Final Report: Exploring Proof of Concept in Ocean Current Energy ExtractionEPA Grant Number: SU835500
Title: Exploring Proof of Concept in Ocean Current Energy Extraction
Investigators: Sundararajan, V , Awakian, Michael , Delgadillo, Raul Delga , Flaggs, Johnathan , Gomez, Roberto , Lou, Paul , Nash, Trent
Institution: University of California - Riverside
EPA Project Officer: Hahn, Intaek
Project Period: August 15, 2013 through August 14, 2014
Project Amount: $14,972
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2013) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Energy , P3 Awards , Sustainability
The goal of this project is to explore the feasibility of extracting energy from ocean currents. The project involves an investigation of the nature and energy content of ocean currents, design and fabrication of technology that can harness this energy, development of evaluation procedures and identification of the challenges involved in commercial deployment.
Ocean energy is available in six forms: 1) waves, 2) tidal ranges, 3) tidal currents, 4) ocean currents, 5) ocean thermal energy conversion (OTEC), and 6) salinity gradients. Of these, only tidal ranges have been commercially exploited whereas tidal currents, waves and salinity gradients have been harnessed in demonstration projects. OTEC may be better suited for thermal processes, for example to provide cooling for power plants. To date, there does not seem to have been an actual demonstration project that exploits ocean currents. This project would be the first demonstration project of ocean current based technology. The energy available from this technology relies strongly upon the current speed. The Gulf Stream is a strong, deep and narrow current with speeds around 2 ms-1 whereas the California Current is a slower, shallower and wider current with speeds around 0.2 - 0.3 ms-1. Given its proximity to the California Current, the team will initially focus on the California Current.
The purpose of this project is to create a hardware prototype of an ocean energy extraction device. The prototype consists of three subsystems: 1) turbine, 2) generator, and 3) structure. The turbine subsystem consists of a turbine and a shroud. The turbine is the primary means of converting the energy in the ocean current to rotational energy of a turbine shaft. The shroud accelerates the flow in the vicinity of the turbine and thus increases the energy output of the turbine. The generator converts this rotational energy into electrical energy. The structure consists of a buoy and an anchor. The buoy houses measurement systems and the generator, and provides support to the turbine. The anchor provides support to the buoy so that it does not drift away with the current or with winds.
In Phase I, the scope was restricted to a smaller scale model that could be tested in a water channel in a laboratory. The objective was to design and test various turbine and shroud configurations to determine the influence of geometrical parameters on torque and speed of the turbine shaft. A further objective was to identify a set of optimal design parameters that provide the most power output.
The team first conducted a review of oceanography to understand 1) the origin and the characteristics of ocean currents such as speed, direction, and seasonality 2) the ocean floor 3) marine life. Next the team performed rough calculations to determine the technical feasibility of the project. It then developed conceptual designs and used ranking methods to evaluate them and to select the most promising solutions. It subsequently developed mathematical models and computer-aided design (CAD) models for the two highest ranked solutions. These solutions were the vertical axis Savonius turbine and the vertical axis Darrieus turbine. In addition, the team also designed shrouds that will be placed around the turbines to accelerate the flow into the turbine.
A Savonius turbine is an S-shaped drag turbine and operates on the difference between the forces on the two surfaces facing the flow. The drag on the concave side is higher than that on the convex side. This difference creates an unbalanced moment about the central axis and causes the turbine to spin. A Darrieus turbine operates on the principle of lift and consists of airfoil blades. As the fluid flows past the airfoil, it generates drag and lift forces on the blade. The net component of the force along the direction of blade motion (i.e. tangent to the circle at the blade’s location) provides the turning moment for the turbine.
Several versions of the Savonius turbine, the Darrieus turbine and the shrouds were fabricated using machining and 3D printing. The turbines were tested in a water channel that can provide uniform flow with speeds upto0.5 ms-1. To decrease turbulence, two sets of honeycombs made of straws are used at the inlet end of the tank. Uranine dye is injected into the flow to study the flow patterns.
Testing the Savonius turbine: The goal of the tests was to determine the relationship between swept area, blade positions and torque output. The overlap of the blades and their diameters were varied, and the torque required to stall the turbine in the water channel was measured. Results showed that a 9 inch configuration with no overlap between the blades produced the maximum torque of 29 mN-m.
Testing the Darrieus turbine: The goal of the tests was to determine the combination of airfoil type, blade pitch (i.e. the angle of the airfoil with respect to the tangent of the mounting circle) as and the diameter at which the blades are mounted can be varied. Three different types of airfoil with 3 blade pitches and 3 diameters were tested. These turbines did not turn and did not yield power. However careful experimentation showed that the turbine has two stable equilibrium points 180 degrees apart. When the turbine is disturbed from these positions, there is a restoring force that returns the turbine to these positions. An analytical calculation confirmed that there are restoring moments present in the turbine that would prevent it from spinning. Despite the negative result for this turbine, the team has a better analytical and experimental understanding of the design considerations for this type of turbine. Further research will focus on two solutions increasing the diameter of the blade so that the tip-speed ratio increases 2) modifying the blade configuration.
Testing of shrouds: The goal of these tests was to determine the parameters that result in the greatest increase in fluid velocity at the throat of the shroud. The shrouds were varied in length, curvature and outlet area, where the length determines the distance for the flow, the curvature controls the curvature and outlet area controls the exit velocity. The best performing shroud is with length of 12”, curvature of 12” and outlet area of 6”.
The findings provide promising directions for further development of the ocean current energy extraction device. The Savonius turbine combined with the shroud will yield energy. Modifications and redesign of the Darrieus turbine may also provide an alternative design to harness energy.
This project shares with other alternative energy projects the goals of reducing this dependence on fossil fuels, reducing greenhouse emissions, harnessing a renewable source of energy source and providing an environmentally benign source of energy. Furthermore, a consideration of life cycle analysis shows that the OCEE excels in the stages of pre-manufacture, use and disposal whereas in the phases of manufacture and transportation, it is comparable to other renewable and non-renewable technologies.