The Electrostatic Energy Harvesting Engineering Essay

this paper presents a general thought of the electrostatic energy reaping devices. Their on the job rule, reaping method and basic designs will be expounded. And another two new attacks, 2D energy reaper and non-resonant energy reaper with turn overing mass will be shown. The 2D energy reaper can reap energy in arbitrary waies in a plane. And non-resonant energy reaper with turn overing mass shows its low frequence capableness. It can reap 0.5 µW at 10Hz. Further betterment of this design may use to energy reaping from human organic structure gesture.


By and large, batteries are the most dependable power beginning for electronic devices. It is powerful, easy to utilize. However, it can merely supply changeless power for a limited period. After that, the batteries have to be changed. Therefore, for those devices that have big sum of figure or in unaccessible place. Batteries are non suited for them. Energy reaping devices are one of the attractive options of these applications. Energy reapers can reap energy from different ambient beginnings such as solar, thermic and quiver. In these instances, solar is non a dependable beginning, temperature gradients are modest, quiver seems the more abundant, stable and predictable picks.

Presently, three major methods apply to vibration energy harvest home, electromagnetic, electrostatic and piezoelectric mechanisms. Each technique has its ain advantages. Lot of articles did research and provided good decision on them. [ 1-3 ]

Electrostatic energy reaping device has the lowest energy reaping capablenesss in these three mechanisms, but it has the most specific advantages. It facilitates CMOS integrating [ 4 ] . That means it can recognize self-power incorporate circuits as an on-chip power beginning. It besides environment protects. Unlike piezoelectric and electromagnetic opposite numbers that require alien stuffs. Electrostatic devices are chiefly made of Si. [ 5 ]

This paper will concentrate on the current electrostatic harvest home research. Its working rule and harvest home procedures will be discussed in the first portion. Some new attacks will besides be presented.

electrostatic harvest home

Operating Principle

The electrostatic reapers harness the work done against the electrostatic force of a variable capacitance. In other words, the quivers cause the spread distance or overlap country of a parallel home base capacitance to change under changeless charge or electromotive force status. This causes the electrical capacity alteration of parallel home base capacitance and produces electrical energy.

The cardinal definition is given by the expression below.

C=Q/V ( 1 )

Where C is electrical capacity of variable capacitance in Fs, Q is the charge on the home base in C and V is the electromotive force on the home bases in Vs

C=? ( A/d ) ( 2 )

Where A is the overlap country of the home bases in and vitamin D is the distance between the home bases in m. This equation shows the electrical capacity is relative to A and reverse proportional to d.

( 3 )

Tocopherol is the work done in Js.

If the charge Q is held changeless, so V will change as C alterations because of their reverse relative relationship. Then from, the relation between electromotive force and capacitance energy is squared instead than linear. As a consequence, the work done will increase as the C lessening. That provides the harvested energy. Similar thing happens when the electromotive force V is held changeless and Q varies. [ 1 ]

They 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 excess charge reservoir to maintain the electromotive force in a changeless value, while the charge-constrained method merely requires one. [ 4 ]

For charge forced system, as shown in Figure 1 the energy transition rhythm starts as the variable electrical capacity reaches it maximal Cmax. The bear downing procedure is represented by the way from point A to Point B in figure 1. At point B, the energy stored can be shown as,

( 4 )

From point B to indicate C, an external charge reservoir is connected in order to maintain the charge changeless. The electrical capacity is get downing diminishing as the overlap country A decreases or the distance between the home bases d additions. The electromotive force is reverse relative to the electrical capacity which is why the electromotive force increases in this period. This period is the existent mechanical to electrical transition period. The energy stored at point C is now,

( 5 )

The way from point C to indicate A is the discharging of the charge on the variable capacitance back into the charge reservoir

The whole procedure forms a proper energy transition rhythm. And the sum of energy addition is,

( 6 )

Normally there is a parallel capacitance is connected parallel with the variable capacitance in order to restrict the maximal electromotive force that might damage the system during the harvest home. Then the energy equation is going,

( 7 )

Figure 1. Charge-constrained energy transition rhythm.

B. Stairss of energy harvest home

The quiver rhythm in an electrostatic energy reaper has three stairss, pre-charge, crop and reset. Figure 2.

Figure 2. quiver rhythm of electrostatic reaper.

In the system, the variable capacitance is pre-charged to the battery electromotive force, and so the capacitance is connected to the battery. The circuit has no current flow at first since the capacitance and battery have same electromotive force degree. But with the separation of the capacitance plates or the lessenings of overlap country, the electromotive force increases with the lessening of electrical capacity. Charge therefore flows into the batteries and energy is harvested. When the electrical capacity reaches minimal value, the energy left in the capacitance will be driven back to the batteries and ready for the following rhythm.

C. Basic Designs

These three mechanisms in the figure 3 are the three basic design constructions of the electrostatic reapers, in-plane convergence convertor, in-plane gap-closing convertor and out-of-plane gap-closing convertor. The in-plane convergence convertor varies its electrical capacity by altering the overlap country between comb fingers ; the in-plane gap-closing convertor varies its electrical capacity by alteration the supplanting between comb fingers and the out-of-plane spread shutting convertor varies its electrical capacity by alteration the spread between the Centre cogent evidence mass and two electrode home bases.

The most of the current designs of electrostatic reapers are based on these three basic designs.

Figure 3. ( a ) in-plane overlap convertor. ( B ) in-plane gap-closing convertor. ( hundred ) out-of-plane spread shutting convertor.

D. Comparison in these three designs

Ye Mei Lim [ 8 ] did a survey on the end product energy for these three designs. First the in-plane convergence and in-plane spread shutting convertors were compared. The Cmax for the one set of comb fingers were 0.122pF and the Cmin can be treat as nothing since the application of Si nitride insulator coating which is a really thin bed of chemical ( up to 0.1µm ) that can electrically insulate the electrodes even the home bases contacts with each other [ 4 ] . While utilizing the same set of comb fingers, the Cmax were 0.149nF and Cmin were 0.122pF. By using equation ( 7 ) , the in-plane convergence convertor harvests 1000 times less than in-plane spread shutting convertor. Then with the simulation of both in-plane spread shutting convertor and out-of-plane spread shutting convertor. The consequences were found out that the in-plane spread shutting mechanism is about 1.8 times that of the out-of-plane spread shutting mechanism for burden volumes between 5 and 50.


A. 2D Electrostatic Harvester

Most of the past electrostatic reapers are merely one grade of freedom. They can merely reap energy via one way of gesture. Y. Zhu fabricates a 2 grade of freedom electrostatic transducer for energy reaping with resonance frequences of 38520 Hz and 38725 Hz. It can scavenge energy in arbitrary waies in a plane with two resonance frequence extremums. Besides an ultrasound-based method for powering the device is presented.

Y. Zhu ‘s design includes a 2 grade of freedom gesture mechanism. The seismal mass is coupled with both frames as shown in figure 3 with elastic flections. This design makes the device be able to observe both motions in X and Y frames and besides decouples the Ten and Y motions of the mass.

Figure 4. Two grade of freedom gesture mechanism to reap any way in-plane quiver energy

Figure 5 shows the SEM image of the 2-DOF electrostatic transducer. And table I are the cardinal parametric quantities of this design. The width difference of X frame and Y frame gives the transducer two different resonance frequences. The primary resonance frequence at 39238 Hz and 2nd at 39266 Hz. That gives a 302 Hz of -10dB bandwidth. It is twice of the 1D resonating chamber. This device can obtain 10mV through a 1M ohm resistive burden and harvest 0.1 nW power. Since this transducer can be power by an supersonic generator of frequence near to its resonance frequence. Since the supersonic is comparatively safer than other power beginnings. This design may be utile for maps in medical environment.

Figure 5. SEM image of the 2-DOF electrostatic transducer.

Table 1. cardinal parametric quantity of the 2-DOF energy reaper

B. Non-Resonant electrostatic reaper with rolled mass

M.E.Kiziroglou ‘s design [ 10-11 ] focuses on maximising the cogent evidence mass. In this design, an external free turn overing proof mass is introduced. The mechanical energy is relative to the cogent evidence mass, bigger mass generates more energy. This design is a non-resonant device. This belongings gives it wider applications.

Figure 6. ( a ) Device construction. ( B ) Equivalent circuit of the device

The Device construction is shown in figure 6 ( a ) . Figure ( B ) is the tantamount circuit of the device. The chromium steel 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 off from the strip electrode and reduces the electrical capacity at changeless charge. The rod so disconnects with the input contact and makes the contact with a discharge electrode. The energy will be transferred as a high electromotive force pulsation. The trial of the current paradigm of this device reveals a electrical capacity ratio of 4 and demonstrates a electromotive force addition of 2.4.

Subsequently Kiziroglou provides an advanced design of that [ 12 ] . This clip the glass substrate is form in a cylindrical form. Figure 7 is fiction and optical images of the device. The first paradigm is characterised with home base size 1 ten 10 millimeter and SiO2 dielectric thickness of 50 nanometers. A 10 mm-long, 2.5 mm-diameter steel rod was used. A minimal electrical capacity of 2 pF and a upper limit of 9 pF are observed. The electromotive force addition is 3 corresponds to a priming electromotive force 30V. The power coevals is 0.5µW when the rod oscillation frequence at 10 Hz. The biggest advantage of this device is the capableness of low frequence. That makes the human organic structure gesture as a suited gesture beginnings for it.

Figure 7. Fabrication and optical images


The focal point of this paper is to show the general thought current accomplishment of electrostatic energy harvest home. And it gives a related mention for the group undertaking. For most of the electrostatic reaper designs, a comparatively high resonating frequence comparing with human organic structure gesture is need. However, the low frequence capableness of the non-resonant energy reaper with turn overing mass shows the possibility of the application of this engineering in the undertakings. Additionally, most of the current devices merely have one grade of freedom. The 2D energy reaper design can reap arbitrary waies gesture in a plane, which sufficiently increases the power end product of device. However, it needs a high frequence. That makes it non suited for the demand of the undertaking.