal Society A, Vol 370, No 1959, pp 246-277, 28 January 2012. (http://dx.doi.org/10.1098/rsta.2011.0249).]

page 429, lines 5-6 contain some false information: The citation on line 6 does not exist in the mentioned reference (Falnes 2002). Moreover, on line 5 there is an incorrect historical statement. In relation to acoustic or electromagnetic waves, it is classical to consider wave absorption by a microphone or a receiving antenna as destructive interference between a generated/radiated wave and the incident wave. In relation to conversion of ocean-wave energy, -- certainly not Falnes, but -- Budal may have been the first to express this principle, when he in 1973 took the initiative to start research on point absorbers. To correct false information, the following amendment is proposed:

On page 429, replace lines 5-6 by:

"absorber may have been first expressed by Budal, who considered wave absorption as destructive interference of a radiated wave with the incident wave. Thus, a wave absorber needs to be a wave generator. To explain it in some more detail, this may be expressed as follows: ‘a secondary, ring-shaped, outgoing wave is generated, which interferes with the incoming wave in such a way that the resulting transmitted wave carries with it less energy than the incoming wave does.’ [Citation from (Budal and Falnes, 1975)] -- [The reference for this is: Budal, K. and Falnes, J. (1975) ‘A resonant point absorber of ocean-wave power’. Nature, Vol 256, pp 478-479.]"

page 429, line 14: Replace ‘Falnes' principle’ by ‘Budal's principle’ or by ‘the above principle’. This comment applies also to line 7 above the bottom of the same page 429.

page 429, line 20 above the page bottom: Replace ‘of the `point´).’ by

‘of the `point´). A row of two or more point absorbers located behind each other may serve as one `attenuator´.’

page 429, line 19 above the page bottom: Replace ‘a line attenuator’ by ‘an array of attenuators’.

page 429, line 8 above the page bottom: Replace ‘waves).’ by

‘waves). Budal (1977) derived a similar result for an array of point absorbers’. -- [The reference for this is: Budal, K. (1977) "Theory for absorption of wave power by a system of interacting bodies". Journal of Ship Research, 21 (4), pp 248-253.]

On page 429, it seems that the three first lines of the last paragraph need to be rewritten, because it is not directly clear -- merely from the principle of considering wave radiation as a necessity for wave absorption -- that there may be a need or desire for tuning an oscillating wave-energy absorber to resonance. It is better to refer to equation (11.56), which shows that tuning to resonance, corresponds to maximum (or approximate maximum) absorbed wave power, because the first term in the denominator of (11.56) vanishes at resonance. I could propose the following correction:

Replace the three lines 7-5 above the bottom of page 429 by:

‘Most types of wave power devices should preferably be tuned to resonance with the incident wave. This concerns, in particular, devices for which the denominator of equation (11.56) is dominated by its first term except near resonance. The term vanishes if the device is tuned to resonance with the wave. This tuning is one of the conditions for optimum energy conversion. Yet this criterion is far from easy’.

page 430, Replace the text on the last five lines, including the first word on page 431, by the following text:

‘wide. The amplitude increases progressively as the incoming waves propagate from open sea, through a 90 m long collector, into the, 80 m long, horizontal, narrowing (tapered) channel, whose 10 m tall, vertical, concrete walls reach 3 m above mean sea level. Where wave crests reach a level exceeding 3 m, water spills into the reservoir. The collector is a natural, but partly dynamite-shaped, cove, 55 m wide at the entrance. The width of the concrete channel is 1.7 m at the entrance, and 0.2 m at the innermost end. Additional information on the Tapchan converter may be found on http://brage.bibsys.no/xmlui/handle/11250/246693.

page 431, figure 11.14: This drawing does not give a correct impression of the Tapchan. It does not show the lowering of wave troughs as the wave moves further into the horizontal tapered cannel. The drawing gives the impression that there is an inclined ramp, which is not true for the Tapchan. Assuming that this illustration is valid for the Wave Dragon, the figure caption should be revised correspondingly, and the words ‘Cliff face’ should be removed from the drawing.

page 431: Comments to the two first lines of the last paragraph of § 11.6.1: Firstly, it was a nearby shore-based OWC structure, including an air turbine and an electric generator, that was destroyed by a violent storm. Secondly, the Tapchan was damaged during several storms, during which blasted rock pieces were wave-transported from the wave collector sea bed into the Tapchan or into the elevated water reservoir. After the storms, some of these rock pieces had to be removed from the Tapchan concrete structure. It was decided to rebuild the Tapchan to replace its vertical concrete walls by inclined ones. However, this reconstuction of the Tapchan has not been completed due to lack of financing.

page 432, line 1: Replace ‘structure floating at sea.’ by ‘structure floating at sea, and with an inclined ramp instead of a horizontal tapered channel.’

page 435. The three last lines before Fig. 11.18 should better be replaced by: ‘power, over-damping is more preferable than under-damping, mainly because this broadens the resonance bandwidth. This is very important for wave-power systems where the first term in the denominator of (11.56), in off-resonance cases, dominates over the second term. In such cases it may be desirable to tune the device to another resonance frequency, which better matches the incident wave.’

page 436, line 5 of the second paragraph: Replace ‘anchored’ by ‘moored’.

page 436, lines 5-6 of the second paragraph: Delete ‘as explained by the Falnes principle of §11.5.2’. [Comment: Observe that it is not an easy task to explain, in detail, or quantify the capture width for any wave power device, and, in particular not for a many-degrees-of-motion system as the Pelamis device.]

page 440, line 2 of question 4: ‘same speed as the wave.’ could preferably be replaced by ‘same speed as a single wave.’

page 441: Comment to Problem 11.4, first paragraph: It seems that these experimental results agree reasonably well with ideal-fluid theory developed, independently, by Evans (1976), by Mei (1976) and by Newman (1976): For such a symmetric system (Fig 11.21a), at optimum oscillation, 50 % of the incident wave energy is extracted, and the remaining 50 % contribute to the reflected and transmitted wave energy. This theory does not account for real energy loss, which seems to amount to about 15 % in the reported experiment.

[References for this are: Evans DV (1976) A theory for wave-power absorption by oscillating bodies. Journal of Fluid Mechanics; Vol. 77, part 1, pp 1-25. Mei CC (1976) Power extraction from water waves. Journal of Ship Research; Vol.21, No.4, pp 248-253. Newman JN (1976) The interaction of stationary vessels with regular waves. Proc. 11th Symposium on Naval Hydrodynamics; pp 491-501.]

page 444, last two lines above the ACKNOWLEDGMENT section: ‘http://folk.ntnu.no/falnes/teach/wave/JF_introduction2010-06-28.pdf’ could preferably be replaced by ‘http://folk.ntnu.no/falnes/teach/’.

page 444: Comment to the last line (the ACKNOWLEDGMENT): It does not appear to me, Johannes Falnes, that I have provided significant contribution to this Wave Power chapter 11, apart from my list of corrections and comments to the Wave Power chapter of the 2nd edition (2006) of the book ‘Renewable Energy Resources’; see http://folk.ntnu.no/falnes/teach/wave/TFY4300/correctionsTW2006ch12.htm.

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JF 2015-09-01
e-mail: johannes.falnes(a)ntnu.no