LIFT & DRAG TYPE TIDAL STREAM TURBINES

   MANY QUESTIONS ARE BEING ASKED ABOUT TIDAL STREAM ENERGY AND THE VARIOUS TYPES OF TURBINE DEVICES BEING PUT THROUGH R&D PROGRAMMES TO EVALUATE THEIR POTENTIAL

 THE STANDARD UNDERSHOT WATERWHEEL, IN USE STILL AROUND THE WORLD  FOR SEVERAL THOUSAND YEARS IS A SIMPLE, EFFECTIVE , FAIRLY EASY TO MAINTAIN AND TO MATCH TO ANY POSSIBLE WATER SOURCE, COULD BE CALLED A DRAG FORM OF WATER TURBINE

( ALTHOUGH TECHNICALLY THE DEVICE HAS TO BE FULLY IMMERSED WITHIN THE DRIVE FLUID TO BE CLASSED AS A TURBINE.)
 

                            ( SORRY FOR THE QUALITY OF THE SKETCH ABOVE)
  
  THE WATER FLOW PUSHES AGAINST THE LOWER BLADES INDUCING ROTATION AROUND THE CENTRAL AXIS SHAFT .
  IT IS POSSIBLE TO ACHIEVE GOOD EFFICIENCY, AS SHOWN ABOVE, BUT THE DEPLOYMENT IS LIMITED TO THE VERY TOP SECTION OF THE WATER COLUMN.
THE DEEPER THE BLADES ARE IMMERSED IN THE WATER FLOW, THE GREATER THE DIAMETER OF THE CARRY WHEEL.  ( THE ADVANTAGE OF THE HALES TURBINE TYPE DEVICES WHICH OPERATE IN THE VERTICAL AXIS, ARE THEY CAN BE MODULAR STACKED TO FORM A TIDAL FENCE SYSTEM ETC.)

THE  ADVANTAGES OF DRAG TYPE TURBINES =:

 HIGH TORQUE OUTPUT FROM SLOW RPM ( FISH FRIENDLY).
 LESS EFFECTED BY MARINE GROWTH ON DRIVE FACES.
 LESS EFFECTED BY TURBULENT FLOWS ON DRIVE FACES.
 CAN BE OPERATED IN LOW WATER FLOWS.
 DUCTING AND VENTURI SYSTEMS CAN IMPROVE PERFORMANCE.

THE DISADVANTAGES OF DRAG TYPE TURBINES =:

 SLOW RPM REQUIRE HIGH GEARBOX RATIOS.
 SLOW RPM ALLOW GREATER MARINE GROWTH ATTACHMENT.

LIFT TYPES OF TIDAL STREAM TURBINES ARE WELL UNDERSTOOD BY THE COMPARING WITH STANDARD WIND TURBINES.
  THE BASIC THREE FORMS ARE PROPELLER, DARRIUS FOIL AND OSCILLATOR.

   
 

  JUST LIKE AN AIRCRAFT WING, THE PRINCIPLE OF LIFT IS WELL UNDERSTOOD, FLUID PASSING OVER THE  WING (FOIL SHAPE) TRAVELS FURTHER, HENCE FASTER ACROSS THE TOP THAN BENEATH, CREATING A UPWARD PULLING OR LIFTING EFFECT.


ADVANTAGES OF LIFT TYPE WATER TURBINES =:

 PRINCIPLE WELL UNDERSTOOD AND RECOGNISED.
 MANY FORMS AND DESIGNS POSSIBLE.
 HIGHER RPM ALLOWS DIRECT DRIVE GENERATOR SYSTEMS TO BE USED.
 DUCTING AND VENTURI SYSTEMS IMPROVE PERFORMANCE.
 OFTEN LESS CONSTRUCTION MATERIALS REDUCES COST.

DISADVANTAGES OF LIFT TYPE WATER TURBINES =:

  SIZE CONTROLLED BY  BLADE TIP SPEED  ( OFTEN CAVITATION ISSUES).
  MARINE GROWTH CAN GREATLY REDUCE LIFT EFFECT ON BLADES.
  WATER TURBULENCE CAN REDUCE LIFT EFFECT ON BLADES LIMITING SITES.
  OFTEN REQUIRE FASTER WATER FLOWS TO BE EFFECTIVE,  LIMITING SITES.
  CLEAVER ACTION OF MANY BLADES/FOILS NOT FISH FRIENDLY.
  HIGH STRESS LOADS ON SINGLE POINT FIXING HUB TO BLADE A WEAKNESS.
  

 THIS POST HAS BEEN MADE TO HOPEFULLY EXPLAIN SOME OF THE ISSUES AND CHALLENGES FACED BY TIDAL STREAM TURBINE DEVELOPERS IN TAKING THIS VAST  RENEWABLE SOURCE INTO THE  CONSUMER WORLD.