Intermittent Power Source Research Articles

Investigation of an Intermediate Temperature SOFC Operation in Series with a CPOx Reformer

Solid oxide fuel cells are known for their fuel flexibility which makes them a great candidate for decentralized power generation and off-grid applications. Depending on the target application, there have been different system configurations such as steam reforming vs. CPOx reforming, external reforming vs. internal reforming, anode recirculation vs. single pass. The CPOx reforming in series with SOFC with single pass configuration opens up the intermittent power source market, due to its unique benefit of low cost, short startup time and easy maintenance. In this scheme fuel cell electrochemical performance is directly related to the syngas composition, i.e. H2 and CO fractions in the fuel stream, as well as any unreformed methane.In this study a computational fluid dynamics (CFD) model has been developed to study the dynamic and steady state behavior of a commercial intermediate temperature SOFC stack operating in series with a CPOx reformer. The CFD model of the SOFC accounts for the charge, species, momentum and energy balances, whereas a reduced order model for the CPOx incorporates equilibrium thermodynamics. CPOx is an exothermic catalytic reaction with an overall balance of:CH4+1/2O2 → 2H2+CO (1)which is defined in stoichiometric ratios. In practice the air flow is regulated to control the reactor temperature and syngas yield. Hence, along with temperature and pressure the ratio of oxygen to carbon (O-C ratio) in the left hand side of Eq. 1 is a critical parameter for determining the overall performance and efficiency of the system. Moreover, the CPOx operation typically has different routes to achieve desired products which constitutes a system with various reaction equilibria [i]. However, apart from Eq.(1) the dominant reaction paths can be summarized as [ii]:Reformation CH4+CO2 ↔ 2H2+2CO (2)Water gas shift CO+H2O ↔ H2+CO2 (3)Boudouard CO2 + C ↔ 2CO (4)CH4 Combustion CH4+2O2 → 2H2O+CO2 (5)CO Combustion 2CO+O2 → 2CO2 (6)H2 Combustion 2H2+O2 → 2H2O (7)Of these, combustion reactions can be practically considered as irreversible. Overall system equilibrium is a result of these reactions and is a function of temperature, pressure and O/C ratio. Any changes in these parameters shift the reaction equilibrium, hence the conversion efficiency and syngas yield. As a result, SOFC performance will be directly dependent on these parameters.In this study different operating regimes of CPOX and SOFC have been investigated. The model is validated experimentally with polarization data and gas chromatography analysis. The model predictions show good agreement with the GC data for both the CPOX and SOFC outlet gas compositions.Performance of the SOFC running on syngas obtained as a result of different CPOx equilibria is compared and a fuel cell efficiency map as a function of temperature and O-C ratio has been constructed. Also, dynamic response of SOFC to the changes in CPOx operating parameters is estimated with the transient simulations. Moreover, selectivity of SOFC toward H2 and CO has been investigated by outlining the different energy conversion paths inside the fuel cell electrode. It is found that SOFC at intermediate temperatures has an overall tendency for CO conversion due to enhanced WGSR reaction inside the electrodes although the standalone CO oxidation overvoltage is higher than that of H2 oxidation. Finally, SOFC performance is analyzed for injecting various amounts of unreformed CH4 into the syngas stream.[i] Bjørn Christian Enger, Rune Lødeng, Anders Holmen, A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts, Applied Catalysis A: General 346 (2008) 1–27.[ii] Ruoshui Ma, Bang Xu, Xiao Zhang, Catalytic partial oxidation (CPOX) of natural gas and renewable hydrocarbons/oxygenated hydrocarbons—A review, Catalysis Today 338 (2019) 18–30. Figure 1

High Charge Storage of Amorphous Ni-Doped VOx-Modified Ni(OH)2 Substrate on a Ni Foam Cathode in a Base Solution

Green energy of solar light has been harnessed to overcome the energy and environment crisis. However, solar energy is an intermittent power source. This work demonstrates a promising amorphous Ni-doped VOx/Ni(OH)2 (a-NVO-x)-layered Ni foam with different contents of VOx as the energy material to store the intermittent energy. a-NVO-x is electrodeposited on Ni(OH)2 lamellar-grown Ni foam and used as a cathode material in a supercapacitor cell. The as-prepared a-NVO-x cathode materials with different electrodeposition times are characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Simultaneously, the electrochemical properties are examined by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) analyses. a-NVO-5 achieves the highest specific capacitance of about 7500 F/g or 3000 C/g at 1 A/g, with energy and power densities of about 167 Wh/kg and 199 W/kg, respectively, in a 3 M KOH solution. The capacity retention can reach >90% after 10k cycles at 20 A/g. A full-cell supercapacitor is also built by coupling the a-NVO-5 cathode with an active carbon anode, and it can deliver energy and power densities of 286.5 Wh/kg and 1150 W/kg at 1 A/g, respectively. The excellent performance of the a-NVO-x electrode accommodates the pseudocapacitive charge balance in the a-VOx structure during charge–discharge processes.

Modelling, Simulation and Controlling of a Multi-Pump System with Water Storage Powered by a Fluctuating and Intermittent Power Source

In recent years, many pumping systems have begun to be powered by renewable energy generators, including mostly photovoltaic generators and, less frequently, electrical wind generators. Because of the technology’s complexity and novelty (it has not yet reached its maturation), most of those systems consist of single pumps powered by photovoltaic generators or electrical wind generators. For this reason, the current paper proposes a strategy for driving a multi-pump system with water storage powered by a fluctuating and intermittent power source, such as power grids, which are limited by price variation over different periods during the day, or photovoltaic generators and/or electrical wind generators. The current work begins by proposing a model of a multi-pump system with water storage, followed by the design of a control strategy for operating such a system powered by a fluctuating and intermittent power source in an energy-efficient manner, without sacrificing the reliability, robustness and lifetime of the plant. Finally, an analysis of two concrete situations encountered in practice is made: in one, the considered multi-pump system is powered only by a power grid limited by price variation over three periods; in the other, it is powered by a photovoltaic generator.

Solar-driven hydrogen generation coupled with urea electrolysis by an oxygen vacancy-rich catalyst

Urea, an environmental pollutant for both soil and water, is widely present in wastewater. On the other hand, a strategy utilizing renewable electricity to decrease the cost of green hydrogen, which holds the key to a sustainable energy future, is promising but challenging. Gas crossover that generates explosive hydrogen–oxygen mixture becomes very serious with intermittent renewable power source (partial load issue). Herein, we address these issues in one device, i.e., a hybrid electrolyzer where water oxidation that produces oxygen is replaced by urea oxidation which generates inert gases. A self-supported electrocatalyst of nitrogen-doped nickel-iron oxyhydroxide derived from waste rusty iron foam is synthesized via an in situ ‘waste-to-value’ synthetic route followed by an ammonia/argon plasma treatment, which reconstructs the surface of the catalyst to a 3D nanosheet-like porous network with abundant oxygen vacancies. The as-prepared catalyst shows a small potential of 1.45 V vs. RHE at 500 mA cm−2 for urea oxidation. Overall water-urea electrolysis only requires 1.58 V to deliver 100 mA cm−2, which is 0.33 V less than that in urea-free water splitting, and thus lowers the overall energy consumption by 17.3%. Without oxygen evolution, the hybrid electrolysis does not suffer from the safety hazard caused by explosive hydrogen–oxygen mixture. We demonstrate the safe production of green hydrogen (3.1% oxygen in the gaseous product) in the hybrid electrolysis powered by solar energy via a photovoltaic panel. Our work provides a method to address the urea-caused environmental issues and simultaneously generate green hydrogen safely using solar energy.

From room to roof: How feasible is direct coupling of solar-battery power unit under variable irradiance?

Integration of photovoltaics (PV) with electrical energy storage (battery) is a straightforward approach to turn intermittent power source into stable power supply. Power coupling, or power matching, between PV-device, a battery, and a load is most frequently performed with aid of maximum power point tracking (MPPT) electronics. MPPT electronics provides high flexibility as for PV and load impedances, and irradiance, however, it brings in additional cost, and complexity, power overhead, potential reliability issues, and interference signals. On the other hand, direct coupling via preselection of PV and battery parameters is a simple scalable and highly efficient alternative to MPPT for a specific set of conditions. We explore with modeling how far a directly coupled PV-battery unit can stay power-matched under various conditions, and demonstrate feasibility of excellent power matching over orders of magnitude of irradiance and a wide range of load resistances. Both a PV-harvester in an office room with low irradiance, non-demanding load, and high autonomy, and a PV-system on a roof with high irradiance, demanding load, and partial autonomy, can operate efficiently without MPPT electronics if an appropriate battery is included. This result emphasizes the role of a battery as an impedance matching element besides storage functionality in a directly matched PV-system.

Quantitative characterization of uncertainty levels of intermittent power sources

This paper establishes a statistical quantification of the uncertainty levels of intermittent power sources. We first construct a negative exponential function, referred to as a statistical function, to represent the relationship between the statistical regularity of the forecast error of a single intermittent power source and the time ahead of the forecast. Subsequently, we generalize this negative exponential function to a family of statistical functions, namely, the sum statistical functions, the equivalent statistical functions, and the contour statistical functions, which are proposed to characterize the overall statistical forecast uncertainty levels of multiple intermittent power sources and all power sources. Based on historical observations, parameters of these functions are estimated to represent the statistical regularity of the forecast uncertainty levels of all the power sources of interest. Historical data sampled from real wind farms and solar power sites demonstrate the effectiveness of the proposed method.

A Rapid Active Power Regulating Technique for A Virtual Small-hydro Cluster

A rapid active power regulating technique for a virtual small-hydro cluster is studied in this paper. The virtual small-hydro cluster consists of several decentralized small-hydro power stations, which quickly responds to the active power target instructions of real-time dispatch and AGC. Furthermore, the power fluctuation of the delivery cross-section caused by intermittent power source in short-term scale can be efficiently smoothed. When any tie line transmission power reaches its thermal stability limit, the active power target instructions is allocated according to the sensitivity priority of small-hydro power plants to threshold-crossing lines among the small-hydro cluster. When all the tie lines transmission powers are within their thermal stability limits, the active power target instruction is allocated on the basis of the priority of the summation of product of tie line power variations and sensitivity of small-hydro power stations to tie lines. Under the constraint of fulfilling the active power target instructions, the simulation results show that the adopted control strategy and allocation algorithm can effectively decrease the power fluctuation of each tie line.

Design Algorithm for Optimum Capacity of ESS Connected With PVs Under the RPS Program

A distributed power generation system uses an intermittent power source that provides low power. Furthermore, it cannot be centrally controlled from the perspective of the system operator. The photovoltaic (PV) power generation system is installed with an energy storage system (ESS) to increase the power source quality compared to that possible with PV-only power generation. In this paper, an estimation algorithm is proposed for improving battery capacity to construct a suitable and economical system. In addition, an optimal algorithm is proposed that considers different factors. For example, the cost of constructing the battery and the amount of electric power sales reflecting the renewable energy certificate weighting factor of 5.0 are based on the cost of the battery construction, depth of battery discharge, charge/discharge cycle of the battery, cost of the energy conversion device, operation maintenance cost, and charge cycle of the battery. The optimized battery capacity is calculated by considering the correlation between PV power and meteorological parameters, construction cost, and the benefit of the amount of electricity sold, with an error rate of 5.75%.

Rechargeable, flexible and mediator-free biosupercapacitor based on transparent ITO nanoparticle modified electrodes acting in µM glucose containing buffers

We present a transparent and flexible self-charging biosupercapacitor based on an optimised mediator- and membrane-free enzymatic glucose/oxygen biofuel cell. Indium tin oxide (ITO) nanoparticles were spray-coated on transparent conducting ITO supports resulting in a flocculent, porous and nanostructured electrode surface. By this, high capacitive currents caused by an increased electrochemical double layer as well as enhanced catalytic currents due to a higher number of immobilised enzyme molecules were obtained. After a chemical pre-treatment with a silane derivative, bilirubin oxidase from Myrothecium verrucaria was immobilized onto the ITO nanostructured electrode surface under formation of a biocathode, while bioanodes were obtained by either immobilisation of cellobiose dehydrogenase from Corynascus thermophilus or soluble PQQ-dependent glucose dehydrogenase from Acinetobacter calcoaceticus. The latter showed a lower apparent KM value for glucose conversion and higher catalytic currents at µM glucose concentrations. Applying the optimised device as a biosupercapacitor in a discontinuous charge/discharge mode led to a generated power output of 0.030mW/cm2 at 50µM glucose, simulating the glucose concentration in human tears. This represents an enhancement by a factor of 350 compared to the power density obtained from the continuously operating biofuel cell with a maximum power output of 0.086µW/cm2 under the same conditions. After 17h of charging/discharging cycles a remarkable current enhancement was still measured. The entire device was transferred to flexible materials and applied for powering a flexible display showing its potential applicability as an intermittent power source in smart contact lenses.

Performance Analysis of Electric Spring in Reducing the Power Drawn From the Supply System During generation Uncertainties

This article deals about reduction of power drawn from the supply system in smart environment during un-certainties in the power generation. Supply system in this work is the system which consists of an intermittent power source and a storage system to support whenever there is deficit in power generation. A small micro grid system is considered for this study which has critical and non-critical load present in it along with storage system and Electric spring. An attempt is made to study the performance of Electric spring in reducing the power drawn from the supply system under different non-critical loads during un-certainties or power deficit conditions of the source. The effect of reactive component in the non-critical load is examined separately for the considered battery storage system. All the simulation models have been developed in MATLAB.

Intelligent Identification of Voltage Variation Events Based on IEEE Std 1159-2009 for SCADA of Distributed Energy System

Since the renewable energy is intermittent power source, feeding a lot of renewable energy into the grid would cause voltage variations affecting the stability of the power system. As a result, the effective detection and identification of voltage variation events become important tasks for the protection of microgrid integrated different distributed energy systems. In this paper, a rule-based technique based on IEEE Std 1159-2009 is proposed to deal with the estimation of voltage variations, and a prototype of a supervisory control and data acquisition system with filter bank and adaptive filtering is developed. With the help of the implemented technique, the monitoring of voltage variation events can be efficiently performed and practically applied in the microgrid system of the Taiwan Institute of Nuclear Energy Research.

Electric System Cascade Analysis (ESCA): Solar PV system

In a previous work, Electricity System Cascade Analysis (ESCA) was applied to optimize a Distributed Energy Generation (DEG) system consisting of a non-intermittent power generator and an energy storage device. DEG systems consisting of intermittent power source however were not discussed. This work thus extends the application of ESCA for intermittent resources. New ESCA proposed in this work has significance differences in term of execution strategy. The main cause of the differences is due to intermittent power generation which influenced by weather variability. In addition, this work includes the sizing of inverter. In this paper, the new ESCA was applied to optimize a solar PV system for an isolated rural house with daily energy consumption of 5.575kWh.

Household Solar Photovoltaics: Supplier of Marginal Abatement, or Primary Source of Low-Emission Power?

With declining system costs and assuming a short energy payback period, photovoltaics (PV) should, at face value, be able to make a meaningful contribution to reducing the emission intensity of Australia’s electricity system. However, solar is an intermittent power source and households remain completely dependent on a “less than green” electricity grid for reliable electricity. Further, much of the energy impact of PV occurs outside of the conventional boundaries of PV life-cycle analyses (LCA). This paper examines these competing observations and explores the broader impacts of a high penetration of household PV using Melbourne, Victoria as a reference. It concludes that in a grid dominated by unsequestered coal and gas, PV provides a legitimate source of emission abatement at high, but declining costs, with the potential for network and peak demand support. It may be technically possible to integrate a high penetration of PV, but the economic and energy cost of accommodating high-penetration PV erodes much of the benefits. Future developments in PV, storage, and integration technologies may allow PV to take on a greater long term role, but in the time horizon usually discussed in climate policy, a large-scale expansion of household PV may hinder rather than assist deep cuts to the emission intensity of Australia’s electricity system.

Turbulent Character of Wind Energy

Wind turbines generate electricity from turbulent wind. Large fluctuations, and, more importantly, frequent wind gusts cause a highly fluctuating electrical power feed into the grid. Such effects are the hallmark of high-frequency turbulence. Here we show evidence that it is the complex structure of turbulence that dominates the power output for one single wind turbine as well as for an entire wind farm. We illustrate the highly intermittent, peaked nature of wind power fed into the grid. Multifractal scaling is observed, as described initially by Kolmogorov's 1962 theory of turbulence. In parallel, we propose a stochastic model that converts wind speed signals into power output signals with appropriate multifractal statistics. As more and more wind turbines become integrated into our electric grids, a proper understanding of this intermittent power source must be worked out to ensure grid stability in future networks. Thus, our results stress the need for a profound understanding of the physics of turbulence and its impact on wind energy.

Efficient decomposition of NO by ammonia radical-injection method using an intermittent dielectric barrier discharge

Although many NO decomposition systems have been developed using plasmas such as dielectric barrier discharges (DBDs), corona discharges, surface discharges, glow discharges, and microwave discharges, the present system is unique on the viewpoint of the use of an intermittent one-cycle sinusoidal power source to generate DBD plasma. There are several features of the system: (1) easy control of the electric power consumed in the DBD plasma, and (2) DBD-plasma generation used only for the production of ammonia radicals. The system employs a radical injection system, where the radicals are produced in a separate discharge chamber, called radical injector, from NO flow field. This enables an efficient production of ammonia radicals being appropriate for DeNOx. It is shown from the temperature dependence of NO removal (DeNOx) characteristics that the present system is a low-temperature DeNOx system compared to a conventional thermal DeNOx system, and NO decomposition is performed over a wide range of gas temperature containing NO. Surveying parametric characteristics of DeNOx, the energy efficiency is improved by a factor of 30% compared to the previously obtained result.

Optimum Conditions for NO Reduction Using Intermittent Dielectric Barrier Discharge at Atmospheric Pressure

NO in N2 gas was removed by injecting ammonia radicals, which were externally generated by flowing NH3 gas diluted with Ar gas through a dielectric barrier discharge (DBD) with a one-cycle sinusoidal-wave power source. The discharge was intermittently formed between coaxial cylindrical electrodes at an applied peak-to-peak voltage of 2–15 kV. The generated radicals were introduced in a reaction chamber and reacted with NO. In order to find optimum parameters for NO reduction and energy efficiency, the reaction temperature in the mixing zone, the voltage applied to the gap of the electrodes for DBD generation and its repetition rate, the NO gas concentration, and the ammonia concentration and flow rate were varied. A maximum energy efficiency of 140 g/kWh at a NO reduction of over 99% is obtained at a voltage slightly higher than the discharge firing voltage and a repetition rate of 5 kHz, which corresponds to a duty cycle of 5%. Thus it is found that the use of the intermittent power source is an advantage for obtaining a high energy efficiency of NO reduction.

Power/energy: No ill winds for New Mexico utility: Under test in one municipality's power system, an experimental machine supplies up to 15 percent of community needs

Discusses a large wind generator which joined the electric utility system in the town of Clayton, NM, and while much remains to be studied, already it is clear: the results are encouraging. Linked to the town's present system of seven diesel generators, the revolving wind blades can supply 200 kW of output power, nearly 15 percent of Clayton's total power load during off-peak periods. This result has become evident despite many early shakedown problems-most since solved-and operation in a system with significant frequency deviations. Technical evaluation is still under way, and operating and maintenance costs have yet to be fully analyzed. The operation is serving as a testbed for the impact of an intermittent power source on an electric-utility system. The machine in use is the US Department of Energy/NASA Mod 0-A and is rated at 200 kW in a 22.4-mi/h wind for a constant rotor speed of 40 r/min. It went into service in Clayton on Jan. 28, 1978.