The time has come that the technical problems facing the old design of Airship with Helical Vertical Axis for Wind Turbine (Eagle 14) at High Altitude shall be replaced with a new one that meets engineering standards in order to work more efficiently. A team will handle the proposed project based on a research plan made by the members. The project is expected to produce good results because the team will assign a principal investigator or PI who has a wide experience and record of achievements in the fields of experimental and quantitative design of Airborne Wind Turbine at high altitude. This research plan discusses the different methods to be used in achieving the goals of the proposed project and the expected project deliverables.
The subsections on this paper describe the proposed research design including the extensive lab experiments that involve testing and validation.
Objective and Significance
The primary aim of this project is to create a new design of Airship with Helical Vertical Axis for Wind Turbine (Eagle 14) at High Altitude to enhance stability and reduce control. This will produce improved wind equipment that is more efficient than the existing airborne wind turbines designs that are not fully tested and failed to validate at high altitude (10,000 m). The literature review section in this research paper shows that the important aspects listed below were not tested at high altitude:
- Numerical Model Validation: Precision of the numerical model, which is intended to be developed in this project, did not pass prior verification. Thus, physical modeling is required to validate the simulation.
- Analytical Models of Six degree-of-freedom (DOF): The aim of this project is to extend the analytical models in order to assess the six DOF for Eagle 14.
- Computational Analysis: This project is significant in combining different analytical models by the team to study each component of the design pertaining to Eagle 14. Further explanation of this aspect will be done in the computational analysis subsection.
Project Methodology
In achieving the objectives of this proposal, the investigation team is set to utilize a multi-stage tasks methodology. The process involved in this project can be found in Figure 1. As mentioned earlier in the introduction, the flowchart as shown below starts with the team’s design and study concerning the Airship and Helical Vertical Axis Wind Turbine. Thereafter, fabrication of parts will be done in a prototype model scale needed for the Airship and Helical Vertical Axis Wind Turbine (Eagle 14). In the final stage, the prototype will be tested and the results will be validated.
Eagle 14 Design
Under this stage of the project, there will be several sub-stages with each sub-stage having its own expected deliverables. The Eagle 14 components that will be used in the study of the design will include: Airship (wings, balloon and tail), helical turbine, tether, ground wench device and electrical generator. Figure 2 illustrates these parts.
Eagle 14 Assessment Study
The members of the team will be involved in studying all parts of Eagle 14 including the terms of design, use of tools and methods, wind density sensitivity, power generation capacity, sensitivity to conditions at high altitude, decommissioning and maintainability, onsite installation complexity, system vibration and stability, power transmission, turbine efficiency, control and navigation complexity, helium properties and airship design complexity. When all parts of Eagle 14 have been examined by the team, investigation of parts construction material will follow.
Materials Selection
This proposal intends to study the selection of material for each part of Eagle 14 system. The process will cover metallic and composite materials. The materials listed below will be considered by the team:
- Composite materials: Include Carbon Fiber Reinforced Polymers and Glass Fiber Reinforced Polymers.
- Metallic materials: Include Carbon steels, titanium alloys, nickel base alloy, stainless steel and low alloy steel.
- Other materials: Include Kevlar-49.
The team will ensure that all selected materials will meet the following requirements:
- They must meet AIAA, API, ASME and ASTM standards.
- They must be available and easily accessible in the US market.
- They must possess a very good record of effective application.
Selection of the materials for Eagle 14 parts will depend on different factors. Each selected part of the Eagle 14 must have factors related to its role and function. These flying parts factors include machinability, relative density, specific gravity, specific volume, specific weight and corrosion. The cables study factors are: machinability and weldability (if the construction material is metallic). Other factors include: fatigue resistance, design life (involving corrosion and erosion lifespan), mechanical properties (elongation, strain age test, yield stress and tensile strength), operating conditions (operating loads and environments and operating temperature), weight reduction and fracture toughness. After selecting the construction materials for each part pertaining to Eagle 14, the team will then perform designing of numerical model applicable for Eagle 14 system.
Numerical Model Design
The team will be engaged in studying the characteristic and geometric configuration of each part of Eagle 14. The quantitative analysis of the design will involve several repetition attempts to get the best possible characteristics and geometries of the Eagle 14. The overall objective of Eagle 14 design is to maximize control and stability, minimize drag and vibration, minimize maintenance shutdowns, and reduce construction, installation and operational costs. Adequate details in studying aerodynamics will be applied to model designs. Structural modeling and responses will be used as well in this process. In addition, efficient examining of a wide range of environmental conditions such as strong turbulent winds and low temperatures at high altitudes will be achieved in this project. Overall, several assumptions will be considered in designing Eagle 14.
Aerodynamic Modeling: The airborne wind energy Eagle 14 system will be used by the team as a model with the use of hybrid coordinate system, Cartesian and Polar, with an added angle. Fig. 3 shows the composition of the coordinate system; namely, the state variables that represent tether angle, the angle of attack pertaining to the horizontal axis, and the tether length pertaining to the vertical axis. Traditional aircraft longitudinal models and blimp usually use a Cartesian coordinate system, but only utilize the angular positions and linear velocity while disregarding the linear positions. The airborne wind energy system position is considered more critical in its operation as compared to its velocities because it is stationary by nature. Furthermore, the hybrid polar coordinate system is efficiently designed to allow the tether easily in exhibiting mass and inertia. On the other hand, it is extremely difficult to use a Cartesian coordinate system to model the tether inertia and mass. There is a close resemblance between the hybrid coordinate system and the movement of the airborne wind energy system in rotating on the ground tether point and oscillating in tether length. In utilizing a Cartesian-only coordinate system, it was observed that it produced an erratic tether behavior in some conditions and failed to model the system properly.
Structural Modeling: In this aspect, the team will be involved in quantitative investigation of structural responses pertaining to Eagle 14. This will be done by creating a wind-structure interaction model that applies to a Boundary Element Model (BEM) intended for the wind-body interaction. The team will also examine the cable dynamics model to be used for tethering the system, and investigate a Finite Element Model (FEM) applied to structural response. In describing the finite segment method, the cable used is divided into several finite segments. Each segment may appear to be rigid and straight, extensible and straight, or otherwise curved. After designing the numerical model intended for each part of Eagle 14 system, the next step is to examine the computational analysis stage by the team.
Stability and Control Modeling: Aimed at stabilizing and controlling the movement of Eagle 14, the team is determined to apply the best in class technology. The team members are set to use a system that will serve several purposes like regulating the altitude for efficient power delivery, regulating pitch angle intended for alignment with wind and aerodynamic lift, ensuring that no tether turns slack, and achieving an entirely autonomous operation. To achieve these goals, several components aimed at regulating the altitude, tether tensions, roll, and pitch will make up the primary control system. This control system will assume a structure of hierarchy in which an upper level controller will compute desirable set points present in the system’s flying envelope. Moreover, a lower level controller will compute control input commands with the purpose of achieving these set points. The basis for utilizing this system is the model created by Alteros Energies system that shows an AWT which is lighter than air, and demonstrated excellent control and stability results.
Computational Analysis
The team will use four computer software programs to model and study Eagle 14 system and its structural behavior:
- Vorticity Transport Model or VTM: This code involves wind turbine aerodynamics as well as performance.
- Riflex: Includes the Finite Element or FEM solver that will be used to model the tether.
- NASTRAN: This refers to the Finite Element Analysis or FEA program originally intended for NASA. This is important for use in structural dynamic modeling applicable for flying objects.
- AeroDyn: This code is applied to the forces as BEM option, moments and wind dynamics motion in Eagle 14. The study relevant to this aspect will cover the use of appropriate models pertaining to each operating conditions and wind speed.
Through investigation of numerical design applicable to all geometric and characteristic variables, identification of some design trends will be made possible for use in guiding further AWT development. Once the computational analysis has been completed, the team will go to stage 3, and begin developing a small-scale prototype design of Eagle 14 for extensive studying and testing.
Prototype Fabrication
Considering the limited budget allocated for this project, the team is bound to study each part of Eagle 14 system in a prototype scale. The materials needed for all parts of Eagle 14 will be ordered in advance from a local supplier in the US. In order to get accurate results, the Eagle 14 should be scaled with precise weight, mass distribution and dimensions geometrically. This means that the model must be undistorted geometrically. The small-scale models can be possibly fabricated in the Mechanical & Aerospace Engineering Department at the University. As studied before, selected material will be used to make the prototype. Before fabricating, a drawing of the final design of Eagle 14 parts will be made based on a solid model in SIMPACK. Expected duration of fabrication is from 6 to 9 months for all models. Once the fabricated models are completed, the team will perform several tests of the Eagle 14 prototype that will be subject for further studies.
Prototype Tests
The objectives of prototype testing are to validate the design produced, and to identify erroneous performances of the system. Prototype testing and analysis will be conducted both at the University fluids lab and NASA labs. There will be two phases of testing that will be performed in this research project. The first one involves wind tunnel and buoyancy testing that are required to validate the CFD or computational fluid dynamics results. In these tests, wind tunnel will be determined if it has the capability to generate a turbulent and laminar wind flow with different speeds. In the second phase using a vibration test, examination of the models under different vibration modes will be done to assess the dynamic stability and behaviors of the system. Under the third phase, verification of Eagle 14 responses with regard to wind frequencies and amplitude will be performed to determine the relation of aerodynamic damping to the flow and density of wind at different altitudes.
Project Deliverables
Eight major outcomes of this proposed project are expected:
- Eagle 14 Assessment: The plan of the team is to consider all the parameters necessary in their study. These parameters include: design tools and methods, wind density sensitivity, power generation capacity, high altitude conditions sensitivity, decommissioning and maintainability, installation complexity onsite, system stability and vibration, power transmission, turbine efficiency, complexity of control and navigation, helium properties and airship design complexity.
- Materials Selection: The criteria for selection by the team for the best material to be used for each part of the Eagle 14 are mentioned in section 2.2.1.2.
- Eagle 14 design characteristics and geometries: Examples of the desired outputs include airship volume and size, span of wings and tails, tether length and diameter, size and number of twisted turbine blades, helium gas pressure and volume, center of buoyancy, center of gravity pertaining to the airborne body, and blades geometry.
- Tether Design Variables: The expected outputs in this study include the diameter of cables, cable characteristics, total cable forces, pre-tension of a cable, surge frequency, bending moment and pitch frequency.
Timeline and Milestone
The expected duration of this proposal is 36 months. In order to meet the target timeline in accomplishing this proposed project, several tasks will be rendered back to back. Table 1 shows the summary of milestones including the timeline of this proposed research. The team is set to hold monthly meetings to discuss the results and expected deliverables of the previous period, and discussing highlights of the coming period. In the absence of any member of the team, video conference (WebEx) may be utilized. Minutes of meeting (MOM) will be put on record and saved for tracking purposes. The project’s PI will call for semiannual meetings at the duration of the project with NSF to convene about the progress of the funded project. Upon completion of the project period, an annual report will be issued to NSF that will include results, team outcomes and accomplishments. Moreover, publication of one journal a year by the team will be done by the team for the next three years. The journals will follow the content and objectives of the American Wind Energy Association.
