Adaptive Control Strategy for Efficient Conversion of Photovoltaic Energy to Mechanical Work An Innovative Scheme for Maximum Power Point Tracking and Optimal Load Modulation Luís S. Rodrigues, Jorge A. F. Ferreira, Fernando Neto, Nelson Martins Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro Aveiro, Portugal lrodrigues@ua.pt Abstract — Naturally occurring fluctuations in solar irradiance (W/m2) – predictable (e.g., time of day) or not (e.g., clouds) – pose challenging difficulties for the optimal conversion of photovoltaic energy to mechanical work, since the output voltage at the source correlates very poorly with the instantaneously available maximum power point (MPP) [1]. Unfortunately, the open circuit voltage (Voc) remains almost unchanged throughout a wide range of MPPs and is mostly influenced by the temperature of the photovoltaic cells and, to a lesser extent, by the age of the cells and the spectral distribution of the incident radiation [2]. Ideally, to maximize the utilization of the available energy, the load should always work at the MPP voltage and current (Vmp and Imp, respectively), tracking the respective variations. This cannot be achieved with a fixed or passive load. Fortunately, in solar-powered applications where mechanical work is the goal (e.g., water pumping, cooling, etc.), the load power can be modulated by controlling the speed and/or torque of an electrical motor by means of a variable frequency drive circuit (VFD). Fortunately, the types of motor that allow this type of control – e.g., brushless DC, synchronous AC and asynchronous/induction – are also the most efficient types, with efficiencies comfortably above 90% (even 96% in some cases). However, two problems remain: (i) determining if there is enough power available to start and sustain the motor (intermittent starts can be problematic in some applications, e.g., cooling), and (ii) finding the optimal working point. Hunting for this point by changing the driving frequency by trial and error is far from an optimal strategy and can result in motor stall conditions (which, again, can be critical in cooling applications). To solve both these problems, the instantaneous MPP should be, ideally, continuously measured. However, the MPP cannot be calculated from Voc and can only be roughly estimated by measuring the short-circuit current (Isc). To accurately find the MPP, the range of output current-voltage combinations should be scanned. Executing this measurement without interrupting the supply to the load presents yet another challenge. The implemented solution is comprised of a specially designed electronic circuit for measuring the power curve of a photovoltaic source while maintaining a constant supply to the load – without resourcing to an electrochemical battery – and an adaptive control algorithm for modulating the driving frequency of a brushless motor to maximize energy utilization. A practical prototype was built for optimizing the efficiency of a solarpowered chest freezer equipped with an asynchronous AC motor and a variable frequency drive (VFD).
Keywords — Photovoltaic Energy, Efficiency Optimization, Adaptive Control, Maximum Power Point Tracking (MPPT), Load Modulation. ACKNOWLEGEMENTS
This work was supported by the project «UFA+EE – Investigação e Desenvolvimento de Unidades de Frio Autónomas e Energeticamente Eficientes» (Research and Development of Autonomous and Energy Efficient Cold Units), financed by the European Union through its Structural and Investment Funds, and through the programs “Portugal 2020”, “Programa Interface”, “Sistema de Incentivos à Investigação e Desenvolvimento Tecnológico (SI I&DT)” (System of Incentives for Research and Technological Development) with the reference 03/SI/2017. TOPIC 2) Technologies for the Wellbeing b. Innovative Technologies for Smart Cities. REFERENCES [1] J. Cubas, S. Pindado, and F. Sorribes-Palmer, “Analytical calculation of photovoltaic systems maximum power point (MPP) based on the operation point,” Appl. Sci., 2017. [2] H. Mäckel and A. Cuevas, “The spectral response of the open-circuit voltage: A new characterization tool for solar cells,” Sol. Energy Mater. Sol. Cells, 2004.
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