Several Distributed Energy Systems Engineering Essay

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Introduction

Several distributed energy (DE) systems are expected to have a significant impact on the California energy market in near future. These DE systems include, but are not limited to: photovoltaics (PV), wind, microturbine, fuel cells, and internal combustion (IC) engines (Byron 2002). In addition, several energy storage systems such as batteries and flywheels are under consideration for DE to harness excess electricity produced by the most efficient generators during low loading.

This harvested energy can be released onto the grid, when needed, to eliminate the need for high-cost generators. Inclusion of storage in the distributed generation system actually provides the user dispatchability of its distributed resources which generally are renewable energy sources, like PV and solar, having no dispatchability by their own. In the future, using hybrid electric vehicles along with the utility grid in the form of plug-in hybrid electric vehicles (PHEV) and vehicle-to-grid systems (V2G) will be a very promising option to be included in the DE classification.

All of these DE technologies require specific power electronics capabilities to convert the power generated into useful power that can be directly interconnected with the utility grid and/or can be used for consumer applications. Because of similar functions of these power electronics interfaces, the development of scalable, modular, low cost, highly reliable power electronic interfaces will improve the overall cost and durability of distributed and renewable energy systems.

Although most DE systems are not new in a technological respect, they are receiving increased attention today because of their ability to provide combined heat and power, peak power, demand reduction, backup power, improved power quality, and ancillary services to the power grid. Out of all DE systems, the visibility of renewable energy sources are increasing significantly due to several state governments' adoption of Renewable Portfolio Standards (RPS) that require a certain percentage of energy be produced using renewable energy sources. A large factor in whether or not DE systems are installed is the initial capital cost.

Although power electronics are the integral part of most of the DE technologies, in order to convert the power generated into useful power that can be directly used on the grid, they can cost up to 40% of the costs of a distributed energy system (Blazewicz 2005). Therefore, the improvement of the DE economics strongly requires decreased costs for the power electronics. Another important aspect to the life-cycle cost of the DE systems is reliability.

Many of the power electronics used for DE applications have a low reliability rate, typically operating less than five years before a failure occurs.

This rate can be improved with modern reliability testing techniques and needs to be fully examined to improve the economics of DE systems.

This report presents a summary providing a convenient resource to understand the current state-of-the art power electronic interfaces for DE applications. It also outlines the power electronic topologies that are needed for an advanced power electronic interface.

This article focuses on commercially available DE systems and is organized into eight pplication-specific areas:

•Photovoltaic Systems

•Wind Systems

•Microturbine Systems

•Fuel Cell Systems

•Internal Combustion Engine Systems

•Battery Storage Systems

•Flywheel Systems

Plug-In Vehicles.

Different power electronics topologies are discussed for each of these DE systems and a generalized topology is selected for understanding the control design. An interesting section on plug-in hybrid vehicles is also included in the report that can help to explain the new vehicle technologies that are viable to be included in the DE framework. Figurediagram of general p

The current plans of the California Energy Commission Public Interest Energy Research (PIER) program are to implement projects to accelerate the use of DE systems, in part by addressing the cost and reliability of the common element of all of the distributed and renewable technologies: the power electronics interface. This objective is being accomplished through a recently announced Advanced Power Electronics Interface (APEI) initiative. The objectives of the APEI initiative include (Treanton 2004):

Developing an architecture for standardized, highly integrated, modularized power electronics interconnection technologies that will come as close as possible to "plug-and-play" for distributed energy platforms;

Reducing costs and improving the reliability for DE and interconnections by developing standardized, high production volume power electronic modules; and

Improving the flexibility and scalability for power electronic modules and systems to provide advanced functionality at a range of power levels.

The purpose of this report is to provide a consolidated resource describing the current state-of-the-art in power electronic interfaces for DE applications and outline possible power electronic topologies that will lead to a low-cost, reliable APEI.

Power electronics interfaces

Power electronics interface for use with DE systems and can be subdivided into four major modules.

These include:

the source input converter module,

an inverter module,

the output interface module, and

the controller module.

The blue unidirectional arrows depict the power flow path for the DE sources whereas the red arrows show the bidirectional power flows for the DE storages. The input converter module can be either used with alternating current (AC) or direct current (DC) DE systems and is most likely to be specific for the type of energy source ES-1 shows a general block or storage.

The DC-AC inverter module is the most generic of the modules and converts a DC source to grid-compatible AC power. The output interface module filters the AC output from the inverter. The fourth major module is the monitoring and control module that operates the entire interface and contains protection for both the DE source and the utility at the point-of-common-coupling (PCC).

Due to many inherent similarities in these modules, it is possible that a modular and scalable APEI could allow each of the energy source technologies to use the same power electronic components within their system architectures. The future requirements for modular design, such as standard interfaces, are also discussed. These requirements can lead to modular and flexible design of the power electronics converters for the DE applications.

General Topology

A generalized block diagram representation of power electronics interface associated with DE systems is shown in Figure 1. The power electronics interface accepts power from the distributed energy source and converts it to power at the required voltage and frequency. For the storage systems, bidirectional flow of power between the storages and the utility is required. Figure 1 illustrates a design approach to organize the interface into modules, each of which can be designed to accommodate a range of DE systems and/or storages.

For this example, four major modules for a power electronics interface are depicted. They include the source input converter module, an inverter module, the output interface module and the controller module. The blue unidirectional arrows depict the power flow path for the DE sources whereas the red arrows show the bidirectional power flows for the DE storages.

The design of the input converter module depends on the specific energy source or storage application. The DE systems that generate AC output, often with variable frequencies, such as wind, microturbine, IC engine, or flywheel storage needs an AC-DC converter. For DC output systems like PV, fuel cells, or batteries, a DC-DC converter is typically needed to change the DC voltage level. The DC-AC inverter module is the most generic of the modules and converts a DC source to grid-compatible AC power. The output interface module filters the AC output from the inverter and the monitoring and control module operates the interface, containing protection for the DE and utility point-of-common-coupling (PCC).

The power electronic (PE) interface also contains some level of monitoring and control functionality to ensure that the DE system can operate as required. The monitoring and control module also contains protective functions for the DE system and the local electric power system that permit paralleling and disconnection from the electric power system. These functions would typically meet the IEEE 1547-2003 interconnection requirements (Basso and DeBlasio 2004), but should have the flexibility for modifications of the settings depending on the application or a utility's interconnection requirements.

In addition, the monitoring and control module may also provide human-machine interface, communications interface, and power management. Monitoring functions typically include real-power, reactive power, and voltage monitoring at the point of the DE connection with the utility at the PCC. These functions are necessary because, in order to synchronize the DE system, its output must have the same voltage magnitude, frequency, phase rotation, and phase angle as the utility. Synchronization is the act of checking that these four variables are within an acceptable range before paralleling two energy sources.

Every power electronics circuit consists of different semiconductor devices fabricated with appropriate impurities (known as "doping") in order to achieve particular electrical properties such as conduction, resistance, turn on/turn-off times, power dissipation, etc. The fundamental power electronics device is the semiconductor-based switch, a technology that has existed for many decades, but is continuously being improved in terms of power density and reliability. In general, the term "power electronics" refers to the device switches (e.g., IGBT and SCR), and the various modules that they comprise. In power applications, these devices are most often used to convert electrical energy from one form to a more usable form. Benefits of power electronic devices include increased efficiency, lower cost, and reduced packaging size.

A rectifier is a power electronics topology that converts AC to DC. Rectifier circuits are generally used to generate a controlled DC voltage from either an uncontrolled AC source (i.e., microturbine, wind turbine) or a controlled AC source (i.e., utility supply) (Kroposki et al. 2006). When converting from a utility supply, a rectifier application is usually for linking DC systems or providing DC voltage for specific load applications such as battery regulators and variable frequency drive (VFD) inputs.

Some DE systems like photovoltaics and fuel cells produce DC power. In order to make this power useful for grid-tied applications, it must be converted to AC; therefore, inverters are used to convert DC to AC. Inverter circuits generate a regulated AC supply from a DC input. They are commonly found in systems providing standalone AC power, utility-connected DE systems, and on the motor side of a VFD.

There are a number of applications for DC-to-DC systems. These systems are used to convert the DC voltage magnitude from one level to another with or without galvanic isolation. They take an uncontrolled, unregulated input DC voltage and condition it for the specific load application. An example for such topology can be found in PV applications, where the dedicated DC-DC units are often designed to extract the maximum power output of the PV array.

AC-to-AC converters can be used convert the AC source voltage magnitude and frequency to a fixed amplitude and frequency, making it compatible with the utility grid. The AC-to-AC converters are not typically used in modern DE applications due to some inherent disadvantages. A summary of the different power converters that are used for DE applications are given in Table 1 (Shepherd et al. 2004; DeBlasio et al. 2006).

Table 1. Power electronics systems for different power conversions

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