[attachment=10506]
Energy Harvesting from Passive Human Power
ABSTRACT
Portable equipments are the first evolution from fixed equipments to make possible that some day computers are part of our everyday lives. The trends in technology allow the decrease in both size and power consumption of complex digital systems. This decrease in size and power gives rise to the concept of wearable devices in which digital systems are integrated in everyday personal belongings, like clothes, watch, glasses, etc. Power is a limiting factor in this kind of devices. Wearable computers are distributed devices in clothes and therefore the power must be distributed and supplied over the body.Human power is defined as the use of human work for energy generation to power an electronic device. One possible division is to distinguish between active and passive harvesting energy method. The active powering of electronic devices takes place when the user of the electronic product has to do a specific work in order to power the product that otherwise the user would not have done. The passive powering of electronic devices takes places when the user doesn't have to do any task diferent to the normal tasks associated with the product. The energy is harvested from the user's everyday actions (walking, breathing, body heat, blood pressure, finger motion). Once the power is harvested it must be stored and there are many possibilities (capacitors, rechargeable batteries, etc.)
I.INTRODUCTION
Energy harvesting, scavenging, and harnessing – these are all almost analogous terms related to research and engineering activities aimed at extracting energy in electric form from various ambient energy reservoirs, which generally cannot be scaled up for full-size, power-plant energy generation schemes.There are many ambient energy pools that could be exploited by autonomous, low-power fuel-less generators: waste heat,vibrations, localized air movement or human-generated power. In addition, traditional “renewable energy resources” like water flow, tidal and wind energy or sun radiation can also be exploited at the miniature scale by energy micro-harvesters. All environments are now being populated by miniaturized, micro-power electronic devices working in wireless sensor networks or just as mobile gadgets. A dream to power all those devices without batteries making them perpetual or at least to supplement mobile supply scheme is the driving force of the research oriented on ambient energy harnessing. Serious interest in those problems, partially motivated by climate change and global warming, is also reflected by several R&D organizations (e.g. DARPA, CEALITEN) and companies aiming at harnessing energy from ambient vibrations, leg and arm motion, shoe impacts, and blood pressure for self-powered systems.
II. KINETIC ENERGY HARVESTING
Different techniques are being proposed for powering up remote, unattended, implantable or wearable sensors, short distance wireless communication stations, RFID tags or personal health monitors. Generating electricity from ambient vibrations or kinetic motion is one of those approaches applicable as a renewable substitute for batteries in micropower electronic products making timed use of accumulated energy. Kinetic energy in the form of motion or vibrations is generally the most versatile and ever-present. Operating principles of motion-driven ambient energy microscavengers relay on utilization of inertial forces acting on a proof mass fitted with a damper which simultaneously serves as an electromechanical transducer converting kinetic into electrical energy. Vibration stimuli are extensively present but vary widely in amplitude and frequency thus micro-generators are designed as resonant systems matched to vibration spectrum of the source. Non-resonant or dynamically tunable devices are also being designed, as broadband operation is required for maximizing output power when the source has a complex or time-variable vibration spectrum.
III. PIEZOELECTRIC HARVESTERS
When a mechanical vibration stimulates a piezoelectricmaterial, the internal charge configuration changes to generate a voltage across the surfaces ; in other words, an ac current charges and discharges the capacitance between the surfaces . The purpose of a piezoelectric harvester is to transfer the energy in the form of charge to an intermediate reservoir, such as a capacitor or battery. The harvester does not supply the load directly because the mechanical input is unpredictable and therefore unreliable for on-demand loading events . Considering its aim, the system must therefore condition and rectify an ac source into a dc output without losing considerable energy, which is why efficient rectifiers and rectifiers with the conditioned input and output voltages that produce higher power are the subject of ongoing research.
A. Rectifier-Free, Switched-Inductor System
While the efficiency of rectifiers can be high, the power they draw is not because the rectifier only transfers energy when the input voltage exceeds its output. In other words, the rectifier can only harvest for a fraction of the vibration cycle,when the piezoelectric cantilever bends enough to generate a voltage that surpasses the rectified output. To circumvent this fundamental limitation, the harvester, as shown in Fig.1,can temporarily store the transduced energy in an inductor before delivering it to the storage capacitor or battery.
The rectifier-free, switched-inductor harvester in Fig. 1first allows the half of the vibration to induce the transducer to source current iPZT into piezoelectric capacitance CPZT. Once CPZT’s voltage reaches its peak, which corresponds to the transducer’s maximum displacement point, the system transfers CPZT’s stored energy into harvesting inductor LH,after which point the circuit reconfigures its switches to de-energize LH into the battery. Because energizing and delivering LH’s energy to the battery only requires a few μs and the vibration period is on the order of ms, the position of the cantilever practically remains unchanged through this LH’s entire energy-transfer process. Similarly, after the other half of the vibration cycle induces the transducer to maximally charge CPZT in the other direction, the harvester discharges CPZT into LH and then redirects LH’s energy into the battery.CPZT stores the electrical energy produced by the piezoelectric effect each half cycle, so input energy per cycle EIN is
where vPZT(PEAK+) and vPZT(PEAK–) are CPZT’s positive and negative peak voltages, respectively. Without the harvester,the quarter of the vibration cycle after the positive and negative peak points would be used to discharge CPZT from their respective peaks. In contrast, since the harvester extracts all the stored energy in CPZT and resets the voltage to zero at the peaks, the whole vibration cycle is exploited to generate the higher peak voltages compared to the open-circuited counterparts, the maximum input voltage a rectifier-based system can experience. Higher peak voltages thus indicate the harvester draws more energy from the environment.
The driving force behind adopting a switched-inductor topology is LH and its accompanying switches, which conduct with close to zero voltages across them, dissipate little power.Unfortunately, harvested power can also be low, so parasitic energy losses ELOSSES in LH’s equivalent series resistance (ESR), the switches’ turn-on resistances, driving parasitic capacitances of switches, and controller quiescent current IQ can use a considerable fraction of the energy harvested:
where REQ+/– represent the equivalent resistances that conduct peak inductor current IL(PEAK+/–) during conduction time TC+/– for positive and negative half cycles, and CEQ is the total equivalent parasitic capacitance present that must be charged to and discharged from battery voltage VBAT during the vibration period TVIB [7]. Thus, the net energy harvested ENET is necessarily below the energy the transducer avails (EIN):
B. Circuit Embodiment
In the circuit shown in Fig. 2, for example, after iPZT charges CPZT across half the vibration cycle to its positive peak voltage, switches SI and SN first energize LH, and SI and diode-switch DN then steer LH’s current iL into VBAT. Similarly, after iPZT’s negative phase charges CPZT to its negative peak voltage, SI and SN again energize LH but now SN and DI channel iL into VBAT. Notice asynchronous diodes DN and DI stop conducting when the system depletes LH: when iL attempts to reverse.

send me railway mini projects