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Conversione energetica del moto ondoso: il convertitore di moti orbitali Seaspoon (in lingua inglese)

Lo sfruttamento del moto ondoso rappresenta un'opportunità per la produzione di energia da fonte rinnovabile.
Il convertitore di moti orbitali Seaspoon è un innovativo dispositivo per la conversione del moto ondoso che cattura l'energia cinetica dei moti turbolenti. Per lo studio delle prestazioni del dispositivo è stato messo a punto un modello numerico basato sull'equilibrio dinamico delle forze ed è stata avviata una campagna sperimentale su modello fisico in canale ondogeno.

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Articoli tecnico scientifici o articoli contenenti case history
La Termotecnica maggio 2013

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da Alessio Rampini
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Estratto del testo
INTRODUZIONE
This article focuses on wind-driven waves and their energy
exploitation.
The worldwide wave power resource potential is huge (Figure1).
The global power potential has been estimated to be around 8.000
- 80.000TWh/y (1-10TW). The best wave climates are found in the
temperate zones (30 - 60 degrees latitude), where strong storms
occur. However, attractive wave climates are also found within +30
degrees latitude the lower power levels being compensated by the
smaller wave power variability [1]. WavE ENERgy IN mIlD clImaTE sEas
Wave energy refers to the kinetic energy and potential energy in waves
on ocean surface. According to linear waves theory and equation (1)
a wave energy flux is proportional to its period of motion T and to the
square of its height H, and it is one of the most stable energy resources [3]. When evaluating a potential site for wave energy project
development, dealing with the forces imposed by a dense fluid
like water makes extremely important the consideration of extreme
conditions occurring during storms and associated to the highest
energy flux entering a wave energy converter (WEC),: in fact, the
extreme wave heights may be under-predicted in some locations
due to certain limitations of forecast tools [4].
The properties of a WEC correspond with the climate of the sea. A
perfect technology for a sea around UK may not be successful in
a milder climate.
An important parameter at the beginning of a energy wave conversion
project is the wave energy development index (WEDI) value of the site,
obtained by dividing the average annual wave energy flux by the storm
wave energy flux (occurring in extreme storm conditions). A high WEDI
reflects a severe design penalty that has to be paid in terms of capital
cost for a wave power plant to harness the annual average wave energy
resource available at a particular site [5]. In fact, during the worst expected
storm, wave power plant structures and foundations (or moorings for a
floating device) must be able to absorb the storm wave energy which highly
exceeds the energy under routine operating conditions.
As highlighted by Hagerman [5], for example, the most promising stretch
of United States coastline for wave power development, from Newport,
Rhode Island, to Nantucket Shoals, has WEDI values ranging from 2 to
2.5, which means that a wave power plant in this coastal stretch would
have to survive storm wave energies that would be only 40 to 50 times
the annual average wave energy flux.
Hence, many future developers of wave energy projects may seek
out locations where the wave energy resource is moderate and fairly
steady throughout the year and where the extreme waves are relatively
benign [4], even though this will imply to address some efforts in
designing devices of smaller size choosing to exploit minor energy
fluxes and extended operational time. In fact, many wave energy Tecnica Energie Rinnovabili la TERmOTEcNIca maggio 2013 di Di Fresco L., Traverso A. Ing. Di Fresco l., Dottore di ricerca in Ingegneria della Macchine a Fluido, Università di Genova.
Prof. Ing. Traverso a., Ricercatore Universitario di Sistemi per l''Energia e l''Ambiente, Università di Genova Ocean wave energy conversion:
Seaspoon orbital motion converter Ocean waves power, around the coasts worldwide, has been estimated to be in the order of 8.000 - 80.000TWh/y. The conversion of this
resource into sustainable electrical power represents a major opportunity to nations endowed with such a kind of resource.
The Seaspoon device catches the kinetic energy of ocean waves with promising efficiency conversion performance, according to specific
''wave-motion climate'. 67 cONvERsIONE ENERgETIca DEl mOTO ONDOsO: Il cONvERTITORE DI mOTI ORbITalI sEasPOON Lo sfruttamento del moto ondoso rappresenta un''opportunità per la produzione di energia da fonte rinnovabile.
Il convertitore di moti orbitali Seaspoon è un innovativo dispositivo per la conversione del moto ondoso che cattura l''energia cinetica dei moti turbolenti.
Per lo studio delle prestazioni del dispositivo è stato messo a punto un modello numerico basato sull''equilibrio dinamico delle forze ed è stata avviata
una campagna sperimentale su modello fisico in canale ondogeno. FIgURE 1- World''s Wave Energy Potential: annual average
power per front width [kW/m]
(1) Tecnica Energie Rinnovabili 68 la TERmOTEcNIca maggio 2013 devices that have been and are being developed to be competitive
off the Atlantic coasts of Europe might not be attractive in other
locations with lower average energy sea climate.
At first this may seem discouraging, but it is important to remember
that in addition to the wave energy development work that is being
done in Europe, there also continues to be a strong wave energy
development program in other countries, where prototype devices
in the output range of tens to hundreds of kilowatts like the Japanese
Kaimei, and Mighty Wales are more suitable.
Furthermore, any wave energy device that can be value-engineered
to be economically feasible in milder sea climate will have a much
larger global export market than a device that is economically
attractive in the high wave energy environments of Western Europe
only.
The Pacific coastlines of East and SouthEast Asia, for example, have
wave power densities in the range of 5 to 10 kW/m, which would
be out of range for devices designed for climates of 30 kW/m or
more [5].
In China, central region of Zhejiang, Taiwan, North to Haitan Island
in Fujian in the South eastern coast of the country, and Bohai Strait
in the North eastern coast have a resource in wave energy density
up to 5 - 8 kW/m [3].
The wave energy is being increasingly regarded as a major and
promising resource in South Korea because the Korean peninsula
is surrounded on all sides by Yellow Sea, East China Sea and East
Sea, nevertheless the wave energy potential of South Korea has only
been evaluated by global wave modeling [4].
Details on the wave energy resources around the Korean peninsula
remain poorly defined, but an annual mean wave power was
found to be 11 kW/m in the Southwestern side, 4 kW/m in the
southern side, and 6 kW/m in the eastern side of the peninsula
with the greatest monthly averaged wave powers of 25 kW/m in
winter, whereas the wave power was less than 10 kW/m around
the peninsula in summer season [6].
Considering European regions overlooking Mediterranean basin,
Greece has relatively low wave energy potential. It ranges from
3kW/m to 7kW/m. So the interest for sea wave energy exploitation
is moderate compared to regions in the North Sea and the Atlantic
Ocean where the wave energy flux is two to three times higher.
In the Maltese Islands, the very restricted space available on land is
compensated by the highest ratio (>12) amongst European nations
of available marine space with respect to land territory, considering
the extent of territorial waters up to 12 nautical miles. Sea depths
increase rapidly around most of the coast, especially on the southern
perimeter of the Maltese Islands, and this poses a constraint on the
location of offshore energy production installations.
Energy from offshore sea waves can be a realistic option for the
Maltese Islands, offering opportunities not only from direct benefits
deriving from local installations, but also in developing tradeable
expertise in the related technologies [7].
Similar conditions can be found off the Turkish coasts where although
power levels are low in general, some points can be found between
4-17 kW/m mostly in the southwestern parts of Turkish coasts that
are open to the Mediterranean sea. The regions in the west of the Black Sea in the North of Istanbul
Straits and off the Southwestern and western coasts of Aegean
Region have been suggested as the best sites to harness the wave
energy.
Excluding possible areas for shipping lanes, regions for submarine
training, etc., and ignoring most of the sites on the eastern and
southern Anatolia coasts, where the wave power levels are low for
commercial exploitation, wave farms that can be set up at three main
regions off the coasts of Turkey can harness totally about 10 TWh/
year with an annual wave power between 3 and 17 kW/m [8].
For the Italian seas, while the regions of the Adriatic, the Ionian
and the Southern and Middle Tyrrhenian seas don''t exceed average
energy content of 4 kW/m yet the western part of the North
Tyrrhenian shows an interesting energy availability with average
yearly values up to 13 kW/m . A first assessment for wave energy
around Italy highlights that in some location the energy extraction
could be feasible. In particular, the Alghero potential wave energy
power of 13 kW/m encourages the development/installation of
pilot WEC and its commercial use [9].
In conclusion, it is important to remark that, while a wave energy
resource on the order of kilowatts per meter of device width seems
low when compared with European wave energy fluxes, it actually
is a much more energy dense resource than either solar or wind
energy at similar latitudes in the regions considered. WavE ENERgy cONvERsION TEchNOlOgy
The energy content stored in oceans and carried by ocean waves can
be converted by devices based on different principles of conversion,
these devices are commonly called Wave Energy Converters (WEC).
These devices are generally categorized by the installation location
and the Power Take-Off (PTO) system. Locations are shoreline, near
shore and offshore. Besides, most devices can be characterized
according to their positioning respect to the incoming waves and to
their principle of operation, providing information on the geometry
of the device [2]. The classification system above is not sufficient to
encompass all device categories, though.
A commonly accepted classification system includes the following
six categories of devices, reported in Figure 2 from left, clockwise: FIgURE 2 - WEc''s classification - courtesy of aqua-RET
www.wavec.org
Point absorber, Submerged pressure differential, Oscillating wave
surge, Attenuator, Oscillating water column, Overtopping.
More than a thousand ideas have been patented in the recent years,
but at this stage, only few technologies have reached a large-scale
sea trial state. The research and development activities in the
wave energy conversion field is paying attention to the opportunity
offered by regions where the wave climate is moderate although
conveniently exploitable by devices expressly designed.
The Seaspoon is an orbital motion particle converter aiming to
convert the energy potential of Mediterranean sea-waves adopting
economic and robust wind-derived technology. ThE sEasPOON: PaRTIclE ORbITal mOTION cONvERTER
Particle motion converters obtain energy from the moving water
particles at the origin of ocean wave propagation according to the
linear wave theory [10].
The Seaspoon device (Figure 3) captures the wave energy by mean
of a horizontal axis rotor orthogonal respect to the incoming wave
direction [11]. The ''spoon' device, a plate stiffly coupled to the
rotor, boosts the conversion efficiency of the rotor immersed in the
particle flux. The rotor is shaped like a Savonius drag-type wind
turbine (but could be also a Darrieus type turbine), matching simple
and low cost construction, high starting torque, low operating speed
and low maintenance. This device represents an optimal choice for
small power applications [3].
The power P that can be obtained from a Savonius turbine operating
in a flux with relative speed V is according to equation (2):
P = 0,5 x CPs x As x r x V3
Where:
C Ps= Power Coefficient of Savonius rotor P = output Power [W]
r = water density [kg/m3]
A S = swept area of Savonius rotor (Diameter x Lenght) [m 2 ] V = relative speed of the fluid particle flux [m/s]
The Seaspoon device originates after in-depth review of existing
Wave Energy Converter technologies, as well as up to date wind
energy turbines. The Seaspoon is conceived with innovative
distinctive features, which allow overcoming current technological
limits. Its peculiar features are: - Increased wave energy is captured as the rotor is placed with its spinning axis perpendicular to the incoming flow direction
maximising the equivalent cross section of the blades. - Higher rotational speed since the device runs with the vector sum of the wave particles velocity and the rotational velocity of the
entire device. - High Operational Time. - Sea State Power adaptable: the device sets its working point according to the current sea-state, allowing extended operational
time, survivability, invisibility. - Robustness due to the implementation of standard manufacturing components. Referring to Figure 3a the simplest layout of the Seaspoon consists of
a rotor (1) whose spinning axis (2) is submerged at an appropriate
depth, considering the sea conditions (wave period, wave height and wavelength), perpendicular to the wave propagation direction.
The wave particles, while in motion, hit the rotor blades (3), which
start spinning in one direction independently on the incoming water
direction.
A connecting rod (5) ties stiffly the rotor axis to a main spinning
axis (4). The main spinning axis is placed parallel to the rotor axis.
It is submerged at an appropriate depth likewise the rotor axis.
The main axis is hinged to a flat plate (6), which acts as a rudder for
the overall rotation. In turns, the entire device is free to spin around
the main axis (4) in the same direction of the propagating waves. At
the same time, the blades are set to move and spin around (2) and
(4) at once. The Energy conversion efficiency is increased due to
the flat plate: its presence increases the relative velocity at which the
blades (3) are set to move, compared to simple case configuration
where one simply deals with (1), (2) and (3). Indeed the blades are
already moving at the speed u (rotational speed with respect to
(4)) when they are hit by the particles, moving at a certain speed v.
The resultant of the vector sum gives the relative velocity w, whose
module is higher than v.
The device can be installed in deep water by mean of a slack
mooring system and buoyant body or an elastic beacon hinged to
the sea bottom, in any case the mooring system allows the Seaspoon
to adjust to the suitable depth and position considering the occurring
sea state, in order to follow the designed operating conditions in
terms of orbital motion specifications. ThE sEasPOON NUmERIcal mODEllINg
The performance of the device have been assessed through
numerical model simulation performed in MATLAB environment in
order to define an optimal geometry of the device for given wave
parameters of height and period (H,T), and set the design for a scale
model to be tested with regular waves in a wave flume conveniently
equipped at the Department of Mechanical Engineering at the
University of Genova (Figure 5) facilities.
Kinematics and dynamics of the sea waves represented the starting
point to develop the model able to simulate the actual behavior of
the Seaspoon, and allowing a better comprehension of the way the
Seaspoon works.
The model is mainly based on a dynamic balance equation,
involving the torques due to the different forces acting on the device.
The solution of this equation brings the rotational acceleration α'
[rad/s2] of the Seaspoon. Tecnica Energie Rinnovabili 37 la TERmOTEcNIca maggio 2013 69 FIgURE 3 - The seaspoon particle motion converter:
a) components scheme; b) a possible rendering
(2) Integrating the rotational acceleration α' allows to retrieve α''
[rad/s] (angular Seaspoon speed) and α [rad] (angle swept by
the Seaspoon).
The model is developed for deep-water conditions (i.e. perfectly
circular wave particle motion). WavE DaTa
The Seaspoon motion is determined by the action of the incident
waves. Two are the main independent variables provided to the
simulation model: - the wave period T [s] (required time for two successive wave crests or troughs to pass a fixed point); - the wave height H [m] (difference between crest elevation and trough elevation). These two parameters are arbitrarily assigned to the model.
The Seaspoon performance is investigated considering two different
applications. A first application is the prototype to be tested using
the wave flume. The second application is an hypothetical offshore
demo plant.
The goal is to provide design specifications of the Seaspoon for
both cases.
Within the performed simulations, T and H were chosen according to: - the range values to be tested in the wave tank (T = [1; 3], H = [0.1; 0.2]); - the Alghero RON (Rete Ondametrica Nazionale) data - open water conditions (T = [3; 6], H = [1; 3]); - A Sav = b''D [m 2 ] rotor wet cross section; - 'α rot [rad] angle between rotor and flat plate ThE sEasPOON sImUlaTIONs
A simple Savonius rotor has been modeled within the Matlab model
to allow a comparative test with the Seaspoon device.
Two different Savonius rotors are set up: - one with the same dimensions of the Seaspoon rotor but located closer to the sea surface; - the other one placed at the same operating depth of the Seaspoon (z 0center), but with the same dimensions of the entire Seaspoon. A velocity profile distribution along the blades of the Savonius, is
considered.
For both the Savonius, the blades are divided in ten parts, thus ten
different depths defined with the following equation (3):
z(m) = z0Savonius - pRcos(ym)
where m is the loop index, p is a number ranging from 1 to 0,1 with
step size 0,1; y (m) is an array, updated at every loop, containing the angle swept by the blade while spinning.
The y (m) array varies in the range [0-2']. For each depth z (m), there is a free wave velocity (exponential profile).
An arithmetic average is performed among these ten velocities, so
that a single average speed is picked to estimate the power output
of the Savonius.
The two equations to estimate the two simple Savonius configuration
power output are of the kind:
PSav(m) = 0,5rH2OASavhelhmechcptVSav_avg3(m) Throughout the energy plot (Figure 4), the small Savonius fixed in
space is set as reference (Savonius energy =1).
In all the simulations, either Wave flume or Alghero ones, the results
always showed the higher efficiency guaranteed by the Seaspoon
compared to the simple rotor-based configurations.
The plot of figure 4, for example, shows how the average Seaspoon
energy is around four times the one provided by the ''small'
Savonius (rotor fixed in space) and two times the one provided by
the ''big' Savonius (rotor fixed in space with radius equal to the
spoon tip radius). ExPERImENTal TEsT
In order to validate the results of the numerical simulation and be
able to later measure the conversion performance of the Saspoon
device: - a wave flume has been designed and built at the Savona University campus; - a physical model of the Seaspoon device was realized according to the geometric features supplied by the numerical simulation; - wave train with parameters of wave height and wave period equivalent to the MATLAB simulations were generated - an experimental campaign aimed to verify the Seaspoon concept was driven. The tests done confirmed the ability of the Seaspoon model to turn
in phase with the wave period, and the increase of the revolution
speed of the Savonius rotor by the effect of the spoon plate.
However the experimental results obtained are merely qualitative,
due to the first stage of development of the designed technology
and the limited performance of the experimental equipment, thus
a improved equipment for further investigation is being designed. Tecnica Energie Rinnovabili 70 la TERmOTEcNIca maggio 2013 FIgURE 4 - seaspoon energy compared to the two savonius
ones, with the small savonius set as energy reference (=1)
(alghero simulations)
(3) (4) cONclUsIONs
The innovative WEC (Wave Energy Converter) called Seaspoon has
been first represented in a dynamic and kinematic model in order
to fully describe and simulate the Seaspoon operating conditions.
The numerical model gave a possible Seaspoon device configuration
for a future open water deployment.
The results of the numerical simulations showed that the Seaspoon
(Savonius rotor coupled to spoon plate) performs better than a
simple Savonius rotor, in all conditions. This means that the basic
idea on which the Seaspoon is based on can work. The spoon
plate eventually coupled to a rotor with even higher efficiency of
the Savonius (e.g. Darrieus type rotor) can improve its efficiency
converting the energy associated to gravitational waves.
At the end of this first stage of development of the wave energy
converter Seaspoon, the research activity gave the followings
results:
1. the spoon plate coupled to a Savonius rotor operating conveniently placed below a water surface interested by
gravitational waves increase the conversion efficiency of
the rotor itself by mean of an additional speed induced on
the rotor blades; 2. the wave energy converter device called Seaspoon, made by the coupling of an horizontal axis rotor with a spoon plate of
proper geometry is able to rotate around its main axis, working
conveniently fixed at a specific depth, with an angular speed
equivalent to the period of the incoming waves moving the
device (Figure 5); 3. experimental tests on a model in a wave flume generating regular sinusoidal waves proved the statements above,
showing that the ability of the device to behave as it was
conceived is strongly dependant on the correct match
between device geometry and wave parameters. The results obtained so far gave the premises to go on with the
investigations until the definition of the conversion capability of
the Seaspoon in ideal condition first, and eventually in open sea. REFERENcEs
1. Cruz J. Ocean Wave Energy, Current status and Future perspectives, Springer 2008 2. BERR, Atlas of UK Marine Renewable Energy Resource 2008, www.renewables-atlas.info ; 3. Wang Shujie, Yuan Peng, Li Dong, Jiao Yuhe''An overview of ocean renewable energy in China', Elsevier, Renewable & Sustainable Energy Reviews, Vol. 15, No. 1. (January 2011),
pp. 91-111. 4. A.M.Cornett, A global wave energy resource assessment. Canadian Hydraulics Centre, National Research Council. 11
Jul 2008 www.isope.org/publications/proceedings 5. Hagerman G.,''Southern New England Wave Energy Resource Potential.' Building Energy 2001. Greenfield, Massachusetts:
Northeast Sustainable Energy Association. March 2001. 6. Gunwoo Kim, Weon Mu Jeong, Kwang Soo Lee, Kicheon Jun, Myung Eun Lee, ''Offshore and nearshore wave energy
assessment around the Korean Peninsula', Energy, Elsevier,
volume 36, number 3, pages 1460 - 1469, year 2011 7. A- Drago , IOI-Malta Operational Centre, ''Introduction to the Blue Energy Project'http://oceania.research.um.edu.mt 8. M. Saglam, ''Wave Energy And Technical Potential Of Turkey' , Journal of Naval Science and Engineering 2010, Vol. 6 , No.2,
pp. 34-50 9. D. Vicinanza, L. Cappietti, V. Ferrante, P. Contestabile, ''Estimation of the wave energy in the Italian offshore', Journal
of Coastal Research, Special Issue 64, 2011 , pp. 613 - 617. 10. R.G. Dean, R.A. Dalrymple, Water wave mechanics for engineers and scientists, Advanced Series on Ocean Engineering, 2,
World Scientific, Singapore, 1991 11. A.Traverso, L. Di Fresco, Seaspoon: dispositivo per la conversione energetica del moto ondoso del pelo libero di liquidi, Domanda
di Brevetto Italiano GE2011A000020, 25 febbraio 2011 NOmENclaTURE
r = water density [kg/m3]
g = gravity acceleration [m/s2]
H = wave height [m]
T = wave period [s]
C Ps= Power Coefficient of Savonius rotor P = output Power [W]
A S = swept area of Savonius rotor (Diameter x Lenght) [m 2 ] V= relative speed of the fluid particle flux [m/s]
R = Savonius rotor radius
A Sav = rotor wet cross section [m 2 ] 'α rot = angle between rotor and flat plate [rad] h el = electric efficiency h mec = mechanical efficiency h cpt = capturing efficiency (Savonius blades) z 0Savonius = depth displacement of the rotor axis [m] FIgURE 5 - seaspoon spinning ''in-phase' with the incoming waves during waveflume trials Tecnica Energie Rinnovabili 37 la TERmOTEcNIca maggio 2013 71


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