A NEW OPTIMUM POWER CONTROL SCHEME FOR LOW ENERGY HARVESTING SYSTEMS. ARUNDAS S7E CET 1

OVERVIEW Introduction Energy harvesting system Maximum energy harvesting scheme Design guidelines Simulation results PV cell energy harvesting system Maximum power point tracking Duty cycle vs. power output Advantages and Disadvantages Conclusion References 2

INTRODUCTION Energy harvesting is of greater importance due to developing low power “perpetual” devices that do not require any replacement. Energy can be harvested from a number of sources like vibration, sunlight, wind, thermal gradient etc. Energy harvesting source depends on the nature of application and power requirement. We deal with energy harvesting for two completely different fixed excitation s/m.  Low voltage Vibration based EM micro generator  Miniature Solar cell array 3

ENERGY HARVESTING SYSTEM Two types of energy harvesting systems. 1. Vibration based EM micro generator. 2. Miniature Solar cell array. ELECTROMAGNETIC ENERGY HARVESTING SYSTEM A typical electromagnetic micro generator consist of a moving magnet which induces an emf in a stationary coil. A resonance-based electromagnetic micro generator can be modeled as a spring-mass- damper system. 4

Fig 1: Model for electromagnetic micro generator The basic Equation for such a system can be presented as mz”(t)+ D total z’(t)+kz=my”(t) (source: accessed on ) 5

To achieve maximum energy harvesting, the electrical damping coefficient must be adjusted to match with the mechanical damping and coil losses of the system. Switching power converter can perform this function by offering a resistive load that varies with duty cycle. It operates in discontinuous conduction mode(DCM). Advantages of DCM are: 1) constant duty cycle for voltage regulation which enables simple control. 2) Simple power factor correction operation which allows the converter to offer a resistive load to the micro generator. 6

MAXIMUM POWER POINT TRACKING It is mainly utilized in solar cells & generally limited to low & medium power PV cells due to high associated power requirements and cost. The proposed control is based on classic “perturb and observe” mechanism. P&O is to change the duty cycle of the converter,& power generated by PV cell is always positive. In steady state, duty cycle of converter oscillates around maximum power point. 7

P OWER CURVE Fig 2: typical power vs voltage curve of a solar cell. (Source: assessed on ) 8

S OLAR ENERGY HARVESTING SYSTEM Buck converters is designated for the power processing of the miniature solar cell. Mode of operation Continuous mode(CM) Discontinuous mode(DCM) 9

A p-MOSFET Mp is utilized for switching to avoid the need of a floating gate driver. Based on P&O mechanism Fig 3: low power Buck Converter (source: accessed on ) 10

MAXIMUM ENERGY HARVESTIG SCHEME Fig 4: Set-up for maximum energy harvesting scheme (source: accessed on ) 11

Value of duty cycle is governed by voltage across capacitor Cs-controlled by switches Sp & Sn. The gate of these switches are controlled by T flip- flop output. When Sp is on, the duty cycle increases as the DC source E1 charges the capacitor Cs through resistor R1 & when Sn is on duty cycle Vk & Vk-1 is fed to comparator Cp. When the output voltage is increasing the output of comparator is zero allowing the T flip-flop to retain its state and vice versa. In steady state, duty cycle will oscillate around maximum power point. 12

DESIGN GUIDELINES Designed to offer proper impedance to energy harvesting source. The switching frequency=50kHz, and impedance offered depends on the duty cycle of converter & value of boost inductor. The resistance offered by the converter decreases with increase in duty cycle.(duty cycle generally limited to D~0.9) POWER CONVERTERS 13

Fig 5: (a) impedance plot for AC-DC boost converter (b) Impedance plot of buck converter for different load conditions. The minimum impedance, buck converter can offer equal to load impedance when duty cycle equal to unity when MOSFET is ON. The maximum impedance that the converter can offer is infinite when MOSFET is OFF. 14

SIMULATION RESULTS Simulation results to validate the proposed control system using the software SABER. The various components of the controller are: 15

The simulation results for the PV cell are: two solar cells in series, AM-5610 from Sanyo Semiconductor are used as the source. The buck converter is designed to offer optimum impedance to the PV cells. From graph it can be seen that the maximum power achieved is around 27mW For a duty cycle of D=.52 Fig:6 open loop plot 16

E XPERIMENTAL RESULTS ELECTROMAGNETIC MICROGENERATOR A shaker s/m is used to produce vibrations. 17 Duty cycle(D) Fig 7: output voltage vs duty cycle of converter

P V CELL ENERGY HARVESTING SYSTEM The set-up consists of 2 solar cells from Sanyo Semiconductors connected in series. The solar cells are placed inside the closed box with high brightness LEDs mounted inside. The lighting condition inside the box are controlled by changing the current through these LEDs. The different lighting conditions are depicted by the current flowing through the LEDs. The value of optimum resistance required for maximum energy harvesting changes with lighting conditions 18

19 Fig 8: (a) Assembled set up for PV cell harvesting using power LEDs (a) (b) b) Disassembled structure – box, power LEDs and solar cell. (source: accessed on )

DUTY CYCLE V S. POWER OUTPUT 20  Converters provide the optimum impedance to the solar cell at different duty cycles depending on lighting condition.  At higher duty cycles the power curve has a more flattened Fig 9: power curves for o/p power vs duty cycle (source: )

ADVANTAGES AND DISADVANTAGES 21

CONCLUSION A new method for implementing maximum power scheme for low power energy harvesting system is presented using EM micro gen. and Solar cell array. Impedance matching is very simple and requires low power. Can be used with different harvesting sources and diff types of power converters. The controller circuit only requires around 200µW of power even for discrete circuit implementation. 22

REFERENCES Rohan Dayal, Kumar Modepalli,” A new optimum power control scheme for low energy harvesting systems”,IEEE Trans. On Industry applications, Volume:PP, Issue: 99, 21 june M. El-Hami, P. Glynne-Jones, N.M. White, M. Hill, S. Beeby, E. James, A. D. Brown, andA J. N. Ross, "Design and fabrication of a new vibration-based electromechanical power generator," Sensors and Actuators A: Physical, vol. 92, Issues 1-3, pp , August S. Meninger, J. O. Mur-Miranda, R. Amirtharajah, A. P. Chandrakasan, and J. H. Lang, "Vibration-to-electric energy conversion," IEEE Trans. on VLSI Systems, vol. 9, pp , February