Stand-Alone and Hybrid Wind Energy Systems

Technology, Energy Storage and Applications

J K Kaldellis

• Publisher: Woodhead Publishing
• Year: 2010
• ISBN:
  9781845699628
• Topic: Tecnología e ingeniería
• Subtopic: Recursos de energía renovable

Wind power is fast becoming one of the leading renewable energy sources worldwide, not only from large scale wind farms but also from the increasing penetration of stand-alone and hybrid wind energy systems. These systems are primarily of benefit in small-scale applications, especially where there is no connection to a central electricity network, and where there are limited conventional fuel resources but available renewable energy resources. By applying appropriate planning, systems selection and sizing, including the integration of energy storage devices to mitigate variable energy generation patterns, theses systems can supply secure reliable and economic power to remote locations and distributed micro-grids.Stand-alone and hybrid wind energy systems is a synthesis of the most recent knowledge and experience on wind-based hybrid renewable energy systems, comprehensively covering the scientific, technical and socio-economic issues involved in the application of these systems.Part one presents an overview of the fundamental science and engineering of stand-alone and hybrid wind energy systems and energy storage technology, including design and performance optimisation methods and feasibility assessment for these systems. Part two initially reviews the design, development, operation and optimisation of stand-alone and hybrid wind energy systems – including wind-diesel, wind -photovoltaic (PV), wind-hydrogen, and wind-hydropower energy systems – before moving on to examine applicable energy storage technology, including electro-chemical, flywheel (kinetic) and compressed air energy storage technologies. Finally, Part three assesses the integration of stand-alone and hybrid wind energy systems and energy technology into remote micro-grids and buildings, and their application for desalination systems.With its distinguished editor and international team of contributors, Stand-alone and hybrid wind energy systems is a standard reference for all renewable energy professionals, consultants, researchers and academics from post-graduate level up.

• Provides an overview of the fundamental science and engineering of stand-alone hybrid and wind energy systems, including design and performance optimisation methods
• Reviews the development and operation of stand-alone and hybrid wind energy systems
• Assesses the integration of stand-alone and hybrid wind energy systems and energy storage technology into remote micro-grids and buildings, and their application for desalination systems

Part 1

Fundamental science and engineering of stand-alone and hybrid wind energy systems and energy storage technology

1

Overview of stand-alone and hybrid wind energy systems

J.K. Kaldellis, TEI of Piraeus, Greece

Abstract:

This chapter introduces the reader to the definition and development of stand-alone and hybrid energy systems. Emphasis is given to the description of wind-based stand-alone hybrid energy systems as well as to the use of energy storage for the support of such configurations. Accordingly, the most established applications of similar systems are presented, including some representative real-life examples. Future prospects of such systems are discussed.

Key words:

stand-alone system

hybrid system

wind power

energy storage

remote consumer

1.1 Introduction

At the beginning of the twenty-first century, almost every inhabitant of the industrialized world has access to a constant electricity supply and thus electricity may be viewed as a significant aspect of contemporary societies, similar to fresh water and clean air. Nevertheless, this is not the case for the planet’s entire population. According to official statistics (European Commission, 1999), almost two billion people worldwide have no direct access to electrical networks with 500 000 of them living in the European Union and other financially developed countries. Afar from decision centres and having limited political influence, isolated consumers are usually abandoned, facing a dramatically insufficient infrastructure (Jensen, 2000; Kaldellis et al., 2001a). In this context, autonomous stand-alone wind-power systems have proven to be one of the most interesting and environmentally friendly technological solutions for the electrification of remote consumers, especially in the presence of high wind potential (Kaldellis, 2002, 2004). Small wind turbines are able to produce an annual total of only few MW h which, although limited in absolute numbers, makes a considerable difference in upgrading living standards in the remote areas of our planet.

The required investment cost, however, may be quite high, especially in cases of medium quality wind potential regions and no-load rejection operational conditions, i.e. the entire load demand must be met. One of the most expensive components of a stand-alone system is the energy storage device, necessary to guarantee the required system reliability. Thus, in cases of increased system autonomy the energy storage contribution to the initial or the total operational cost is found to be dominant (Kaldellis, 2003, 2008a). In addition, energy storage systems are usually land-intensive, need a lot of maintenance and often need to be replaced every specific time period, thus increasing the operational cost of the system. To avoid oversizing of energy storage configurations, wind-based stand-alone systems are augmented with another available energy source, such as solar energy, hydropower or biomass. Such a stand-alone hybrid energy system is an option worth considering (Muselli et al., 1999; Kaldellis and Kavadias, 2001; Kaldellis et al., 2006a).

Recapitulating, stand-alone wind energy systems are electricity-generating systems, based on the operation of one or more wind turbines, being also remote (not connected) from the central electrical grids. In this context, import or export of electricity is not permitted, but there are occasions where a stand-alone system can be connected to an existing electrical network, e.g. emergency status (Bueno and Carta, 2006).

Accordingly, hybrid energy systems incorporate two or more electricity generation options, based either on the exploitation of renewable energy sources (RES) or on small thermal power units, e.g. diesel-electric generators or even micro-turbines. Note, however, that with regards to the case currently studied, the first electricity generation option is by definition wind energy.

1.2 Description of a wind-based stand-alone energy system

A typical wind energy stand-alone system (see Fig. 1.1) includes:

1.1 Typical wind energy stand-alone system.

• one or more (usually small) wind converters of N_o kW;

• an appropriate energy storage device, e.g. a lead–acid battery storage array, able to guarantee h_o hours of autonomy, or equivalently with energy storage capacity Q_max and maximum permitted discharge capacity Q_min;

• an AC/DC rectifier of N_r kW in case the energy storage installation operates on DC current;

• a charge controller of N_c kW;

• a UPS (uninterruptible power supply) of N_p kW in order to guarantee high quality AC electricity generation;

• a DC/AC inverter of N_pkW.

1.2.1 Wind turbine

The rated power of the selected wind-turbine(s) depends on the system electricity demand, the available wind potential and the operational characteristics of the machine (Vlachou et al., 1999). Keep also in mind that the wind-turbine output curves are given at standard-day conditions, without air humidity. Thus, in real-day conditions, the output of the turbine depends (Kaldellis et al., 2004) on the wind speed value V at hub height, the manufacturer’s power curve N_W = N_W* (V) at standard day conditions and the air density ρ at the installation area, thus:

1.1

Finally, note that the air density value depends on the ambient temperature and pressure as well as on the corresponding air humidity (Houghton and Brock, 1980).

1.2.2 Energy storage

There are several different energy storage alternatives, such as flywheels, hydraulic storage, pumped hydro, battery storage and even fuel cells (Kaldellis and Zafirakis, 2007; Kaldellis et al., 2009a), with lead-acid batteries being one of the most widely applied solutions adopted in similar size applications. The operational principle of an energy storage installation in this kind of systems is based on the accumulation of available energy surplus in order for it to be used during periods of inadequate wind. More precisely, the energy storage size is given in units of the time-period that the storage can cover the average load without the contribution of other power sources. Hence, the energy storage system used is defined by the installation’s hours of energy autonomy h_o, the corresponding operational characteristics, e.g. the output voltage U_b for battery storage systems, the maximum permitted depth of discharge DOD_L and the overall efficiency of the energy storage branch. Note that the latter includes the energy storage process (e.g. rectifier and charge controller losses), the standing losses owing to the energy storage self-discharge, the losses of the line connecting the storage branch apparatus and finally any electricity generation losses (e.g. inverter).

1.2.3 System electr...