Fixed-speed Operation Research Articles

Experimental investigation on a closed-cycle single-stage turbo-refrigerator for deep freezing

Turbo-refrigerator based on reverse Brayton cycle is an attractive solution for providing refrigeration at ultra-low temperatures below −60 °C for deep freezing. Employing a motor-driven turboexpander compressor (MTEC) not only allows the turbo-refrigerator to achieve high compactness, reliability and flexibility, but also leads to new operating characteristics especially in a closed cycle. This paper aims to investigate the operating characteristics of a closed-cycle single-stage turbo-refrigerator with a MTEC. A high-speed turbo-refrigerator system with the design refrigeration temperature of −86 °C was manufactured and then tested under variable rotating speeds and refrigeration conditions. Test results show that a refrigeration temperature below −96.4 °C was obtained in a pull-down progress. Compared with a fixed-speed operation, variable-speed operation enables the independent control of the refrigeration temperature and the cooling capacity. With the decrease in the refrigeration temperature (or in the cooling capacity), the rotating speed has a greater effect on the COP than that on the cooling capacity (or on the refrigeration temperature). The mass flow rate along with the compression and expansion work shows a decreasing trend as the refrigeration temperature decreases at a fixed speed of the MTEC. The advanced exergy analysis shows a total exergy efficiency of 0.42 (with a COP of 0.62 at refrigeration temperature of −86 °C) could be expected with component improvement in the near future, and it is of the priority to increase the efficiency of the turboexpander.

Impact of waste-plastic-derived diesel on the performance and emission characteristics of a diesel engine under low load conditions

In recent years, hybrid (e.g. diesel-renewable) power systems have been implemented in many isolated regions (e.g. remote areas and islands) to reduce dependency on diesel generation. Nevertheless, low load (up to 30% of the maximum rated power) at variable speed operations is considered flexible and relevant for maximum renewable energy penetration because of the high cost and complexity of the conventional mode with fixed speed operation in the hybrid system. Given the increasing emphasis on fossil fuels in power generation sectors, plastic-derived diesel can be a suitable alternative. In this study, plastic-derived diesel was used for engine testing. It was produced from plastic crude oil from high-density polyethylene, polypropylene and polystyrene in a 1:1:1 ratio by using a vacuum distillation technique via the pyrolysis process. The pyrochemical properties of plastic-derived diesel were tested after distillation. The present work focused on validating the two-zone thermodynamic model for investigating engine performance and emissions at variable speeds (i.e. 1200, 1500, 1800 and 2100 rpm) at 5, 10, 15, 20, 25 and 30% load conditions fuelled with ultra-low sulphur diesel and plastic-derived diesel blends (5, 10 and 20). The simulation and experimental results showed good agreement. The maximum variation of 17.2% was found between the experimental and simulated results at mentioned speeds and loads for all tested fuels. Compared with ultra-low sulphur diesel, plastic-derived diesel showed a significant improvement in brake power, torque, brake thermal efficiency and brake-specific fuel consumption. The maximum brake power variation was 9.85% for plastic-derived diesel (20%) at 1200 rpm at 5% load. Moreover, the minimum brake specific fuel consumption variation was 4.15% for ultra-low sulphur diesel at 1500 rpm and 25% load. Compared with the commercial ultra-low sulphur diesel, 20% of plastic-derived diesel blends exhibited maximum reductions of 4.75, 5.95, 4.45 and 4.35% in NOx, CO2, CO, and HC emissions, respectively, under any tested conditions. Plastic-derived diesel blends showed less particulate matter emission compared to ultra-low sulphur diesel. Therefore, up to at least 20% of the plastic-derived diesel blends tested in this study can be used as an alternative fuel for ultra-low sulphur diesel. Further study should be carried out to test more than 20% plastic-derived diesel blends and reach a conclusion.

Modeling and design optimization of a hybrid power generator for full-electric naval propulsion

The purpose of this work is to build a model of diesel engine useful for studying new hybrid solutions for power generation. This kind of study is of significant interest particularly for the marine sector, due to recent limitations on fuel consumption and constraints imposed on the minimum powertrain efficiency. In this paper a variable-speed diesel generator model is presented. The model simulates both the dynamic behavior of the engine and the fuel consumption during the operating cycle. This model was developed using experimental results of bench tests conducted on the VL1716C2-MLL engine by Isotta Fraschini Motori (IFM) SpA. Diesel engine dynamics is based on the formulation found in the technical literature on mean value models and then adjusted through experimental data. Special attention was paid to developing the procedure to find the minimum specific fuel consumption for a given load. Initially, the presented resutls concern the correct response of the model under different load conditions. Then, the improvement in specific fuel consumption (obtained by adjusting the engine speed according to the load), in comparison with fixed speed operation, is shown.

A variable wind harvesting based induction generator using variable voltage and variable frequency converter for renewable energy applications

ABSTRACT This paper proposes the design and development of a power electronic converter intended for induction generator (IG) to harvest the power generated from variable wind in a wind energy conversion system (WECS). The squirrel cage induction generator (SCIG) is anticipated for fairly fixed speed operation, while its speed should be super-synchronous with respect to the synchronous speeds corresponding to the frequency of the receiving alternating current bus bar. In the existing WECS, SCIG requires a minimum wind velocity below which generation is not possible. The novelty of the proposed method is that the range of wind velocities can be selected over which generation is possible. To achieve this, the proposed system makes use of a variable voltage and variable frequency converter (VVVFC). Depending upon the wind velocity, the frequency of the VVVFC is adjusted such that the speed of the turbine and hence the IG are virtually maintained at super synchronous speeds with respect to the frequency of the AC side of the VVVFC. In addition, the perturb and observe-based maximum power point tracking (MPPT) scheme is also implemented. The proposed methodology also includes a series compensated buck–boost converter (SCBBC) inserted between the battery and the direct current link of the final inverter that feeds an RL load. Simulation in the MATLAB Simulink environment is carried out, and an experimental prototype has also been developed to validate the proposed method. Based on the results, it is found that the proposed variable speed system with the integration of VVVFC, SCBBC, and MPPT harvests is 78% greater than the conventional fixed speed system.

Influence of Reynolds number on the performance of small horizontal axis wind turbine with fixed speed operation

ABSTRACT The Blade Element Momentum (BEM) theory is most commonly utilized to quickly assess the performance of horizontal axis wind turbine. However, the BEM theory requires the aerodynamic characteristic of the airfoil to evaluate the performance. Generally, the aerodynamic characteristics of airfoil in BEM theory for performance prediction of wind turbine with fixed rotor speed operation is obtained at an approximated fixed Reynolds number which may not accurately correspond to the operating Reynolds number over the blade. In the present work, the performance of a small wind turbine, with fixed rotor speed operation, and considering a range of approximated fixed Reynolds number (i.e., 0.5, 0.75, 1.00, and 1.25 million) and the operational Reynolds number over the blade for the aerodynamic characteristic of the airfoil has been evaluated and compared for a range of wind speeds i.e., 6, 8, 10, and 12 m/s. To incorporate the operational Reynolds number in the study, the aerodynamic characteristics of the airfoil have been mathematically represented as function of Reynolds number and angle of attack using an artificial neural network (ANN)-based model. The results suggest that inadequate Reynolds number consideration can led to high discrepancies up to 18.6% in power coefficient prediction.

Regulation Characteristics and Load Optimization of Pump-Turbine in Variable-Speed Operation

In order to improve the operating and regulation characteristics of the hydropower unit and to stabilize the load fluctuations, variable-speed pumped storage technology based on converters has been proposed and given more attention recently. However, different from the conventional units, due to the variability of operation conditions, variable-speed units need to develop a load optimization strategy in terms of operating parameter identification to ensure state matching for operation. Therefore, this paper proposes an optimization search step based on the model test curve, and the process of parameter optimization search is elaborated and calculated in the turbine operating condition and pump operating condition, respectively. A mathematical model of the turbine regulation system is established to analyze the influence of speed and guide vane related parameters on the regulation characteristics, and the achievable operating range and regulation capacity in the variable-speed condition is pointed out based on pump-turbine model test, as well as the advantages over the fixed-speed operation. The results show that by applying the load optimization method, the variable-speed unit can be significantly improved in terms of operating efficiency, especially at low head and low power conditions. Meanwhile, a certain range of active power regulation can be realized by the decoupling control of the converter and measuring the guide vane opening in both modes. The analysis of the model test verifies the effectiveness of the variable-speed regulation operation of pump-turbine and provides a reference for the design and operation of the variable-speed hydropower units.

Optimal scheduling of variable speed pumped storage, solar and wind energy system

ABSTRACT The entry of renewable energy sources in the power sector has significantly affected the inter-connected power system’s safety and stability as renewable energy sources tend to generate variable power. The variability of power inflicts severe challenges for the scheduling of renewable sources. A pumped storage hydro plant (PSP) can help in integrating the variable renewable energy and balance the demand and generation on a day-ahead (DA) or real-time basis through fast response generation. An optimal PSP operation schedule has been obtained to increase the power producer’s profit by formulating it as a mixed-integer problem. The mathematical problem formulation for the hybrid system with solar, wind, and PSP operation has been made, considering the next day’s market schedule. The revenue has been computed by utilizing the energy of solar, wind, and PSP. A comparative study for the profit earned depending upon the power is regulated or not, using variable speed technology, during the pumping mode has been presented. The grid power has been utilized for the pumping operation of the pumped storage plant. A penalty factor has been utilized for the mismatch in the demand and generation. The analysis shows that the variable speed operation of PSP during pumping mode has increased the net profit by and reduced the charges paid toward the pumping operation by compared to the fixed speed operation and a rise of in profit and decline in pump charges have been recorded compared to two-speed operation.

Variable-Speed Hydropower Generation: System Modeling, Optimal Control, and Experimental Validation

The variable-speed hydropower generation technology of small hydropower plants has great potential because of abundant water resources and the low overall efficiency of the traditional constant speed plants. In this article, we investigate a converter-fed synchronous generator system capable of variable-speed operation (VSO) in the full power range. First, the hydraulic turbine, permanent magnet synchronous generator, and power electronic converter in the system are modeled, respectively. Next, based on the system model, a three-layer control system is proposed to realize the multiobjective control. To automatically track the optimal speed under different inflow rates, a modified “perturb and observe” algorithm is proposed, which is easy to implement and able to track the maximum power point quickly and accurately compared with the classical method. In addition, the proposed system is used to reconstruct one of the two parallel hydropower sets with the same hydrological conditions. And the performance tests and comparative experiments between the fixed-speed operation and VSO sets were extensively performed. It shows that a considerable increase in energy production is achieved in the VSO set.

Performance Testing of a Dual Rotary Compressor with a Variable Speed PSC Motor

A new system solution has been developed and tested by the authors and has been shown to improve the Seasonal Energy Efficiency Rating (SEER) of fixed-speed condenser split-system residential heat pumps. This is achieved through the use of a low power electronic drive that allows components with PSC motors (such as fans and compressors) to operate at multiple low speeds or variable speeds when partial capacity is required. At full capacity, the electronic drive is disabled and the PSC motors are operated at fixed speeds dictated by the frequency of the power source. Previous research by the authors has shown that fixed-speed heat pumps driven by rotary compressors with PSC motors achieve significantly greater improvements in performance and in overall SEER when operated with the electronic drive than similar heat pumps driven by scroll compressors with PSC motors. The current research measures the compressor and drive performance through calorimeter testing. This paper presents the results of testing a dual rotary PSC compressor in variable speed mode in association with the new electronic drive. The purpose of this testing is to understand how an off-the-shelf compressor with a PSC motor intended for fixed-speed operation performs under variable speed operation. Preliminary results of testing a PSC scroll compressor in variable speed mode with the electronic drive are also presented for comparison with the rotary compressor. Future research will focus on testing other common HVAC compressor technologies and employing mechanistic compressor models in order to better understand the parameters which affect the performance of PSC driven compressors under variable speed operation.

Particle Swarm Optimization-Based Power and Temperature Control Scheme for Grid-Connected DFIG-Based Dish-Stirling Solar-Thermal System

Variable-speed operation of a dish-Stirling (DS) concentrated solar-thermal power generating system can achieve higher energy conversion efficiency compared to the conventional fixed-speed operation system. However, tuning of the controllers for the existing control schemes is cumbersome due to the presence of a large number of control parameters. This paper proposes a new control system design approach for the doubly-fed induction generator (DFIG)-based DS system to achieve maximum power point tracking and constant receiver temperature regulation. Based on a developed thermo-electro-pneumatic model, a coordinated torque and mean pressure control scheme is proposed. Through steady-state analysis, the optimal torque is calculated using the measured insolation and it serves as the tracking reference for direct torque control of the DFIG. To minimize the tracking error due to temperature variation and the compressor loss of the hydrogen supply system, four optimal control parameters are determined using particle swarm optimization (PSO). Model-order reduction and the process of the pre-examination of system stability are incorporated into the PSO algorithm, and it effectively reduces the search effort for the best solution to achieve maximum power point tracking and maintain the temperature around the set-point. The results from computational simulations are presented to show the efficacy of the proposed scheme in supplying the grid system with smoothened maximum power generation as the solar irradiance varies.

Impact of Unbalance Grid Voltage on the Performance of Doubly-fed Induction Machine at Motoring Mode

Hydro-turbines are usually optimized for an operating points defined by speed, head, and discharge. At fixed-speed operation, limited deviations from head and discharge are allowed. However, to improve operational efficiency of hydro turbines with partial loads and varying heads, variable-speed turbine and generator operation offers an attractive solution. Variable-speed operation is achieved using a doubly-fed induction machine with power electronics. During hybrid power generation, stable interconnected system is achieved with pumped storage schemes as renewable energy sources are highly erratic, especially solar and wind. Application of doubly-fed induction machine in pumped storage scheme is equivalently significant in both motoring and generating modes. In practice, the doubly-fed induction machine experiences power quality problems, depending upon the location of pumped storage scheme and the length of the distribution feeder. The objective of this study is to analyze the effects of voltage unbalance on a 2.2-kW doubly-fed induction machine at motoring mode through computer simulations and laboratory experiments. The test results and subsequent analysis confer hidden damage of negative-sequence voltage and effects of positive-sequence voltage magnitude on the performance of doubly-fed induction machine, particularly ripples in torque and speed and reduction in overall machine efficiency, which makes pumped storage scheme economically unviable.

Adaptive Variable Speed Control of Wind Turbines

The uncertain and random characteristics of wind energy make the problem of wind turbine control interesting and challenging. This work investigates an adaptive method for variable speed control of wind turbines under varying operation conditions. For fixed-speed operation of wind turbines, maximum power conversion can be achieved only at a particular wind speed, thus variable speed control of wind turbines is of practical interest in enhancing wind turbine operating efficiency over wide wind speeds. Based on the nonlinear dynamic model of wind turbine, adaptive algorithms are developed in accommodating unknown system parameter uncertainties. This method is shown to be able to achieve smooth and effective tracking of rotor angular speed to capture maximum wind energy. The effectiveness and adaptation of the proposed approach is validated via numerical simulation.

Real-time algorithm for cone crusher control with two variables

Cone crushers are used in the mineral, mining, and aggregate industry for fragmentation of rock materials, minerals and ores. Systems used for controlling the Closed Side Setting (CSS) on cone crushers, and thereby the size reduction, are widely used to compensate for wear of the manganese crushing liners and to protect the machines from overloads. With a frequency converter also the eccentric speed in a cone crusher can be adjusted in real-time in addition to the CSS. The eccentric speed affects the dynamic interaction between the rock material and the crusher liners. Especially the number of compressions the material is exposed to is affected and also the local compression of the rock material is affected, thus the particle-size distribution of the product. Eccentric speed also affects crusher capacity. Real-time feedback data on the sellable product streams can be obtained by applying mass-flow sensors to the process. The adjustment of these two online parameters in real-time can result in an increased potential for production yield; however, a nontrivial optimization problem with a large solution space also arises. As the feed material also varies, the optimal setting for the parameters varies in time. Herein, we report the development of a monitoring and control system including a two variable online algorithm for the selection of the setpoint for eccentric speed with respect to the current CSS. The different product yields from the crushing plant were monitored by mass-flow meters and continuously evaluated by a fitness function. A model for the outcome of the crushing stage, with the two parameters eccentric speed and CSS, was fitted mathematically to the measurement data. However, since the process varies continuously, due to the wear of crushers and screens and feed material variations, the performance landscape is also continuously varying. Therefore, an Evolutionary Operation (EVOP) approach was adopted, wherein the variations are instead used to continuously find an operating point closest to the optimal. The developed algorithm was tested and evolved at a crushing plant for aggregates that produces around 400,000 tonnes aggregates per year. The algorithm was implemented in a computer that communicated with the frequency converter and retrieved data from ten mass-flow meters in the process. The operator was able to interact and supervise the system through a Human Machine Interface (HMI). The result is an algorithm that can determine the position and direction of a dynamic speed control to continuously improve the process-operation point. The magnitude of the improvement potential compared to a fixed speed operation is from 5% to 20%.

A particle swarm optimization for wind energy control problem

The control problem of a wind turbine involves the determination of rotor speed and tip-speed ratio to maximize power and energy capture from the wind. The problem can be formulated as a nonlinear programming problem with the annual energy generation as the objective function. The wind speed distribution is modeled as the Weibull distribution. The Weibull shape and scale parameters are assigned to be stochastic in response to limited wind data and variability nature of the wind. It is proposed to apply particle swarm optimization to solve for optimum rotor speed under fixed-speed operation and optimum tip-speed ratio under variable-speed operation. The optimum rotor speed varies with the wind speed distribution, while the optimum tip-speed ratio does not depend on the wind speed distribution. It can be concluded from the simulation results that both the wind power and energy are more dependent of the Weibull scale parameter than the Weibull shape parameter. This implies that the wind power and energy are more dependent of the mean wind speed than the speed distribution.

Predictive field-oriented control of PMSM with space vector modulation technique

This paper is concerned with two popular and powerful methods in electrical drive applications: field-oriented control (FOC) and space vector modulation (SVM). The proposed FOC-SVM method is incorporated with a predictive current control (PCC)-based technique. The suggested method estimates the desirable electrical torque to track mechanical torque at a fixed speed operation of permanent magnet synchronous motor (PMSM). The estimated torque is used to calculate the reference current based on FOC. In order to improve the performance of the traditional SVM, a PCC method is established as a switching pattern modifier. Therefore, PCC-based SVM is employed to further minimize the torque ripples and transient response. The performance of the controller is evaluated in terms of torque and current ripple and transient response to step variations of the torque command. The proposed method has been verified with MATLAB-Simulink model. Simulation results confirm the ability of this technique in minimizing the torque and speed ripples and fixing switching frequency, simultaneously. However, it is sensitive to parameter changes.

404 風速変動下における系統連系用大型風力発電システムの出力変動評価(新エネルギー(1))

Power output fluctuation of 2 MW-wind turbine-generator systems, which apply fixed speed operation and variable speed operation, under wind speed fluctuation is analyzed by numerical simulation. The fixed speed operation has two levels of operating rotational speeds in response to the wind speed. The variable speed operation has two operating modes : constant tip speed ratio operation at low wind speeds and constant speed operation at high wind speeds. For any operation method, the pitch angle is controlled to maintain the rated electric power output. The simulation result carried out by using an observed field wind data shows that the variable speed operation can reduce the power output fluctuations with both long and short periods as compared with the fixed speed operation due to rotational inertia of the rotor. Furthermore, the influence of the pitch angle control at high wind speeds on the power output fluctuations is revealed.

Control design of an atmospheric solid oxide fuel cell/gas turbine hybrid system: Variable versus fixed speed gas turbine operation

This study is focused on a hybrid fuel cell/gas turbine (FC/GT) system with an atmospheric pressure solid oxide fuel cell (SOFC). The impact of the gas turbine rotational speed on dynamic performance and controllability of a hybrid system is investigated. The transient response of the FC/GT system to perturbations in the power demand has been investigated. Two operational strategies of gas turbines are compared: (1) fixed speed operation, and (2) variable speed operation. For both operation strategies, a wide range of power production is numerically simulated. The results show that variable speed operation is superior for the FC/GT hybrid configuration studied. Variable speed operation allows a 50% turn down in power with no additional balance of plant equipment required. The system efficiency is maintained above 66% for variable speed operation compared to 53% for fixed speed operation with auxiliary combustion.

Continuous turbo-mixer tackles EPS application for Czech firm

Various technical standards are concerned with the efficiency of electric motors for fixed-speed operation or of motors combined with frequency converters (CDMs) to power drive systems for variable-speed operation. Both can serve as drives for pumps. Also, technical standards that concern the efficiency of special types of pumps (circulators, clean water pumps) exist. A standard concerning the energy efficiency of complete pump units is currently in preparation. The standards give definitions of efficiency classes or efficiency indicators and describe methods for their determination. Based on the EcoDesign Directive, the European Union implemented several regulations regarding requirements on minimum efficiency values. These regulations concern electric motors, circulators in buildings, and clean water pumps for a range of specified attributes. For part of the technical standards and EU regulations, revisions are now being prepared.

Soft-stall control for variable-speed stall-regulated wind turbines

A variable-speed, fixed-pitch wind turbine control strategy was investigated to evaluate the feasibility of constraining rotor speed and power output without the benefit of active aerodynamic control devices. A strategy was postulated to control rotational speed by specifying the demanded generator torque. By controlling rotor speed in relation to wind speed, the aerodynamic power extracted by the blades from the wind was manipulated. Specifically, the blades were caused to stall in high winds. In low and moderate winds, the demanded generator torque and the resulting rotor speed were controlled and the wind turbine operated near maximum efficiency. Turbine models were developed and simulations of operation in turbulent winds were conducted. Results indicated that rotor speed and power output were well regulated. Preliminary investigations of system dynamics (E. Muljadi, K. Pierce, P. Migliore, A conservative control strategy for variable-speed stall-regulated wind turbines, AIAA-2000-31 A Collection of the 2000 ASME Wind Energy Symposium Technical Papers presented at the 38th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, January 2000). Showed that, compared to fixed-speed operation, variable-speed operation caused cyclic loading amplitude to be reduced for the turbine blades and low-speed shaft and slightly increased for the tower loads. This result suggests that implementation of the proposed control strategy will have a favorable impact on the turbine's fatigue life. The concept was implemented on a 275 kW wind turbine test bed (K. Pierce, P. Migliore, Maximizing the energy capture of fixed-pitch variable-speed wind turbines, AIAA-2000-0032 ASME Wind Energy Symposium Technical Papers presented at the 38th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, January 2000). Test data show that the wind turbine performance matches the predicted simulation results. The control concept was shown to operate the wind turbine near maximum efficiency in low-to-moderate wind speeds, while stalling the rotor in the high winds to regulate speed and output power.

Performance of a variable-speed inverter/motor drive for refrigeration applications

Presents the results of an experimental investigation into the application of inverter-based variable-speed drives to positive displacement rotary vane refrigeration compressors. The investigation considers the effects of the inverter on a number of operating parameters such as the harmonic currents and voltages on both the supply side and motor side, power consumption and power factor, starting current and overall system efficiency. The results indicate that the use of off-the-shelf inverters for variable-speed control of refrigeration compressors can inject harmonics to the supply system which exceed the maximum limits imposed by BS5406. The inverter may also cause a reduction in the power factor and in the overall efficiency of the drive. Variable-speed operation of a rotary vane compressor may not offer efficiency advantages over fixed-speed operation but can provide other benefits such as better temperature control and faster response to sudden changes in load.