Wave Power - Supply and Crop          updated 17 Dec 2012

"You may fight against the waves - or let yourself be carried by them into the future" (unknown source)

Procedure of estimation

The estimation of the amount of conversation into useful energy by the mobile wave power station of the Eco-Trimaran is done in principal in the same way as a wind power station:
  1. Determination of the supply of primary energy  - in this case that of the waves,
  2. calculation of the width of the approaching wave field, which may be used by the wave power station.
  3. Multiplication of the produkt of (1) and (2) by the efficiency factor of the wave power station.
  To (1): Maps of wave energy show the power of an intersection of a wave with crest length of 1 meter.
The unit is kW * m-1 (Kilowatts per 1 meter wave crest length).
Because waves move orthogonal to the coast line in most cases, sometimes the denotation "Kilowatts per meter coast line" is used; but this means the same. For the predominant part of our scenarios we could use the wave atlas of the Koninklijk Nederlands Meteorologisch Instituut. It shows no items of wave power, but of heights and periods of waves. (Height: Difference in meters between lowest point in the wave hollow and highest point of the wave crest. The period is the time in seconds between the arrival of a wave crest and the arrival of the next. These data stem from measuring buoys, which gather these figures since about 1970 and so deliver a longtime average.
By the formula  P = 0,5 * T * H2 , where T = period in sec. and H = wave height in meters, one may calculate the wave power P in kW per meter wave crest.
 To  (2): According to a conservative procedure, it is supposed that the power of an approaching wave field may be used with the front width of the wave power station (analog to the procedure with wind turbines).
With the three floats of the Eco-Trimaran this would be 6.45 m overall.

To (3): By the wave power station Pelamis  (2nd Generation P2), which functions with similar principles as the Eco-Trimaran, an average  efficiency factor of 0.70 has been found out1. This figure we may disclaim for the Eco Trimaran as well. Both wave converters are  Offshore devices and belong to the power station type "line absorber" or "Attenuator" . Both use the wave height as power source.

The product of the figures, which are calculated as described in the 3 steps above, leads to the amount of average conversion of primary energy of waves into useful energy in terms of kW (kilo Watts).

At the before mentioned calculation we had assumed that the ship is adjusted vertically to the wave crests. That is the case when the ship anchors at the open sea (if applicable with a drift anchor or in shallow areas). In this case, the ship turns automatically in a position parallel to wave movement and consequently vertically to the wave crests. The normal case, however, is a free course of the ship, where a vertical adjustment to wave crests happens only occasionally. At a course parallel to the wave
(1) Yemm & Pizer 2011, Figure 11 
Energie kursabhängig

crests the wave power station is not working at all. At an acute-angled course, the power output is reduced. This is shown in the drawing above. The black lines with  specifications of angle degrees represent the orientation of wave crests. The green arrows show the power output when the ship anchors at the open sea - it has the same amount for all directions of wave crests, because the ship pivots into the optimal orientation automatically. The red arrows stand for the power output when the ship is on the way on a free course. The more
acute-angled the orientation of the vessel is towards the direction of incoming waves the lower the output of wave converter gets. While the ship is cruising the converted wave power PR  has a relation to the converted wave power PA during anchoring at open sea like the area of the half circle to the area of the rectangle, that is  0,5π r2 : 2r2 ; so results a correction factor of 0,7854 for the converted power while the ship is cruising.
To calculate the conversion over the course the year  we must include the scenarios for the mode and place of operation of the ship - see the  table at right hand:
scenarios wave power


The outcome for the different scenarios is as follows:
Particularly striking are the differences between the northern and the southern scenarios, because the average wave energy in the Mediterranean 
with 3 - 4 kW per 1 meter wave crest is much smaller than in the North Sea and especially in the northern Atlantic; the latter we have taken for the "best case scenario".
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