基于超临界CO2循环的地热与太阳能混合系统研究

超临界CO₂循环可以耦合较低温度的地热和较高温度的太阳能热组成混合热源发电系统。相比能量分析方法,火用分析方法更便于分析混合系统对提高能量利用率的作用,以及识别造成可用能损失的设备和过程。115℃地热和200℃地热分别与采用槽式聚光集热技术的太阳能热组成混合热源,构成简单回热超临界CO₂循环。分析结果表明：混合系统的火用效率比单纯太阳能热的循环系统提高了5% ~ 10%;太阳能聚光集热器的?损失最大,占80%以上,其次是除预冷器以外的各类换热器以及透平;相比之下,压缩机和预冷器的火用损失较小。减少?损失的关键是提高太阳能聚光集热器和换热器的性能,包括提高集热管运行温度,以及提高换热器效能。     

Modeling and performance of microscale thermophotovoltaic energyconversion devices

We analyze the feasibility of energy conversion devices that exploit microscale radiative transfer of thermal energy in thermophotovoltaic devices. By bringing a hot source of thermal energy very close to a receiver fashioned as a pn-junction, the near-field effect of radiation tunneling can enhance the net power flux. We use the fluctuational electrodynamic approach to microscale radiative transfer to account for the spacing effect, which provides the net transfer of photons to the receiver as a function of the separation between the emitter and receiver. We calculate the power output from the microscale device using standard thermophotovoltaic device relations. The results for the performance of a device based on indium gallium arsenide indicate that a ten-fold increase in power throughput may be realized with little loss in efficiency. Furthermore, we develop a model of the microscale device itself that indicates the influence of semiconductor band-gap, energy, carrier lifetime and doping     

Miniaturized piezoelectric energy harvester for battery‐free portable electronics

This paper describes a miniaturized energy harvester based on the interaction of two permanent magnets placed on both ends of a folded cantilever made with piezoelectric thin films. A unique design of the folded cantilever structures is developed to efficiently collect ambient vibrational energy over a wide frequency bandwidth for the operation of battery‐free electronic devices. The output performance of the two energy harvesters is observed to improve significantly through the use of the mutual coupling technique. The folded cantilever structure also demonstrates improved space efficiency compared with the conventional rectangular cantilever design. A wide frequency band and sufficient energy‐harvesting ability were successfully achieved by the optimally manufactured device. The maximum output power of the miniaturized energy harvester was 41.6 μW with an impedance of 0.3 MΩ. Furthermore, a pedometer was powered completely by the energy harvester without the need for any battery and external power source, demonstrating the potential of the proposed design for self‐powered electronics applications in a vibrational environment.     

Optimal energy use of the collector tube in solar power tower plant

As one of the most important parts of the solar power tower plant, the receiver plays an important role in the high-efficiency operation of the solar power tower system. Obtaining the maximum available energy in the receiver is highly desired in real-world operations. In this paper heat transfer and exergy transfer methods are used to model the energy transfer process in a collector tube. Different from common analysis methods, in order to ensure the molten salt to obtain the maximum available energy, an objective function is proposed to convert the task into a constrained optimization problem. The gravitational search (GS) algorithm is employed to search for the optimal solution of the proposed objective function. Numerical results indicate that the proposed optimization method can find the optimal operation parameters under different conditions. The heat transfer and exergy transfer characteristics along the collector tube under the optimal working condition are revealed, which quantifies the available energy along the collector tube, as well as reveals the location of energy degradation in the tube. The research findings will provide a beneficial reference for the effective use of the solar energy.     

Energy storage for enhancing transmission capacities and trans-regional reserves of a UHV AC/DC power grid

探索应用储能为弱受端电网提供故障后大功率支撑以提升特高压交直流输电线稳态输送能力,同时兼作送端电网调峰的跨区备用源以增加新能源发电空间,以有效促进新能源跨区外送降低弃风弃光率。基于特高压交直流馈入受端电网的功率缺额与频率变化间的对应关系,提出在电网频率最低点满足要求的前提下,电网受电能力提升程度与储能容量配置间的数学模型;依据送端电网典型日各时段负荷特性建立可释放的新能源发电空间与所需储能备用调峰容量间的数学模型。结合考虑事故功率支撑与跨区调峰备用的容量需求特性及优先级别,探索两种功能联合应用下的容量需求方案和技术经济可行性。以交直流馈入的弱受端河南电网及新能源富集的送端新疆电网为例,验证了该新能源消外送纳能力提升方案的有效性。     

Performance improvement study of an integrated photovoltaic system for offshore power production

Sustainable energy is one of the main options for resolving energy problems and climate change issues. Solar energy is one of the main promising renewable energy sources, which can be captured and converted to electrical energy through photovoltaic (PV) panels. In the open literature, it is shown that having two PV panels integrated into a back‐to‐back configuration placed on naturally reflective surfaces provides the potential of doubling the total power produced by a single‐faced PV panel with the appropriate location and orientation. This paper presents a case study of two‐PV panel systems for offshore power production. The relevance to offshore has the water surface as the reflective surface to produce power from the back facing panel. The city of Ottawa in Canada is selected as the location for a case study. Various conditions and operating parameters are considered in assessing the performance of the proposed system, including solar radiation intensity, system orientation, time of year in terms of months, and the variations in parameters throughout the day. The assessment of the proposed system is carried out through modeling and simulating the proposed double PV panels in the COMSOL Multiphysics software. It is found that the minimum improvement in the total power production over the single face conventional PV is 38% in January for the east‐facing PV front face. For the two PV systems, the optimal overall power production for the various time conditions and orientations, at the specified location, is found to be the north orientation of the PV panel. In this case, the power it produces is 89% of that of the east orientation. A similar trend is observed for the single‐faced PV panel, where the north‐facing PV provides 62% of what it could produce in the east‐facing orientation.     

Smart grid adds to renewable resources hosting capacity: Collaboration of plug‐in hybrid electric vehicles in Volt/VAr control process

This paper develops an efficient energy management approach to increase the renewables share in energy provision of smart distribution grids (SDGs). Voltage violation ends in curtailment of renewables generations and, hence, decreases the economic success of distribution companies. To avert such deficits, this study fosters the collaboration of SDG components in an intelligent Volt/VAr control process. The investigated SDG is characterized with high penetration of photovoltaics (PVs), dispatchable distributed generations (DDGs), plug‐in hybrid electric vehicles (PHEVs), and infield control devices say as under‐load tap‐changing transformers (ULTCs). In charge stations, PHEVs are coupled to the SDG through bidirectional inverters which are offering simultaneous exchanges of active and reactive powers. Thus, regarding the PHEV aggregators, optimal schedules of active power charge/discharge signals with their inductive/capacitive reactive power provisions are determined. This notion effectively increases PV power injections and, consequently, provides significant monetary savings. Besides, this mechanism reduces ULTC tap operations in Volt/VAr control process maintaining its nominal lifetime. The proposed approach is formulated as a mixed‐integer non‐linear programming (MINLP) and solved based on DICOPT solvers in general algebraic modeling system (GAMS). Effectiveness of the proposed approach is explored on a typical distribution test system. The obtained results show 8.94% increment in harvested PVs power and hence 5.24% reduction on daily operation cost of SDG.     

Vibration energy harvesting in vehicles by gear segmentation and a virtual displacement filtering algorithm

In this study, vibration straight‐line displacements occurring during electric vehicle operations are changed into reversely rotating displacements and divided into small segments by a gear unit. Piezoelectric bending elements are used to form a cantilever beam–based energy feedback mechanism for converting vibration energy into electrical power within an allowable displacement range. A virtual vibration displacement filtering algorithm is proposed to effectively filter virtual displacements that cannot excite the energy harvester and generate electrical power. The average speed of the gear exciting the bender has an accuracy 30%, which is increased using this algorithm compared with 2 other algorithms and directly affects the accuracy of calculating the average power generated by the calculation of piezoelectric bending elements. Theoretical and experimental analyses are conducted for the impact of gear pitch on the regeneration power value by changing the gear pitch at a fixed driving speed. Experiments show that when a vehicle is operated at a fixed speed, the proposed method can be used to obtain the maximum average power of a single piezoelectric bending element through determination of a rational gear pitch. Specifically, when the test vehicle operated at 20 and 60 km/h, the gear pitch should have been 7 and 10 mm.     

Polymer-based nanopiezoelectric generators for energy harvesting applications

Abstract

Energy harvesting from ambient vibrations originating from sources such as moving parts of machines, fluid flow and even body movement, has enormous potential for small power applications, such as wireless sensors, flexible, portable and wearable electronics, and bio-medical implants, to name a few. Nanoscale piezoelectric energy harvesters, also known as nanogenerators (NGs), can directly convert small scale ambient vibrations into electrical energy. Scavenging power from ubiquitous vibrations in this way offers an attractive route to provide power to small devices, which would otherwise require direct or indirect connection to electrical power infrastructure. Ceramics such as lead zirconium titanate and semiconductors such as zinc oxide are the most widely used piezoelectric energy harvesting materials. This review focuses on a different class of piezoelectric materials, namely, ferroelectric polymers, such as polyvinlyidene fluoride (PVDF) and its copolymers. These are potentially superior energy harvesting materials as they are flexible, robust, lightweight, easy and cheap to fabricate, as well as being lead free and biocompatible. We review some of the theoretical and experimental aspects of piezoelectric energy recovery using Polymer-based NGs with a novel emphasis on coupling to mechanical resonance, which is relevant for efficient energy harvesting from typically low frequency (<1 kHz) ambient vibrations. The realisation of highly efficient and low cost piezoelectric polymer NGs with reliable energy harvesting performance could lead to wide ranging energy solutions for the next generation of autonomous electronic and wireless devices.     

Effect of a longitudinally positioned solenoid coil on electronic descaling

The present study introduces a new method in electronic descaling (ED) technology. In the proposed method, the ED apparatus consisted of a longitudinally positioned solenoid coil installed on an outer surface of a pipe. When electric current flows through the electric coils a magnetic field is generated whose direction is perpendicular to that of the flow direction. Experiments were performed at various Reynolds numbers. In order to monitor fouling at the heat transfer test section, the pressure drop across the test section and the overall heat transfer coefficient were measured as a function of time. The present study demonstrated an enhanced descaling effect of the longitudinally positioned solenoid coil while effectively inhibiting a formation of scales at slow flow conditions.     

Holographic diffraction‐through‐aperture spectrum splitting for increased hybrid solar energy conversion efficiency

A holographic module is designed to split light into two spectral bands for hybrid solar energy conversion. Incoming light is either transmitted to a large subsystem receiver or diffracted through an aperture in this receiver toward a second subsystem receiver. The holographic element is simulated using rigorous diffraction and ray‐tracing methods. Two applications of the design are described and simulated. A photovoltaic/thermal system with 93% optical efficiency and adjustable subsystem power output ratio is designed to address solar intermittency and provide energy storage. A photovoltaic system added to an alga biofuel operation significantly increases energy output while maintaining 92% of the original algae yield. The energy return on investment of this photovoltaic/biofuel system is 2.4× greater than that of the biofuel system alone, leading to economically viable operation. Modifications to the standard holographic lens provide additional increases in spectrum‐splitting capability, optical efficiency, and energy conversion efficiency. The diffraction‐through‐aperture concept is demonstrated as a successful approach to spectrum splitting for hybrid solar applications. Copyright © 2014 John Wiley & Sons, Ltd.     

Damping subsynchronous resonance using superconducting magneticenergy storage unit

A novel damping scheme using superconducting magnetic energy storage (SMES) unit is proposed in this paper to damp subsynchronous resonance (SSR) of the IEEE Second Benchmark Model, system-1 which is a widely employed standard model for computer simulation of power system SSR. The studied system contains a turbine-generator set connected to an infinite bus through two parallel transmission lines, one of which is series-capacitor compensated. In order to stabilize all SSR modes, simultaneous active and reactive power modulation and a proportional-integral-derivative (PID) damping controller designed by modal control theory are proposed for the SMES unit. A frequency domain approach based on eigenvalue analysis and time-domain approach based on nonlinear model simulations are performed to validate the effectiveness of the damping method. It can be concluded from the simulation results that the proposed damping scheme can effectively suppress SSR of the studied system     

Modelling of wind energy‐based microgrid system implementing MMC

Nowadays, renewable energy systems have come up with more potential in power generation so as to meet the power demand. Among all the renewable systems, the wind energy generating system is believed to be at the peak. However, the wind energy‐based microgrid system is associated with many problems such as fluctuations in output voltage due to the fluctuated wind speeds and harmonics generations in the system. To address these issues, this article proposes a new method in order to achieve harmonic mitigation across its output by maintaining constant voltage. Nevertheless, particular attention has been given to the form and function of modular multilevel converter with multi‐winding transformer connected to the grid. Modular multilevel converter has been implemented with an advanced voltage controller tuned to control the voltage at its output. Also, a new system topology has been introduced with two wind turbines that are interconnected to multi‐winding transformer through asynchronous generators. The proposed system has been implemented with constant and variable wind speeds, and their respective results have also been analysed. The proposed scheme shows its effectiveness by theoretical calculations, verified by simulation and experimental results. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermodynamic optimisation of the integrated design of a small‐scale solar thermal Brayton cycle

The Brayton cycle's heat source does not need to be from combustion but can be extracted from solar energy. When a black cavity receiver is mounted at the focus of a parabolic dish concentrator, the reflected light is absorbed and converted into a heat source. The second law of thermodynamics and entropy generation minimisation are applied to optimise the geometries of the recuperator and receiver. The irreversibilities in the recuperative solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In a small‐scale open and direct solar thermal Brayton cycle with a micro‐turbine operating at its highest compressor efficiency, the geometries of a cavity receiver and counterflow‐plated recuperator can be optimised in such a way that the system produces maximum net power output. A modified cavity receiver is used in the analysis, and parabolic dish concentrator diameters of 6 to 18 m are considered. Two cavity construction methods are compared. Results show that the maximum thermal efficiency of the system is a function of the solar concentrator diameter and choice of micro‐turbine. The optimum receiver tube diameter is relatively large when compared with the receiver size. The optimum recuperator channel aspect ratio for the highest maximum net power output of a micro‐turbine is a linear function of the system mass flow rate for a constant recuperator height. For a system operating at a relatively small mass flow rate, with a specific concentrator size, the optimum recuperator length is small. For the systems with the highest maximum net power output, the irreversibilities are spread throughout the system in such a way that the internal irreversibility rate is almost three times the external irreversibility rate. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental investigation of a new thermal energy storage system using thermo‐sensitive magnetic fluid and porous medium

In this paper, a novel thermal energy storage (TES) system based on a thermo‐sensitive magnetic fluid (MF) in a porous medium is proposed to store low‐temperature thermal energy. In order to have a better understanding about the fluid flow and heat‐transfer mechanism in the TES system, four different configurations, using ferrofluid as the basic fluid and either copper foam or porous carbon with different porosity (90 and 100 PPI, respectively) as the packed bed, are investigated experimentally. Furthermore, two thermal performance parameters are evaluated during the heat charging cycle, which are thermal storage velocity and thermal storage capacity of the materials under a range of magnetic field strength. It is shown that heat conduction is the primary heat‐transfer mechanism in copper foam TES system, while magnetic thermal convection of the magnetic fluid is the dominating heat‐transfer mechanism in the porous carbon TES. In practical applications in small‐scale systems, the 90‐PPI copper foam should be selected among the four porous materials because of its cost efficiency, while porous carbon should be used in industrial scale systems because of its sensitivity to magnetic field and cost efficiency.     

Energy harvesting from sediment microbial fuel cell to supply uninterruptible regulated power for small devices

Sediment microbial fuel cell (SMFC) is a bio‐electrochemical device that generates direct current by microbes present in the soil. The main drawback of SMFC is the low voltage and fluctuations. Therefore, a suitable scheme is required to obtain sufficient voltage with insignificant fluctuation. This paper proposes an energy harvester power management system (PMS) to get rid of low voltage and fluctuation problem of SMFC. The proposed PMS is composed of a dc‐dc boost converter, switches, and super capacitors. The boost converter (using LTC3108 IC) successfully steps up the voltage up to 2.658 V and provides it to the load for 1.5 minutes. Four SMFCs connected with four individual super capacitors and a single boost converter has been used to implement this scheme. In this strategy, the charging and discharging time of the SMFCs are controlled in such a way that the continuous power will be supplied to the load with the optimum number of SMFCs. This scheme is tested on an experimental setup. It is found that the energy harvester PMS supplies a continuous voltage of 2.658 V with the efficiency of 85.46%, which is sufficient to power for small devices such as remote environment sensors, temperature sensors, LED lighting, and submersible ultrasonic receiver.     

Setting a fostered energy network by decarbonizing the grid: Hybridization,control, and future solutions upon storage

According to recent energy scenario, the abundancy of noncarbonaceous resources (NCRs) is capable of satisfying the energy demand of residential, commercial, and domestic in near future. This paper portrays hybridization of these sources, along with its control and hence presenting a novel framework for finding the operational hours of the storage device. This in turn will help in developing a fostered energy network (FEN) across the globe. Instead of connecting the NCRs concurrently, the proposed system is integrated in a segment fashion that yields three subsystems like solar PV‐wind, wind‐fuel cell, and solar PV‐fuel cell. The controller thus designed for the voltage source converter (VSC) has its own uniqueness in operating at any system combination under healthy as well as various fault conditions. The electrolyzer is used for storing the surplus energy generated by the system and supplying energy during energy crisis and operates on a bidirectional controller. The reliability of the controller is tested by examining its operation for healthy as well as faulty conditions such as single line to ground (SLG) and three phase fault. Finally, the useful lifespan of the electrolyzer is estimated by using the probability approximation method. It relies upon obtaining the number of electrolyzers that will work for stipulated time period and how many of them will be discarded among all the installed. The system under study is modeled, simulated, and validated in MATLAB/SIMULINK environment.     

Energy harvesting in wireless sensor networks: A taxonomic survey

Wireless sensor networks (WSNs) have aroused the conspectus attention of scholars due to their extensive deployment in the emerging fields of the Internet of Things (IoT's) and self‐driven devices. But WSNs technologies having a major bottleneck has been associated with limited energy. Mostly research in WSNs has been focused on minimizing energy usage to extend the survival time of limited power source in a network. Energy harvesting can be addressing its energy‐scarcity problem of WSNs, so it is giving popularity to Energy Harvesting in Wireless Sensor Networks (EH‐WSNs). The paper presents a comprehensive taxonomic survey on recently energy harvesting techniques and algorithms that proposed by various authors and also examined the work done by the various researchers in the field of EH‐WSNs. For the ready reference of the researchers, a concise summary and comparative analysis of various promising techniques for energy harvesting have also been included in the systematic survey. However, many equipment developed using the hybridization method in a singular package to get full advantages of available free energy, are explored in this review. The review on hybrid energy harvesting (HEH) systems can be considered as the originality of this article. However, the outdoor photovoltaics have been provided maximum power density about ≈100 mW/cm³, and the piezoelectric harvesters have been given maximum voltage about 325 V but the current in very minute amount. The thermoelectric, rectenna and hybrid energy harvesters (EHs) have been given high efficiency more than 80%. Additionally, hybrid EHs have location/time‐independent characteristics which harnessed power from more than one source that can be became more popular for upcoming leading technologies of self‐driven or autonomous devices shifting from battery operated devices. Finally, the survey also identifies often challenges and various significant issues that still essential to be addressed to develop a cost effective, efficient, long‐lasting, and almost maintenance‐free energy harvesting systems for WSNs along with trail to their possible solutions for future perspectives.     

Power control optimization of a new contactless piezoelectric harvester

In this study, we propose an optimization scheme for the control of a piezoelectric wind energy harvester. The harvester is constructed by a blade in front and a magnet in the rear in order to sustain a magnetic repulsion by another magnet located on the stable harvester body in a contactless manner. For such a new harvester, the control scheme is missing in the literature in the sense that the harvester is new and an overall optimization study is required for such a device. In that context, the optimization has been realized by using a new current control law based on the harvester piezoelectric terminal voltage and the layer bending. The proposed control law can impose a second order linear dynamics although the magnetic effects can yield to nonlinear magnetic force relation. In order to improve the new control strategy, a Particle Swarm Optimization algorithm (PSO) has been applied, since there is a nonlinear dependency among the control parameters, the collected energy and the bending force mean values. According to results, the captured electrical power has a high increasing trend with respect to the only-voltage-based (OVB) control as the current study proves. On the contrary, the artifact of the method is that the obtained power is too low to increase the mean bending forces and it requires much complicated control system.