Showing papers on "Electricity generation published in 2006"

Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey

Abstract: The use of distributed energy resources is increasingly being pursued as a supplement and an alternative to large conventional central power stations. The specification of a power-electronic interface is subject to requirements related not only to the renewable energy source itself but also to its effects on the power-system operation, especially where the intermittent energy source constitutes a significant part of the total system capacity. In this paper, new trends in power electronics for the integration of wind and photovoltaic (PV) power generators are presented. A review of the appropriate storage-system technology used for the integration of intermittent renewable energy sources is also introduced. Discussions about common and future trends in renewable energy systems based on reliability and maturity of each technology are presented

Control of permanent-magnet generators applied to variable-speed wind-energy systems connected to the grid

Abstract: Wind energy is a prominent area of application of variable-speed generators operating on the constant grid frequency. This paper describes the operation and control of one of these variable-speed wind generators: the direct driven permanent magnet synchronous generator (PMSG). This generator is connected to the power network by means of a fully controlled frequency converter, which consists of a pulsewidth-modulation (PWM) rectifier, an intermediate dc circuit, and a PWM inverter. The generator is controlled to obtain maximum power from the incident wind with maximum efficiency under different load conditions. Vector control of the grid-side inverter allows power factor regulation of the windmill. This paper shows the dynamic performance of the complete system. Different experimental tests in a 3-kW prototype have been carried out to verify the benefits of the proposed system.

Analysis of power output for piezoelectric energy harvesting systems

Abstract: Power harvesting refers to the practice of acquiring energy from the environment which would be otherwise wasted and converting it into usable electric energy. Much work has been done on studying the optimal AC power output, while little has considered the AC–DC output. This article investigates the optimal AC–DC power generation for a rectified piezoelectric device. In contrast with estimates based on various degrees of approximation in the recent literature, an analytic expression for the AC–DC power output is derived under steady-state operation. It shows that the harvested power depends on the input vibration characteristics (frequency and acceleration), the mass of the generator, the electrical load, the natural frequency, the mechanical damping ratio and the electromechanical coupling coefficient of the system. An effective power normalization scheme is provided to compare the relative performance and efficiency of devices. The theoretical predictions are validated and found to be in good agreement with both experimental observations and numerical simulations. Finally, several design guidelines are suggested for devices with large coupling coefficient and quality factor.

A simplified wind power generation model for reliability evaluation

Abstract: Renewable energy sources, especially wind turbine generators, are considered as important generation alternatives in electric power systems due to their nonexhausted nature and benign environmental effects. The fact that wind power penetration continues to increase has motivated a need to develop more widely applicable methodologies for evaluating the actual benefits of adding wind turbines to conventional generating systems. Reliability evaluation of generating systems with wind energy sources is a complex process. It requires an accurate wind speed forecasting technique for the wind farm site. The method requires historical wind speed data collected over many years for the wind farm location to determine the necessary parameters of the wind speed models for the particular site. The evaluation process should also accurately model the intermittent nature of power output from the wind farm. A sequential Monte Carlo simulation or a multistate wind farm representation approach is often used. This paper presents a simplified method for reliability evaluation of power systems with wind power. The development of a common wind speed model applicable to multiple wind farm locations is presented and illustrated with an example. The method is further simplified by determining the minimum multistate representation for a wind farm generation model in reliability evaluation. The paper presents a six-step common wind speed model applicable to multiple geographic locations and adequate for reliability evaluation of power systems containing significant wind penetration. Case studies on a test system are presented using wind data from Canadian geographic locations.

Residential cogeneration systems: Review of the current technology

Abstract: There is a growing potential for the use of micro-cogeneration systems in the residential sector because they have the ability to produce both useful thermal energy and electricity from a single source of fuel such as oil or natural gas. In cogeneration systems, the efficiency of energy conversion increases to over 80% as compared to an average of 30–35% for conventional fossil fuel fired electricity generation systems. This increase in energy efficiency can result in lower costs and reduction in greenhouse gas emissions when compared to the conventional methods of generating heat and electricity separately. Cogeneration systems and equipment suitable for residential and small-scale commercial applications like hospitals, hotels or institutional buildings are available, and many new systems are under development. These products are used or aimed for meeting the electrical and thermal demands of a building for space and domestic hot water heating, and potentially, absorption cooling. The aim of this paper is to provide an up-to-date review of the various cogeneration technologies suitable for residential applications. The paper considers the various technologies available and under development for residential, i.e. single-family (

Output power leveling of wind turbine Generator for all operating regions by pitch angle control

Abstract: Wind energy is not constant and windmill output is proportional to the cube of wind speed, which causes the generated power of wind turbine generators (WTGs) to fluctuate. In order to reduce fluctuation, different methods are available to control the pitch angle of blades of windmill. In a previous work, we proposed the pitch angle control using minimum variance control, and output power leveling was achieved. However, it is a controlled output power for only rated wind speed region. This paper presents a control strategy based on average wind speed and standard deviation of wind speed and pitch angle control using a generalized predictive control in all operating regions for a WTG. The simulation results by using actual detailed model for wind power system show the effectiveness of the proposed method.

Wind Turbine Reliability: Understanding and Minimizing Wind Turbine Operation and Maintenance Costs

Abstract: Wind turbine system reliability is a critical factor in the success of a wind energy project. Poor reliability directly affects both the project's revenue stream through increased operation and maintenance (OM increased risk, or at least the perception of increased risk, is generally accompanied by increased financing fees or interest rates. This paper outlines the issues relevant to wind turbine reliability for wind turbine power generation projects. The first sections describe the current state of the industry, identify the cost elements associated with wind farm O&M and availability and discuss the causes of uncertainty in estimating wind turbine component reliability. The latter sections discuss the means for reducing O&M costs and propose O&M related research and development efforts that could be pursued by the wind energy research community to reduce cost of energy.

Can microgrids make a major contribution to UK energy supply

Abstract: Almost all the electricity is generated in the UK as part of a centralised power system designed around large fossil fuel or nuclear power stations. This power system is robust and reliable but the efficiency of power generation is low, resulting in large quantities of waste heat. The principal aim of this paper is to investigate an alternative concept: the energy production by small scale generators in close proximity to the energy users, integrated into microgrids. Microgrids - de-centralised electricity generation combined with on-site production of heat - bear the promise of substantial environmental benefits , brought about by a higher efficiency and by facilitating the integration of renewable energy sources such as photovoltaic arrays or wind turbines. By virtue of good match between generation and load, microgrids have a low impact on the electricity network, despite a potentially significant level of generationby interemittent energy sources. The paper discusses the technical and economic issues associated with this novel concept, giving an overview of the generator technologies, the current regulatory framework in the UK, and the barriers that have to be overcome if microgrids are to make a major contribution to the UK energy supply. The focus of this study is a microgrid of domestic users powered by small Combined Heat and Power generators and photovoltaics. Focusing on the energy balance between the generation and load, it is found that the optimum combination of the generators in the microgrid - consisting of around 1.4 kWp PV array per household and 45% household ownership of micro-CHP generators - will maintain energy balance on a yearly basis if suplemented by energy storage of 2.7 kWh per household. We find that there is no fundamental technological reason why microgrids cannot contribute an appreciable part of UK energy demand. Indeed, an estimate of cost indicates that the microgrids considered in this study would supply electricity at a cost comparable with the present electricity supply if the current support mechanisms for photovoltaics were maintained. Combining photovoltaics and micro-CHP and a smallbattery requirement gives a microgrid that is independent of the national electricity network. In the short term, this has particular benefits for remote communities but more wide-ranging possibilities open up in the medium to long term. Microgrids could meet the need to replace current generation nuclear and coal fired power stations, greatly reducing the demand on the transmission and distribution network.

Control and Performance Evaluation of a Flywheel Energy-Storage System Associated to a Variable-Speed Wind Generator

Abstract: The flywheel energy-storage systems (FESSs) are suitable for improving the quality of the electric power delivered by the wind generators and for helping these generators to contribute to the ancillary services. Supervisors must be used for controlling the power flow from a variable-speed wind generator (VSWG) to the power grid or to an isolated load. This paper investigates the control method and the energetic performances of a low-speed FESS with a classical squirrel-cage induction machine in the view of its association to a VSWG. A test bench is developed, and experimental results are presented and discussed

Distributed generation: challenges and possible solutions

Abstract: This contribution starts from the observation that there is a renewed interest in small-scale electricity generation. The authors start with a discussion of the drivers behind this evolution indicating the major benefits and issues of small-scale electricity generation. Attention is paid to the impact of a massive penetration of distributed generation in the grid on the system safety and protection. An overview of the impact on voltage quality and stability is given, both static and dynamic. A practical example is discussed in order to show the problems and indicate solutions. Different types of generators and grid interfaces are treated. In a final chapter, an attempt is made to correctly define small-scale generation also commonly called distributed generation, embedded generation or decentralized generation.

Single-phase single-stage photovoltaic generation system based on a ripple correlation control maximum power point tracking

Abstract: A maximum power point tracking algorithm for single-stage converters connecting photovoltaic (PV) panels to a single-phase grid is presented in this paper. The algorithm is based on the application of the "ripple correlation control" using as perturbation signals the current and voltage low-frequency oscillations introduced in the PV panels by the single-phase utility grid. The proposed control technique allows the generation of sinusoidal grid currents with unity power factor. The algorithm has been developed to allow an array of PV modules to be connected to the grid by using a single-stage converter. This simple structure yields higher efficiency and reliability when compared with standard solutions based on double-stage converter configurations. The proposed maximum power point tracking algorithm has been numerically simulated and experimentally verified by means of a converter prototype connected to a single-phase grid. The results are presented in the paper, showing the effectiveness of the proposed system.

Impact of Plug-in Hybrid Vehicles on the Electric Grid

Abstract: Plug-in hybrid vehicles (PHEVs) are being developed around the world; much work is going on to optimize engine and battery operations for efficient operation, both during discharge and when grid electricity is available for recharging. However, there has generally been the expectation that the grid will not be greatly affected by the use of the vehicles, because the recharging would only occur during offpeak hours, or the number of vehicles will grow slowly enough that capacity planning will respond adequately. But this expectation does not incorporate that endusers will have control of the time of recharging and the inclination for people will be to plug in when convenient for them, rather than when utilities would prefer. It is important to understand the ramifications of introducing a number of plug-in hybrid vehicles onto the grid. Depending on when and where the vehicles are plugged in, they could cause local or regional constraints on the grid. They could require both the addition of new electric capacity along with an increase in the utilization of existing capacity. Local distribution grids will see a change in their utilization pattern, and some lines or substations may become overloaded sooner than expected. Furthermore, the type ofmore » generation used to recharge the vehicles will be different depending on the region of the country and timing when the PHEVs recharge. We conducted an analysis of what the grid impact may be in 2018 with one million PHEVs added to the VACAR sub-region of the Southeast Electric Reliability Council, a region that includes South Carolina, North Carolina, and much of Virginia. To do this, we used the Oak Ridge Competitive Electricity Dispatch model, which simulates the hourly dispatch of power generators to meet demand for a region over a given year. Depending on the vehicle, its battery, the charger voltage level, amperage, and duration, the impact on regional electricity demand varied from 1,400 to 6,000 MW. If recharging occurred in the early evening, then peak loads were raised and demands were met largely by combustion turbines and combined cycle plants. Nighttime recharging had less impact on peak loads and generation adequacy, but the increased use of coal-fired generation changed the relative amounts of air emissions. Costs of generation also fluctuated greatly depending on the timing. However, initial analysis shows that even charging at peak times may be less costly than using gasoline to operate the vehicles. Even if the overall region may have sufficient generating power, the region's transmission system or distribution lines to different areas may not be large enough to handle this new type of load. A largely residential feeder circuit may not be sized to have a significant proportion of its customers adding 1.4 to 6 kW loads that would operate continuously for two to six hours beginning in the early evening. On a broader scale, the transmission lines feeding the local substations may be similarly constrained if they are not sized to respond to this extra growth in demand. This initial analysis identifies some of the complexities in analyzing the integrated system of PHEVs and the grid. Depending on the power level, timing, and duration of the PHEV connection to the grid, there could be a wide variety of impacts on grid constraints, capacity needs, fuel types used, and emissions generated. This paper provides a brief description of plug-in hybrid vehicle characteristics in Chapter 2. Various charging strategies for vehicles are discussed, with a consequent impact on the grid. In Chapter 3 we describe the future electrical demand for a region of the country and the impact on this demand with a number of plug-in hybrids. We apply that demand to an inventory of power plants for the region using the Oak Ridge Competitive Electricity Dispatch (ORCED) model to evaluate the change in power production and emissions. In Chapter 4 we discuss the impact of demand increases on local distribution systems. In Chapter 5 we conclude and provide insights into the impacts of plug-ins. Future tasks will be proposed to better define the interaction electricity and transportation, and how society can better prepare for their confluence.« less

Resistor Emulation Approach to Low-Power Energy Harvesting

Abstract: This paper presents an approach and associated circuitry for harvesting near maximum output from low power sources in the 100 μW range for miniature wireless devices. A set of converter topologies and control approaches are presented together with detailed efficiency analysis and a design example for a buck-boost based energy harvesting converter using commercially available discrete circuitry. Experimental results are presented for harvesting energy from miniature RF and wind power sources operating over a 10:1 input power range from 500 μW to 50 μW. A closed-system efficiency of 65 % at 50 μW output power is achieved, including all control and converter losses. The results demonstrate that useful energy, sufficient to power typical wireless sensors, can be harvested from miniature sources that output less than 100 μW at a few hundred mV.

Operation planning of hydrogen storage connected to wind power operating in a power market

Abstract: In this paper, a methodology for the operation of a hybrid plant with wind power and hydrogen storage is presented. Hydrogen produced from electrolysis is used for power generation in a stationary fuel cell and as fuel for vehicles. Forecasts of wind power are used for maximizing the expected profit from power exchange in a day-ahead market, also taking into account a penalty cost for unprovided hydrogen demand. During online operation, a receding horizon strategy is applied to determine the setpoints for the electrolyzer power and the fuel cell power. Results from three case studies of a combined wind-hydrogen plant are presented. In the first two cases, the plant is assumed to be operating in a power market dominated by thermal and hydropower, respectively. The third case demonstrates that the operating principles are also useful for isolated wind-hydrogen systems with backup generation

A Hybrid Energy Storage System Based on Compressed Air and Supercapacitors With Maximum Efficiency Point Tracking (MEPT)

Abstract: In addition to the high-capacity storage facilities based on hydropower technologies, electrochemical solutions are today's candidate for storage for renewable energy sources support. However, sustainability and limited life cycles of batteries are often inhibiting factors. This paper presents a hybrid energy storage system with high life cycle, which is mainly based on compressed air, where the storage and discharge are done within maximum efficiency conditions. As the maximum efficiency conditions impose the level of converted power, an intermittent time-modulated operation mode is applied to the thermodynamic converter to obtain a variable converted mean power. A smoothly variable output power is achieved with the help of a supercapacitive auxiliary storage device used as a filter. This paper describes the concept of the system, the power-electronic interface circuits, and especially the maximum efficiency point tracking (MEPT) algorithm and the strategy used to vary the output power. In addition, this paper presents the characteristics of high-efficiency storage systems where the pneumatic machine is replaced by an oil-hydraulic and pneumatic converter, which is used under isothermal conditions. Practical results are also presented, which are recorded from a low-power pneumatic motor coupled to a small dc generator as well as from the first prototype of the final hydropneumatic system