Developing embedded electronic technology
Over the past decade increased functionality and reduction in size are important factors in developing embedded electronic technology, as a result small but powerful handheld devices such as music players, mobile phones, wireless sensors etc.. are available in the market. The size of these devices is further reduced that instead of portable devices they are becoming wearable devices. At present these devices are powered by using batteries with finite power supply, once this power is finished batteries needs to be replaced which is tedious and expensive when devices placed in remote locations. Research advances in the field of low power electronics, wireless technology and sensors to operate continuously has led to increasing demand of self powered electronics.
One possible technique to potentially alleviate these issues is harvesting energy from surrounding environment to power electronic devices. Energy harvesting (or power scavenging) refers to capture the energy from environment or energy surrounding a system or other energy sources and convert it into usable energy for electronic devices. Harvesting energy from environment is a promising alternative to reduce system mass and volume, increase functioning and reduce maintenance cost. The obtained electrical energy from kinetic energy, electromagnetic energy and thermal energy is used to develop electronic devices that can be recharged using a secondary battery or to power electronic devices directly.
The main objective of this project is to discuss and investigate on various methods of energy harvesting technologies. This paper presents a detailed study on Piezoelectric power generators (PEG). Energy conversion using Piezoelectric power generators (PEG) is more advantageous as they can be fitted comfortably in small micro electromechanical systems. PEG's can work continuously for a infinitely long period of time, if the force applied and the temperature outside are with in the range of the system. Piezoelectric technology has gained a lot of interest in the recent years due to the advancement in the development of low power electronic devices and portable devices. In order to meet the weight and power generation requirements to these devices design issues and electromechanical characteristics of piezoelectric materials should be studied.
This paper also presents previous works done on piezoelectric energy harvesting techniques, a detailed study on modelling and design issues, their stability and power generation.
The two major technologies must be considered when developing energy harvesting electronic devices with less weight, low power consumption and reduced maintenance cost. Two technologies play an important role to overcome these issues.
2.1) Battery Evolution
The technological advancement has led to decrease in the size, weight and increase in performance and also to recharge batteries. Energy necessary for majority of portable devices like music players, cellular phones, digital cameras ect,, is provided by batteries. Batteries act as a storage place of energy also called as reservoir energy source. Batteries can be characterized by calculating their energy density, self discharge and life cycle. Energy density is classified with respect to volume and weight also called as volumetric energy density and gravimetric energy density. Below table1 discusses typical values of energy density, self discharge and number of cycles of available commercial batteries.
A vast research is on its way to further reduce the size and increase the performance of batteries which in turn directly affects portable electronic devices. Different types of battery technologies like thin film Lithium-ion or Lithium polymer cells, power generation using fuel cells ( Methanol or hydrogen), Ni-Cd or Ni-MH batteries are available commercially which has limited life time and needs to be refuelled alternatively. The concept of secondary battery plays better role for portable or ubiquitous electronic systems, because these batteries can be recharged or refuelled in many number of ways without removing the battery from the device, one such possibility is through capturing energy available in the environment also called energy harvesting.
2.2) POWER CONSUMPTION
The trend in integrated circuit technology resulted in shrinking the size of transistor which in turn resulted in reduced supply voltage (VDD). Due to advancement in low power electronics, electronic devices are manufactures with different modes (inactive, sleep mode and active mode) to save the energy. As a result a discontinuous operation method is used to manage between these modes to refill the energy storage reservoir during inactive or sleep modes using energy harvesting technology. The model of the device and power consumption factors is explained.
Maximum performance of the device reduces the time per cycle to give higher number of cycles (or services), which results in increased energy consumption from the battery. Maintaining same number of cycles (or services) reduces the time per service which results in constant power supply from the device.
3. ENERGY HARVESTING DEVICES
Energy harvesting devices capture the untapped abundant energy available in the environment and convert different forms of captured energy into electrical energy to power electronic devices.
Energy harvesting devices are broadly classified into two types
3.1) Human energy devices
3.2) Environment energy devices
3.1) Human energy devices
Energy is generated by humans in different forms (movement, heat, vibration) is captured using various devices attached to human body. The amount of energy captured from human body is very small.
Human energy devices are divided into two types first is Human active energy and the other one is Human passive energy.
Human active energy devices basically work on the principle of ‘increasing the ratio of time of use with respect to time of charge' . These energy sources are controllable as they provide harvested energy when required. Devices such as radio receivers, electric torches and mobile phone batteries are powered by using only human energy by winding a hand crank, finger motion, paddling, by shaking the device, and walking. All these movements come under kinetic energy. Human active energy finds its application mainly in industrial area.
One example where human active energy can be implemented effectively is remote control. The device is powered by applying the mechanical force to push the switch which bends the cantilever piezoelectric ceramic.
Another example of human active energy is scavenging energy with Shoe-Mounted Piezoelectrics, where energy is extracted while walking helped to power RFID tag system.
3.1.2 Human Passive Energy
Human passive energy concept is mainly applicable to portable and wearable electronic devices. These energy sources cannot be controlled, energy is harvested only when ever it is available from sources such as blood pressure, body heat etc. Basic wearable devices to implement human passive energy are wristwatches as they have very low power consumption. Wristwatches can be powered by using both kinetic and thermal energy. Wristwatches powered with kinetic energy have power output of 5µW in normal condition and 1mW when forcibly shaken. Wristwatches powered with thermal energy have power output of 1.5µW when the temperature difference is 1-3ºC.
Human passive energy concept is also implemented in several other applications such as attachable medical devices, self powered heat sensors and mobile electronics.
3.2) ENVIRONMENT ENERGY DEVICES
Severe approach and research is going on from the past few decades on to capture surplus ambient energy available in the environment in various sources like solar energy, wind energy, RF etc. The energy required for low power electronic is in the order of hundreds of micro watts. Extracting energy from environmental energy sources is explained in chapter 4.
4. ELECTROMAGNETIC RADIATION
Electromagnetic radiation is an important from of energy source available abundantly in the environment in the form of solar energy, RF energy etc.
4.1) SOLAR ENERGY HARVESTING
Solar energy is available abundantly in the environment surrounding us. However in designing an efficient and mature technology to harvest this solar energy, requires an in-depth analysis of several factors. Availability of solar energy is highly time varying, mostly depends on sunlight, weather and location (latitudes and longitudes). As a result storage elements like batteries and capacitors are required to harvest energy. The storage components all together must work simultaneously in different characteristics (voltage and current) to achieve maximum efficiency.
Among various methods of modalities available, solar energy harvesting through photovoltaic system provides highest power density . The average lifetime of photovoltaic cells is around 20 years. The main source of energy for photovoltaic systems is solar radiation. Photovoltaic systems are found from Megawatt to the milliwatt rage producing energy from a wide range of applications like grid connected PV systems and wristwatches . Various factors like inclination angle and orientation of photovoltaic cells affect the amount of solar radiation on solar cells outdoors . Indoor irradiances widely vary in the range of 3.5 to 20W/m² as the solar cells are made of semiconductor materials . “The power conversion efficiency of photovoltaic solar cell is defined as the ratio between the solar cell output power and the solar power impinging the solar cell surface” . Photovoltaic solar cells
technology is implemented in radios, solar lanterns, wristwatches, battery chargers etc.
This paper discusses the design issues of harvesting, battery issues, implementation and power management. This paper also explains Heliomote and Prometheus architectures and their design, implementation and performance evaluation.
4.1.1)STORAGE TECHNOLOGY & CELL CHARACTERISTICS
Due to uncertainty in the climatic conditions, energy storage plays an important role in storing the available harvested energy using energy storage mechanism. There are two available technologies in storing harvested solar energy, ultracapacitors (also known as electromechanical double layer capacitors) and batteries . Batteries have very high energy density when compared with ultracapacitors, where as ultracapacitors have very high power density when compared with batteries. Different types of batteries are available commercially in the market among them four of them are commonly used for storing the energy. 1) Nickel Metal Hydride (NiMH), Lithium based (Li+), Nickel Cadmium (NiCd), Sealed Lead Acid (SLA). Among the four NiMH and Lithium based batteries are commonly used and are more efficient where as Sealed Lead Acid and Nickel Cadmium are rarely used because of relatively low energy density and “temporary capacity loss caused by shallow discharge cycles”. Lithium based batteries are more efficient with longer lifetime and are used vastly among the four technologies available even though they are more expensive.
Solar cell characteristics vary vastly from battery characteristics. Solar cells are arranged in series and parallel combination to form a solar panel, which are characterized by open circuit voltage (Voc) and short circuit current (Isc) parameters. Solar panel acts as a voltage limited current source with optimal operating point (power extracted from panel is maximized). Figure 2 shows V-I characteristics of 4-4.0-100 solar panel where Voc and Isc acts as x and y intercepts simultaneously . Observing the Figure 2 clearly show that, the value of short circuit current decreases as the incident solar radiation decreases and vice versa, where as open circuit voltage remains constant.
4.1.2) CIRCUIT DESIGN AND POWER MANAGEMENT
The most important step in achieving an efficient solar energy harvester is to design a harvesting circuit that can manage energy drawn from solar panels and effectively transfer the captured power to the target systems. However, while designing a circuit the concept of maximal power output should be considered. In order to over come this problem maximal power point tracker (MPPT) is used continuously to track the optimal operating point and also by clamping the output terminals of the solar panel to fixed voltage . Adding to this MPPT, depending upon the operating voltage and supply voltage required a DC-DC converter is used to continuously supply voltage to low power electronic systems. In all the first and foremost target of any solar energy harvesting circuit is to capture all the harvested energy from solar panel and effectively utilize the total power (even mA of power).
Power management is another important aspect in solar energy harvesting design other than circuit design. The concept behind the power management technique is distributing energy to sensor networks “instead of minimizing the total energy consumption” . An efficient power management system should satisfy all the conditions like, the information about availability of energy for future use obtained by developing parameterized macro models for power source and routing of solar power to more than one node.
4.1.3) HELIMOTE DESIGN
The word Heliomote had its origin from a Greek word Helioswhich means sun. By understanding the name we can say that this system is powered continuously by solar radiation. Heliomote system is built using MICA2 platform with single storage energy scavenging system. The system is designed to be user friendly where the user can simple plug in the Heliomote system (consisting of solar panel, batteries etc.) to the target system for its function. While designing a Heliomote harvesting system several factors like hardware, software and performance of system should be considered.
Heliomote system uses a solar panel with an area of 4-4.0-100 with open circuit voltage of 4.0V and short circuit current of 100 mA, the values vary slightly depending on time of the day. The power captured from solar panels is used to recharge two AA-sized Ni-MH batteries each with a capacity of 1800 mAh . The voltage of the batteries varies between 2.2V and 2.8V. The amount to which batteries should be recharged is monitored to a particular operating point (called maximal power point) as overcharging is hazardous and also causes instability to batteries performance. Similarly, undercharging of NiMH battery leads to the damage of rechargeable batteries as the load continues to draw power from batteries even after the voltage has dropped below threshold . In order overcome this problem a threshold voltage is created. Electronic systems should be provided with continuously supply of power for its proper functioning. In order to provide a constant supply of voltage specific DC-DC converter is used depending on the system supply voltage and operating voltage range of the batteries used. Also a boost converter is used to improve power supply if the supply voltage falls below the battery voltage. The amount of harvested solar energy available and its usage should be monitored continuously this can be done by energy monitor component. Information gathered from energy monitor is used to perform harvesting aware performance applications in microcontroller.
Careful steps should be taken while designing a PCB (printed circuit board) of Heliomote system. Electromagnetic interference (EMI) radiated from switching supply source should be decoupled by placing the ground plane low in the stack which helps in preventing EMI affecting other circuits . Small filter components are used when PWM (pulse width modulation) frequency of DC-DC converts value is above the operating frequency. Due to effective filtering of peak currents the noise frequencies fall drastically into filters stop bands. Noise can be reduced by placing the EMI components away from the RF circuits.
Performance evaluation (or efficiency) of Heliomote system is tested by connecting a potentiometer in the place of load in order to vary load current. The efficiency of DC-DC converter is estimated to be 90%, however the total efficiency of Heliomote system is reduced to 80% and 84% for typical operating values due to reduction in the efficiency of filters, overcharge and undercharge protection units, diodes, board and battery monitor IC . A complete mathematical analysis of Heliomote is presented in .
4.1.4) PROMETHEUS SYSTEM
Prometheus scavenging system uses double storage energy technology built on a TelosB platform for harvesting solar energy . Harvested energy is stored in two buffers (primary and secondary buffer). Initially primary buffer charged by available solar energy is used to power Prometheus node. The remaining extra energy available from primary buffer is used to charge secondary buffer. However a shift of node from primary buffer to secondary buffers occurs when the primary buffer energy falls below threshold level, node returns back to primary buffer until it is completely recharged .
Prometheus energy harvesting architecture shown in the figure above has the following components like solar energy source, primary buffer (super capacitor), secondary buffer (Li-ion battery), power switch enclosed to Telos sensor node and charge controller . Super-capacitor is used to power the node whenever solar energy is available. However Li-ion battery comes into use only when there is no solar energy i.e during nights, on cloudy days etc.. so, super-capacitors are used as first storage component whereas second storage consists of Li-ion battery. Prometheus harvesting system differs from Heliomote harvesting system with an additional storage component, also charging control is accomplished in software. A software driver is used to manage the switching of node between both energy storage buffers during charging to power Telos. The concept of two storage elements is implemented in Prometheus energy systems because the battery technology used in Heliomote has a limited number of deep recharge cycles.
A model state diagram is shown below which explains the logic implemented by the driver to switch between power sources. The main function of SWITCH block shown in the above figure is used to switch between super-capacitor and Li-ion battery.
By observing the state diagram in Figure we can state that, a high threshold is created to draw a barrier between super-capacitor and Li-ion battery. Super-capacitor is used to charge the battery as long as the charge is above the threshold point when compared with Li-ion battery as a result battery is charged from super-capacitor. Whereas the super-capacitor is charged (recharge option is available) when the charge is below the threshold point, i.e., recharge opportunity is available. Node is powered by Li-ion battery when the recharge opportunity is not available. The node switches returns back to the super-capacitor when recharge option is available back again. The process repeats continuously depending on the availability of solar power.
4.2) RF ENERGY HARVESTING
Radio Frequency Identification is a new technology developed to harvest energy from the potential RF sources like TV stations, radio stations, mobile telephony, wireless networks. Rectifying antenna (rectenna) is used collect all the radiated energy and convert into useful energy with the help of Schottky diode located between the antenna dipoles. Rapid growth in RFID (Radio Frequency Identification) systems has led increased applications in public transportation, logistics, airline baggage tracking, locate and track people, assets etc.
A Radio Frequency Identification system generally consists of readers (or interrogators) and a number of transponders (or tags). Rectenna (receiving antenna) are used to collect the time varying electromagnetic RF wave transmitted by reader to power RFID tag .There are two different types of tags in general, one is Active Tag and the other one is Passive Tag. Active tags are used in long distance applications. Active tags are provided with their own battery supply, so their functioning doesn't depend on external RF energy. Passive tags/transponders uses RF energy transmitted to power its functioning under lower frequencies with in shorter arrange. As Passive tags don't include battery, they are cheaper and perform to greater longevity when compared to Active tags. A typical RF energy harvester is shown in the figure below.
Energy harvesting devices must have an energy receiver or antenna to collect the transmitted RF power by source. The area of receiving antenna is called effective area. RF signal is also called carrier signal, this signal when passed through an antenna coil located on RFID tag generates AC voltage across the coil . The effect of mutual inductance in loop antennas results in magnetic coupling between RFID tag and RFID reader. The amount of power generated depends on the effect of mutual inductance between reader and tag. This effect is shown in the figure below.
The amount of power received from the antenna can be calculated as
PR = S*Aeff [ m^2]
Aeff = G^2
PR = PS*G1*G2*
PR = Received power
PS = Transmitted power
Aeff = Effective antenna area
G = Antenna gain
r = distance
The RFID tag modulates and changes the amplitude of the received carrier signal. The reflected RF signals transmitted by RFID reader give details of changes in amplitude to denote the presence of RFID tag. A HIGH Q crystal resonator is used for impedance transformation. The sensitivity and operational distance frequency are -30dBm and 24 MHz respectively.
The key benefits of RF energy harvesting are power over distance, one source to many receivers and power is controllable. RF energy harvesting can be practically implemented in building automation in areas like indoor sensors, low light areas, behind walls and above ceiling. Other application examples are industrial monitoring and location tracking. But at present the amount of energy levels actually are too low that no electronic device is able to use them. Future advancement of harvesting technologies allows the development of low power devices to recycle the RF energy emitted from different sources.
5. KINETIC ENERGY
Kinetic energy is one of the most promising sources of energy available in our day to day life. “The main principle behind generating kinetic energy is the mechanical deformation of a structure or some stress or strain applied on the structure or displacement of a moving part inside energy harvesting device. All these actions can be converted to electrical energy. Kinetic energy is available from both Human and environment energy harvesting devices. Energy can be harvested using Kinetic energy principle using three methods
5.1) Piezoelectric energy harvesting.
5.2) Electrostatic energy harvesting.
5.3) Harvesting energy by Magnetic induction.
Two types of conversion technologies are used to harvest energy. One possible approach is to respond to vibration or displacement of a proof mass, the energy obtained by this process is dependent on mass. Another possible solution is the stress applied from an external element causes a deformation (elastic energy) which is converted into electric energy.
5.1) PIEZOELECTRIC ENERGY HARVESTING
Piezoelectric energy can be generated by causing a deformation or applying a stress by different means to a piezoelectric element is directly converted to electric energy via piezoelectric effect (Piezo means pressure). Piezoelectric effect was discovered in 1880 by Jacques and Pierre Curie. Many sources like vibration of machines, structural strains, human motion etc are harnessed to collect piezoelectric energy. Piezoelectric energy generators (PEG's) are not complex systems and can easily be fitted in any closed material with no rotating shafts to generate energy.
Two types of Piezoelectric materials are used to convert mechanical force to electrical energy.
5.1.1) Piezo-electric films
5.1.2) Piezo-electric ceramic
5.1.1) Piezoelectric films
The first type of piezoelectric material i.e piezo-electric films are flexible and exhibit piezoelectric effect due to the intertwined long chain molecules attracting and repelling each other . An example of piezo-electric films is PolyVinylidene Fluoride (PVDF). PVDF's exhibits stability over time and does not depolarize. PVDF material when stretched or bent produces an electric potential. When bent the external part of PVDF sheet is expanded, whereas the internal surface of the sheet is contracted. The result of both these actions produces voltages across terminals. The expansion and contraction effect is shown in the figure below
5.1.2) Piezo-electric ceramic
On the other hand the second type of piezoelectric material is Piezo-elecrtric ceramic. The rigid and crystal structure of piezo-electric ceramic is responsible for piezoelectric effect. An example of piezo-electric ceramic is Lead Zirconate Titanate (PZT). The expansion and contraction of PZT sheet (two PZT unimorph sheet attached on either side of the metal bachplate form a PZT dimorph) develops charge similarly as PVDF. Due to ceramic nature of PZT, it is not flexible as compared to PVDF. PZT unimorph using a metal backplate used to prevent damage is shown in figure below.
The efficiency of Piezoelectric energy harvesting device depends on several factors like piezoelectric material, circuit design, storage methods, type of batteries used etc.. Piezoelectric material and interface circuit design are identified as two main parts of harvesting device. The energy harvested from piezoelectric energy harvesting device is stored from future use using available storage methods. Rechargeable batteries and super capacitors (ultra capacitors) are the two available technologies for energy storage. Rechargeable batteries such as Ni-Cd/Ni-Mn have different characteristics with a limited capacity of charging and discharging. To overcome this problem super capacitors are used as storage buffers. Super-capacitors act as resistive loads.
The above gives an example of piezoelectric harvesting system. An interface circuit consisting of load or storage buffer with a rectifier is connected to piezoelectric material to convert energy. The interface circuit acts as AC/DC converter. The concept of piezoelectric energy harvesting is practically implemented in industrial sector, transportation, human applications and structural applications. An example of practical implementation is Shoe-powered RF tag system. The energy required to transmit 12-bits of wireless information through RFID tag is generated when the bearer walks with shoe. Another example is Self powered push button controller, where energy is generated with the push motion of the button. When the button is pushed the pressure is applied on PZT element
-oscillates at a particular resonant frequency. Generated electrical energy is stored in a capacitor.
All the above systems mentioned convert mechanical energy into electrical energy. A vast research is to be done to harvest energy from human movements for long standing power supply to the electronic devices. Integrating piezoelectric systems with wireless sensor networks is another area where research focus is to kept for future advancement.
A detailed study on Piezoelectric energy generators is carried on in the chapter 7.
5.2) ELECTROSTATIC ENERGY HARVESTING
Electrostatic energy harvesting system principle mainly depends on the mature MEMS (Micro ElecroMechnical Systems) processes. The key principle behind generation of electrostatic energy is that the moving part of the transducer moves against an electrical field. When the plates are separated at constant charge with the charge between two electrodes increases the volume of the electric field results in the increased electric potential energy. Different designs are developed to harvest energy, one among them is, coulomb-damped resonant generators (CDRGs) which is based on electrostatic damping. In CDRGs parallel capacitor is operated with a constant charge and a comb capacitor operated with a constant voltage. Voltage, charge and capacitance of CDSGs is interrelated. Voltage will increase and capacitance will decrease if the charge on the capacitor is maintained constant, whereas both charge and capacitance decreases if the voltage on the capacitor is maintained constant. A model design of electrostatic energy harvester is shown below.
An equivalent circuit diagram of the above harvesting model is shown below. By observing the circuit diagram capacitance Ce is in series with voltage source Ve. The design discussed is often referred to as in-plane overlap converter as the transduction is quite insensitive to stray capacitances between the proof mass and package. At the initial point of starting energy harvesting cycle, the capacitor must be previously charged to a maximum capacitance point. Two different harvesting schemes are discussed in this paper. One of them is Charge Constrained harvesting in which charge is maintained constantly by open circuiting a capacitor as its plates separated in response to vibrations. The conversion cycle moves from point A to B when the system capacitance is maximum Cmax , MEMS capacitor is charged by an voltage source to an initial voltage (Vstart) which is less than maximum voltage (Vmax). The relation between both voltage and capacitor is squared rather that linear. The area B-D show in the below figure explains the shift of plates from maximum capacitance to minimum capacitance with constant charge Qo. Voltage increases as the charge is maintained constant and the capacitor value decreased results in the charge return back to D-A. The process results in the output energy equal to area A-B-D. The energy gained from charge constrained conversion cycle is less when compared to voltage constrained conversion cycle. One way to overcome this defect is to add a capacitor in parallel with a MEMS capacitor. One of the main disadvantages in adding a capacitor in parallel is to increase the value of initial voltage source.
Another type of conversion technique discussed is voltage constrained conversion cycle. The conversion cycle starts when the capacitance reaches a maximum point with voltage source charging the MEMS capacitor moving the cycle from A-C, as shown in the above figure. The process of plates moving from Cmax to Cmin is shown using the path C-D. Capacitor is discharged when the capacitance is decreased and parallel plate distance increases. This process is explained in the above diagram with the path D-A. In order to move the plates some sort of energy is required, we consider this energy as mechanical energy. The harvested energy during this process is either stored in a capacitor or a rechargeable battery. Path C-D shows the mechanical vibration during the harvesting process. DA path is used as a common path for both voltage constrained conversion cycle and charge constrained conversion cycle to transfer the gained energy from MEMS capacitor along the path. A constant voltage is maintained across the plates to convert mechanical vibrations into electrical energy.
Greater amount of energy can be harvested if the voltage across the capacitor is constrained rather than charge across the capacitor. In order to achieve better efficiency results, the operation of switches should be done effectively. More care must be taken to synchronize the operation of switches with the mechanical oscillation as the frequency of the mechanical operation depends on the resonance frequency of the mechanical structure. The frequencies are operated in the order of KHz for miniature components.
5.3) ELECTROMAGNETIC INDUCTION
Electromagnetic Induction is the process of converting the available ambient kinetic energy into electric energy. Electric current is generated by the relative movements of magnet and coil or due to changes in the magnetic field. The variation of magnetic flux across the electric circuit causes electric field. The coil typically acts as a conductor is placed within the magnetic field. The amount of electric current generated mostly depends on several factors like strength of magnetic field, number of turns of the coil, magnetic flux density of magnets, excitation frequency, geometry configuration and relative motion. Electromagnetic Induction was first discovered by Faraday in 1831. Faraday's law states that a time varying magnetic field incident normal to the surface of a coil will produce a voltage proportional to the number of coils (N), the area of the coils (A), the velocity of the spatially varying magnetic field (2πf), and magnitude of the time varying magnetic field (Bo). Both the equations given below explain the Faraday's concept clearly.
The amount of electricity generated depends on the size of the coil, this shows that the big transducers with large area coils generate more energy than the one with small transducers with small area coil.
Initially this paper discusses a basic model of magnetic induction transducer. This model works on basic principle, the vibrations created in the generator effects the seismic mass (m) connected to a spring (k) moves through a constant magnetic field to oscillate in out of phase with respect to housing to create a relative displacement. This displacement created is assumed to be in sinusoidal form, so that magnetic induction generator is able to convert into electrical energy. During the conversion process of energy from mechanical to electrical energy, electromechanical force (fe) in transducer damps the mass movement of the magnet with a factor Bm. Basic model of magnetic induction transducer is shown below.
The movement of mass (m) creates a displacement (z (t)). L and Rc shown in the above figure corresponds to inductance and parasitic resistance of the coil respectively, whereas l and R corresponds to length of coil and load resistance respectively. Relation between electromechanical force and voltage is given by the following equation given below. Newton's second law of motion is applied to obtain transfer function.
This paper also discusses two types of Electromagnetic Induction systems usually implemented in market.
5.3.1) Magnet through coil Induction.
5.3.2) Magnet across coil Induction.
The basic principle in a magnet through a coil induction system is, a “magnet is translating through a coil with no relative rotation” . This system consists of cylindrical coil of N turns if wire with a length lc and inner and outer diameter of dmax and dmin respectively. Bar magnet used in this system is of length lm, with radius rm. The power generated using this system is low as the coil never experiences complete flux reversal. The average radius of the coil and the cross sectional area is calculated as
rc = .
Ac = πrc^2
and the average diameter dc is 2rc. When placed in a co-ordinate axis, the midpoint is placed at origin with longitudinal axis of the magnet fixed along the y-axis.
The main advantage of this type of system is that it is easy to build
5.3.2) Magnet Across Coil Induction
In magnet across coil induction system the, magnet is moved across coil, generates high voltage as the coil experiences complete flux reversal. The system consists of a layer of magnets with air gap in between the magnets. Voltage is generated across the coil, as the movement of the magnet across the coil causes a change in the magnetic flux. Development of this type of systems is complicated to build.
6. THERMAL ENERGY HATVESTING
Thermoelectric energy harvesting is one class of energy harvesting system which converts thermal energy directly into electrical energy. Thermal energy is abundantly available in the environment in different forms or sources like human body, animals, machines and natural resources. Seebeck in 1821 discovered Seebeck effect which is implemented in thermoelectronics. “Seebeck devices produce a relatively low voltage output in the presence of small thermal gradient” [Ujihara]. Due to small thermal gradient, the power output and efficiency of the devices is low. To overcome this problem researches are conducted to develop super-lattice structures.
The design of a thermoelectric generator is basically done by sandwiching thermopiles between a hot and a cold plate. Thermocouples basically made of p-type and n-type of semiconductor connected electrically in series and thermally in parallel when arranged in large number from a thermopile . “Seedbeck coefficient is positive for p-type materials and negative for n-type materials” .An electric circuit is formed by connecting a load connecting to electricity. An efficient system with maximum power can be generated only when, both the load matches electrical resistance of system and thermal conductance of the thermocouples equals that of the air between plates [vullers]. Temperature gradient at the junction and peltier effect are the two factor effecting the transmission of heat in thermoelectric systems.
Converting ambient thermal energy into electrical energy using Seebeck effect can be fundamentally explained in the following discussion. An electric potential difference is created when two conductive materials (A and B) attached to a load (L). Two temperatures T1 and T2 are taken to form a temperature difference T2-T1. A charge (Je) flows across the resistive load when a temperature difference (T2-T1) is applied across the conductive materials. Equation governing the Seebeck effect is taken into consideration.
Єb - Єa = Єabo - JeRAB
RAB = Total resistance of isothermal loop measured in ohms.
Єabo = Open circuit voltage.
Je = Induced current flowing through load measured in amperes.
The above equation describes the electric potential difference in volts across load.
Єab0 = πAB (1/ T) dT
Єab0 is a function of Peltier coefficient (πAB).
Peltier coefficient is different for different materials and is measured in joules per columb-Kelvin (J/(C*K)).
Thermoelectric energy generator is implemented in many practical areas like human body, machines and natural resources etc. Seiko thermic wristwatch is one example of system implemented to human body. Thermic wristwatch uses 10 thermoelectric modules to generate sufficient microwatts for its functioning.
A thermo energy harvester of 0.5 cm^2 in area by 16 mm thickness with Bi2 Te3 generates 10 µA at 3 V with 5ºC temperature difference. An airplane engine has typical temperatures of around 100ºC to 2000ºC. When the engine is functioning all the energy is wasted in the form of heat. When a thermoelectric generator is implemented in airplane engine, wasted heat is converted into electrical energy.
7 Piezoelectric Energy Generators (PEG's)
Piezoelectric energy generators convert mechanical energy into electrical energy. Piezoelectric energy generators (PEG's) are simple in structure and design consisting of piezoelectric ceramics (Lead Zirconate Titanate or PZT) and electrodes. The simplicity in their design helps them to fit in any small devices for power generation. A detailed study on stacks, beams and circular plates is carried in this chapter. Lead Zirconate Titanate or PZT is used as Piezoelectric material for energy generation in the following discussion through out this chapter.
A lot research is carried on PEG's based on various conditions. The performance of PEG's is lowered when impact and resonance are applied on the PEG system. Another area of research is to control noise using acoustic isolator, which used two diaphragms one generates energy while the other controls the noises using energy from other diaphragm . Another area of research is the generation of energy from human motion using polymer piezoelectric materials and thunder actuators .
The basic theory used in piezoelectric effect is Young's constant. Young's constant measures the ratio between stress and strain of the particular element.
An electric potential ue is observed as the piezoelectric element is vibrated.
Energy generated by piezoelectric element (E) = Piezoelectric coefficient (ue)
The conversion units from mechanical vibrations to electrical energy is shown below,
E = ue =>
Energy is measured in Joules = Nm
Power from Piezoelectric element =>
The numerical relation between electric field and stress is given by
Ei = -gij σ (xi)
gij = Piezoelectric constant
xi = thickness direction axis
7.1) Basic Configurations of Piezoelectric Material
The basic configurations of the piezoelectric energy generators (PEG's) taken from many of the existing actuator designs are helpful in explaining the working functions of mathematical models, also helpful in separating different design concepts. Initially basic configurations of the piezoelectric material are studied.
7.1.1) Charge Generation
In order to shift the free electrons available in the coil around a fixed magnetic rotator electromagnetic force is applied using electromagnetic generators. Where as the piezoelectric ceramic used as piezoelectric material is a non conductive material, so free electrons are not available in the coil avoiding the free movement of electrons. The crystal nature of the PZT has some fixed electrons. When external pressure is applied on the crystal, the deformation of the crystal causes slight movement of the fixed electrons generates electric force affecting the equilibrium of the conductive material.
The open circuit charge generation relation is given by
Dij = d ijk σ lk
d = constant when static load is applied
D= charge per area
σ = applied stress
i and j = both range from 1 to 6
7.1.2) Dielectric Properties
“Dielectric properties are observed on the piezoelectric ceramics”. A charge dipole is created when the positively charged atoms moves away from the centre of the crystal. The direction of movement of these atoms from the centre determines the poling direction which can be altered by changing the heat and voltage values. Over heating of the piezoelectric ceramic results in a new poling direction affecting its polarity, this can be observed along the applied voltage seen in figure 21. The threshold in the heating is measured using Curie temperature.
The input and output relations are affected by the polling direction. Both 6mm and 4mm ceramic crystals are considered to be as common piezoelectric materials with 5 constants d31, d32, d33, d15, d24. When we consider a 4mm crystal,
d31 = d32 and
d15 = d24
Therefore the overall magnitude relations of other three constants is given by, d15 > d33 > d31
7.1.3) Stacked piezoelectric Device
Several individual plates are stacked to form a multilayered d33 device in order to combine the effects of individual plates. Whereas single layered piezoelectric material doesn't support effectively for actuator purpose. Stacked piezoelectric device produce the same strain as single plate device with low electric field. Stacked piezoelectric devices currently available are d33 devices which are stack actuators and interdigital actuators. With this concept the performance can be increased by n times where nis the total number of layers.
The main disadvantage of stacked piezoelectric device is, power can be generated only when the force is applied in the longitudinal direction as shown in the figure.24 i,e it cannot generate electrical energy with lateral force. In order to convert lateral forces to longitudinal forces an external system for conversion is required which in turn makes the system complex. Another disadvantage of the device is the size of the device is relatively large the small sized stacked devices are hard to manufacture.
7.1.4)Sliced Stack Piezoelectric Device type-33
Stacked piezoelectric material is broken with an external twisting force to form an sliced stack piezoelectric material. This type is advantageous when used with an bender, but long plate or beam cannot be made due to brittleness of the piezoelectric layer. One disadvantage of this device is sliced plate is only used for rectangular patch on cantilever bender, but it is not applicable for diaphragm applications.
This problem can be solved by using a piezoelectric plate with interdigitated electrodes.
7.1.5) Interdigitated Piezoelectric Device
Interdigitated piezoelectric devices technology is evolving newly to increase the performance just by changing the polling direction. This type devices use the concept of polarization of the piezoelectric material. Below figure.26 show the interdigitated configuration, polling direction is parallel to the stress direction and electrodes cover only a portion of the surface.
Mostly the polling direction is done perpendicular to the stacked piezoelectric plate's surface. Figure 27 explains the fabrication process of interdigitated piezoelectric patch. The initial electrodes present on the stacked piezoelectric plate are removed and rearranged in interdigitated pattern on surface. After this step maintaining constant high temperatures a high voltage is applied, make sure that the temperatures should be above Curie temperature. With this process polling direction is changed according to electric field conditions.
The main disadvantage of this device is because of the non uniform poling field directions and incomplete depth of poling field exact 33 poling direction cannot be achieved . Another disadvantage is 33 poling doesn't exit exactly beneath the electrodes, also generated power less. This problem can be solved by surface electrode effect with very thin electrodes.
7.1.6) Surface Electrode Effect
In this concept the PZT devices are covered with electrodes to generate different electrical energy characteristics and collect the electrical energy generated when stress is applied. This concept has its application in cantilever and diaphragm bending structures. This effect is observed in figure 28 below
From the above figure we can clearly observe that when a force is applied in the upward direction, the inner region expands in the upper surface and the outer region is compressed in the upper surface. When the stress is distributed the power generation will be small, because the fully covered electrodes on the surface acts as external electric field reduce the system stiffness. In order to overcome this problem electrodes present on the surface of the different stressed regions should be disconnected.
7.2) Modelling of Piezoelectric Energy Generators (PEG's)
In this chapter a detailed study is carried on various electric power generation models and their equations. Below figure 29 shows various methods of PEG's branched together.
While selecting the material for PEG's the relation between stress, strain, electric field and charge are calculating with specified equations considering the effect of strong electro mechanical coupling.
The power generation equation of a piezoelectric material is divided into two parts, first part is elastic energy and second part is electric energy, this is shown in the equation given below,
Energy = *stress*strain + *charge*electric field
7.2.1) Cantilever Beam
Cantilever beam is a commonly available piezoelectric generator. Multilayered stacked cantilever beams are widely used for mechanical bending of materials. The distribution of the applied stress is constant through out the material when we consider the concept of cantilever. In the cantilever beam model stress is applied only in the longitudinal direction.
The charge calculation is done by differentiating the total structure energy with respect to voltage. The energy generation for unimorph cantilever beam is clearly studied by Smith J G . By observing the above figure 30 a force Fo is applied at the edge of the cantilever beam, this force bends the beam longitudinally, now we calculate the momentum of the material.
M (x1) = Fo (L- x1)
The strain of the beam is calculated with the equation given below,
Ε(x1,z) = - = -ρ (x1,M, E3)
Where R(x1,M, E3) = Radius of the curvature.
ρ (x1,M, E3) = curvature
The momentum of the material should be same through out the beam, momentum of the cantilever is calculated as,
M = ƪσ(ε (x1,z) E3)z dz
The applied stress inside the piezoelectric layers is coupled with strain and electric field. Now the momentum equation is a function of curvature and electric field.
M = ƪσ(ρ (x1,M,E3) E3 )z dz
By calculating the above two equations the curvature equation is obtained in terms of electric field and force,
Ρ(x1, Fo, F3) = f1 (x1, Fo ) + f2 (x1, E3 )
Taking all these equations into consideration total charge generated can be calculated by differentiating with respect to voltage.
7.2.2) Cantilever Bender
Cantilever benders are considered as better energy generation devices because the conversion of applied force to high stress is easily done. Cantilever benders are able to generate significant deflection, they are found in applications such as micro valves. The working mechanism of cantilever bender is not complex, they are easy to fabricate. When a force is applied one layer is in tension and the other layer is in compression.
Cantilever benders are classified into types,
Unimorph cantilever bender
Interdigitated unimorph cantilever bender
Triple-morph cantilever bender
Interdigitated triple-morph cantilever bender.
Unimorph Cantilever Bender
When there is a change in the applied temperature in the unimorph cantilever bender piezoelectric layer expands or contracts while the non piezoelectric layer doesn't have any change.
Neutral surface can be calculated using the following equation
Where A= cross sectional area of each layer
h = thickness of each layer
E = Young's moduli of each layer
z = distance from the centre
subscripts 1 and 2 show upper and lower layer, p shows piezoelectric parameters, m shows non-piezoelectric parameters.
Using the equation of the neutral surface strain can be calculated in terms of curvature,
ε1 = - (z-zc)/R è ρ(z-zc)
Mostly used unimorph cantilever bender is type-31, it is made up of a piezoelectric layer operated in mode 31. The electric field direction of type-31 unimorph cantilever bender is shown figure 32.
Electric field in the unimorph cantilever bender is calculated by using the following principle.
Electric Field (E)=(Voltage across electrode (V))/ (PZT thickness between electrode)
(ii). Interdigitated Unimorph Cantilever Bender
Interdigitated unimorph cantilever bender varies slightly with type-31 unimorph cantilever beam at the areas with no electrodes. Basic model of interdigitated unimorph cantilever bender is show in the figure 33 below
(a) side view (b) top view
For a piezoelectric layer there is a change in poling direction, but for non-piezoelectric layer the relation between both stress and strain is same as for 31-type. With the change in the poling direction the elastic properties also change as a result the neutral surface also changes. The overall energy of the interdigitated system is calculated by summing each segment between electrodes.
(iii) Triple-morph Cantilever Bender
Triple morph cantilever bender consists of three layers, outermost two layers are piezoelectric layers and the middle layer is non-piezoelectric layer. Due to this arrangement the polarities of the two layers are in opposite direction. Due to symmetric nature along the cross section their neutral surface lies in the middle of the beam.
Using this basic concept type-31 triple-morph cantilever bender is designed. In type-31 bender the top and bottom piezoelectric layers are connected to electric fields, i,e lower piezoelectric layer is connected to positive terminal and top layer is connected to negative terminal. Overall energy generated by this device is calculated by summing the three layers. The total energy generated from the force Fo is calculated using the following equation,
Where U = position of the layer (upper Uup, middle Um, lower Ulo)
Vgen = generated voltage
Qgen = generated charge from the applied force.
(iv) Interdigitated triple-morph cantilever bender
Interdigitated triple-morph cantilever bender consists of three layers, the top and bottom layers are piezoelectric and the middle layer is non-piezoelectric. The polarities of both the top and bottom layers are in opposite direction. In interdigitated triple-morph cantilever bender the piezoelectric material is poled in type-33 direction. The basic view of the polarities of interdigitated triple layered PEG is shown in the figure below,
By observing the above figures the bottom layer should be connected to positive electric field to obtain the positive polarity and vice versa.
The total energy generated from the applied force Fo can be calculated as,
Where U = position of the layer (upper Uup, middle Um, lower Ulo)
Vgen = generated voltage
Qgen = generated charge from the applied force
7.2.3) Diaphragm with Constant Pressure
Diaphragm structure is same as circular plate structure, commonly used to detect acoustic pressure and hydraulic pressure. The elasticity of the thick plate shows that there is no stress and strain along the thickness direction. Also the deflection generated is much smaller that the thickness of the plate.
When a constant pressure is applied to the diaphragm, we get two moment terms, one term is in radial direction and the other term is in angular direction. As a result there will be two curvatures each in the form of moment terms. The elastic properties are same for both moment terms. Three types of cases are considered in diaphragm with constant pressure,
- Unimorph Diaphragm
- Unimorph Inrterdigitated Diaphragm
- Triple layered Diaphragm
(i) Unimorph Diaphragm
In the unimorph diaphragm the neutral surface is not located in the middle of the surface because due to change in the electric field neutral surface moves from its position. In order to calculate the neutral surface the elastic properties of PZT material are taken into account. A schematic view of the unimorph PZT diaphragm is shown in the figure 35 below,
An example of this type is type-31 unimorph diaphragm. In this type of diaphragm the electric field is constant through out the structure. The strain curvature of the type-31 diaphragm can be calculated using the following equation,
εr = -ρr (Z-Zc)
εϴ = -ρϴ (Z-Zc)
(ii) Unimorph Inrterdigitated Diaphragm
In unimorph interdigitated diaphragm overall energy generated can be obtained by integrating over the volume where no electrodes are present on the surface. For the poling process to occur the electrodes position should be far and narrow from each other. A schematic view of the unimorph interdigitated diaphragm show in the figure 36 below,
By observing the above diagram, there are n number of clusters with n+1 number of electrodes located on the surface, a is the radius of the diaphragm and b is the width of the electrode.
The total energy generated after an external pressure is applied is calculated by using the following equation,
Ugen = Qgen * Vgen
(iii) Triple Layered Diaphragm
The electric field direction of PZT layers has opposite polarities. It consists of three layers, in a triple layered diaphragm the top piezoelectric layer is connected to negative electric field and bottom layer is connected to positive electric field. An example of this type is type-31 triple morph circular plate.
In this type of diaphragm the total energy generated with the application of constant pressure is zero.
8 Model Preparation
Various steps are carried out while the specimen preparation. Aluminium substrate and a PZT layer is used while preparing a unimorph diaphragm. A PZT layer of thickness 0.127mm is used. The fabrication of the model is done in different steps like,
- Etching the PZT electrodes
- Poling and bonding
The etching process is carried mainly to modify the poling direction in certain areas. In order to etch nickel electrode Ferric Chloride acid is used and for masking acid resistive ink is used.
The poling process is done at high temperature and high electric field as discussed earlier in the chapter 7. the curie temperature should be 250 ºC, with electric fields of up to 2000 V/mm. After finishing the process of poling it is tested for poling direction with the help of PZT transformer (Phase test).
In the recent years a vast area of research and investigation is continuously carrying on a number of ways to extract energy from environment and surrounding activities and convert it into electrical energy to power low power electronic devices. Researches have showed that harvesting energy from day-to-day activities plays an important role in developing future microelectronic devices. Evolution in the technology of batteries and reduced power consumption has closed gap between generated energy and energy spent. Recent developments in nanotechnology, microelectronics and communication techniques add to reduced power consumption.
This paper has provided a review on state of art energy harvesting technologies to power low power electronics. This paper also discuses various types of energy harvesting systems, their architecture, types of energy sources available, battery technology, power consumption. Practical implementation of power harvesting technologies and advancement in low power electronic reduces the emission of green house gases to some extent.
7. Risk Assessment
Risk assessment table shows the risks which are likely to come in the course of project which is evaluated for the factors like Importance of the risk and Likelihood to happen.
RISK(L-Low, M-Medium and H-High)
2)Poor System Performance
4)Software License Ends
5)Software doesn't work
7)Personal Reasons like illness
The Risk management evaluates the consequences and the control measures to overcome the risks presented in the Risk assessment table.
1)Entire System fails
Use another Hardware
2) The proceeding system will not work
Improve the system performance using proper implementation.
3) The System can't be implemented
Try to get the Software working
Use another Software(worst case)
4) Cannot use the Software
Buy the License
5) The User Interface will not work
Correct Implementation of the Software
6) Lack of experienced person to evaluate.
Seek the help of other experienced guy
Follow the Supervisor(worst case)
7) Cannot continue the work
Try to work if possible
Apply extension for submission(worst)
9. Professional, Legal, Ethical and Social Issues
Professionalism and avoiding plagiarism as key factors in every research project must be considered bit in this project there are no professional, ethical or social issues.