Chris Vermillion

Model-Based Plant Design and Hierarchical Control of a Prototype Lighter-Than-Air Wind Energy System, With Experimental Flight Test Results

This paper presents the modeling, control system design, and experimental results for a prototype lighter-than-air wind energy system being pioneered by Altaeros Energies. This unique design features a horizontal-axis turbine that is elevated to high altitudes through a buoyant shroud, which is tethered to a ground-based platform. The systems altitude can be adjusted to maximize power production, and because the system is both functional and economical in a stationary position, it circumvents many of the controls challenges faced by kite-based wind energy systems. However, the need for generation of energy introduces pointing, efficiency, and autonomy requirements, which are not faced by conventional aerostats, thereby requiring a careful model-based control design. In this paper, we provide a dynamic model of the Altaeros system, then show how this model is leveraged in the plant design and in the design of the control system, which provides full autonomy, from takeoff, through power production, to autonomous landing. We provide simulation and experimental results that demonstrate the performance of the prototype and point to important areas where Altaeros will focus its efforts moving forward.

Modeling and control design for a prototype lighter-than-air wind energy system

This paper presents the hardware configuration, modeling, and control design for a lighter-than-air wind power system being pioneered by Altaeros Energies. This unique design features a horizontal-axis turbine that is elevated to high altitudes via a buoyant shroud, which is tethered to a ground-based platform. Because the system is based on proven aerostat technology and is designed to remain substantially stationary, it circumvents many of the controls challenges faced by so-called aerodynamic (e.g. kite-based) wind energy systems. However, the need to generate energy introduces pointing, efficiency, and autonomy requirements that are not faced by conventional aerostats, thereby requiring a careful model-based control design. In this paper, we first provide a detailed description of the system and controls hardware for the Altaeros 2.4 kW proof-of-concept prototype. We provide a detailed 3-dimensional dynamic model for the Altaeros system, which is used in the design of a fully autonomous control system. This paper details the control system design and shows simulation results that substantiate the systems performance.

Lighter-Than-Air Wind Energy Systems

Several wind energy concepts utilize airborne systems that contain lighterthan-air gas, which supplements aerodynamic lift and expands these systems’ available operating regimes. While lighter-than-air systems can incorporate the traction and crosswind flight motions of their heavier-than-air counterparts, several lighterthan-air concepts have also been designed to deliver large amounts of power under completely stationary operation and remain aloft during periods of intermittent wind. This chapter provides an overview of the history of LTA airborne wind energy concepts, including the design drivers and principal design constraints. The focus then turns to the structural and aerodynamic design principles behind lighterthan air systems, along with fundamental flight dynamic principles that must be addressed. A prototype design developed by Altaeros Energies is examined as an example of the application of these principles. The chapter closes with suggestions for future research to enable commercially-viable LTA systems.

Real-time altitude optimization of airborne wind energy systems using Lyapunov-based switched extremum seeking control

This paper applies a Lyapunov-based switched extremum seeking (LSES) control algorithm to the application of altitude optimization of airborne wind energy systems. We evaluate the stability and convergence of this control algorithm for the application at hand. Achievement of the optimal altitude to maximize energy production requires use of energy in order to search for the optimal altitude, which is continually and randomly varying. The simultaneous desires to optimize altitude and minimize control energy consumption are balanced in this paper through a variant of extremum seeking control, where the periodic perturbation signal is reduced when convergence upon an optimal altitude is detected. The signal is reinstated when the wind speed begins to deviate from its instantaneous optimal value. Because the wind shear profile (wind speed vs. altitude) is subject to continual variations, this application represents a challenging case study in LSES control. Using real wind shear data, the results presented in this paper show that the LSES controller is successful in significantly increasing the net energy production over fixed-altitude and standard extremum seeking strategies.

Evaluation of a Water Channel-Based Platform for Characterizing Aerostat Flight Dynamics: A Case Study on a Lighter-Than-Air Wind Energy System

Aerostat development and testing costs often suffer from a lack of scalability. In particular, it very difficult to fabricate an inexpensive lighter-than-air system that can be evaluated in a lab environment, since the maximum allowable mass of the aerostat becomes prohibitively low for small length scales. This paper presents an evaluation of a novel water channel-based platform for assessing the flight dynamics of aerostats at a very small scale, in a lab environment, for a very low cost. Altaeros Energies’ buoyant airborne turbine (BAT) is used as a case study to demonstrate the effectiveness of the proposed approach. Specifically, we identify important dynamic scaling properties and show how the water channel experiments are run to match these properties closely in the water channel vs. full-scale settings. We then show how the water channel results can be used in concert with a simulation model to predict the performance of the full-scale system. The ultimate result is a design which, after an inexpensive evaluation process, can proceed to a larger-scale prototype stage with a high degree of confidence in its success.

Model Predictive Longitudinal Control of a Lighter-Than-Air Wind Energy System

This paper presents the design and simulation results of a model predictive controller (MPC) applied to the longitudinal dynamics of a lighter-than-air wind energy system being pioneered by Altaeros Energies. The unique Altaeros design features a traditional horizontal axis wind turbine that is held aloft by a buoyant shroud, which is tethered to a ground based platform. This structure provides access to strong, high-altitude winds, requires minimal setup, and builds upon proven aerostat components, making the system an attractive component in expanding wind energy throughout the world. However, because the system replaces a conventional tower with tethers, its dynamics are highly susceptible to variations in the wind. In particular, the control system must keep the shroud pitch angle and tether tensions within acceptable bounds in order to maintain stable operation and remain within structural limitations of the system. In this paper, we apply MPC to achieve desirable longitudinal system performance while simultaneously enforcing the constraints. We describe the longitudinal dynamic model of the system, detail the linear MPC design, and provide simulation results on both the linearized and nonlinear system for a variety of real-world wind conditions, including a Dryden turbulence model and data acquired from the Altaeros functional prototype test site at Loring Air Force Base in Limestone, Maine.Copyright

Development and Full-Scale Experimental Validation of a Rapid Prototyping Environment for Plant and Control Design of Airborne Wind Energy Systems

Airborne wind energy systems present great promise for inexpensive, clean energy at remote locations, but have only been demonstrated through short-duration flights in very limited wind conditions. Because of the time and money that is required to implement full-scale airborne wind energy prototypes, convergence toward designs that achieve longer-duration flight in adverse weather has been slow. This paper presents an inexpensive rapid prototyping approach for improving the flight dynamics and control of airborne wind energy systems, which has been implemented and validated on Altaeros Energies most recent full-scale flight prototype. The approach involves the 3d printing of lab-scale water channel models of airborne wind energy lifting bodies, which enable prediction of dynamic flight characteristics, rapid iteration between the designs, identification of unknown or poorly known parameters, and improved control design. By applying this approach to its last prototype design cycle, Altaeros demonstrated robust operation in double the wind speeds sustained by its previous prototype (reaching a maximum of 21.2 m/s, with sustained 10–15 m/s winds), with demonstrably improved flight characteristics.© 2014 ASME

Altitude Optimization of Airborne Wind Energy Systems via Switched Extremum Seeking—Design, Analysis, and Economic Assessment

This paper applies a Lyapunov-based switched extremum seeking (LSES) control algorithm to the application of altitude optimization of airborne wind energy systems. We perform an economic analysis to evaluate the effectiveness of the control scheme. Finding the altitude with the highest energy yield requires energy to be consumed in the search for this optimal altitude. The simultaneous desires to optimize altitude and minimize control energy consumption are balanced in this paper through a variant of ES control, where the periodic perturbation signal is reduced when convergence upon an optimal altitude is detected. The signal is reinstated when the wind speed begins to deviate from its instantaneous optimal value. Because the wind shear profile (wind speed versus altitude) is subject to continual variations, this application represents a challenging case study in LSES control. Using real wind shear data acquired over a 25-day period, the results presented in this paper show that the LSES controller is successful in significantly increasing the net energy production over fixed-altitude and standard ES strategies. The economic advantage of the approach is illustrated through a comparison of achievable wind energy penetration with and without LSES-based altitude optimization in place, using real load demand data.

A case study in experimentally-infused plant and controller optimization for airborne wind energy systems

This paper presents a combined plant and controller optimization process for airborne wind energy systems (AWEs) that fuses numerical optimization with lab-scale experimental results. The methodology introduced in this paper, referred to as experimentally-infused optimization, addresses several challenges faced by AWE system designers, including a strong coupling between the controller and plant design, significant modeling uncertainties (which require the use of experiments), and high costs associated with full-scale experimental prototypes. This paper presents an initial case study of the proposed experimentally-infused optimization, where experiments were conducted on a 1/100th-scale model of Altaeros Buoyant Air Turbine (BAT), which was tethered and flown in the University of North Carolina at Charlotte 1m × 1m water channel. The lab-scale experimental platform reduced the cost of evaluating flight dynamics and control by more than two orders of magnitude, while resulting in substantially improved flight performance, quantified by a 15.2 percent improvement in an objective function value, as compared to a purely numerical optimization.

Stable hierarchical model predictive control using an inner loop reference model and λ -contractive terminal constraint sets

This paper proposes a novel hierarchical model predictive control (MPC) strategy that guarantees overall system stability. This method differs significantly from previous approaches to guaranteeing overall stability, which have relied upon a multi-rate framework where the inner loop (low level) is updated at a faster rate than the outer loop (high level), and the inner loop must reach a steady state within each outer loop time step. In contrast, the method proposed in this paper is aimed at stabilizing the origin of an error system characterized by the difference between the inner loop state and the state specified by a full-order reference model. This makes the method applicable to systems with reduced levels of time scale separation. This paper proposes a framework for guaranteeing stability that leverages the use of the reference model, in conjunction with λ -contractive constraint sets for both the inner and outer loops. The effectiveness of the proposed reference model-based strategy is shown through simulation on an existing stirred-tank reactor problem, where we demonstrate that the MPC optimization problem remains feasible and that the system remains stable and continues to perform well when time scale separation between the inner and outer loops is reduced.