Adaptive MPP tracking control applied to
Faculty of Electronics, Communication and Computers
In this presentation, a Maximum Power Point (MPP) tracking technique is proposed for the Fuel Cell (FC) stacks based on a Extremum Seeking (mES) control that slightly improves the performances obtained. A higher value of the search speed is obtained for the same tracking accuracy for the proposed modified ES (mES) control in comparison with the classical ES control schemes (PART I). The analysis made for the advanced ES (aES) control in frequency domain reveals interesting relationships to design the control parameters, the values of closed loop gain and the dither amplitude (PART II). Thus, the imposed performances related to the search speed and tracking accuracy are easy to be The search speed will increase proportionally with the product of both control parameters, so it is practically limited for safe reasons. The tracking accuracy will be proportional with the magnitude of the first power harmonic, so the power ripple will be negligible after the MPP is caught. Simulations show that the performances mentioned above are effective for the mES and aES control based on a band pass filter (BPF) scheme. The dither persistence is improved for the BPF scheme having a large frequencies band that cover at Hydrogen is widely recognized as a potentially viable energy source via the Proton Exchange Membrane FC (PEMFC) systems that supply Hybrid Power Sources (HPS). The PEMFC systems have a price which continues to drop each year, but still requires expensive It is very important to extract as much energy as possible from the PEMFC stack in order to reduce the hydrogen consumption especially from a limited fuel tank used for example in portable and automotive applications. So, the PEMFC systems must operate at or close of the MPP, even if the FC efficiency is with about 5 % lower than the highest efficiency, which is obtained at the maximum efficiency point. It is known that the operating point of a FC stack depends by a huge numbers of parameters related to each subsystems: the feeding and humidification subsystems, cooling circuit, temperature control logic, electrical interface and so on. The FCHPS must have an Energy Storage System (ESS) and eventually other energy sources to assure the power flows’ balance on the DC bus under a dynamic load. Hybridization of the FC system with an ESS is an effective technology to overcome the disadvantages of the FC-alone-powered vehicles. Many MPP tracking techniques have been proposed during last decades, which differ in complexity, hardware implementation, popularity, convergence speed and sensed parameters. Usually, the static characteristic of the FC system is modeled using mathematical equations or numerical approximations, considering the loading and fuelling conditions, stoichiometric air-fuel ratio, temperature, and so on. The P&O method and its improved variants are most used algorithms that are implemented in MPP tracking controllers. The P&O methods are based on periodically changing of the operating point for the FC stack and observing the resulting change in the FC power. It is obvious that lower oscillations obtained during the MPP tracking phase increases the harvested FC power, beside that the FC lifetime also increases by avoiding the high mechanical and electrical stress of the PEMFC membrane. Although MPP tracking control is a well established algorithm, certain instability may appear when the control parameters change rapidly, as happens in the effort to increase the overall FC system efficiency. Thus, an optimal management of all FC subsystems to load dynamic is required. In some cases, it is actually desirable to operate the FC stack at the MPP because the corresponding fuel efficiency is close to the maximum value obtained in the maximum efficiency point, which usually it is difficult to track. In this presentation, two real-time optimization methods based on ES control schemes were analyzed in order to compare both performance indicators for PEMFC systems operating under different conditions. It was demonstrated that proposed ES control scheme improves the performances related to energy harvesting from the FC system, in addition to the performances inherited from the basic ES control schemes: a guaranteed convergence and internal robustness. As it is known, the ES control is a method of adaptive closed-loop control used for searching of unknown extremum on a static input-output characteristic. The accuracy of the MPP finding (usually named as tracking accuracy) is reported in literature to be higher of 99.98 % for the PV inverters. A lower accuracy (<99 %) is reported for the FC inverter. So, other ES control scheme must be used to If the dither period of the ES control is chosen higher than the FC time constant (in general, this option is not recommended), then large oscillations (> 0.2 % of the MPP power) on the net FC power appear. Lower oscillations (about 0.02 %) are obtained if the dither period is chosen lower than FC time constant. Moreover, if the dither frequency is selected too high, then the convergence speed will become too slow and other ES control scheme must be used to overcome this issue. Part I: FC energy harvesting using the MPP tracking based on modified extremum seeking control Part II: FC energy harvesting using the MPP tracking based on advanced extremum seeking control Part III: On the Dither Persistence in the Extremum Seeking control This presentation is mainly based on the following copyrighted materials: REFERENCES
1. N. Bizon (Ed.), Distributed Generation systems integrating Renewable Energy Resources, Nova Science Publishers Inc., USA, 2012, 978-1-61209-991-0 (hardcover), 978-1-61209-991-2 (ebook). 2. Bizon N., Energy harvesting from the FC stack that operates using the MPP tracking based on modified extremum seeking control, Applied Energy 2012, 10.1016/j.apenergy.2012.11.011 3. Bizon N., FC energy harvesting using the MPP tracking based on advanced extremum seeking control, International Journal of Hydrogen Energy 2012, 10.1016/j.ijhydene.2012.10.112 4. N. Bizon, On tracking robustness in adaptive extremum seeking control of the fuel cell power plants, Applied Energy 87(10) (2010) 3115–3130, doi: http://dx.doi.org/10.1016/j.apenergy.2010.04.007 . 5. N. Bizon, M. Oproescu, M. Raducu, 2012, On the Dither Persistence in the Extremum Seeking control - Part I: ESC loop based on Band-Pass Filter, World Congress on Sustainable Technologies (WCST-2012), London, 6. N. Bizon, M. Oproescu, M. Raducu, 2012, On the Dither Persistence in the Extremum Seeking control - Part I: Signal harmonics’ persistence for large filtering pass band, World Congress on Sustainable Technologies (WCST-2012), London.
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