MARINE CURRENT TURBINES AN EFFICIENT RENEWABLE SOURCE OF POWER

MARINE CURRENT TURBINES AN EFFICIENT RENEWABLE SOURCE OF POWER

Just as there were a no. of water mills across several countries in the medieval period that had helped power the rural economy and then the industrial revolution. And just as wind power was taken to be a savior for greener energy, this new technology known as Marine Current Turbine is posed to power the world in the 21^st century. This paper essentially deals with this new form of power generation as an effective alternative to conventional sources of power. These turbines are cost effective as they employ reinforced plastics as the building material. This form of power generation is more reliable than its counterpart on land which, is of course, wind power generation. This form of power generation is not affected by tides, cyclones and tsunamis since the operation is usually done in continental reef region of the sea. Further, its operation has been declared successful in the North Sea. Further onsite testing of the prototype is done in and around Sweden and Britain. The power generated from this source can also be used for desalination of sea water for domestic supply.

KEYWORDS

PROTO TYPE

TIDAL TURBINE

SOLAR POWER

WIND MILL

PLASTIC BASED MATERIAL

MARINE CURRENT TURBINES

INTRODUCTION:

During the past few decades the advanced technology nations of the world have been engaged in an energy and resources race that has brought us to the position of a global energy crisis. The most intriguing fact is that even the so-called third world countries have also caught up with this race and the situation is getting worse than ever. 'This has brought us to a very strange scenario that makes us believe as if our dooms day is not far off. To prevent this avoidable mishap, many scholars have taken up research to find greener sources of power which has brought us to the cross-roads of technologies. To keep pace with the ever increasing power demand and detoriating environmental conditions and depleting resources we have developed many promising new ideas some of which are solar power, wind power, tidal power, fusion energy, geo­thermal power, ocean thermal power, MHD, etc.

The most promising of these is however a radical new look to the older watermills known as Marine Current Turbine (MCT) or Tidal Turbine. We are going to see how this new technology could be used to produce a greener source of power, in this paper.

MARINE CURRENT TURBINE:

Harnessing power from flowing water in the 21 S' century may be anything between one & two decades behind wind power, but the technology has moved beyond the conceptual stage, has shown that it works in pilot installations and is poised to progress to initial commercial exploitation. Companies in the forefront in this field have already commenced the construction of prototypes in many parts of the world & even a prototype in northern Norway has begun production on commercial lines. This paper essentially deals with the principles &various bottle necks in establishing economically & ecologically profitable operation of these MCTs.

PRINCIPLE:

MCT basically involve the same concepts & principles involved in the construction of wind turbines. The Kinetic energy of per unit volume of water is 'h pv² (p is the density of water molecules, v is their velocity).For a surface area A, this energy means an energy of 1/2 pv²A per unit distance in the direction of current. The energy is converted along the direction of v This means the energy per second i.e., power is '/2 pv³ But one thing we can be sure of is that not all this energy can be extracted .To extract all the energy the current velocity has to be brought to zero. This would in turn mean an accumulation of water around the turbine which is something next to impossible. Conventional turbines can be made to operate with an efficiency anywhere between 35-45 % which would mean an extraction of .45 Kw/m² for a current velocity of 10 mands which is the current velocity at its mean.

WIND VERSUS SEA:

We can definitely say that both wind flow & underwater-current flow have much to share as common-properties. Both follow the laws of fluid mechanics. Both convert kinetic energy from a moving fluid (or gas, in air's case), first into mechanical energy and then into electrical energy. Both have a rotor with two or more blades driving a generator via a shaft and a gearbox. Most `windmills' and so far `watermills' too, are horizontal -axis machines. In both cases, the cube law operates, power extractable being proportional to the velocity of the fluid medium passing through the rotor, raised to the power of three (v³). Although the rotors may visually suggest scaled air-craft propellers, they turn relatively slowly are driven by the, flow rather than driving it, and operate in very different order of flow speed. One similarity however, is that it is important to optimize the blade's aero/hydro-dynamic shape for efficient operation. Both wind and water rotors require pitch control so that their blades can be adjusted to flow conditions. Both also need support structures so that the rotors can be positioned in a favour regime. In short a very simple overview may give us the fact that MCT are nothing but new avatars of successfully proven models of wind turbines.

BOTTLE-NECKS:

An irrefutable fact is that there are also many great differences between wind and MCT. One is that marine current turbines are designed to capture the flow in just two directions. Since tides are lunar driven and reverse their direction every six and a quarter hours, rotors may simply be reverse to face the new current, or accommodate the change with a blade pitch `reversal'. Wind rotors, on the other hand, must generally yaw so as to face the wind from any direction.

The most striking dissimilarity, though, is due to massively higher density -832 times that of air. In the long, slender blades of wind turbines, centrifugal forces dominate and these tend to restrict bending. In water-turbines, however because they operate in a much denser medium, bending ' loads dominate, and these tend to bend the blades of the turbines. For example, a 1 MW tidal flow machine might have anything up to 100 tones of thrust on it, about double that it would be for a similarly rated wind machine. To keep these bending loads manageable, marine turbine blades are relatively short and flat. Because marine rotors are of limited diameter and rotate slowly, there is little centrifugal force to react the bending.

Further , water flows at a lower rate than wind -for example, a MCT might be rated to a maximum power in a current of 2.3 m/s (or 4.5 knots), compared with a typical 12-13 m/s flow for wind. But this in no way compensates for the density difference. Whereas a 1MW water turbine would have some 30-40 tonnes of air pass through the cross-section of its rotor when at full power, a 1 MW water turbine operating at 4.5 knots tide sees over 900 tonne of water pass through its rotor. Designing to accommodate these flow rates over a lifetime of 20-30 years is a challenge. Structures for

marine turbines must also be stressed to withstand transient forces caused by turbulence as by passing surface waves.

MATERIAL CHOICE:

Sub-marine structures also have to withstand the notoriously aggressive marine environment with its corrosive salt water, fouling growth and abrasive suspended particles. Designers first considered producing the required stiff, unyielding marine rotors in steel. However, achieving the necessary compound-curved profile in steel proved to be expensive. Moreover steel is very heavy, prone to fatigue and susceptible to corrosion induced by salt water. These disadvantages prompted a decision to adopt composites instead. Plastic-based materials ease the fatigue problem, both through their inherent fatigue tolerance and by reduced blade weight. Calculations also showed that appropriately applied, they could deliver the required stiffness. However, some wind blades rely solely on the form of stiffness of a sandwich envelope structure, a robust internal 'skeleton' was considered essential for their marine counterparts.

A two-blade configuration was chosen for the l lm diameter rotor of the Sea flow prototype. This was partly for practical reasons -a two-blade rotor attached to the turbine shaft and drive train forms a flat 'T' that can be readily laid on the deck of an installation barge and handled by cranes .

A RADICAL NEW METHOD

The blades can be built so as to derive most of their strengths from a robust carbon fiber main spar that can be bonded with a glass-fiber composite envelope that will give the blade its hydrodynamic shape. Carbon-reinforced ribs, attached perpendicularly to the spar, will help in transmitting loads from the load. Here glass is chosen as it is impact resistant (also shatter-proof) and affordable. Also foams impregnated with carbon fibers & epoxy cloths can be used for the same purpose.

Keeping blistering at bay for longer periods is a very difficult task for which high quality laminate with low void content and incorporating non-hydrophilic epoxy resin is a good starting point, while innovative finishes that also resist marine growth are being considered.

THE POTENTIAL:

This new technology has a lot of potential to light the entire planet with the power it generates. As we all know ours is a blue planet with a lot of potential places where such MCT can be established. For example an assessment study conducted by a private firm in and around Britain has found that the country has got more power than it could ever consume. Such a study along the coastal creeks and under­water hills around our country could easily establish the fact that even we've got equal potential. Since this source of power doesn't consume any kind of fuel, operating these turbines will also prove to be much more economical.

ECOLOGICAL IMPACT:

This technology represents a novel method for generating electricity from a huge energy resource in the sea. It is rare enough for an entirely new energy resource to be developed but even rarer if the technology, as in this case:

• produces no pollution and has negligible environmental impact
• delivers energy to a predictable timetable
• has the potential to make a major contribution to future energy needs

Although the relentless energy of marine currents has been obvious from the earliest days of seafaring, it is only now that the development of modern offshore engineering capabilities coinciding with the need to find large new renewable energy resources makes this a technically feasible and economically viable possibility.

The rationale for developing this business is based on three robust arguments:

• The world needs more energy, yet this will have to be clean renewable energy in line with the Kyoto Protocol and "Agenda 21" (most governments world-wide are committed to this) and Marine Current Turbines can deliver this

• The scope for meeting future energy requirements solely from land-based resources will be constrained by conflicts over land-use; so large renewable energy projects will need to move away from crowded land areas - it is significant that the wind

industry is also moving offshore, yet many potentially energetic marine current sites are not far from large electricity markets

• the nearest competitor is wind-electricity generation, which has grown from almost nowhere ten years ago, into a multi-billion dollar industry; the marine current energy resource has the potential for exploitation to equal or exceed wind energy in its future importance
• MCTs do not emit or produce any greenhouse gases, harmful waste, or pollutants during operation (e.g. C02, NOx and SOx).

• Compared to offshore wind farms visual disturbance at sea will be low. The only indication of the presence of MCTs would be the surface piercing structures with navigational-buoyage equipment, and this will be located some distance from the coastline.
• The low frequency sub-sea noise that would be emitted during operation would not interfere with the echolocation capability of cetaceans and also serve to warn marine mammals to keep away.
• The minimal effects from installation are only temporal and any sea life displaced would soon return.
• As fishing and recreational and commercial access within MCT zones would be prohibited, schemes may act as protective man-made reefs in which wildlife may flourish. This could bring serious benefits to endangered species and fragile fish stocks.

Environmental Impact Analyses completed by independent consultants have confirmed our belief that the technology does not offer any serious threat to fish or marine mammals. The rotors turn slowly (10 to 20 rpm) (a ship propeller by comparison typically runs 10 times as fast and moreover our rotors stay in one place whereas some ships move much faster than sea creatures can swim). There is no significant risk of leakage of noxious substances and the risk of impact from our rotor blades is extremely small bearing in mind that the flow spirals in a helical path through the rotor and that nature has adapted marine creatures so that they do not collide with obstructions (marine mammals generally have sophisticated sonar vision). Another advantage of this technology is that it is modular, so small batches of machines can be installed with only a small period between investment in the technology and the time when revenue starts to flow. This is in contrast to large hydro electric schemes, tidal barrages, nuclear power stations or other projects involving major civil engineering, where the lead time between investment and gaining a return can be many years.

Further, independent analysis by American Fisheries Society which earlier had some reservations regarding tidal power generators, has given an almost clean chit for this type of projects. Another encouraging fact is the potentially less retention of sea water when compared to that of tidal water turbines.

CONSTRAINTS:

• The main visual disturbance would be land-based electrical substations and over­head lines.
• Public may be concerned with the risk of the turbine rotor colliding with fish or mammals. However this is unlikely because small and medium sized creatures would tend to be pushed out of the way by the blades (due to fluid dynamics) and larger mammals, such as seals, whales and dolphins, are known to avoid ships and ships propellers and could be expected to exhibit that same behavior with MCT. Also, a ship's propeller, which is accepted without question for widespread use at sea, is a far greater hazard as it rotates at far greater speeds, e.g. up to 10 times faster, and it is attached to a moving vessel that may travel faster than the fauna.
• Seismic surveys and blasting prior to installation could create frequency waves that may have negative effects on the health of fish and marine mammals.

• Installation of cables may also cause sediment displacement, which in turn could cause dangerous particles to be dispersed and contaminate the local environment e.g. radioactive particles discharged from Dounreay. Proposed sites would have to be analyzed for contaminants.
• During operation of the MCT, the pile would experience increased scouring around its base caused by increased turbulence affecting the seabed morphology.
• Possible discharge of oil from the gearbox or the anti-fouling paint used to prevent bio-fouling may pollute.
• Current flow and coastal processes could be affected because an oversized scheme could act as a barrier to the oncoming current. Attempting to extract too much energy from a tidal stream could cause the tide to divert to an easier path.

The exact environmental impact of a Marine Current Turbine has not '!et been assessed therefore it is not possible to quantify the exact impact an array of turbines may have on the ecology, environment and existing users. however. installing a monopole -mounted to-meter diameter turbine in water up to 50 meters deep would inevitably have some impact, possibly negative or positive on the surrounding environment. The level of impact would be determined by the quantity of units installed and the packing density.

The second strategy, which selected the maximum number of units that could be installed thus maximizing the storage required, would result in maximum environmental impact. The first strategy, which compromised the number of installed units with the aim of reducing the storage required to an acceptable level, would have a less impact.

The technology used to secure the turbine to the seabed may also have differentimpacts. For example, the proposed first-generation of MCT uses mono piles which could destroy seabed habitats during installation. A second-generation design could use mooring systems, which may have less of an effect on seabed wildlife whilst allowing the turbines to operate in deeper waters.

The density in which units are installed may also influence any environmental effects. For example, drilling monopoles, of a significant distance apart may reduce impact of erosion caused by scouring around the base of the monopoles on wildlife. The MCT parametric model and packing density selected in this project means that across the tidal flow, turbines with diameters of 15.85-metres would be spaced out some 60-metres apart.

This would leave a minimum gap of 44 meters from blade tip-to-tip. The turbines would be positioned 1000-metres downstream from each other in order to reduce the negatives effects on performance caused by turbulence and allow for the tidal streams to restore themselves, The horizontal and axial spacing would result a packing density of 18 units per km2, which could allow for tidal stream, the various wildlife and larger mammals to pass through unaffected.

The layout of arrays of MCT could also influence the impact. For example, rows of turbines located besides each other may act as a larger barrier to oncoming tidal stream compared to farms where the turbines are installed in scattered arrays.

Waters of depths from 20 to 80 meters chart datum would be used; therefore any wildlife or marine activities using water in depths outside this range would not be directly affected.

CONCLUSION:

With increasing pressure from the environmentalists and acute shortage of power due to uncertainty in the political status of the middle-east, the world cannot simply thrive on the hope that these conventional sources of power will continue to supply all our needs for posterity. In the wake of this crisis we can be sure that this new MCT technology will power the world in the near future as is the thermal power in the present day world. For, no one believed that day, when Faraday told that we can collect revenue by generating power. It is because of these new and innovative ideas that the present world is as it is today. The day is not far away, for