Low Output Power Research Articles

Optimal electrode coverage based on a new criterion for piezoelectric energy harvesters

Piezoelectric energy harvesters (PEHs) are a promising alternative to conventional electrochemical batteries with the advantage of being self-powered and maintenance-free, but their application is severely restricted by the low electric power output. It has been reported that the power output of PEHs can be enhanced by well-designed electrode coverage. A common design criterion for beam-like PEHs is based on the strain node to avoid electrode charge cancellation. This criterion, however, is not feasible for PEHs subject to complex spatio-temporal excitation patterns, where strain nodes change their position. This work proposes a new design criterion for optimal electrode coverage of beam-like PEHs based on the closed-form solution of the circuit equation that expresses voltage as a function of the beam’s dynamic response, specifically the cross-section rotation. The new criterion maximizes the averaged curvature of the beam segment covered by the electrode using data on the instantaneous rotation field. The improved physical significance and reliability of the presented criterion are discussed. The associated electrode optimization procedure is then exemplified for PEHs driven by fluid flow, which helps to realize a complex excitation pattern. Two numerical studies, both including a variety of combinations of fluid densities and inlet velocities, are performed to demonstrate that an optimal electrode configuration can be obtained with the proposed criterion. Comparison of different electrode configurations in above studies finally leads to useful conclusions on the power output and electrode configuration.

Microwave generation from betatron oscillation of slow electron beam in DC cross-fields configuration

A high-efficiency mechanism of microwave (MW) generation from slow electron beam (e-beam) is investigated in details. Space-inhomogeneous density profile of the e-beam enables its current vortex profile ∇ × j easy to be modified significantly by external fields. Suitable DC cross-fields configuration can modify time behavior of the ∇ × j of an e-beam from monotonically-varying to oscillatory. Under available not-too-severe values of beam peak density and beam drifting velocity, such a time-oscillatory current vortex can be tuned to have a narrow-spectrum around a desired central frequency within a broad range from MHz to Thz. The mechanism is applicable to slow non-relativistic e-beam and hence does not need bulky cooling component and high-voltage component. This favors the whole vacuum electron devices (VEDs) to be compact enough without at the cost of lower output power.

An Efficient Photon Utilization Radioisotope Thermophotovoltaic Based on Curled Reflectors

For radioisotope thermophotovoltaic (RTPV) to produce higher output power, it is often required that the irradiance incident on the thermophotovoltaic (TPV) cell array be more uniform. However, the irradiance received by each cell of the TPV array is relatively different under the conventional cell array surround structure of RTPV, resulting in a serious overall electrical output mismatch loss and a lower output power utilization rate. Herein, an RTPV with curled reflectors optimized by finite‐element method is proposed. It is shown in the simulation results that the irradiation uniformity of the cell array can reach 87.5%, the total output power reaches 88.12 W, which is 39.8% higher than that prior to optimization, and the mismatch loss is reduced by 69.2%. In addition, the impact of the reflectivity of reflectors is also targeted from the perspective of practical applications, and it can still produce higher output power than that of the conventional structure even at reflectivity as low as 70%. Ultimately, the RTPV prototype with reflectors is developed, and the electrical performance is tested to verify the effect of irradiance improvement. Herein, reflectors with positive gain for RTPV are proposed, which provides a new idea for the development of high‐efficiency power supplies.

Stack cooling system coupled with secondary heat pump in fuel cell electric vehicles

Fuel cell electric vehicles (FCEVs) requires proper thermal management of a proton-exchange membrane fuel cell (PEMFC) stack, but conventional thermal management systems (TMSs) cannot manage the thermal requirements of PEMFCs under high-power density at high ambient temperatures. In this study, a stack cooling system coupled with a secondary heat pump in an FCEV is suggested. The enhanced thermal flexibility of the secondary heat pump system enabled extensive cooling with various TMS configurations. The cooling performance with different TMS configurations was evaluated, considering the thermal and electrical coupling of the heat pump system and the PEMFC stack. The results show that the stack temperature was lowered up to 2.6 °C when the excessive cooling load of the stack radiator was distributed to the cabin radiator under a relatively low-power output condition. To dissipate a large amount of waste heat from the stack, TMS requires a temporary shutdown of the cabin cooling system. In this case, utilizing the inactive outdoor heat exchanger as an additional radiator decreased the temperature up to 18.4 °C by extending the heat transfer area for stack cooling. This study proposes a cooling scheme for an FCEV and suggests appropriate cooling methods and configurations under various operating conditions.

Preliminary Experimental Research on Split-Cathode-Fed Relativistic Magnetron With Bidirectional Emission Structure

Simulations and experiments of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}$ </tex-math></inline-formula> -band TEM-output relativistic magnetron (RM) driven by split cathode (SC) are proposed. In the previous researches on SC, as a result of the space-charge-screening effect of squeeze-state electrons accumulated nearby the emission surface, the diode current is limited, which finally results in low output power. To improve the output performance, a bidirectional emission SC (BSC) is introduced. Experimental results on EPA-90 high-voltage electron beam accelerator demonstrate that when the voltage is around 480 kV and the axial magnetic field is 0.49 T, comparing with a conventional unidirectional emission SC, the output power can be enhanced from 124 to 400 MW by adding an extra downstream emitter. This work lays a foundation for further researches to make the SC-fed RM more practical.

Implementation of Modified SEPIC Converter for Renewable Energy Built DC Microgrids

The nonavailability of fossil fuels and the shortcomings of nonconventional energy sources taking place in the environment lead the research and development towards alternative and clean energy sources such as renewable energy sources. Renewable or nonconventional energy resources are being used to meet ever-increasing energy demand. The photo voltaic (PV) energy is the right choice of renewable energy for small voltage DC distribution systems, due to their advantages. But this energy source can produce low output power at the utility grid. Hence, to step up this low input voltage to high value for a range of high-voltage applications, DC-DC converters are integrated to the DC microgrids by means of PV system. The present work elaborates the modified SEPIC converter (MSC) designed based on the traditional SEPIC with a boost-up module. In comparison with conventional or traditional SEPIC converter, the proposed MSC produces high voltage gain and continuous current to the DC microgrids. Furthermore, MSC is operated with only one controlled switch. The proposed converter design improves the efficiency, output voltage, and continuous output current of the DC microgrids. This entire work is completed with PSIM, and finally, numerical simulation results prove the possibility of the MSC with PV-powered DC microgrids, and also the dynamic response of MSC for DC microgrid loads enhances the regulated output voltage and continuous output current of DC loads.

Ultrastrong-polar cyano-Prussian blue analogs hybrid tribomaterials for biomechanical energy harvesting and self-powered sensing

Triboelectric nanogenerators (TENGs) are green energy devices that can convert mechanical energy into electrical energy. However, lower output power density currently hinders its realistic applications. Introducing functional materials into dielectric polymers layer is an efficient approach to enhance TENG output. Herein, a low-cost, biocompatible Prussian blue analog (KxMn[FeCN)6]y·zH2O, MnFePBA) is proposed as a functional component for high-performance hybrid tribomaterials of TENGs. Numerous highly polar CN groups found in MnFePBA can interact dipole-dipole with the C-F bond in PVDF, promoting the formation of electroactive β-phase PVDF and enhancing the capacity for charge induction and charge capture. Simultaneously, the weak interface fusion between the inorganic material and the organic polymer can be improved as the O-H bond in MnFePBA can establish a stronger hydrogen bond with the C-F bond in PVDF. Additionally, high dielectric MnFePBA can enhance the dielectric properties of PVDF/MnFePBA to boost the TENG output. Consequently, the PVDF/MnFePBA can achieve evidently increased short-circuit current of 31.2 µA, output voltage of 1580 V and ultra-high power density of 12.8 W/m2 (4 ×4 cm2). Moreover, the assembled TENG could also be efficiently utilized for human motion capture and sensing applications. This work enriches the range of functional components for high-performance tribomaterials of TENGs.

Numerical simulations of ice accretion on wind turbine blades: are performance losses due to ice shape or surface roughness?

Abstract. Ice accretion on wind turbine blades causes both a change in the shape of its sections and an increase in surface roughness. These lead to degraded aerodynamic performances and lower power output. Here, a high-fidelity multi-step method is presented and applied to simulate a 3 h rime icing event on the National Renewable Energy Laboratory 5 MW wind turbine blade. Five sections belonging to the outer half of the blade were considered. Independent time steps were applied to each blade section to obtain detailed ice shapes. The roughness effect on airfoil performance was included in computational fluid dynamics simulations using an equivalent sand-grain approach. The aerodynamic coefficients of the iced sections were computed considering two different roughness heights and extensions along the blade surface. The power curve before and after the icing event was computed according to the Design Load Case 1.1 of the International Electrotechnical Commission. In the icing event under analysis, the decrease in power output strongly depended on wind speed and, in fact, tip speed ratio. Regarding the different roughness heights and extensions along the blade, power losses were qualitatively similar but significantly different in magnitude despite the well-developed ice shapes. It was found that extended roughness regions in the chordwise direction of the blade can become as detrimental as the ice shape itself.

Microbial Fuel Cells as a Promising Power Supply for Implantable Medical Devices

The Future of Energy is focused on the consolidation of new energy technologies. Among them, Fuel Cells (FCs) are on the Energy Agenda due to their potential to reduce the demand for fossil fuel and greenhouse gas emissions, their higher efficiency (as fuel cells do not use combustion, their efficiency is not linked to their maximum operating temperature) and simplicity and absence of moving parts. Additionally, low-power FCs have been identified as the target technology to replace conventional batteries in portable applications, which can have recreational, professional, and military purposes. More recently, low-power FCs have also been identified as an alternative to conventional batteries for medical devices and have been used in the medical field both in implantable devices and as micro-power sources. The most used power supply for implantable medical devices (IMD) is lithium batteries. However, despite its higher lifetime, this is far from enough to meet the patient’s needs since these batteries are replaced through surgeries. Based on the close synergetic connection between humans and microorganisms, microbial fuel cells (MFCs) were targeted as the replacement technology for batteries in IMD since they can convert the chemical energy from molecules presented in a living organism into electrical energy. Therefore, MFCs offer the following advantages over lithium batteries: they do not need to be replaced, avoiding subjecting IMD users to different surgeries and decreasing medical costs; they do not need external recharging as they operate as long as the fuel is supplied, by the body fluids; they are a more environmentally friendly technology, decreasing the carbon dioxide and other greenhouse gases emissions resulting from the utilization of fossil fuels and the dependency on fossil fuels and common batteries. However, they are complex systems involving electrochemical reactions, mass and charge transfer, and microorganisms, which affect their power outputs. Additionally, to achieve the desired levels of energy density needed for real applications, an MFC system must overcome some challenges, such as high costs and low power outputs and lifetime.

Design and analysis of a d15 mode piezoelectric energy generator using friction-induced vibration

Research works have been conducted on transverse and longitudinal mode piezoelectric energy generation to collect energy from ambient vibrations. However, the inconsistency with the frequency of the energy source and low output power density remain problems for high energy output. In this work, we propose a shear mode piezoelectric energy generator, which utilizes the friction-induced vibration (FIV) and high shear mode piezoelectric coefficient to improve the energy output. A piezoelectric coupled FIV mathematical model is developed to accurately calculate the dynamic vibration response and voltage output. The dynamic voltage response is validated by experiment, and it proves the possibility of continuous friction-induced high-frequency vibration. The energy generation process is evaluated by transient charging simulation of a storage capacitor through an iteration process, which was experimentally validated in the literature. Parameter studies have been conducted to investigate the influences of the piezoelectric patch dimensional parameters, vibration system parameters, friction model parameters, methods of electrical connections, and different piezoelectric materials on the energy generation performance to provide guidance for better design. Under ideal experiment conditions with proper parameters, a volume of m3 PZT4 piezoelectric material indicates root mean square (RMS) charging power density of Wm−3 and Wm−3 with electrically in parallel and electrically in series, respectively. While using the same amount of material and structural setup, the single crystal PMN-PT piezoelectric material shows RMS charging power density of Wm−3 and Wm−3 with electrically in parallel and electrically in series, correspondingly. These promising results demonstrate that close to W-level RMS charging power output may be realized by structure optimization of energy generator design and incorporating multiple generators together for operation. Possible incorporation into vehicle braking systems can be considered to utilize the wasted friction energy, and it may offer an energy supply for low-power wireless devices.

Scalable, high-performance, yarn-shaped batteries activated by an ultralow volume of sweat for self-powered sensing textiles

One-dimensional sweat-activated batteries (SABs) provide a practical solution to generating electricity for textile electronics due to their excellent flexibility, biocompatibility, and compatibility with conventional textile techniques. However, the low power outputs and slow, inefficient activation during the initial perspiration stage greatly hamper their applications in textile-based, self-powered sensing systems. To address this issue, we developed a core-sheath-structured sweat-activated yarn battery (SAYB) with a Zn-wire core, a cotton-yarn sheath, and an outermost carbon yarn that served as the anode, sweat-wicking separator, and cathode, respectively. The battery has a power density of 1.72 mW cm−2 and an energy capacity of 15.3 mAh, which are much greater than those of the previously reported cotton-yarn-based SAB. The SAYB could be activated by only 1 μL of 100 mM NaCl solution within 3 s, thanks to the thin, hydrophilic cotton sheath, which may shorten the distance and accelerate liquid penetration between the electrodes. The batteries could tolerate 10,000 cycles of bending, 2800 cycles of twisting, and 20 cycles of washing without significantly reducing their power outputs. Multiple batteries connected in proper configurations could power headlights or charge portable gadgets like a smart watch and a smart phone. The SAYB was produced on a large scale (60 m long) to create a sweat-activated energy fabric (5 m long and 0.5 m wide) using conventional weaving techniques. The fabric was further combined with mini wireless analyzers and strain-sensing yarns to prepare a self-powered sensing T-shirt. The T-shirt can be triggered to wirelessly monitor the arm swing frequency and breathing rate in real time after wicking up the sweat created by a volunteer while he is jogging. The SAYBs are expected to contribute to the production of self-powered smart garments for wearable healthcare and exercise monitoring as reliable one-dimensional power sources.

Short-term prediction for distributed photovoltaic power based on improved similar time period

A short-term prediction method for distributed PV power based on an improved selection of similar time periods (ISTP) is proposed, to address the problem of low output power prediction accuracy due to a large number of influencing factors and the large difference in the degree of influence of various factors. First, the simple correlation coefficient (SCC) based on path analysis is used to screen the main influencing factors with stronger correlation with PV output power, and these factors are classified into three categories. Second, correlations of the three dimensions are calculated, respectively: (i) TOPSIS (with weights optimized by the SCC) determines meteorological correlation, (ii) linear weighting (based on the fuzzy ranking) obtains time correlation, and (iii) load correlation is quantified with existing current parameters. Third, the combined impact correlation (CIC) is obtained by weighting the three correlations above to establish criteria for the selection of similar periods, and a short-term PV power prediction model is established. Finally, experimental results based on real data of Australian Yulara Solar System PV plant demonstrate that errors of proposed ISTP method are respectively improved by 47.06% and 46.09% compared with the traditional ELMAN and similar day method.

Output characteristics of series-parallel triboelectric nanogenerators

To tackle the problem of the low output power of triboelectric nanogenerators (TENGs), the output characteristics of multiple TENGs in series and parallel were explored. An experimental device for multilayer TENGs was developed. Through the method of experimental modeling, the equivalent circuits and mathematical models in series and parallel are established respectively, and the working principle is expounded. The experiment found that when multiple TENGs are connected in parallel, the voltage, current, and output power can be regularly increased. When the TENGs are connected in series, the output power is only related to the performance of the first and last two triboelectric generators and has nothing to do with the number of series-connected TENGs. This finding has a certain reference value for the design of high-efficiency triboelectric nanogenerators.

Gallium Arsenide and Gallium Nitride Semiconductors for Power and Optoelectronics Devices Applications

The advancement in technology in semiconductor materials significantly contributed in improvement of human life by bringing breakthrough in fabrication of optoelectronics and power devices which have wide applications in medicine and communication. The Gallium Arsenide (GaAs) and Gallium Nitride (GaN) are versatile materials for such applications but with relative merits and demerits. GaAs transistors are suitable for both narrowband and wideband applications due to very wide operating frequency range (30 MHz to millimetre-wave frequencies as high as 250 GHz). They are highly sensitive, generate very little internal noise and have power density typically around 1.5 W/mm. But low break down voltage (5x105V/cm), low output power (5-10W) and inability to withstand higher temperatures are the main limitations. On the other hand, GaN possess the improved physical and chemical characteristics, with high output power, high operating temperature (1000°C in vacuum), fast heat dissipation, high breakdown voltage (4x106V/cm), high power density (5-12W/mm), high frequency characteristics and large band gap (3.4eV) which allow significant reduction of devise size. Also high breakdown voltage increases the overall impedance which make it suitable in matching process and enables efficient operation in broad band region. The present paper critically analyses the GaAs and GaN semiconductors in relation to their significant physical and chemical properties, which make them suitable to make efficient power and optoelectronics devices for applications in communication, space and medicine.

Part-load performance analysis of a dual-recuperated gas turbine combined cycle system

To undertake peak shaving tasks, gas turbine combined cycle (GTCC) units often deviate from the design operating point. In order to improve the off-design performance of GTCCs, a dual-recuperated gas turbine combined cycle (DRGTCC) system based on partial recuperation and compressor inlet air heating is proposed in this paper. The operating characteristics of the novel system using the proposed operation strategy are investigated, and the effects of pressure drop in recuperators and ambient temperature on system performance are discussed. The results indicate that, compared with the GTCC system, the DRGTCC system has higher turbine inlet temperature (TIT) and turbine expansion ratio at the same power output, and the performance has been improved especially at lower power outputs. When the inlet temperature of the compressor is increased from 0 °C to 45 °C, the air mass flowrate can be regulated between 46.0% and 100% of the design value. The combined cycle efficiency of the DRGTCC system is improved by 2.55 percentage points at 45.5% design combined cycle load. Moreover, the sensitivity analysis shows that the system performance decreases with the increase of the pressure drop in recuperators, and the ambient temperature affects the load regulation range and efficiency.

Investigating the Activity of Carbon Fiber Electrode for Electricity Generation from Waste Potatoes in a Single-Chambered Microbial Fuel Cell

Microbial fuel cells (MFCs), a technology that converts chemical energy into electrical energy, have been regarded as the most suitable method for sustainable energy production. However, most MFCs generate a low power output, limiting their large-scale industrial applications. Here, we introduce a high-performancesingle-chambered microbial fuel cell (SCMFC) based on carbon fiber anode and zinc oxide nanoparticle (ZnO NP)-fabricated copper cathode. Furthermore, best optimized conditions of temperature, pH, substrate, external resistance, and cathode fabrication were also considered to evaluate the performance of this SCMFC in treating potato wastewater along with bioenergy production. Results indicated that chemical oxygen demand (COD) could be effectively removed (80%) and maximum voltage, current density, and power density were 1.58 V, 0.235 mA/cm2, and 0.3714 mW/cm2, respectively. Hence, the SCMFC used in this study has a higher potential to treat potato wastewater along with high power production.

Two-orders of magnitude enhanced droplet energy harvesting via asymmetrical droplet-electrodes coupling

The collection and utilization of water power as clean energy have made tremendous progress in recent years, where the droplet-based triboelectric power generation is one of the most studied fields. Current researches mainly focus on the droplet-based single-electrode triboelectric generation mode; however, the low power output greatly limits the practical applications. By contrast, the droplet-based bi-electrode freestanding TENG (DBE-TENG) show great potential for enhancing the output, while the charge transfer mechanism of droplets evolution on the DBE-TENG is still ambiguous. Here we demonstrate that for a DBE-TENG, the maximum current output of the droplet at different droplet impact positions exhibits a sinusoidal-like variation, and the current output can be improved by two orders of magnitude by simply controlling the droplet impacting positions. Furthermore, an asymmetrical-capacitance-induced charge transfer mechanism is proposed, which well elucidates the prominent current output enhancement and the sinusoidal-like variation. In addition, the current output enhancing strategy is applicable under different kinds of dielectric materials and conditions (inclined conditions and different Weber numbers), with the maximum enlargement up to 263 times. The findings deepen the understanding of the charge transfer mechanism for bi-electrode triboelectric nanogenerators, and will be of significance for improving the efficiency of sustainable energy harvesting.

Low-Grade Thermal Energy Harvesting and Self-Powered Sensing Based on Thermogalvanic Hydrogels.

Thermoelectric cells (TEC) directly convert heat into electricity via the Seebeck effect. Known as one TEC, thermogalvanic hydrogels are promising for harvesting low-grade thermal energy for sustainable energy production. In recent years, research on thermogalvanic hydrogels has increased dramatically due to their capacity to continuously convert heat into electricity with or without consuming the material. Until recently, the commercial viability of thermogalvanic hydrogels was limited by their low power output and the difficulty of packaging. In this review, we summarize the advances in electrode materials, redox pairs, polymer network integration approaches, and applications of thermogalvanic hydrogels. Then, we highlight the key challenges, that is, low-cost preparation, high thermoelectric power, long-time stable operation of thermogalvanic hydrogels, and broader applications in heat harvesting and thermoelectric sensing.

Achieving milliwatt level solar-to-pyroelectric energy harvesting via simultaneous boost to photothermal conversion and thermal diffusivity

Pyroelectric technology is an effective strategy to harvest ambient waste heat into electrical energy to tackle global energy and environmental crises. However, current pyroelectric generators are often limited by low effective power output. Herein, we develop a non-contact, solar-induced pyroelectric nanogenerator (S-PENG) which integrates Au@CNT as solar-thermal layer and polarized PVDF film as pyroelectric layer. The high thermal conductivity of CNT accelerates the heat transfer process, while its strong solar-thermal effect can be coupled with the plasmonic effect of Au nanoparticles to obtain a hybrid ensemble with superior light absorption and conversion. Notably, the solar-thermal temperature of Au@CNT/PVDF rapidly increases from 38 °C to 79.6 °C within 30 s under sunlight irradiation, with a corresponding temperature change rate reaching a maximum of 14.3 °C/s. The drastic temperature fluctuation is crucial to improve the output performance of our S-PENG. More importantly, our S-PENG successfully generates a notable 1.5 mW/m2 output power under a 200 MΩ load (at 20 ℃), thereby overcoming the performance bottleneck of traditional S-PENG designs with micro-watt power output. Our design offers a promising approach to efficiently utilize green solar energy to alleviate our demand on limited energy resources and reduce carbon footprint.

A high-performance mini-generator with average power of 2 W for human motion energy harvesting and wearable electronics applications

The sustainability of power supply for mobile electronic devices is a problem that needs to be solved urgently. The energy from human motion is an optimal solution for this problem. As for the traditional energy harvesters based on electromagnetic, triboelectric, and piezoelectric induction, energy management is the largest challenge because of the low conversion efficiency and output power. In this paper, a high-efficiency and high-power energy harvester (EPEH) is presented based on the intersecting beam array structure. It can effectively harvest the pressure energy of humans under the soles at the states of walking, running, and weight-bearing walking. The EPEH does not impair the flexibility of humans compared with joint energy harvesters. The intersecting beam array structure is key component, which can magnify the compression distance in the vertical direction of 3.4 times in the horizontal direction, facilitating maximum energy harvesting within a limited height. On the other hand, the design of rolling friction structure can effectively reduce energy loss and improve energy conversion efficiency. The result shows that the maximum average output power can reach 2 W at 7 km/h. Because of the high output power, the energy management circuits and voltage regulator circuits can be derived and stable work, the energy harvested by the EPEH can be stored in the lithium battery (3.7 V, 50 mAh). Furthermore, the generated energy could supply mobile phones, smart watches, light bulbs, flashlights, children's anti-lost devices, and so on. Through the energy management circuit and voltage regulator circuit (1.5 V and 5 V), the energy conversion efficiency of the device can reach 65.2 % at a compression velocity of 2500 mm/min and a frequency of 1.25 Hz. It is of great significance for the practical application of wearable energy harvesting technology and has multiple application prospects.