Photovoltaic reverse osmosis (PVRO) systems can provide water to many underserved communities. These systems can be constructed from modular components for individual applications. To be most efficient, PVRO systems should be custom configured for the water demand, solar insolation and water characteristics of a specific location. Designing a custom system composed of modular components is not a simple task. The process is challenging due to the complex non-linear behavior of the system, the uncertainty in the energy source, and the enormous number of design choices that need to be made for even a small inventory. Even for a designer with significant expertise, selecting the right components and configuration is often difficult. For remote, small communities, this becomes a daunting task.

To accommodate the large number of design choices, a hierarchical design method has been developed, shown in Figure 1. The method uses a series of filters to limit the design space based on engineering principles and calculations. The system is then configured from the reduced design space using optimization methods and detailed system models. The design method also accounts for uncertainty in factors which affect the system performance, such as the solar radiation and water demand. A set of detailed physics-based system models were developed to enable this process. A novel method of representing a PVRO system using a graph was developed to enable rapid evaluation of different system configurations. This modeling technique was validated using the MIT Experimental PVRO system, shown in the video below, which was designed and constructed as part of this research.

Figure 1: Modular design architecture.

MIT experimental PVRO system.

Precision mirrors are required for effective solar energy collectors. Manufacturing such mirrors and making them robust to disturbances such as thermal gradients is expensive.  In this research, the use of parallel binary actuation to control the shape of mirrors for solar concentrators is explored. The approach embeds binary actuators in a compliant mirror substructure. Actuators are deployed in a specified pattern to correct the mirror shape.

Modelling and controlling these binary actuated structures is difficult and is the focus of this research.  Analytical models have been developed for one-dimensional and two-dimensional compliant structures with embedded binary actuators. In addition, a model-predictive control approach has been developed to determine which binary actuator configuration corrects a given mirror deformation. These analytical models and control approaches are validated using finite element analysis and experimental studies. The experimental system built to validate the modelling approaches and the control techniques is shown in the video below.

Previous Research

Vibration Estimation of Large Space Structures using Vision and Inertial Sensors

Future space structures such as solar power stations and telescopes are expected to be very large. These structures will require on-orbit construction. Due to the risks and costs of human extravehicular work, teams of robots will be essential for the on-orbit assembly of the large space structures. There are a number of technical challenges presented by such robotic construction. The structures will need to be made of lightweight materials and will be very flexible. Autonomous robots will require information about the vibrations of the flexible structures and their dynamic parameters in order to perform the construction efficiently. Often models of the structures are imperfect, therefore the magnitude of the vibrations of the structure must be estimated on-orbit. This thesis presents a method for estimating the shape and dynamic parameters of a vibrating large space structure. This technique is a cooperative sensing approach using remote free-flying robot observers equipped with vision sensors and structure-mounted accelerometers. This approach exploits the complementary nature of the two types of sensors.

Vision sensors are able to measure structure deflections at a high spatial frequency but are bandwidth limited. Accelerometers are able to make measurements at high temporal frequency, but are sparsely located on the structure. The fused estimation occurs in three steps. First, the vision data is condensed in a modal decomposition that results in coarse estimates of modal coefficients. In the second step, the coarse estimates of the modal coefficients obtained from vision data are fused with the accelerometer measurements in a multi-rate nonlinear Kalman filter, resulting in a refined estimate of the modal coefficients and dynamic properties of the structure. In the final step, the estimated modal coefficients are combined with the mode shapes to provide a shape estimate of the entire structure. Simulation and experimental results demonstrated that the performance of this fused estimation approach was superior to the performance achieved when using only a single type of sensor.

Optimal Shape Design of a Solar Powered Vehicle

The shape design of a solar vehicle is a constrained multi-disciplinary optimization problem. The shape of the vehicle not only determines a vehicles aerodynamic performance but also affects the energy which is collected by the solar array. In this research, an optimization strategy was developed to design the shape of a solar powered vehicle. A set of candidate designs were generated using CAD software. The aerodynamics and light capturing capabilities were analyzed using Ansys CFX and custom written code. A gradient-based search was then utilized to determine the optimum vehicle shape parameters.

Publications & Patents

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Journal Papers

Bilton, A.M., Lee, S.J., and Dubowsky, S., “The Kinematic Control of Hyper-Redundant Binary Actuated Compliant Systems with Application to Solar Mirrors with Experimental Validation,” submitted to the ASME Journal of Mechanisms and Robotics, 2012.

Bilton, A. M., and Dubowsky, S., “The Modular Design of Photovoltaic Reverse Osmosis Systems – Making Technology Accessible to Non-Experts,” Desalination and Water Treatment, Vol. 51, pp. 702-712, 2012.

Bilton, A. M., Wiesman, R., Arif, A. F. M., Zubair, S. M., and Dubowsky, S., "On the Feasibility of Community-Scale Photovoltaic Powered Reverse Osmosis Systems for Remote Locations." Renewable Energy, Vol. 36, No. 12, pp. 3246-3256, December 2011.

Bilton, A. M., Kelley, L. C., and Dubowsky, S., “Photovoltaic Reverse Osmosis – Feasibility and a Pathway to Develop Technology,” Desalination and Water Treatment, Vol. 31, pp 24-36, July 2011.

Bilton, A. M., Ishijima, Y., Lichter, L., and Dubowsky, S. "A Sensor Architecture for the Robotic Control of Large Flexible Space Structures." The Journal of Field Robotics, Vol. 24, No. 4, pp. 287-310, April 2007.

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Book Chapters

Lienhard, J. H., Antar, M. A., Bilton, A. M., Blanco, J., Zaragoza, G., "Solar Desalination," in Annual Review of Heat Transfer, Vol. 15, New York: Begell House Inc., 2012.

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Conference Proceedings

Reed, E. A., Bilton, A. M., and Dubowsky, S., “Controllable Energy Recovery for a Smart PVRO System,” Submitted to the IEEE/ASME International Conference on Advanced Intelligent Mechatronics 2013, July 9-12, 2013, Wollongong, Australia.

Meng, L., Bilton, A. M., You, Z., and Dubowsky, S., “Adaptive Heliostat Solar Arrays using Shape-Optimized Compliant Mirrors,” Proceedings of the ASME 2012 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference IDETC/CIE 2012, Aug. 12-15, 2012, Chicago, Illinois, USA.

Bilton, A.M., and Dubowsky, S., “Inverse Kinematics for the Control of Hyper-Redundant Binary Mechanisms with Application to Solar Concentrator Mirrors,” Proceedings of the International Symposium on Advances in Robot Kinematics 2012, June 24-28, 2012, Innsbruck, Austria.

Kelley, L. C., Bilton, A. M., and Dubowsky, S., “Enhancing the Performance of Photovoltaic Powered Reverse Osmosis Desalination Systems by Active Thermal Management,” Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition, November 11-17, 2011, Denver, Colorado, USA.

Bilton, A. M., Kelley, L. C., and Mazzini, F., “Design Optimization of Sustainable Off-Grid Power Systems for the Developing World,” Proceedings of the ASME 2011 Energy Sustainability Conference, August 7-10, 2011, Washington, DC, USA.

Bilton, A. M., Kelley, L. C., and Dubowsky, S., “Photovoltaic Reverse Osmosis – Feasibility and a Pathway to Develop Technology,” Proceedings of EuroMed 2010, Desalination for Clean Water and Energy, October 3-7, 2010, Tel Aviv, Israel.

Lee, S. J., Bilton, A. M., and Dubowsky, S., “On the Kinematics of Solar Mirrors Using Massively Parallel Binary Actuation,” Proceedings of the ASME 2010 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference IDETC/CIE, August 15-18, 2010, Montreal, Quebec, Canada.

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Patents

Dubowsky, S., Bilton, A. M., and Lee, S. J., “System for Discretely Actuated Solar Mirrors,” US Patent Application Number 12/839479, July 20, 2010.

Dubowsky, S., Bilton, A. M., and Kelley, L. C., “Photovoltaic Reverse Osmosis System with Thermal Management,” US Patent Application Number 13/032896, February 23, 2011.