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M agazine agazine Andreea Samoila Politehnica University, Bucharest, Romania
[email protected]
Ocean Energy Conversion C aptarea aptarea energiei oceanelor Tidal Systems
Sisteme mareomotrice
Water covers more than 74% of the Earth’s surApa acopera mai mult de 74% din suprafata face gathering enormous Renewable Energy re- Pamantului, insumand cantitati impresionante de sources and offering the marvellous blue sight of the resurse energetice regenerabile si in acelasi timp planet –viewed from the space (fig.1) dand Terrei minunatul aspect de planeta albastra – There are moving water resources (ocean waves , vizibil din spatiu (fig.1). In prezent sunt identificate, tides, ocean and studiate si river currents ) experimentate, as well as noncel putin pana moving water la stadiul de r e s o u r c e s demonstrator, (offshore wind, tehnici de offshore solar, captare a ocean thermal, e n e r g i e i salinity gradient, cinetice a apei marine biomass, (valurile oceanelor , geothermal). m a r e e , C o n s e quently, differcurentii din rauri si mari ) ent types of applications have precum si a Wave attenuator principle Wave terminator principle been designed Principiul atenuatorului de unda altor tipuri de Principiul capatului de unda for waves e n e r g i e (terminators, atregenerabila tenuators, point a s o c i a t a absorbers, etc) intinderilor de and for currents ape (curentii de (ocean or river) coasta, energia or tides (fig.2 a, solara din zona b, c, d). de coasta, Even if tides e n e r g i a were observed, Point absorbers Tidal energy harvesting principle c a l o r i c a , Puncte absorbante Principiul colectarii energiei mareelor e n e r g i a managed and used as power rezultata din Fig. 2 Applications for f or ocean waves and currents source, in accord gradientul de Fig. 2 Aplicatii pentru utilizarea valurilor si curentilor oceanului with the technosalinitate, logical state –of-the art of different periods, a more biomasa, energia geotermala din zonele active ale effective energy management of this complex phe- oceanului planetar). nomenon became possible only recently, as a result Chiar daca mareele au fost observate, stapanite si of the complex studies regarding mapping, model- utilizate ca sursa de energie- conform nivelului ling and simulation. If for the vertical move dams tehnologic al diverselor epoci- exploatarea eficienta and estuary infrastructure were already used, for the a acestei resurse naturale a devenit posibila doar in tide currents’ conversion there are used horizontal ultimii ani, ca rezultat al studiilor complexe care
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M agazine agazine Ocean Energy - Tidal Systems Tidal Systems or vertical axes turbines or newly designed devices acting as turbines.
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.
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). Istoricul si evolutia That makes tides to be reliably predicted for years to captarii energiei mareelor Energia mareelor este centuries ahead. As a physical principle tidal power facilities determinata de miscarea harness the energy from the maselor de apa sub influenta rise and fall of the tides- as fortei centrifuge – ca efect al vertical motion, and from the miscarii de rotatie a double-sense daily horizontal Pamantului- si de atractia water currents flow detergravitationala a Lunii (ca mined by tides (flood currentssursa primara) si a Soarelui moving in the direction of the (ca sursa secundara) (fig.3 a, coast- and ebb currents, the b, c). Aceasta face ca mareele currents receding from the sa fie predictibile, putand fi coast). calculate pentru periode de So, the existing two high Figure 3 Gravitational and Centrifugal ani si chiar de secole. and respectively two low tides, Principiul fizic al conversiei force influence are related to the daily Earth energiei mareelor este relativ Figura 3 Influenta fortei gravitationale si a fortei centrifuge rotation and positioning tosimplu: se capteaza energia ward the Moon, while the so called Spring and cinetica a maselor de apa aflate in miscare (fluxNeap tides are related to a more complex relation- reflux) atat pe directie vericala cat si pe directie ship established among the force fields of the Sun, orizontala si se transforma in energie electrica prin Moon and Earth. intermediul diferitelor tipuri de turbine. The highest one, the Spring tide, occurs when Prin urmare exista doua maree inalte si doua the Sun and the Moon line up with the Earth – ei- maree joase, care sunt in stransa legatura cu ther on the same side or on opposite sides (fig.4); miscarea de rotatie a Pamantului si cu pozitia consequently, the lowest tide, the Neap, occurs Lunii; in timp ce mareele extreme, numite mareea when the Sun and the Moon are at 90 deg related to vie si mareea moarta (in functie de amplitudinea the Earth (fig.4).The construction principle of a lor) sunt rezultatul interactiunii campurilor fortelor tidal system is rather simple, as long as either a dam gravitationale ale Lunii si Soarelui asupra vie) or a section of an estuary are going to run as a reser- Pamantului (fig.4). Fluxul cel mai inalt (mareea vie) voir in the action area of the tide. se produce atunci cand Soarele si Luna se gasesc There are two moments of the process: the pe aceeasi dreapta cu Pamantul, fie de aceeasi flooding one- when the highest level tide floods and parte, fie pe parti diferite (fig.4). Refluxul cel mai moarta)) se produce atunci cand fill the reservoir having only one –way access gate; scazut (mareea moarta and as soon as the water withdraws at the lowest Soarele si Luna formeaza un unghi de 90° fata de tide level the existing water from the reservoir is Pamant. released toward a water wheel. Principiul constructiv al unui sistem de captare a For harvesting this type of energy, the ideal mareelor este relativ simplu, atat timp cat un
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M agazine agazine Ocean Energy - Tidal Systems Tidal Systems 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 typical energy conversion installations consists of tidal dams and underwater turbines grid structure. The global potential of such energy harvesting systems (only from tides) is about 1800 TWh/ year, and it is meant to remain 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 effective alternative to the river water mills. Several installations, 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, modelling, 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 order to capture tide energy having at least 50% efficiency 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
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M agazine agazine Ocean Energy - Tidal Systems 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 harvesting 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 harvesting is the same for centuries, and the average efficiency of the mechanical tide energy conversion on electricity is not surpassing yet 50% signifytypes of devices for the waves and currents harvesting 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 harvesting 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 advancements in material science and proper distribution 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 ecosystem 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 utilizarea mai 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 printro 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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M agazine agazine Ocean Energy - Tidal Systems Tidal Systems of the tidal system –presently being obvious that sometimes fishes are fatally fascinated by the turbine vortexes. Dams and turbines separates estuary from the rest of the body of water and changes salinity of habitat. Researchers found that only after
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
a. Individual tide turbine arrays Aranjament de turbine individuale (pentru maree)
α. β.
Horizontal Axis Turbine Turbina cu ax orizontal
b. Vertical Axis Turbine Turbina cu ax vertical
c. Oscillating Hydrofoil - Hidrofolie oscilanta
d. Venturi effect Efect Venturi Figure 7 Tide Energy Conversion Technologies Figura 7 Tehnologii de convesie a energiei mareelor
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
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 b. Tidal Turbine Farms - Ferma de turbine pentru maree Figure 8 Tidalcurrent turbine arrays Figura 8 Aranjamente de turbine pentru maree
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 Fundamentals of marine current turbines, Renewable Energy 28, L E (2003)
