Power conditioning optimization for ultra low voltage wearable thermoelectric devices using self-sustained multi-stage charge pump

Waste heat energy recovery from human body utilizing the thermoelectric generator (TEG) has shown potential in the generation of electrical energy. However, the level of heat source from the human body restricts the temperature deviation as compared to ambient temperature (approximately 3~10 °C in difference), thereby yielding an ultra-low voltage (ULV) normally less than 100 mV. This research aims at generating power from the TEG by harnessing human body temperature as the heat source to power up wearable electronic devices realizing a self-sustain system. However, power conversion of the TEG has typically low efficiency (less than 12%), requiring proper design of its power regulation system. The generated ULV marked the lowest energy conversion factor and improvement is therefore required to validate the use of ULV generated from human body temperature. This problem was addressed by proposing an improved solution to the power regulation of the ULV type TEG system based on the DC-DC converter approach, namely a multi-stage charge pump, with specifications restricted at the ULV source. Performances of the TEG connected in multiple array configurations with the generated source voltage fed into fabricated charge pump circuit to boost and regulate the voltage from the ULV into the low voltage (LV) region were analyzed. The maximum source voltage (20 mV) was referred and simulated in the LT Spice software and used as a benchmark to be compared with the voltage generated by the fabricated charge pump circuits. Error performances of the fabricated charge pump circuits were further analyzed by manipulating the circuits’ parameters, namely, the switching frequency and the capacitance values. It was found that the proposed method was able to handle the ULV source voltage with proper tuning on its component parameters. The overall power conversion efficiency of 26.25% was achieved based on the performance evaluation values for components applied in this research. Hence, this proved the viability of thermoelectric applications in ULV using the proposed power regulation system

Matching Network Elimination in Broadband Rectennas for High-Efficiency Wireless Power Transfer and Energy Harvesting

Impedance matching networks for nonlinear devices such as amplifiers and rectifiers are normally very challenging to design, particularly for broadband and multiband devices. A novel design concept for a broadband high-efficiency rectenna without using matching networks is presented in this paper for the first time. An off-center-fed dipole antenna with relatively high input impedance over a wide frequency band is proposed. The antenna impedance can be tuned to the desired value and directly provides a complex conjugate match to the impedance of a rectifier. The received RF power by the antenna can be delivered to the rectifier efficiently without using impedance matching networks; thus, the proposed rectenna is of a simple structure, low cost, and compact size. In addition, the rectenna can work well under different operating conditions and using different types of rectifying diodes. A rectenna has been designed and made based on this concept. The measured results show that the rectenna is of high power conversion efficiency (more than 60%) in two wide bands, which are 0.9-1.1 and 1.8-2.5 GHz, for mobile, Wi-Fi, and ISM bands. Moreover, by using different diodes, the rectenna can maintain its wide bandwidth and high efficiency over a wide range of input power levels (from 0 to 23 dBm) and load values (from 200 to 2000 Ω). It is, therefore, suitable for high-efficiency wireless power transfer or energy harvesting applications. The proposed rectenna is general and simple in structure without the need for a matching network hence is of great significance for many applications

Optimized Multi-input Single-output Energy Harvesting System

The energy harvesting sources has been introduced as a promising alternative for battery power. However, harvested energy is inherently sporadic, unstable, and unreliable. For this reason, a non-volatile processor has been previously proposed to bridge the intermittent executions in frequent power losses. Nonetheless, recurrent power failures reduce overall system performance which has forced researchers to look into multi-input energy harvesting systems. The purpose of this study is to investigate the possible solutions to improve the reliability and functionality of battery-less devices. This study has two major objectives: (1) implementing periodic checkpointing on WISP5, and (2) proposing optimized multi-input single-output energy harvesting system. The WISP5 was acquired from the Sensor Systems Laboratory, University of Washington, as a viable RFID energy harvesting system to implement software checkpointing techniques. We performed the periodic checkpointing every 50ms based on the RFID power fluctuation style. Then, we explored a number of possible maximum power point tracking techniques to extract maximum power from harvesters. As a result, we verified that the open circuit voltage control is the most cost effective and efficient technique for both thermoelectric (TEG) and photovoltaic (PV) . Also, we revealed that in low-level input voltages, following the fact that the maximum power extraction can be achieved at half of open circuit voltage does not result in maximum possible efficiency. Therefore, by adjusting the converter input voltage at about 66% of open circuit voltage, we improved power efficiency by about 18%.Electrical Engineerin

Micro/Nano Technologies for Achieving Sustainable Microbial Electrochemical Cell Systems

Microbial electrochemical cell systems (MECSs), such as microbial fuel cells (MFCs) and microbial electrolysis cells (MECs), are promising clean and renewable energy sources. MFCs employ exoelectrogenic bacteria to convert organic matter in wastewater into electricity, and biogas (hydrogen, methane) is generated from organic matter by applying electricity in MECs. This emerging technology requires better performance by decreasing the material cost to bring it into practical application. Therefore, the main focuses of this research are fabricating nanomaterial based anode to improve the power production and developing micro devices to analyze real-time performance of MECSs. Physical and electrochemical interactions between microbes and anode are critical to performance. Systematic studies on how different lengths, packing densities, and surface conditions of carbon nanotubes (CNTs) affect MFC power output revealed that long and loosely packed CNTs without any amorphous carbon show the highest power production performance. Furthermore, fabricated 3D sponges composed of interconnected CNTs showed better performance compared to commercially available carbon felt anode. Due to the configuration, monitoring of biofilm development is hard in macro-sized MFCs. Microfluidic laminar flow MFC with interdigitated anode was fabricated to monitor the real-time optical and electrochemical activity of Shewanella oneidensis MR-1 in situ. Power density and impedance were measured to understand the relation between biofilm development and power production of biofilm over time. Expensive and labor intensive equipment such as gas chromatography is commonly used to analyze the biogas produced in MECs. A ZnO nanowires based gas sensor was fabricated to measure H2 concentration in real-time without using any other expensive equipment. Low power and low voltage output of MFCs do not allow them to power most electrical applications. Proposed power management systems (PMSs) can overcome this limitation by boosting the MFC output voltage and managing the power for maximum efficiency, regardless of the power and voltage fluctuations from MFCs over time. Overall, the limitations of the MECSs technology have been identified and possible solutions have been proposed to improve the overall performance of this sustainable renewable energy source

RF Power Transfer, Energy Harvesting, and Power Management Strategies

Energy harvesting is the way to capture green energy. This can be thought of as a recycling process where energy is converted from one form (here, non-electrical) to another (here, electrical). This is done on the large energy scale as well as low energy scale. The former can enable sustainable operation of facilities, while the latter can have a significant impact on the problems of energy constrained portable applications. Different energy sources can be complementary to one another and combining multiple-source is of great importance. In particular, RF energy harvesting is a natural choice for the portable applications. There are many advantages, such as cordless operation and light-weight. Moreover, the needed infra-structure can possibly be incorporated with wearable and portable devices. RF energy harvesting is an enabling key player for Internet of Things technology. The RF energy harvesting systems consist of external antennas, LC matching networks, RF rectifiers for ac to dc conversion, and sometimes power management. Moreover, combining different energy harvesting sources is essential for robustness and sustainability. Wireless power transfer has recently been applied for battery charging of portable devices. This charging process impacts the daily experience of every human who uses electronic applications. Instead of having many types of cumbersome cords and many different standards while the users are responsible to connect periodically to ac outlets, the new approach is to have the transmitters ready in the near region and can transfer power wirelessly to the devices whenever needed. Wireless power transfer consists of a dc to ac conversion transmitter, coupled inductors between transmitter and receiver, and an ac to dc conversion receiver. Alternative far field operation is still tested for health issues. So, the focus in this study is on near field. The goals of this study are to investigate the possibilities of RF energy harvesting from various sources in the far field, dc energy combining, wireless power transfer in the near field, the underlying power management strategies, and the integration on silicon. This integration is the ultimate goal for cheap solutions to enable the technology for broader use. All systems were designed, implemented and tested to demonstrate proof-of concept prototypes

Power Management ICs for Internet of Things, Energy Harvesting and Biomedical Devices

This dissertation focuses on the power management unit (PMU) and integrated circuits (ICs) for the internet of things (IoT), energy harvesting and biomedical devices. Three monolithic power harvesting methods are studied for different challenges of smart nodes of IoT networks. Firstly, we propose that an impedance tuning approach is implemented with a capacitor value modulation to eliminate the quiescent power consumption. Secondly, we develop a hill-climbing MPPT mechanism that reuses and processes the information of the hysteresis controller in the time-domain and is free of power hungry analog circuits. Furthermore, the typical power-performance tradeoff of the hysteresis controller is solved by a self-triggered one-shot mechanism. Thus, the output regulation achieves high-performance and yet low-power operations as low as 12 µW. Thirdly, we introduce a reconfigurable charge pump to provide the hybrid conversion ratios (CRs) as 1⅓× up to 8× for minimizing the charge redistribution loss. The reconfigurable feature also dynamically tunes to maximum power point tracking (MPPT) with the frequency modulation, resulting in a two-dimensional MPPT. Therefore, the voltage conversion efficiency (VCE) and the power conversion efficiency (PCE) are enhanced and flattened across a wide harvesting range as 0.45 to 3 V. In a conclusion, we successfully develop an energy harvesting method for the IoT smart nodes with lower cost, smaller size, higher conversion efficiency, and better applicability. For the biomedical devices, this dissertation presents a novel cost-effective automatic resonance tracking method with maximum power transfer (MPT) for piezoelectric transducers (PT). The proposed tracking method is based on a band-pass filter (BPF) oscillator, exploiting the PT’s intrinsic resonance point through a sensing bridge. It guarantees automatic resonance tracking and maximum electrical power converted into mechanical motion regardless of process variations and environmental interferences. Thus, the proposed BPF oscillator-based scheme was designed for an ultrasonic vessel sealing and dissecting (UVSD) system. The sealing and dissecting functions were verified experimentally in chicken tissue and glycerin. Furthermore, a combined sensing scheme circuit allows multiple surgical tissue debulking, vessel sealer and dissector (VSD) technologies to operate from the same sensing scheme board. Its advantage is that a single driver controller could be used for both systems simplifying the complexity and design cost. In a conclusion, we successfully develop an ultrasonic scalpel to replace the other electrosurgical counterparts and the conventional scalpels with lower cost and better functionality

Ultra-Low Power Transmitter and Power Management for Internet-of-Things Devices

Two of the most critical components in an Internet-of-Things (IoT) sensing and transmitting node are the power management unit (PMU) and the wireless transmitter (Tx). The desire for longer intervals between battery replacements or a completely self-contained, battery-less operation via energy harvesting transducers and circuits in IoT nodes demands highly efficient integrated circuits. This dissertation addresses the challenge of designing and implementing power management and Tx circuits with ultra-low power consumption to enable such efficient operation. The first part of the dissertation focuses on the study and design of power management circuits for IoT nodes. This opening portion elaborates on two different areas of the power management field: Firstly, a low-complexity, SPICE-based model for general low dropout (LDO) regulators is demonstrated. The model aims to reduce the stress and computation times in the final stages of simulation and verification of Systems-on-Chip (SoC), including IoT nodes, that employ large numbers of LDOs. Secondly, the implementation of an efficient PMU for an energy harvesting system based on a thermoelectric generator transducer is discussed. The PMU includes a first-in-its-class LDO with programmable supply noise rejection for localized improvement in the suppression. The second part of the dissertation addresses the challenge of designing an ultra- low power wireless FSK Tx in the 900 MHz ISM band. To reduce the power consumption and boost the Tx energy efficiency, a novel delay cell exploiting current reuse is used in a ring-oscillator employed as the local oscillator generator scheme. In combination with an edge-combiner PA, the Tx showed a measured energy efficiency of 0.2 nJ/bit and a normalized energy efficiency of 3.1 nJ/(bit∙mW) when operating at output power levels up to -10 dBm and data rates of 3 Mbps. To close this dissertation, the implementation of a supply-noise tolerant BiCMOS ring-oscillator is discussed. The combination of a passive, high-pass feedforward path from the supply to critical nodes in the selected delay cell and a low cost LDO allow the oscillator to exhibit power supply noise rejection levels better than –33 dB in experimental results

Power Management Circuits for Energy Harvesting Applications

Energy harvesting is the process of converting ambient available energy into usable electrical energy. Multiple types of sources are can be used to harness environmental energy: solar cells, kinetic transducers, thermal energy, and electromagnetic waves. This dissertation proposal focuses on the design of high efficiency, ultra-low power, power management units for DC energy harvesting sources. New architectures and design techniques are introduced to achieve high efficiency and performance while achieving maximum power extraction from the sources. The first part of the dissertation focuses on the application of inductive switching regulators and their use in energy harvesting applications. The second implements capacitive switching regulators to minimize the use of external components and present a minimal footprint solution for energy harvesting power management. Analysis and theoretical background for all switching regulators and linear regulators are described in detail. Both solutions demonstrate how low power, high efficiency design allows for a self-sustaining, operational device which can tackle the two main concerns for energy harvesting: maximum power extraction and voltage regulation. Furthermore, a practical demonstration with an Internet of Things type node is tested and positive results shown by a fully powered device from harvested energy. All systems were designed, implemented and tested to demonstrate proof-of-concept prototypes

Untersuchung zur Anwendung der Nanostrukturierung in der Thermoelektrik

Der Wunsch nach umweltfreundlicher und effizient erzeugter Energie gewinnt zunehmend an Relevanz. Insbesondere die Rückgewinnung kleinster Energiemengen ist für das low-power-Segment, beispielsweise für autarke Sensorsysteme oder das Internet of Things, von großer Bedeutung. In diesem Zusammenhang bieten thermoelektrische Generatoren (TEG) die Möglichkeit der direkten Umwandlung von thermischer in elektrische Energie und somit des Energy Harvestings ungenutzter Abwärme. Aufgrund des geringen Wirkungsgrades übersteigen TEG bisher nicht den Status eines Nischenproduktes. Aktuelle Forschungsarbeiten zeigen jedoch die Möglichkeit zur Steigerung des Wirkungsgrades bei Verwendung nanostrukturierter thermoelektrischer Materialien. Im Rahmen dieses Buches wird daher die Anwendung der Nanostrukturierung in der Thermoelektrik untersucht. Dies beinhaltet die Entwicklung eines grundlegenden Prozessablaufs zur Herstellung nanostrukturierter TEG. Hierzu werden mithilfe des Depositions- und Rückätzverfahrens quasieindimensionale Strukturen aus Bismuttellurid hergestellt und charakterisiert. Darüber hinaus wird unter Verwendung von FEM-Simulationen der Einfluss verschiedener Geometrieparameter diskutiert. Hierbei wird insbesondere die Veränderung des Generatorinnenwiderstandes sowie der Thermospannung analysiert. Des Weiteren erfolgt die Simulation und Realisierung einer Spannungswandlerschaltung speziell für hochohmige, nanostrukturierte TEG. Das zweistufige System nutzt einen Meißner-Oszillator als Anlaufschaltung und einen Aufwärtswandler mit Maximum-PowerPoint-Tracking als effektive Endstufe

Maximization of power generation from thermoelectric generators operating under constant heat flux

Thermoelectric generators (TEGs) are used to convert thermal energy into electricity. TEGs present an emissions-free source of power and, despite the low efficiency they offer, with typical values of 5%, they can be used to harvest waste-heat energy in different type of applications. The high robustness presented by TEGs allows their use in low-maintenance applications. TEGs can operate under two different conditions: constant temperature gradient or constant input heat flux. When a TEG operates under constant temperature gradient, the input heat flux varies with the electrical operating conditions of the TEG devices. Under these conditions the TEG is modeled by a constant voltage source with a constant resistance in series with the voltage source. When operated under constant heat flux, the temperature gradient of the TEG changes with the electrical operating conditions of the device. In this situation of constant heat flux, both the equivalent voltage source and the resistance in series with it change their values with the electrical operating point. The location of the Maximum Power Point, or MPP, of the TEG is different in both operating conditions. In constant temperature gradient the MPP is located at half of the instantaneous open-circuit voltage of the TEG, whereas under constant heat flux the MPP is located at an electrical point higher than half of the instantaneous open-circuit voltage. DC/DC converters are mainly used to operate TEGs at the MPP and Maximum Power Point Tracking (MPPT) techniques are used to operate the TEG at the MPP. Due to the difference in the location of the MPP between constant temperature gradient and constant input heat flux, the MPPT techniques will be different between these two operating conditions. This thesis focuses in the study of the location and MPPT techniques for TEGs operated under constant heat flux. A computational model of the TEG for its operation under constant heat flux is first developed. The model of the TEG is then interfaced with the model of a boost, or step-up, converter, which implements a new MPPT algorithm to operate the TEG at the true MPP. The output energy of the power converter is used to charge a lithium-ion (Li-Ion) battery. The complete model of the TEG system is then used to compare the new algorithm proposed in this thesis against two state-of-the-art algorithms: the Fractional Open-Circuit method and the Perturb and Observe method. The comparison is made under three different input heat flux profiles: constant heat flux, ramp-varying heat flux and step-changing heat flux. The last chapter of this thesis presents a hardware implementation of the TEG system and the MPPT power converter. Experimental results are presented for the new and the two state-of-the-art algorithms and a comparison between the three algorithms are presented for the three different input heat flux profiles described previously. The TEG model and the MPPT algorithm presented in this work can be applied to any TEG applications where the TEG operates under constant heat flux