Wind versus Waves for Energy Conversion to Electric Power

This document highlights contrasting characteristics of Horizontal Axis Wind Turbines (HAWT) and Savonius Rotor Ocean Wave Turbines (SavRot) for conversion of wind and ocean wave energy, respectively. The assessment is heuristic and avoids technical details that are peripheral to the central issue of contrast between HAWT and SavRot in respect to size, operation and expense for energy conversion.

The primary requirement for electrical energy is in the vicinity of populated areas. The market for SavRot is restricted to the vicinity of coastlines exposed to wave action. Countries with coastal borders tend to have high population in close proximity to the water. SavRot systems are designed to operate in shallow water which avoids the material cost of submarine cable to deliver ashore electrical energy produced by most other ocean wave energy conversion systems.

The class of HAWT equipment used in this assessment is land based with nameplate capacity of about 1.5 megawatts (MW). This class is a primary fraction of the 50,000 installations in the U.S. Much larger systems are available but have not found favor in the U.S. market, presumably because owner/operators have not perceived economy of scale.

The 1.5 MW rated capacity for power conversion of HAWT systems is at a wind speed of about 30 miles per hour. As wind increases above the rating, HAWT power production holds constant, i.e., the additional energy is rejected. To avoid self destruction, rotation of the blades is stopped altogether at wind speeds approaching 50 miles per hour. The HAWT landmark embodies high technology, precision equipment, heavy construction, concentration of assets, and a hazard to birds in flight.

SavRot technology for ocean wave energy conversion uses multiple submerged rotor-generator assemblies arranged in arrays distributed in close proximity to the coastline. Arrangement and power production capacity is dictated by customer requirements, wave climate and environmental features. DC power from the generators is cable connected to shore stations that use equipment perfected by the wind industry to produce electrical power for delivery. The submerged rotor-generator assemblies embody low technology, simplicity, durability, redundancy, and reliable operation with no threat to marine life.

The equation for calculation of power generated by HAWT and Savonius rotors depends in part on density of the fluid multiplied by the projected area of the turbine that intercepts the flow. For HAWT, the applicable area is within the circle swept out by the three blades rotating on the spinner. There are several models of the 1.5 MW HAWT; for purposes of this discussion we take a swept area of 55,000 square feet. For the Savonius rotor, the projected area is the largest “wing” diameter multiplied by the axial length of the rotor. Water is about 800 times denser than air.

Power generation also depends on the cube of the air or water speed. Few sites are available in reasonable proximity to customers where wind speed is nearly constant at the nameplate velocity of 30 miles per hour. Similarly, orbital motion caused by shallow water ocean waves useful for SavRot is unlikely to exceed about 15 mph. HAWT begins to shed energy above 30 mph and is at risk above 50 mph; SavRot can be designed to perform and survive at and above 15 mph water current speed.

The remaining factor for estimating performance of the two systems is efficiency.  Peak efficiency of SavRot at 0.3 compares to HAWT at about 0.4. Efficiency variance with loading and flow speed are remarkably different. SavRot exhibits high torque at very low flow speed. HAWT typically requires a wind speed of about 5 mph to supply enough torque for self starting. It also must respond to change in wind direction; the SavRot response is independent of vertical arrival angle of orbital particle motion. SavRot efficiency and torque also have the property of falling to zero at a flow speed related to rotation rate that enables avoidance of self destruction when the rotor runs unloaded in high water current speeds.

The following statements are drawn from considerations using the factors described above. At sustained wind speed of about 25 mph the chosen HAWT delivers about one megawatt with its swept area of 55,000 square feet. At a sustained current speed of seven mph SavRot will use several rotor-generator assemblies with an aggregate projected area of about 2,700 square feet. Many of the operational functions performed by HAWT atop a 250-foot pedestal are performed by the SavRot system in easily accessible low-profile facilities ashore. Relatively simple interconnection of multiple rotor-generator assemblies, typically less than 12 feet tall, offers extraordinary economies compared to HAWT. Transportation and installation of SavRot submerged assemblies can be accomplished without access to coastal land. The only footprint ashore is an arrangement of modest low profile facilities for delivery of electrical power to a grid or other user.