Wave Energy Extraction
- Wave Energy Basics
- Development of the “Coordinated Floating Energy Equipment” - (COFEQ) system started with the layout of a water wave as shown by FIG-2 Wave Energy Extraction, Schematic.
- Waves undulate with a large quantity of water flowing up and down at low variable head following wave undulations. The wave energy is manifested in potential and kinetic forms that are equivalent and constantly interchanging.
- A “buoyant float” (BF) at the water surface would act as point absorber, follow wave undulations and develop an upward uplift force in tons equivalent to its volume of the (BF) when completely submerged during the first quarter of the wave cycle.
- Transferring the (BF) uplift to a small diameter reciprocating water pump would raise a small quantity of water to a much higher head fifty (50) meters and above, depending on pump diameter.
- Four (4) floats are provided in line with the incoming wave to extract the wave energy.
- Wave energy is estimated at P = 0.5xH^2xT where P is power in KW, H is the wave height in meters and T is the wave period in seconds.
- The characteristics of the wave are complex and vary from site to site with respective variable heights and duration depending on location and sea state. For this a sine wave is chosen to approximate the wave characteristics to develop the approximate energy extraction curve.
- Wave extracted energy depends upon wave height; while extracted power depends on wave height and period.
- Small scale prototype for testing in a wave tank does not represent system performance. A full scale model of a minimum two (2) sets each consisting of four (4) pumps or generators to be installed and tested at actual project site
- Wave Energy Extraction
- “Floating Construction Unit” (FCU)
- A “Floating Construction Unit” (FCU) measuring 24.5 x 22.5 meters and a height of 14.5 meters is provided to handle ten (10) sets of “buoyant float” (BF)’s, each set consisting of four (4) (BF)’s, installed above the crest line of the wave at the (FCU) deck, to directly drive the following equipment:
- Electric generator to provide AC electric power, convert to DC and feed a common DC bus.
- Rotary water pump to pump a small quantity of water to a high head, collect it into a closed circuit piping system and feed it to a hydro-turbo generator to generate electric power.
- An air/water reservoir partially filled with water is provided to compress the air in the reservoir during the front side of the wave and to discharge high pressure water into the closed circuit water piping system during the lee side of the wave; thus reducing pulsating water flow into the hydro-turbo generator and the resulting power ripple.
- Make-up water is provided by extending a pipe between the suction side of the closed circuit piping system and 50 cm below the wave trough line level.
- The (FCU) provides enclosed dry space and supports the generators, the water pumps, the piping system, the hydro-turbo generator and associated electrical protective and control equipment.
- The (FCU) deck is installed at elevation six (6) meter elevation above the “Mean Sea Level” (MSL) to handle 8m high waves (4m amplitude) including level variations of plus or minus 1m due to ebb/tide (MSL) level variations.
- Fixed lateral and cross floats are installed below the wave trough line provide sufficient uplift force to support the (FCU) and respective equipment.
- The (FCU) is the building block for the (COEE) system. It is assembled at the shoreline, floated and towed to location and moored to the sea bed with all equipment ready for operation.
Buoyant Float (BF)
- The (BF) is a hollow cylindrical shape with a 150 cm diameter and a height of 150 cm, when fully immersed develops a gross uplift of 2.65 tons.
- An alternate variable (BF) diameter of 200 cm for the first (BF) with increasing diameter for each succeeding (BF) by ten (10%) and keeping constant (BF) volume, would extract energy from small height waves; as low as 50 cm high. However, the energy extracted from 50 cm high wave is negligible.
- A power reversing pulley and a power pulley with a continuous “pull line” (PL) fixed to top and bottom of the (BF) converts the gross uplift of 2.65 tons to a torque at the power pulley to rotate the water pump or electric generator to pump water or to directly generate electric power.
- Thread the (PL) through a guide pipe installed as part of (BF) to limit horizontal sway of the (BF).
- The wave height after the first (BF) would be lower, as some energy was extracted by the first (BF).
- The (BF) extracts energy during a period less than quarter of the wave period. Therefore, the wave energy extraction is intermittent and is zero for ¾ of the wave period.
- The layout of the (BF)’s restricts the surface area and attenuates the wave height, resulting in higher extraction efficiency.
- The speed of vertical movement of the (BF) is variable due to the characteristic curve of the sine wave.
Wave Lee Side and Tidal Energy
- A hydro-turbine with vertical shrouds is installed below the trough line of the wave extracts energy from the lee side of the wave when the water flow is downward. The effect of the upward water flow is negligible.
- Two additional horizontal shrouds are provided to channel the ebb/tide water flow through the wave lee side hydro-turbine to drive it in unidirectional rotation and extract ebb/tidal energies.
- The combined rotation of wave lee side and ebb/tidal water flow is transmitted via a rod to a step-up gear located the (FCU) deck to drive an electric generator or a water pump.
Putting it All Together
The system was originally developed as “Coordinated Offshore Energy Extraction” system (COEE) for extracting renewable energies form: waves, tidal, solar, thermal, and wind.
With slight modifications, by adding fixed floats the (COEE) system is used as a floating support for offshore wind turbines.
The fixed floats are used as compressed air storage reservoir. This is enhanced by adding floating compressed air reservoirs attached to the lee side of the (FCU)’s.
For a 100MW wind farm using 3MW wind turbines, six (6) (FCU)’s are required to give a wind turbine spacing of 147m, and eight (8) (FCU)’s are required to give a wind turbine spacing of 196m when using 7MW wind turbines.
Compressed air storage and areas for solar system are shown as follows for a 100MW offshore wind farm.
Wind Turbine Rating No. WT No. FCU Uplift tons Solar sqm C. Air cum
3MW WT 33 6 704,732.53 266,805 898,773
7MW WT 14 8 109,926.73 150,920 509,687
Compressed Air Reservoir
- A 100MW wind farm provides compressed air storage reservoir of 898,773 to 509,687 cubic meters for 3MW to 7MW wind turbines respectively, encourages its utilization as “Compressed Air Energy System” (CAES) developed by others for use with a power plant.
- The above modifications led to changing the name of the system from (COEE) to (COFEQ) as described in the heading.
A 100MW wind farm provides an area for solar energy equipment between 266,805 and 150,920 square meters for 3MW to 7MW wind turbines respectively.
The DC output of the solar system is conditioned and fed into the common DC bus at the (FCU).
Floating Wind Turbines
- Two rows of (FCU)’s are installed with alternating active and dummy floating (fcu)’s to give a foot print area of 1,103 square meters (2x 24.5x22.5).
- Fixed floats are added below the trough line of the wave to provide for each active and dummy (FCU):
- Cross floats; 5x (24.5x1x1) provide an uplift of 122.5 tons.
- Lateral floats; 9.5x (2x (22.5x2x4) provide an uplift of 3,420 tons.
- The total uplift of 3,543 tons for the active and dummy (FCU); that is; 3.21 ton per sqm, partially support the floating wind turbines and energy extraction equipment.
- An additional uplift of 101 to 226 tons prorated per (fcu) from 16.76 to 28.27 are provided for 3MW to 7MW wind turbines respectively, by extending the wind turbine support 8m below the trough line of the wave..
- These fixed floats and wind turbine floats provide compressed air storage volume of 3,559 to 3,571 cubic meters for 3MW and 7MW respectively. The resulting uplift is 3.2 tons per sqm.
- Floating wind turbine spacing is 147m that is; six (6) active and dummy (FCU)’s) for 3MW and 196m spacing for 7MW; that is eight (8) active and dummy (FCU)’s).
- A file with multi spread sheets is developed as follows:
- Page-1 is provided to enter all possible variables of the system as shown with shaded background for wave characteristics; wave heights, buoyant float size, water pump size, (MSL), wave period, MSL level, materials and working head. Sample calculations are provided foe fixed and variable floats float diameters from 20 to 200 cm.
- The uplift force developed by the (BF) should accelerate the weight of the (BF) moving parts and the weight of the water in the pump and develop the total dynamic head (TDH) of the system.
- Page-2 automatically computes the respective response of the system in response to the data entered in Page-1. The formulae are locked to prevent inadvertent entry of data. It computes the incoming wave energy, the water pumped and respective head, the (BF) movement, the energy extracted by each (BF) 1 to 4, individual and total power output in KW for different wave heights for each pump or generator and for the total power for pump sets 1to 4.
Energy and Power Output
The energy and power output per one (FCU) is as shown in the following table-1
Foot Print Peak Output Hour Output/FCU
System Description SQM meters KW/FCU Year KW-H/Year
Wave energy Ø-2m Variable float, 24.5x22.5 1,113.0 (1) 8,700 864,000
Tidal energy, eight (8) Ø-2m 8x4 KW 32.0 7,300 233,600
Solar energy-2 (fcu), 2x24.5x22.5 1,102 330.6 2,920 965,352
Solar energy, floating float 245 73.5 2,920 214,620
Wind turbine 3MW/ 6 FCU 6,615 500.0 3,000 2,250,000
- The above output data are for wave characteristics shown on page-8/page-1 and for the sea state shown on page–9/page-2.
- The peak output per (fcu) varies between 5 and 1,113 kw for wave heights between 0.75 and 8 meters respectively. The output of 1,113 kw is for 8-m high wave.
- The average continuous output for the whole year per (FCU) for is:
- For wave energy extraction 9.863 kw.
- For the whole system (COFEQ) is 520.41 kw.
- The wind turbine output per (fcu) is shown for a wind speed of 12 meters per second and using six (6) (fcu)’s for a 3MW floating wind turbine.
- The output per (fcu) of 4,527,572 kw-hours per year is for the sea state shown on Page – 9, wave durations per year. The actual kw-hours per year per (fcu) depends upon sea and wind states and the available hours of sunshine per year.
Using water pumps results in low system efficiency due to:
- Hydraulic losses in the water pump, in the closed circuit piping system and in the hydro-turbo generator. This choice of system leads to reduced efficiency and the use of an electric generator is proposed.
- The (COFEQ) system looks like an offshore wall far away from the shoreline.
- Openings in the (FCU) are be provided to reduce horizontal wind forces.
(COFEQ) System Advantages and Improvements
- The system uses simple engineering principles and equipment developed by others that are readily available on the market with proven quality and performance.
- A fully coordinated offshore energy extraction (COEE) system for extracting renewable energies from: waves, tidal, solar, thermal and wind.
- The roof of the (FCU) is used as support for solar energy. Battery storage system may be installed at the (FCU) deck to provide energy storage and continuity of DC supply.
- The system (FCU) is assembled on the shore line, floated, towed to location and moored to the seabed with all equipment ready for operation.
- The system provides free floating supports for offshore wind turbines, floating bridges and floating power plants.
- The relocation of the step-up gear from the nacelle to the bottom of the support of the wind turbine, greatly reduces bending moments at the base, reduces construction, operation and maintenance costs,
- The (COFEQ) system provides fixed and floating reservoirs for compressed air energy storage in excess of 898,773 to 509,687 cubic meters respectively using 3MW or 7MW wind turbines for 100MW offshore wind farms.
- The system provides a floating area ranging between 266,805 and 150,920 sqn for a floating solar system.
- The (COFEQ) provides a floating bridge suitable for vehicular traffic for ease of construction, operation and maintenance.
- The (COFEQ) provides for floating power plant installation.
- It provides suitable lodging for operation and maintenance personnel.
Extending to a 100MW Offshore Wind Farm
- The system provides six (6) (active and dummy FCU)’s for 3MW wind turbines and eight (8) (active and dummy FCU)’s for 7MW wind turbines.
- The (COFEQ) system provides an area for solar energy extraction of 266,805 to 150,920 sqm when using 3MW and 7MW respectively.
- The (COFEQ) system provides compressed air reservoir storage capacity between 898,773 and 509,686 cubic meters for 3MW and 7MW respectively.
- The freely provided volume of 898,773 and 509,686 cubic meters reservoir capacity for a 100 MW offshore wind farm, entices developers and owners to use this facility to provide a 100MW gas-turbo power plant with "compressed air energy storage" (CAES) to operate at higher efficiency and provide sizeable energy storage facility. This is achieved due to reduced power needed to compress gas-turbine inlet air.
- Installing an offshore floating 100MW power plant would provide security to the project site and would free the project from political and local influence that may contribute to the failure of the project.
- The above information tend to favor the use of 3MW wind turbines, due to more surface area for the solar system and a bigger compressed air storage reservoir.
Return on Investment (ROI)
- The challenge for owners and developers is to request Developers and Contractors to provide an Alternate Bid based on the (COFEQ) as herein described. Tis would provide a practical way for Bid Analysis and choice of an improved system.
- The cost of the project and the income based on the electrical energy produced per year and the sale price of one (1) KW-Hour would determine the Return on Investment, which is estimated above fifteen (15%) percent.
- The project is practically self- financing.
Shamil Ayntrazi, PE
Bsc. Elect'l-AUB, MBA-NY,
Registered Prof. Engineer, Lb. & N.Y
Hion Joon Kim
<!-- Start of StatCounter Code -->
class="statcounter"><a title="web stats" href="http://statcounter.com/"
src="http://c.statcounter.com/2408527/0/bba82f8c/0/" alt="web stats"
<!-- End of StatCounter Code -->