Ocean Energy Conversion Tidal Systems

Water covers more than 74% of the Earth’s sur-

face gathering enormous Renewable Energy re-sources and offering the marvellous blue sight of the planet –viewed from the space (fig.1)

There are moving water resources (ocean waves, tides, ocean and river currents) as well as non-moving water r e s o u r c e s (offshore wind, offshore solar, ocean thermal, salinity gradient, marine biomass, geothermal).

C o n s e -quently, differ-ent types of ap-plications have been designed f o r w a v e s (terminators, at-tenuators, point absorbers, etc) and for currents (ocean or river) or tides (fig.2 a, b, c, d).

Even if tides were observed, managed and used as power source, in accord with the techno-logical state –of-the art of different periods, a more effective energy management of this complex phe-nomenon became possible only recently, as a result of the complex studies regarding mapping, model-ling and simulation. If for the vertical move dams and estuary infrastructure were already used, for the tide currents’ conversion there are used horizontal

Captarea energiei oceanelor Sisteme mareomotrice

Apa acopera mai mult de 74% din suprafata

Pamantului, insumand cantitati impresionante de resurse energetice regenerabile si in acelasi timp dand Terrei minunatul aspect de planeta albastra – vizibil din spatiu (fig.1). In prezent sunt identificate,

s tud ia te s i experimentate, cel putin pana la stadiul de demonstrator, t e h n i c i d e c a p t a r e a e n e r g i e i cinetice a apei ( v a l u r i l e o c e a n e l o r , m a r e e , curentii din rauri si mari) precum si a altor tipuri de e n e r g i e r e g e n e r a b i l a a s o c i a t a intinderilor de ape (curentii de coasta, energia solara din zona d e c o a s t a , e n e r g i a c a l o r i c a , e n e r g i a rezultata din gradientul de s a l i n i t a t e ,

biomasa, energia geotermala din zonele active ale oceanului planetar).

Chiar daca mareele au fost observate, stapanite si utilizate ca sursa de energie- conform nivelului tehnologic al diverselor epoci- exploatarea eficienta a acestei resurse naturale a devenit posibila doar in

ultimii ani, ca rezultat al studiilor complexe care

European Pupils Magazine

Andreea Samoila Politehnica University, Bucharest, Romania

cristylacatus@gmail.com

Wave terminator principle Principiul capatului de unda

Point absorbers Puncte absorbante

Wave attenuator principle Principiul atenuatorului de unda

Tidal energy harvesting principle Principiul colectarii energiei mareelor

Fig. 2 Applications for ocean waves and currents Fig. 2 Aplicatii pentru utilizarea valurilor si curentilor oceanului

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or vertical axes turbines or newly designed devices acting as turbines.

Basic principle of tides Tidal Energy is produced by the centrifugal

force as an effect of the Earth spinning move and of the gravitational attraction of the Moon (as primary source) and of the Sun (as secondary source) (fig.3 a, b, c).

That makes tides to be re-liably predicted for years to centuries ahead. As a physical principle tidal power facilities harness the energy from the rise and fall of the tides- as vertical motion, and from the double-sense daily horizontal water currents flow deter-mined by tides (flood currents- moving in the direction of the coast- and ebb currents, the currents receding from the coast).

So, the existing two high and respectively two low tides, are related to the daily Earth rotation and positioning to-ward the Moon, while the so called Spring and Neap tides are related to a more complex relation-ship established among the force fields of the Sun, Moon and Earth.

The highest one, the Spring tide, occurs when the Sun and the Moon line up with the Earth – ei-ther on the same side or on opposite sides (fig.4); consequently, the lowest tide, the Neap, occurs when the Sun and the Moon are at 90 deg related to the Earth (fig.4).The construction principle of a tidal system is rather simple, as long as either a dam or a section of an estuary are going to run as a reser-voir in the action area of the tide.

There are two moments of the process: the flooding one- when the highest level tide floods and fill the reservoir having only one –way access gate; and as soon as the water withdraws at the lowest tide level the existing water from the reservoir is released toward a water wheel.

For harvesting this type of energy, the ideal

au permis cartografierea zonelor de coasta si realizarea unor baze de date cuprinzand directiile mareelor si amplitudinile lor pe intreaga suprafata a oceanelor. Baraje si estuare erau deja amanajate pentru captarea miscarii verticale a maselor de apa transportate de maree, la acestea s-au adaugat recent si turbine cu axe orizontale sau verticale pentru captarea curentilor generati de maree.

Istoricul si evolutia captarii energiei mareelor Energia mareelor este

determinata de miscarea maselor de apa sub influenta fortei centrifuge – ca efect al miscarii de rotatie a Pamantului- si de atractia gravitationala a Lunii (ca sursa primara) si a Soarelui (ca sursa secundara) (fig.3 a, b, c). Aceasta face ca mareele sa fie predictibile, putand fi calculate pentru periode de ani si chiar de secole.

Principiul fizic al conversiei energiei mareelor este relativ simplu: se capteaza energia

cinetica a maselor de apa aflate in miscare (flux-reflux) atat pe directie vericala cat si pe directie orizontala si se transforma in energie electrica prin intermediul diferitelor tipuri de turbine.

Prin urmare exista doua maree inalte si doua maree joase, care sunt in stransa legatura cu miscarea de rotatie a Pamantului si cu pozitia Lunii; in timp ce mareele extreme, numite mareea vie si mareea moarta (in functie de amplitudinea lor) sunt rezultatul interactiunii campurilor fortelor gravitationale ale Lunii si Soarelui asupra Pamantului (fig.4). Fluxul cel mai inalt (mareea vie) se produce atunci cand Soarele si Luna se gasesc pe aceeasi dreapta cu Pamantul, fie de aceeasi parte, fie pe parti diferite (fig.4). Refluxul cel mai scazut (mareea moarta) se produce atunci cand Soarele si Luna formeaza un unghi de 90° fata de Pamant.

Principiul constructiv al unui sistem de captare a mareelor este relativ simplu, atat timp cat un

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Ocean Energy - Tidal Systems

Figure 3 Gravitational and Centrifugal force influence

Figura 3 Influenta fortei gravitationale si a fortei centrifuge

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sites are located on estuary next narrow channels, which experience significant differences between high and low tides.

Using the present achievements of Technology and Materials Science typi-cal energy conversion instal-lations consists of tidal dams and underwater turbines grid structure. The global poten-tial of such energy harvest-ing systems (only from tides) is about 1800 TWh/year, and it is meant to re-main a renewable resource as long as Water, Moon, Sun and Earth will exist.

Historic and evolution of tidal power On almost all significant shore areas there are

archaeological clues about the existence of different techniques for the use of tide power, from water mills to irrigations. For the European Middle Ages mills were very familiar on each type of landscape, although these were powered by animal force, wind or water.

In those areas of the Earth where the tides were high enough (fig. 5) tide mills represents an effec-tive alternative to the river water mills. Several in-stallations, raised on the Middle Ages, are still in place in England (Woodbridge Tide Mill) and in France (Rance estuary).

During centuries the technical refinements on tidal energy harvesting have been focused on design and materials. Only the technological achievements of the last decades allowed a systematic study of the tidal phenomenon at global scale, mapping, model-ling, simulating and predicting their effects. The tidal systems emerged as a result, using the most adequate solution for the entire tidal power harvest through more efficient installations.

It is obvious that significantly high tides are on very few shore areas of the Earth (fig.5) so, in or-der to capture tide energy having at least 50% effi-ciency of the conversion, some other kinematic sources should be envisaged. Flood and ebb currents are such additional sources having the advantage

baraj sau o sectiune a unui estuar pot fi utilizate pentru a capta apa dislocata de maree.

Exista doua etape ale procesului: cea determinata de flux – cand apa creste si inunda rezervorul al carui sens de acces este unic - si cea determinata de reflux –cand apa coboara sub nivelul de acces in rezervor, moment in care se elibereaza controlat apa acumulata in rezervor pe un traseu prestabilit continand una sau mai multe mori de apa.

Pentru captarea acestui tip de energie locatiile

ideale sunt in estuare, la gurile canalelor inguste acolo unde se inregistreaza diferente semnificative ale cotelor de nivel intre flux si reflux (fig.5). Sistemele de captare sunt constituite atat din baraje, rezervoare si mori de apa cat si din retele de turbine subacvatice.

Potentialul energetic mediu al mareelor la nivel global este de 1800 TWh/an si va continua sa ramana o resursa regenerabila disponibila atat timp cat vor exista Pamantul, Apa, Soarele si Luna.

Istoricul si evolutia captarii energiei mareelor In toate zonele de coasta (ocean sau mare) s-au

descoperit vestigii arheologice care atesta diverse forme de utilizare a maselor de apa aflate in miscare determinate de existenta mareelor. Pentru Europa evului mediu morile erau un element comun oricarui peisaj, fie ca erau animate de puterea vantului, de tractiunea animala sau de caderile de apa. O varianta eficienta a morilor de apa de pe cursurile raurilor repezi, cu debit semnificativ, o reprezentau in zonele de coasta morile actionate de forta mareelor, acolo unde acesta era suficient de mare pentru a asigura un debit emnificativ. Astfel de instalatii se mai afla inca in Anglia Woodbridge Tide Mill, 1170) si in Franta (estuarul Rance). De-a lungul secolelor imbunatatirile aduse sistemelor de captare a energiei s-au regasit in special in rafinarea solutiilor constructive sau in materialele utilizate la constructia barajelor. Abia in

ultimele decenii, datorita progreselor IT, a fost 38

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Ocean Energy - Tidal Systems

of a predictable and relative stabile flow direction. Undersea turbines, even less efficient than wind

turbines (mainly due to the different densities of the activated fluids -air and water) can put in place more compact grids than windmills farm, thus increasing their effectiveness. There are also other types of devices for the waves and currents harvest-ing from seas and oceans (fig.2), but the existing demonstrators didn’t reached yet ei-ther the energy effi-ciency or the economi-cal efficiency thresh-old for large scale use.

Potential environmental issues Although the basic process of tidal power har-

vesting is the same for centuries, and the average efficiency of the mechanical tide energy conversion on electricity is not surpassing yet 50% signify- types of devices for the waves and currents harvest-ing from seas and oceans (fig.2), but the existing demonstrators didn’t reached yet either the energy efficiency or the economical efficiency threshold for large scale use.

Potential environmental issues Although the basic process of tidal power har-

vesting is the same for centuries, and the average efficiency of the mechanical tide energy conversion on electricity is not surpassing yet 50% significant progress is expected from the computer aided design of the new turbines shape and geometry, new ad-vancements in material science and proper distribu-tion of the tide current turbine grids (fig.7 a, b, c, d). However, the system has some disadvantages as long as the turbine generation, nourished from the flow of the tidal stream, will likely generate a swirl of water downstream of the turbine. This horizontal vortex may cause erosion, if it touches the bottom (fig.8).Neither offshore bottom area, nor its ecosys-tem will be preserved entirely on a long term use

posibila studierea sistematica a fenomenului mareelor la scara globala, cartografierea, modelarea, simularea si prognoza pe baze stiintifice. Sistemele de colectare a energiei mareelor, care au rezultat ca urmare a

acestor studii, sunt proiectate pentru u t i l i z a r e a m a i eficienta a acestei resurse regenerabile complexe. Evident, maree suficient de inalte sunt doar in cateva locuri pe tarmurile oceanelor (fig.5) ca urmare, pentru a colecta energia mareelor cu un randament al conversiei de peste

50%, au fost studiate si alte surse animate de acelasi fenomen. Curentii generati de flux si reflux reprezinta o astfel de sursa cinematica complementara, prezentand avantajul stabilitatii si predictibilitatii directiei de curgere.

Turbinele imersate in zonele de coasta, chiar daca sunt mai putin eficiente decat turbinele eoliene (in special datorita diferentei dintre densitatile celor doua fluide de lucru: aer siapa), pot forma insa retele mult mai dense decat cele ale fermelor eoliene, crescand astfel eficienta instalatiei.

Exista si alte tipuri de dispozitive proiectate pentru colectarea energiei cinetice a valurilor (fig.2) care sa afla inca in stadiul de demonstrator, nereusind sa depaseasca pana in prezent pragul de eficienta si rentabilitate pentru implementarea lor pe scara larga.

Impactul instalatiilor mareomotrice asupra

mediului Desi capacitatea de utilizare a energiei mareelor

exista de secole totusi eficienta medie a conversiei energetice este moderata fiind asteptate imbunatatiri prin proiectarea asistata a formei noilor turbine, progresele inregistrate de stiinta materialelor si printr-o amplasare judicios determinata a retelelor de turbine subacvatice (fig.7 a, b, c, d).

Oricum, sistemele de captare a energiei mareelor (mareomotrice) nu prezinta doar avantaje,

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Ocean Energy - Tidal Systems

of the tidal system –presently being obvious that sometimes fishes are fatally fascinated by the tur-bine vortexes. Dams and turbines separates estuary from the rest of the body of water and changes sa-linity of habitat. Researchers found that only after

10 years, species showed signs of adaptation to

new conditions. Some types of turbine have sonar detectors; when dolphins come by they shut off.

Iconography

www.marineturbine.com www.space.dtu.dk/English/Research/

Scientific_data_and_models/Global_Ocean_Tide_Model.aspx

http://.interestingenergyfacts.blogspot.com/2008/03/ocean-energy-facts.html

http:\\hydrovolts.blogspot.com\2009_04_01_archive.html

fenomenele secundare inregistrate in timpul exploatarii acestora necesitand solutii imediate deoarece pe termen lung atat ecosistemul cat si

configuratia fizica a zonei de coasta pot fi afectate. In spatele turbinelor imersate apare vortexul a carui

directie, daca nu este controlata, poate afecta

configuratia fundului oceanului (eroziune) punand in pericol chiar si integritatea retelei de turbine. De asemenea, cercetatorii au stabilit ca este necesara o perioada de cel putin zece ani pentru reechilibrarea ecosistemului dupa amplasarea unor astfel de constructii, unele elemente din fauna sau flora acvatica fiind de nerecuperat pentru aceste zone.

Bibliography

Bahaj, A S and Myers, Fundamentals of marine current turbines, Renewable Energy 28, L E (2003)

α. Horizontal Axis Turbine

β. Turbina cu ax orizontal

b. Vertical Axis Turbine

Turbina cu ax vertical

Power: 750 kW – 1.5 MW - Putere:750 kW–1.5 MW  

Characteristics: 15 – 20 m rotors 10 – 20 RPM - Caracteristici: 15 – 20 m (diam.rotor) 10 – 20 RPM

Deployed in multi-unit farms or arrays Amplasate ca ferme sau retele de multiunitati

a. Individual tide turbine arrays - Aranjament de turbine individuale (pentru maree)

c. Oscillating Hydro-foil - Hidrofolie

oscilanta

d. Venturi effect - Efect Venturi

Figure 7 Tide Energy Conversion Technologies

Figura 7 Tehnologii de convesie a energiei mareelor

b. Tidal Turbine Farms - Ferma de turbine pentru maree  

Figure 8 Tidal-current turbine arrays

Figura 8 Aranjamente de turbine pentru maree

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Ocean Energy - Tidal Systems