Electrical Load Variations Research Articles

Nafion-SiO2 Hybrids Combined with Pt-Sn/C Anodes for DEFC Operating at High Temperature

Polymer electrolyte fuel cells (PEFCs) are particularly attractive for applications with variable electric load and intermittent operation, such as portable electronics, distributed power sources, and electric vehicles. Furthermore, such devices can operate with diverse fuels such as hydrogen, methanol, and ethanol. In particular, the direct ethanol fuel cell (DEFC) has been considered a promising energetic alternative due to low toxicity of ethanol compared with methanol, and the possibility of fuel production from renewable sources. However, the efficiency of DEFCs is substantially lower than for fuel cells fed with hydrogen or methanol mainly due to the lower anode activity. An alternative to enhance the performance of DEFCs is to increase the operating temperature because the electrode reactions are thermally activated. However, pristine Nafion membranes are not suitable for such operating conditions because increasing the temperature, which decreases the level of hydration, results in severe loss of ionic conductivity [1]. Several approaches were proposed to prevent the conductivity loss of Nafion at high temperature. A straightforward approach is the addition of a hygroscopic phase, such as SiO2 and TiO2, within the perfluorosulfonic acid (PFSA) matrix to form a hybrid nanocomposite [2]. On the other hand, PtSn catalysts are currently considered the most efficient materials for ethanol oxidation, which activity is dependent on the amount of alloyed Sn and oxides [3]. In this context, the objective of this work is to evaluate Nafion-SiO2 hybrids in combination with well-alloyed PtSn/C catalysts produced by a modified polyol method [4] for DEFC operating at high temperature (130 oC). Nafion-SiO2 hybrids were synthesized by in situ sol-gel reaction, by the incorporation of oxide directly into the ionic aggregates of Nafion membranes with different thickness (95, 125, and 175 µm) and conformation (casting and extrusion). The effect of synthesis parameters, such as sol-gel solvent, membrane thickness and silicon precursor concentration on silica incorporation degree, water and ethanol uptake, and mechanical stability, were also studied. Characterization of Nafion-SiO2 hybrids showed an effective incorporation of SiO2 particles in both hydrophilic and hydrophobic domains of Nafion template with a compositional range of 4 and 13 SiO2 wt% depending on the silicon precursor concentration, sol-gel solvent, and Nafion thickness. In comparison to pristine Nafion, the hybrids exhibited higher water and ethanol uptakes. Carbon-supported PtSn nanoparticles with different compositions were synthetized by a modified polyol method using two reducers. Characterization was carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). These studies showed rather large amounts of Sn incorporated into the PtSn alloyed phase compared with values usually reported in literature. The electronic properties of PtSn/C catalysts were studied by in situ X-ray absorption spectroscopy (XAS). Polarization curves were obtained for 2 mL min-1 ethanol flow (1 mol L-1). The oxidant was pure oxygen at 3 atm pressure. For the DEFC having a commercial PtSn/C (75:25) at the anode and unmodified Nafion 117 showed a rather poor performance, with a maximum power density of 40 mW cm-2 at operating temperature of 80oC. Changing the electrolyte to Nafion 117-SiO2 (13 wt%) hybrid and keeping the same anode catalyst resulted in a somewhat better performance at 80oC. The cell performance improved at higher temperatures and a maximum power density of about 100 mW cm- 2 was obtained. Finally, the optimized anode and electrolyte were evaluated in DEFC prototypes operating in the 80 – 130 ºC temperature range. For the DEFC composed by optimized PtSn/C prepared by a polyol method combined with the Nafion 117-SiO2 (13 wt%) hybrid a significant performance improvement was observed, with a maximum power density of 125 mW cm- 2 for the cell operated at 130oC. In summary, the combination of optimized catalysts and electrolytes allowed high operating temperatures that resulted in significant improvement of DEFC performance. The authors thank FAPESP (2014/12255-6; 2013/01822-4), CNPq (407143/2013-0), and CAPES for financial support and fellowships.

Modelling an off-grid integrated renewable energy system for rural electrification in India using photovoltaics and anaerobic digestion

This work describes the design optimisation and techno-economic analysis of an off-grid Integrated Renewable Energy System (IRES) designed to meet the electrical demand of a rural village location in West Bengal – India with an overall electrical requirement equivalent to 22 MWh year−1. The investigation involved the modelling of seven scenarios, each containing a different combination of electricity generation (anaerobic digestion with biogas combined heat and power (CHP) and photovoltaics) and storage elements (Vanadium redox batteries, water electrolyser and hydrogen storage with fuel cell). Micro-grid modelling software HOMER, was combined with additional modelling of anaerobic digestion, to scale each component in each scenario considering the systems' ability to give a good quality electricity supply to a rural community. The integrated system which contained all of the possible elements – except hydrogen production and storage presented the lowest capital ($US 71 k) and energy cost ($US 0.289 kWh−1) compared to the scenarios with a single energy source. The biogas CHP was able to meet the electrical load peaks and variations and produced 61% of the total electricity in the optimised system, while the photovoltaics met the daytime load and allowed the charging of the battery which was subsequently used to meet base load at night.

Performance Analysis of High Speed Domino CMOS Logic Circuits

the demand of low power high performance arithmetic circuits multiplies, during this paper, we aim to introduce a style of latest MT-CMOS domino logic and FTL dynamic logic technique to style adder circuit. The MT-MOS transistors cut back the facility dissipation by minimizing sub threshold run current in the introduced domino logic circuits. The MT-NMOS semiconductor connected in discharging path of output electrical converter may be applied for pipeline structure to scale back the facility consumptions and increase fan-out. Dynamic logic vogue CMOS circuit is employed to enhance the speed and cut back the world of style by decreasing the device count. The introduced FTL dynamic logic circuit improves the performance by evaluating the computational blocks partially before its input signals are formalized and then rapidly perform a final evaluation as soon as the inputs arrive. This dynamic logic formation is like minded for an arithmetic circuit wherever the important path s created of an outsized cascade of inverting gates is created. The mixture of MT-CMOS and dynamic logic circuit provides high fan-out, high change frequencies with each lower delay and dynamic power consumption. The simulation results of those projected low power high performance circuits provide 75% power reduction, three times redoubled high speed operation and active space reduction, whereas revealing lower sensitivity to power provide, temperature, electrical phenomenon load and method variations than the dynamic domino CMOS technologies.

Real-time replication of a stand-alone wind energy conversion system: Error analysis

This paper provides adequate information about the problem of real-time replicating in laboratory conditions. The dynamic behavior of stand-alone low-power wind energy conversion systems (WECS) in response to the wind speed variations and also to the electrical load variations is replicated. The investigated system consists of a variable-speed wind turbine based on a permanent-magnet synchronous generator (PMSG), a diode bridge rectifier, a DC–DC step-down converter and a wide range DC load. Because of reduced noise level and better steady-state accuracy, a speed-driven hardware-in-the-loop physical WECS simulator has been used to accomplish this task. Its significant drawback – that is, a reduced bandwidth – has been significantly alleviated by using an enhanced software simulator structure which uses a feed-forward compensation of the inherent physical disturbance produced by the generator torque variations. Both time-domain experimental results and a thorough frequency-domain error analysis show good replication performance in the frequency range of variation of both wind speed and electrical load.

Verified Stability Analysis for Interval-Based Sliding Mode and Predictive Control Procedures with Applications to High-Temperature Fuel Cell Systems

In previous work, control-oriented models have been derived for solid oxide high-temperature fuel cell systems. In these models, interval variables have been used to describe uncertainty due to a limited knowledge about system parameters and to handle effects of electric load variations on the temperature distribution in the fuel cell stack module as well as bounded measurement uncertainty. To deal with these types of uncertainty both in the design of robust controllers and during their online usage, interval techniques can be employed successfully. These control procedures make use of the basic principles of either sliding mode control or predictive control. The corresponding algorithms and the prerequisites for their real-time capable implementation using software libraries for interval arithmetic and algorithmic differentiation are described in this paper. Experimental results show the efficiency of these control laws for a fuel cell test rig that is available at the Chair of Mechatronics at the University of Rostock.

Compressed air energy storage (CAES) with compressors distributed at heat loads to enable waste heat utilization

Large scale penetration of renewable energies such as wind and solar into the electric grid is complicated by their intermittency. Energy storage systems can mitigate these fluctuations by storing off-peak energy for use at peak-demand times. Compressed air energy storage (CAES) is one of the most promising storage technologies due to the large amount of energy that can be stored at an economical cost. We evaluate the feasibility of improving the economics of CAES by distributing compressors near heat loads to enable recovery of the heat of compression to supply low-grade heating needs such as district heating. Distributed CAES (DCAES) is more efficient; however, it has higher capital costs due to the compressed air pipeline required between distributed compressors and the storage site. We evaluate the project economics of DCAES in a hypothetical scenario with a variable electric and heat load. The size and dispatch of a generation fleet composed of a wind farm, CAES or DCAES plant and conventional gas turbines are optimized to satisfy the annual electricity load at an hourly resolution at the lowest total cost. We find that the total cost of supplying heat and electric loads is less expensive with DCAES given a 50km pipeline when fuel prices exceed $7.6/GJ. The cross-over fuel price depends on the distance as it drives the capital cost of the pipeline. The minimum effective fuel price required for economic superiority of the DCAES system is $7.0/GJ and $8.3/GJ at pipeline lengths of 25 and 100km, respectively.

Modified stochastic estimation of load dependent energy losses in electric distribution networks

This paper proposes a new modified stochastic model of steady-state operating conditions for a load-dependent (current-dependent) energy losses estimation in electric distribution networks of any configuration. This model is based on stochastic simulation of a power covariance matrix and electric power loads. Electric power load variation is presented in the form of a linear combination of some independent random factors. It is able to correctly take into account a variety of power system operating conditions with a small volume of initial information, and to determine energy losses for a certain time-period in one computation of expected values of power loads. Energy losses are determined with sufficient accuracy and reliability for practical purposes.

Ultra-short-term multi-node load forecasting – a composite approach

In the electric power system, grasping active and reactive load variation regularity of each node is a key problem for optimal dispatch, preventive control, security assessment and transmission capacity evaluation. This study presents a novel composite approach of ultra-short-term forecasting for multi-node active and reactive load. It proposes a hierarchy and sub-area idea to construct a framework of self-adapting dynamic load models. With load forecasting at the top layer implemented by the recursive least square support vector machines (RLS-SVM) algorithm, discrete state-space equations are established to describe the dynamic characteristics of the multi-node active load distribution factors and power factors, and then the Takagi–Sugeno (TS) fuzzy control technique is introduced to realise feedback compensation. The application of the proposed technique in the actual power system control centre of Shandong Province is verified and satisfactory results are found.

Environmental consequences of the use of batteries in low carbon systems: The impact of battery production

Adoption of small scale micro-generation is sometimes coupled with the use of batteries in order to overcome daily variability in the supply and demand of energy. For example, photovoltaic cells and small wind turbines can be coupled with energy storage systems such as batteries. When used effectively with renewable energy production, batteries can increase the versatility of an energy system by providing energy storage that enables the systems to satisfy the highly variable electrical load of an individual dwelling, therefore changing usage patterns on the national grid. A significant shift towards electric or hybrid cars would also increase the number of batteries required. However, batteries can be inefficient and comprise of materials that have high environmental and energy impacts. In addition, some materials, such as lithium, are scarce natural resources. As a result, the overall impact of increasing our reliance on such “sustainable or “low carbon” systems may in fact have an additional detrimental impact.This paper reviews the currently available data and calculated and highlights the impact of the production of several types of battery in terms of energy, raw materials and greenhouse gases. The impact of the production of batteries is examined and presented in order that future studies may be able to include the impact of batteries more easily within any system. It is shown that lithium based batteries have the most significant impact in many environmental areas in terms of production. As the use phases of batteries are extremely variable within different situations this has not been included here, instead providing comprehensive data for the production stage.

Dynamic analysis of PEMFC-based CHP systems for domestic application

Fuel cell-based CHP systems for distributed residential power generation represent an interesting alternative to traditional thermoelectric plants. This is mainly due to the high efficiency obtainable in the production of electricity and heat in a decentralised, quiet and environmental friendly way. The current paper focuses on the development, in Matlab®Simulink environment, of a complete dynamic model of a residential cogenerative (CHP) energy system consisting of the Proton Exchange Membrane fuel cell (PEMFC), fuel processor, heat exchangers, humidifier and auxiliary hot water boiler. The target of the study is the investigation through such a model of the behavior of CHP systems based on fuel cell (FC) at variable electrical and thermal load, in reference to typical load curves of residential users. With the aim to evaluate the system performance (efficiency, fuel consumption, hot water production, response time) and then to characterize its better operating conditions with particular attention to air relative humidity, suitable simulations were carried out. They are characterized by the following of a typical electrical load trend and in relation to two different thermal load profiles. The dynamic model presented in this paper has allowed to observe the fully functioning of the FC based system under variable loads and it has permitted to design appropriate control logics for this system.

서해 도서지역의 풍력-디젤 하이브리드 발전에 대한 경제성 분석

본 연구에서는 최대 발전량이 500kW인 서해 도서의 풍력-디젤 하이브리드 발전시스템에 대한 경제성 분석을 수행하였다. 이를 위해 해당 도서에 대한 연간 전력 부하 변동치와 바람 데이터를 수집하고 분석하였으며, 풍력을 포함하여 신재생 에너지에 대한 하이브리드 최적화 모델로 미국의 NREL에서 개발한 Homer 프로그램을 이용하였다. 풍력-디젤 하이브리드 발전시스템의 경제성을 판단하기 위해 풍속과 경유 가격을 변수로 하는 민감도 해석도 수행하였다. 그 결과, 현재의 조건에서는 경제성이 낮았으나, 풍속이 초속 3미터가 넘거나 경유 가격이 리터당 2.4달러를 초과할 경우 대상지역에도 풍력-디젤 하이브리드 발전시스템이 경제성을 갖는 것으로 평가되었다. In this paper, an economic feasibility study of wind-diesel hybrid power systems for an island in the Yellow Sea, where the maximum power generation is about 500kW, was performed. For the study, annual electric load variation and wind resource data of the island were collected and analyzed. HOMER program - a typical hybrid optimization model for electric renewables including wind resource, developed by the National Renewable Energy Laboratory - was used. Wind speed and diesel price were picked out as variables for the sensitivity analysis in order to find the economic accountability for the wind-diesel hybrid power system. As the result, even though it is not feasible economically under the present condition, if mean wind speed is over 3 m/sec. or diesel price goes up to 2.4 $ per liter, the wind-diesel hybrid power system for the island becomes a prospective candidate.

Hydrogen energy system analysis for residential applications in the southern region of Algeria

The purpose of this paper is related to hydrogen energy system analysis for residential applications in Ghardaia (southern region of Algeria). This system is based on proton exchange membrane fuel cell (PEMFC) technology, which is supplied by fuel reforming process, for producing hydrogen fuel starting from natural gas. The exhaust heat is recovered by a Thermal Storage Tank (TST), which is used in an absorption sub-system as a generator for residential cooling system. The feasibility analysis of an absorption cooling device, using thermal energy of PEMFC sub-system, for a residence application located at the unit of applied research in renewable energy in Ghardaia has been studied and performances were analysed. Electrical and thermal powers generated by the PEMFC sub-system with variable electrical loads (Part Load Ratio) have been analysed. The feasibility study shows that using PEMFC for residential cooling in Ghardaia is a promising solution. It is shown that the temperature of the TST is sufficient to supply the absorption sub-system with a coefficient of performance equals to 0.72 and, the efficiency of the HES equals to 97%.

A Degradation Measurement System for Polymer Electrolyte Membrane Fuel Cells Using a Dead-Ended Passive Stack and Time-Varying Load

A degradation measurement system has been developed for polymer electrolyte membrane fuel cells to reveal the relationship between degradation rates and electric load variations. The system involves a dead-ended stack of 10 single cells and an electronic load device that functions as a time-varying load to each cell of the stack. Use of a stack enables the temperatures of the cells to be kept almost the same and the supplied gas conditions to be constant. Experiments were conducted to evaluate the feasibility of this test system. The polarization curves of the multiple cells were simultaneously measured, and the load of each cell in the stack can be individually controlled. Therefore, this system could be applied to reduce the time and cost in measuring the degradation and lifespan of polymer electrolyte membrane fuel cells.

Self-excitation and control of an induction generator in a stand-alone wind energy conversion system

This study presents a rugged and cost-effective scheme for start-up and operation of a stand-alone squirrel cage induction machine (SCIG) for a wind energy conversion system (WECS). A voltage source converter (VSC) directly interfaces the SCIG with an equivalent dc load network, which could also be the DC link of an inverter. The VSC dc bus is supported by an electrolytic capacitor and the proposed scheme ensures reliable start up with these rudimentary components. The dc bus voltage is ramped to the rated value from a small initial voltage produced by remanent magnetism of the SCIG core. Subsequently, the load is applied. The V/f control paradigm for motoring operation is suitably extended to the SCIG for controlling both voltage build-up and dynamic transients. A controller is specifically designed to maintain constant dc bus voltage under wind speed and electrical load variations. Steady-state machine flux is maintained constant up to the base speed, which maximises machine utilisation and power extraction especially at higher wind speeds. The overall system modelling and analytical control design is presented. The proposed control strategy has been validated through simulation and experimentally verified on a low power (2.2 kW) laboratory prototype.

From the electrical shunting of a loudspeaker to active impedance control

Variable acoustic properties can be obtained on an electroacoustic transducer's voicing face by very basic control strategies, among which are the shunting of the loudspeaker (shortcut, variable electric load, or negative resistance disposals). It is proven that better performances with enhanced controllability can be obtained by way of hybrid feedback control, consisting in a double feedback loop, one on acoustic pressure at the diaphragm and the other on its velocity, leading to a global acoustic impedance control. The present work describes the theory of the hybrid feedback control, by way of block diagrams aiming at visualizing the control principle, starting from the shunted loudspeaker. Simulations of performances obtained on Matlab/Simulink are presented, and compared to experimental results obtained on analog prototypes mounted at the end of a dedicated impedance tube. A last, a discussion on stability issues follows, leading to concluding remarks on the disposal behavior and possible means of enhancements.

Model for evaluating impact of battery storage on microgeneration systems in dwellings

Lead-acid batteries are a suitable technology for on site storage of microgenerated electricity. In this paper, an algorithm is developed that will assess, with an appropriate temporal resolution of data, the ability of lead-acid battery storage in capturing AC and DC power generated from photovoltaic cells, combined heat and power and wind turbines. The assessment includes the impact that the storage element has on the import and export of energy from the electrical grid and is a valuable tool in determining an optimum storage capacity. Used effectively, storage can increase the versatility of a microgeneration system by satisfying the highly variable electrical load of an individual dwelling, thus changing usage patterns on the national grid. Empirical electrical load demands are considered with a 1 min temporal resolution and compared with microgeneration (and battery) supply profiles with a similar temporal accuracy. The results show that, when producing on site electricity through microgeneration, suitably sized storage can reduce export substantially (by over 90% in some cases) and store this energy at a typical round trip efficiency of 70–72%. The developed model accounts for typical losses in a battery storage system, including that associated with inverters, power electronics and the efficiency of charge/discharge cycles.

A novel maximum power fuzzy logic controller for photovoltaic solar energy systems

The maximum power tracking problem and efficient energy utilization of a stand-alone photovoltaic array (PVA) feeding voltage controlled linear and nonlinear loads is studied. A novel and simple on-line fuzzy logic-based dynamic search, detection and tracking controller is developed to ensure maximum power point (MPP) operation under excursions in solar insolation, ambient temperature and electric load variations. A computer simulation model of the PVA renewable utilization scheme including the effects of temperature and solar irradiation changes was developed and fully simulated. The load voltage is controlled by a DC chopper and kept constant at the required rated voltage. A permanent magnet DC motor (PMDC) driving a fan-type load was connected in parallel to an RL passive load. A speed control scheme is also used for the PMDC motor drive so that the drive can be operated at specified speeds. Different controllers have been employed in the unified PVA scheme to control three separate loads at MPP tracking condition namely voltage at load bus and speed of the PMDC motor. The main objective of the paper is to present a novel enhanced, cost-effective MPP detector (MPPD) and dynamic MPP tracking (MPPT) controller for a hybrid mix of electric loads.

24-h electrical load data—a sequential or partitioned time series?

Variations in electrical load are, among other things, hour of the day dependent, introducing a dilemma for the forecaster: whether to partition the data and use a separate model for each hour of the day (the parallel approach), or use a single model (the sequential approach). This paper examines which approach is appropriate for forecasting hourly electrical load in Ireland. It is found that, with the exception of some hours of the day, the sequential approach is superior. The final solution however, uses a combination of linear sequential and parallel neural models in a multi-time scale formulation.

Chirp response of an active-controlled thickness-drive tunable transducer

The chirp response of a thickness-drive tunable transducer for wide range time-bandwidth and sweep rates chirp signals is demonstrated experimentally and computationally. The computational evaluation uses recursive digital-filter model based on the z-transform method. The model is limited to simple lossless structure with no front and backing layers. The model and experimental results show that there is no limit on the maximum sweep rate of the chirp signal but practically the limit is determined by the limitation of the circuit that generates the control voltage that simulates a variable electric load.

The Taweelah A2 independent water and power project

Taweelah A2 is the first independent water and power project in Abu Dhabi under the government privatization policy. In response to the Abu Dhabi Water and Electricity Authority (ADWEA) request for proposals, CMS Energy prepared and submitted their offer in April 1998. Following clarifications, the Power and Water Purchase Agreement was signed on 3 October 1998. CMS appointed Mott MacDonald as their technical advisers for the tender and project phases. The Taweelah A2 plant will comprise a 720 MW combined cycle power plant and 50 migd MSF desalination plant. Three Siemens type V94.3a gas turbine generators with exhaust heat recovery boilers will supply steam to two back pressure steam turbine generators. Exhaust steam will be taken to four Hanjung 12.5 migd single deck cross tube MSF distillers, equal in capacity to the largest units ever built. Carbon dioxide gas recovered from the distiller air ejectors will be used for remineralizing the distillate. Seawater will be drawn from an existing intake channel serving the complete complex. The boilers are designed for a high level of supplementary firing to meet the wide variations in electrical load on the Abu Dhabi grid, with full water output. Construction of the US $750 million project, located between the existing Taweelah A1 and B stations, began in April 1999. First power production is scheduled in May 2000, first water production in May 2001 and completion in August 2001. The plant will be owned and operated by Emirates CMS Power, jointly owned by CMS and ADWEA. This paper describes the desalination and power plant and outlines the power and water purchase aspects of the project. The high thermal efficiency of the combined cycle and MSF plant minimizes fuel consumption, heat rejection and atmospheric emissions, making this one of the best plants in the region from an environmental standpoint.