Harvestable Energy Sources

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Electrostatic transducer

This section summarise some of the proposed primary electrostatic harvesters which presented either the Step 1, Figure 2.1a, or Step 2, Figure 2.1b, towards a complete system of e-VEH. In this section we distinguish between implementations that makes use of what is called electret transducers and electret-free transducers. The implemen-tations are presented in chronological order, with a Table 2.1 summarising the list of implementations. The capacitive electrostatic transducers can be classified into two categories according to the charging method:
• Electret-free. These are transducers that requires an external source of charges to harvest the vibrational energy. The external source provides the initial in-vested energy for the transducer before the conversion of energy takes place. They usually require active electronics to synchronise the movement of the ca-pacitor with the movement of the charges[23].
• Electret-based These are transducers that uses an electret material to maintain the charge of the electrostatic converter through time. Electrets are dielectrics able to keep an electric field , as shown in Figure 2.2 in the form of a surface voltage for years by trapping the charges on the surface [36].

Electret-Free transducers

This section presents electret-free transducers implementations. This kind of transduc-ers requires a source of charges and thus would also require some managing electronics to operate. Most of electret-free transducers are either implementing the scheme pre-sented in 2.1b or 2.1c.
The first electret-free e-VEH develop as a comb based VEH was presented in 2001 by Meninger et al; [29]. This implementation presented a prototype that is capable of delivering a 8µW of usable power. The synchronisation mechanism was based on inductor switching as was presented in section 1.9.2. In 20103 Roundy et al. [6] proposed an in-plane gap closing e-VEH that is capable of delivering up to 100µW/cm3 harvested power using an external vibration source with 2.2m/s2 at 120Hz. In 2005 Despesse [37] et al. proposed an e-VEH that is able to provide harvested power of 1mW with a 0.2g vibration source at 50Hz. It must be noted that all the previous examples of electret-free transducers used charge-constrained scheme to harvest the energy with inductive-switching to synchronise the energy extraction, see section 1.9.2, . In 2011 Hoffmann et al [38] developed a triangular electrode electrostatic energy harvester that provides higher capacitance per unit displacement 2.3a. This harvester was able to harvest 123µJ of energy into a 20µF capacitor given a bias voltage of 15V and the acceleration amplitude of 2ms−2. The energy was accumulated then manually release using a wireless transmitter model that was charged to 3.5V in 5.7 minutes. In this work, a first implementation of a simple conditioning circuit CC was intro-duced. This CC was was a full wave rectifier and it implements in a continuous circuit scheme using two complementary variable MEMS capacitors. This scheme of harvest-ing the vibration energy is shown in Figure 2.1d where the transducer movement and charge accumulation is synchronised using a CC and the energy is to a load. The load interface was a simple full-wave rectifier which does not take into account extracting the energy efficiently from the CC. Moreover, the energy was released with manual triggers to the load – wireless transmitter – when enough energy was accumulated on the buffer capacitor as shown in Figure 2.3b. Even-though this implementation was permeative in terms of the a whole e-VEH, yet it indicated the main block necessary to build a complete e-VEH.
(a) (b)
Figure 2.3: E-VEH presented in [38] (a) Microscopic images (b) Voltage characteristic of the storage capacitor using a transmission module.

Electret-based transducers

These are variable capacitors with electret material between its plates. It has the capability to directly transform the mechanical energy into electrical energy [1].
In 2007 Loa et al [46] developed an electret based e-VEH that operates in low frequency. The prototype proposed in Figure 2.4a was capable of producing an output power of 2.26µW through an output load of 40MΩ. The measurement was achieved through at a vibrating source frequency of 60Hz.
Later in 2008, Lao et al [47] improved there prototype through using parylene HT as the electret material and a new design that both electrodes are on the stator plate and the rotor are in an insulator blocks coated with electret material. The device was able to harvest a maximum output power of 17.98µW at 50Hz through a load of 80MΩ .
In 2009, Hoffmann et al [48] developed an electrostatic micro-generator shown in Figure 2.5a. It is capable of extracting vibrational energy and delivering up to 1.58 µW average output power. It is packaged in 0.2cm3 volume using a modified SOI technology developed for inertial sensors at HSG-IMIT. Experiment results highlighted two important results:
– An optimal bias voltage where the output power is maximal exists. Thus, the bias voltage has to be considered as a design parameter with respect to the excitation conditions of the corresponding application [48].
– At larger excitations above a critical level the mechanical stoppers come into effect causing the output power to flatten and weakly decrease, cf. Figure 2.5b [48].
In 2011 Choi et al [43] reported a liquid based electrostatic energy harvester shown in Figure 2.6. The harvester a variable capacitance ranging between 10 nF and 5pf which ensures a high capacitance ratio of 2000. It is capable of theoretical energy gen-eration of 45.3µW at 5Hz periodic vibration excitation. The harvester uses a charge-constrained conversion and an auxiliary voltage of 1V. overall device size was 1 cm2.
In 2013 Altena et al [49] presented a method of an electret-based MEMS vibrational electrostatic energy harvester that is capable of producing up to 175µW of power shown in Figure 2.7. This two order of magnitude high output power is due to a different electrical connection principle of the harvester and an optimised geometrical configuration of the electrodes. The devices was tested under sinusoidal excitation with acceleration of 2.5g at 1187Hz frequency and optimum load of 3.2MΩ. The electret potential is 200V and device size of 1cm2.
In 2013 Chiu et al [50] reported an out-of-plane electret vibration energy harvester made of Copper plates and flexible printed circuit board (FPCB). It can produce output power up to 20.7µW with 50MΩ load. The measurement is given an external vibration source of 110Hz and 2g acceleration. The electret potential was 400V and the whole device size was 4cm2. Wang et al [51] developed advice of 1.43cm2 device. It is capable of producing up to 0.15µW power when connected to external load resistance of 13.4MΩ. The vibration source had 1g at 96Hz.
In 2014, Tao et al; develop a 3 dimensional electret based harvester that supports a multiple vibration mode. They proposed a rotational symmetrical resonator as shown in Figure 2.9. Electret material was charged through corona charging method and the harvester prototype was able to produce 4.8nW of power given an external vibration acceleration of 0.05g[52].
In 2015 J. Hillenbrand et al; proposed a vibration-based electret energy harvesters with soft cellular spacer [53]. The proposed harvesters were designed to work with acceleration of 8g to 23g and surface potentials in the 500V regime. It shows a mea-sured harvested power of up to 8µW at 2 kHz and an acceleration of 1g. This soft cellular spacer design has several advantages over traditional design such as a compact design as the harvesters is slightly larger than the seismic mass. This means that these harvesters can operate with low resonance frequencies (< 100 Hz) and produce with relatively high output power. Figure 2.10 shows an illustration of the proposed design as well as the measured normalized power.
In 2015 Perez et al [54]; an airflow energy harvester was presented using an electret-based conversion to turn the air flow into membrane movement and in turn into electri-cal energy. The device proposed was implemented in 25µm thick Teflon PTFE electret layer as shown in Figure 2.11. The device is able to harvest up to 2.1mW output power with a 3g acceleration vibrating source with of 300Hz. The measured output was the optimal load of 14MΩ.
Figure 2.11: Flutter-and-electret-based airflow energy harvester by [54]
Later in 2017 Tao et al [55] developed an improved version of the electret based vibrational energy harvester. The new design was a sandwich structure MEMS that has two opposite charged electrics integrated into a single electrostatic device. With overall devices package of 0.24cm3 size, it generates a power of 0.22µW with a vibra-tion acceleration of 1g with 122Hz and a load of 30 MΩ. Adopting this new MEMS structure resulted in a direct improve of e-VEH performance compared to the per-viously introduced model with the output voltage increased by 80.9% with an input acceleration of 5ms−2.
Table 2.2 summarises most of the recent implementations of electret-based trans-ducers in the energy harvesting community.

E-VEH with energy management interfaces

This section discuss energy management interfaces for the e-VEH. As was shown in Figure 2.1, we distinguish between four type of e-VEH with a key parameter to dif-ferentiate between these interfaces is the monitoring electronics. The first step, 2.1a, lacks any monitoring electronics that synchronise the charge movement with the in-terface. The second step, Figure 2.1b, includes synchronising electronics allowing a synchronised charge movement from the transducer to a load or a capacitor. The third step, Figure 2.1c, rely on CC to autonomously accumulated the harvested charges on a storage capacitor and uses monitoring electronics to control the energy discharge on a load when enough energy is accumulated.

E-VEH Step 1: Primitive transducer

This is the simplest implementation of the vibrational harvesters implementation where the variable capacitor is directly connected to the load. These implementation are usually used either characterise the MEMS transducer.
Few implementations of such interface were presented in 2009 by Paracha et al.[41], in 2011 by Boisseau et al. [36] and in 2013 by Bu et al. [62]. In this interface the variable MEMS capacitor is connected directly to a reservoir which could be capacitor or a resistive load as shown in Figure 2.13. These kinds of interfaces has several drawbacks including:
– They are not capable of synchronising efficently the charge movement with the capacitor mechanical movement. This results in harvested energy loss [23].
– They are not able to self-increase their internal energy. This lead to eventually stopping of the harvesting process [1].

E-VEH Step 2: Synchronised transducer

In these e-VEH interfaces the main concern is to synchronise the charge movement, thus the interface controller relied on sensing the variable MEMS capacitor voltage to activate the extraction mechanism, cf. Figure2.1b. Examples on such interfaces was reported in [6, 23, 24, 29].
As can been in Figure 2.14a Meninger et al. proposed an switched-inductor in-terface that synchronises the switching event for SW1 and SW2 with the variable capacitor to achieved efficient energy extraction, see section 1.9.2. The drawback of such implementation is that the controller assumes known and fixed operating con-dition prior the harvesting process. This however is not possible when considering harvesting energy from ambient vibration source.
In the proposed implementation by Torres et al. a load interface which precharges, detects, and synchronises to a variable voltage-constrained capacitor was presented [23]. The variable MEMS was an electret-free and thus required a battery which was used as both the voltage-constraining device and precharging source. As can be seen in Figure 2.14b the proposed interface had a state detector to distinguish between the different states of the variable capacitor and generate the appropriate switching signals to either extract or. replenish Cvar charges.
In step 2, the main focus was to achieve proper synchronisation of the variable MEMS capacitor according with the charge extraction through switches and sensing electronics, this however was partially abandon with the adoption of self-synchronised conditioning circuits as will be shown in step 3.
In step 3 most of the research of the community shifted towards investigating self-synchronised conditioning circuits. These were CC based on charge-pump and was able to achieve energy extraction from the variable MEMS capacitor thanks to replacing the switches with a network of diodes.
Such interface first implementation was introduce by Roundy et al. in 2002 [6] It relied on charge pump arrangement and later in 2004 replaced the switches with diodes, cf. Figure 2.15a and section 1.9.3. This type of interface allowed self-synchronisation of the charges with the variable capacitor movement, however as was shown in section 1.9.3 it suffers from saturation after few harvesting cycles. In 2005 Ben Yen et al.
[24] proposed a flyback mechanism as shown in Figure 2.15b and in 2011 Dudka et al. [21, 63] proposed an improved version with a self-calibrated flyback thresholds. In 2011 Querioz et al.[64, 65] proposed a new CC based on Bennets doubler. It has gained the interest of the electrostatic harvesting community since then such as [33, 66–68].
In all of implementations proposed in Step 3, the interface control from step 2 was replaced by improved self-synchronising CC and thus the controller electronics shifted from monitoring the transducer to monitoring the internal voltage of the CC. The load interface in this stage was merely addressed.

E-VEH Step 4: Complete energy harvester system

In this step of load interfaces implementation, a whole system is considered from the transducer synchronisation, conditioning of the voltage and load interfacing as shown in Figure 2.1d. Only few groups have proposed a complete system as such, since the main focus was on the harvester itself. With the transducer and its conditioning circuits becoming more mature and fully characterised and analysed it is now most appropriate to address the implementation of a whole e-VEH system.
One of the preliminary implementation was done by Asantha Kempitiya et al. [25, 69] in 2013. They proposed a low power energy harvesting circuit that performs synchronous energy harvesting on tri-plate variable capacitor. The proposed design was a charge pump CC architecture implementing a charge constrained energy conversion. The power IC control was implemented in AMI0.7µm high voltage CMOS process. The overall harvester implementation was capable of generating a 308nW (at 98Hz vibration). This implementation presented a new class of micro generators with the potential for higher energy conversion than regular electrostatic energy harvesters.
More recent attempt for an efficient electrostatic energy harvesters is done by Stefano et al [70, 71]. The implemented a high voltage and low power inductive DC-DC buck converter for e-VEH interface. The implementation was done using TSMC 0.25µm CMOS technology and included a maximum point tracking algorithm for matching the the internal impedance of the harvester and AC-DC converter. This approach usually has a drawback of complex digital processing or a simple but analog to perform the Maximum power point tracking (MPPT). The reported measured peak end-to-end efficiency is 88% and control average power of 5000nW and does not oper-ate under 25µW available power. An improved version of the harvester interface was proposed later on by the same team on 2015 that is capable of operating with input power under 1µW and with a controller average power of 500nW..

Table of contents :

1 Introduction 
1.1 Overview
1.2 Thesis Outline
1.3 Harvestable Energy Sources
1.4 Vibration Energy Harvesting
1.5 Vibrational energy transduction methods
1.6 Electrostatic vibrational energy harvesters
1.7 Electrostatic Transducers
1.8 Energy conversion using variable capacitor
1.9 Conditioning circuit
1.9.1 Basic conditioning circuit
1.9.2 Charge constrained CC
1.9.3 Rectangular Q-V CC
1.10 Necessity of load interfaces
1.11 Thesis contribution
1.12 Chapters summaries
1.13 Summary
2 State of the Art for E-VEHs 
2.1 Overview
2.2 Electrostatic transducer
2.2.1 Electret-Free transducers
2.2.2 Electret-based transducers
2.3 E-VEH with energy management interfaces
2.3.1 E-VEH Step 1: Primitive transducer
2.3.2 E-VEH Step 2: Synchronised transducer
2.3.3 E-VEH Step 3: Synchronised conditioning circuit
2.3.4 E-VEH Step 4: Complete energy harvester system
2.4 Summary
3 Load Interface for E-VEH 
3.1 Overview
3.2 Review of DC-DC Interfaces
3.2.1 Resistive interface
3.2.2 Buck-Boost DC-DC Load Interfaces
3.2.3 Buck DC-DC Load Interface
3.3 Buck Converters as Load Interface for CCs
3.3.1 Reservoir voltage regulation
3.3.2 Load Voltage Regulation
3.4 Reservoir Voltage regulation Strategies
3.4.1 Defining Vres Regulation Interval
3.4.2 Load Interface Control
3.5 DTVC Control
3.5.1 Reservoir Voltage Regulation Operating Phases
3.5.2 DTVC Sampling Clock
3.5.3 Conduction losses
3.6 Multiple Energy-shot transfer
3.7 Load Interface Behavioural Model
3.8 Strategy of CMOS implementation
3.9 Summary
4 First Implementation of Load Interface System 
4.1 Overview
4.2 AMS 0.35μm CMOS Technology
4.3 First implementation of Load Interface
4.4 Structure of the implemented load interface
4.5 Switch Decision Block
4.5.1 Voltage Divider
4.5.2 6T Comparator overview
4.5.3 6T Comparator Analysis
4.5.4 Adjustable 6T Schmitt Trigger Comparator
4.6 The clock generator
4.7 Switch Control Block
4.7.1 Power Switch – SWLI
4.7.2 Dynamic flip-flop level shifter
4.7.3 Switch Driver
4.8 Simulation Results: First Implementation
4.9 Summary
5 Second Implementation of Load Interface System 
5.1 Overview
5.2 Second implementation modification
5.3 Comparator Modification
5.3.1 RS-Trigger based Hysteresis Comparator
5.3.2 RS-trigger hysteresis comparator operation
5.3.3 RS-trigger comparator design
5.3.4 Comparing 6T and RS-trigger comparators
5.3.5 Adjustable RS-Trigger comparator
5.4 The CCR comparator
5.5 High side power switch – SWLI
5.6 Safe-clock gating block
5.7 CCR using the second LI implementation
5.8 Load voltage Regulator LVR
5.9 Autonomy of second implementation
5.9.1 Autonomous energy management system setup
5.9.2 Connection between on-chip and off-chip components of the LI
5.9.3 Discussions
5.10 Summary
6 Summary, conclusions and perspectives 
6.1 Conclusions
6.2 Perspectives and Future work
6.2.1 Start-up circuit
6.2.2 Diode losses
6.2.3 Integration of CC
6.2.4 Self-adjustability threshold
6.3 Publications
A Analysis of Current and Energy for dc-dc buck load interface
B Second implementation schematic

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