The Electrostatic Energy Harvesting Engineering Essay

The exerciseless Energy Harvesting Engineering Essaythis paper presents a general idea of the unchanging animation harvesting devices. Their working principle, harvesting method and basic designs will be expounded. And another cardinal new approaches, 2D heftiness harvester and non-resonant aptitude harvester with curl concourse will be shown. The 2D get-up-and-go harvester can harvest aught in arbitrary directions in a plane. And non-resonant dynamism harvester with rolling mass shows its low frequency capability. It can harvest 0.5 W at 10Hz. Further improvement of this design may applied to competency harvesting from human body motion.IntroductionGenerally, batteries are the most reliable power source for electronic devices. It is powerful, easy to use. However, it can only provide regular power for a limited period. After that, the batteries construct to be changed. Therefore, for those devices that catch large amount of number or in inaccessible position. Batter ies are not worthy for them. Energy harvesting devices are one of the attractive options of these applications. Energy harvesters can harvest cleverness from different ambient sources such as solar, thermal and vibration. In these cases, solar is not a reliable source, temperature gradients are modest, vibration seems the more abundant, stable and predictable choices.Currently, three major methods apply to vibration energy harvesting, electromagnetic, electrostatic and piezoelectric mechanisms. Each technique has its own advantages. Lot of articles did research and provided good conclusion on them. 1-3Electrostatic energy harvesting device has the last energy harvesting capabilities in these three mechanisms, but it has the most specific advantages. It facilitates CMOS integration 4. That means it can realize self-power integrated circuits as an on-chip power source. It similarly environment protects. Unlike piezoelectric and electromagnetic counterparts that require exotic mat erials. Electrostatic devices are mainly made of silicon.5This paper will focus on the legitimate electrostatic harvesting research. Its working principle and harvesting processes will be discussed in the first part. Some new approaches will also be presented.electrostatic harvestingOperating PrincipleThe electrostatic harvesters harness the work done against the electrostatic force of a multivariate capacitor. In other words, the vibrations cause the cranny distance or crossing area of a parallel plate capacitor to vary under constant steering or voltage condition. This causes the electrical condenser change of parallel plate capacitor and produces electrical energy.The fundamental definition is given by the formula below.C=Q/V (1)Where C is capacitance of variable capacitor in farads, Q is the charge on the plate in coulombs and V is the voltage on the plates in voltsC= (A/d) (2)Where A is the overlap area of the plates in and d is the distance mingled with the plates in m. T his equation shows the capacitance is proportional to A and reverse proportional to d.(3)E is the work done in joules.If the charge Q is held constant, then V will vary as C changes because of their inverse proportional relationship. Then from, the relation between voltage and capacitor energy is square rather than linear. As a result, the work done will increase as the C decrease. That provides the harvested energy. Similar smallg happens when the voltage V is held constant and Q varies. 1They are known as the voltage-constrained method and charge-constrained method. 6 In the recent applications, the charge-constrained method is more popular over the voltage-constrained method as the voltage-constrained method requires an extra charge beginning to keep the voltage in a constant value, while the charge-constrained method only requires one. 4For charge constrained system, as shown in Figure 1 the energy conversion cycle starts as the variable capacitance reaches it maximum Cmax. T he charging process is represented by the elbow room from point A to Point B in figure 1. At point B, the energy stored can be shown as,(4)From point B to point C, an external charge reservoir is connected in nightspot to keep the charge constant. The capacitance is starting decreasing as the overlap area A decreases or the distance between the plates d increases. The voltage is inverse proportional to the capacitance which is why the voltage increases in this period. This period is the actual mechanical to electrical conversion period. The energy stored at point C is now,(5)The path from point C to point A is the discharging of the charge on the variable capacitor back into the charge reservoirThe whole process forms a comely energy conversion cycle. And the amount of energy gain is,(6)Usually there is a parallel capacitor is connected parallel with the variable capacitor in order to limit the maximum voltage that might damage the system during the harvesting. Then the energy eq uation is becoming,(7)Figure 1. Charge-constrained energy conversion cycle.B. Steps of energy harvestingThe vibration cycle in an electrostatic energy harvester has three steps, pre-charge, harvest and reset. Figure 2.Figure 2. vibration cycle of electrostatic harvester.In the system, the variable capacitor is pre-charged to the battery voltage, and then the capacitor is connected to the battery. The circuit has no current flow at first since the capacitor and battery have same voltage level. But with the separation of the capacitor plates or the decreases of overlap area, the voltage increases with the decrease of capacitance. Charge therefore flows into the batteries and energy is harvested. When the capacitance reaches lower limit value, the energy left in the capacitor will be driven back to the batteries and ready for the next cycle.C. Basic DesignsThese three mechanisms in the figure 3 are the three basic design structures of the electrostatic harvesters, in-plane overlap conv erter, in-plane infract- closing curtain converter and out-of-plane gap-closing converter. The in-plane overlap converter varies its capacitance by changing the overlap area between plunder out fingers the in-plane gap-closing converter varies its capacitance by change the displacement between comb fingers and the out-of-plane gap closing converter varies its capacitance by change the gap between the centre proof mass and two electrode plates.The most of the current designs of electrostatic harvesters are based on these three basic designs.Figure 3. (a) in-plane overlap converter. (b) in-plane gap-closing converter. (c) out-of-plane gap closing converter.D. Comparison in these three designsYe Mei Lim8 did a study on the proceeds energy for these three designs. Firstly the in-plane overlap and in-plane gap closing converters were compared. The Cmax for the one set of comb fingers were 0.122pF and the Cmin can be treat as zero since the application of silicon nitride dielectric co ating which is a very thin layer of chemical (up to 0.1m) that can electrically isolate the electrodes even the plates contacts with each other4. While using the same set of comb fingers, the Cmax were 0.149nF and Cmin were 0.122pF. By applying equation (7), the in-plane overlap converter harvests 1000 times less than in-plane gap closing converter. Then with the simulation of both in-plane gap closing converter and out-of-plane gap closing converter. The results were put up out that the in-plane gap closing mechanism is approximately 1.8 times that of the out-of-plane gap closing mechanism for load volumes between 5 and 50.NEW APPRAOCHESA. 2D Electrostatic HarvesterMost of the past electrostatic harvesters are only one degree of freedom. They can only harvest energy via one direction of motion. Y. Zhu fabricates a 2 degree of freedom electrostatic transducer for energy harvesting with vibrancy frequencies of 38520 Hz and 38725 Hz. It can scavenge energy in arbitrary directions in a plane with two resonance frequency peaks. Also an ultrasound-based method for powering the device is presented.Y. Zhus design includes a 2 degree of freedom motion mechanism. The seismic mass is coupled with both frames as shown in figure 3 with elastic flexures. This design makes the device be able to detect both movements in X and Y frames and also decouples the X and Y movements of the mass.Figure 4. Two degree of freedom motion mechanism to harvest any direction in-plane vibration energyFigure 5 shows the SEM image of the 2-DOF electrostatic transducer. And table I are the line parameters of this design. The width difference of X frame and Y frame gives the transducer two different resonance frequencies. The primary resonance frequency at 39238 Hz and second at 39266 Hz. That gives a 302 Hz of -10dB bandwidth. It is twice of the 1D resonator. This device can triumph 10mV through a 1M ohm resistive load and harvest 0.1 nW power. Since this transducer can be power by an ultras onic generator of frequency close to its resonance frequency. Since the ultrasonic is relatively safer than other power sources. This design may be useful for functions in medical environment.Figure 5. SEM image of the 2-DOF electrostatic transducer.Table 1. key parameter of the 2-DOF energy harvesterB. Non-Resonant electrostatic harvester with rolling massM.E.Kiziroglous design 10-11 focuses on maximizing the proof mass. In this design, an external free rolling proof mass is introduced. The mechanical energy is proportional to the proof mass, bigger mass generates more energy. This design is a non-resonant device. This property gives it wider applications.Figure 6. (a) Device structure. (b) eq circuit of the deviceThe Device structure is shown in figure 6(a). Figure (b) is the equivalent circuit of the device. The stainless steel rod acts as the contact switches and comb finger. When the steel rod is aligned with one of the strip electrodes, it connects with a Cu input Contacts to pre-charge the rod. That generates an electrostatic force between the rod and the strip electrode. That pulls the rod outside from the strip electrode and reduces the capacitance at constant charge. The rod then disconnects with the input contact and makes the contact with a discharge electrode. The energy will be transferred as a high voltage pulse. The test of the current model of this device reveals a capacitance ratio of 4 and demonstrates a voltage gain of 2.4.after Kiziroglou provides an advanced design of that 12. This time the glass substrate is form in a cylindrical shape. Figure 7 is fabrication and optical images of the device. The first prototype is characterised with plate size 1 x 10 mm and SiO2 dielectric thickness of 50 nm. A 10 mm-long, 2.5 mm-diameter steel rod was used. A minimum capacitance of 2 pF and a maximum of 9 pF are observed. The voltage gain is 3 corresponds to a priming voltage 30V. The power generation is 0.5W when the rod oscillation frequency at 10 Hz. The biggest advantage of this device is the capability of low frequency. That makes the human body motion as a suitable motion sources for it.Figure 7. Fabrication and optical imagesconclusionThe focus of this paper is to present the general idea current achievement of electrostatic energy harvesting. And it gives a related reference for the group project. For most of the electrostatic harvester designs, a relatively high resonant frequency comparing with human body motion is need. However, the low frequency capability of the non-resonant energy harvester with rolling mass shows the possibility of the application of this technology in the projects. Additionally, most of the current devices only have one degree of freedom. The 2D energy harvester design can harvest arbitrary directions motion in a plane, which sufficiently increases the power output of device. However, it needs a high frequency. That makes it not suitable for the requirement of the project.