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SWTs have traditionally been used for remote small off-grid applications, this being the bulk of the market both in the developed and in the developing world. Only in the last few years has this trend changed, due to the growth of grid connections from SWTs. The potential market for grid-connected SWTs is accelerating the development of SWT technology as the anticipated large-scale production justifies the higher financial investments required for development of the technology.
As mentioned before, most of the existing systems that include SWTs are isolated applications, as this has been the most traditional use for them. Among the possible isolated applications, the most common are rural electrification, professional applications (telecommunications, etc.) and pumping.
From the technological point of view, three groups of isolated applications using SWT can be distinguished. These are covered in the following sections.
These usually have a generating capacity smaller than 1 kW. The best-known applications for these configurations are mobile applications, such as boats and caravans, and wind home systems (WHSs): the wind version of solar home systems (SHS) used for rural electrification. This configuration is based on DC connection, where the battery (usually a lead-acid battery) is the main storage and control component. Usually the system supplies DC loads as the consumed energy is very low. In terms of number of systems, this is the most frequently used configuration. Manufacturers include Marlec, Ampair, and Southwest.
The term ‘hybrid’ has different meanings in the context of off-grid systems with renewable energy. In this case, ‘hybrid systems’ refers to systems including wind generation and other generation sources (usually photovoltaic). The power generation capacity for this configuration is in most cases less than 50 kW. A diesel generator (gen-set) is used in many systems in this configuration to supply back-up power. Traditionally, these systems have also been based on DC connection (Proven, Bornay, Windeco), with a battery (lead-acid in most cases) also playing the role of storage and control, and an inverter to generate AC power for the loads (common practice is to use only AC loads in this configuration). However, in recent years some solutions have been proposed using AC connection. This solution has been possible through the development of bi-directional converters (SMA, Conergy, Xantrex) that allow the flow from the DC bus to the AC bus and vice versa using only one stage of power electronics. Developers using this second solution have proposed the use of SWTs with asynchronous generators directly connected to the AC bus (SMA, Conergy), which was a concept rarely used for these systems (except for Vergnet). The trend of technology for these systems is mainly in the development of modular and flexible power electronics, able to provide both the power quality and the supervisory control of the system.
Even though some hybrid systems include only wind and diesel generation, the configuration described as ‘wind-diesel’ (W/D) refers to those systems where the gen-set plays a key role, not only as a back-up source but also as an essential component for the correct control and functioning of the system. This configuration is typical for larger isolated applications (> 50 kW), and some systems in the MW range have been reported. The storage system this configuration uses (if any) is a short-term storage one, commonly batteries or flywheels, which is used for power quality and control purposes only, but not for long-term energy balance.
Three different types of wind-diesel systems can be distinguished, according to the proportion of wind use in the system:
- Low penetration W/D systems, which do not require additional modifications to the diesel-only grid (usually an existing system) as the diesel engine runs continuously and its controls can cope with the control of the system in the W/D mode of operation without significant modification
- Medium penetration W/D systems, which require the inclusion of some control capabilities (usually the regulation of deferrable loads or the regulation of the wind generation) for the moments when wind generation is higher than load consumption
- High penetration W/D systems, which require the addition of complex control strategies and devices in order to guarantee the stability of the system in the wind-only mode, (in other words when the diesel gen-set has been shut off).
Low penetration systems can be found at a commercial level, whereas solutions for high-penetration W/D systems are at a demonstration level (being ENERCON and POWERCORP the main developers). Technology trends for this configuration include the development of robust and proven control strategies. Prospects for this configuration (mainly for high-penetration W/D systems) are very promising, as the cost of fuel has recently increased dramatically.
Another market with great potential for small wind turbines is in grid-connected applications for residential, industrial or even, lately, urban environments. The so-called distributed wind applications are poised for rapid market growth in response to continuing energy price increases and increased demand for on-site power generation. However, in order for distributed wind to reach its mainstream market potential, the industry must overcome several hurdles, primarily in system costs, quality of design, grid interconnection, and installation restrictions.
Presently, the major share of development of this market is in the US, UK and Canada in parallel with new trends in the development of distributed generation systems. This emerging market provides a new impulse to the development of SWT technology.
Wind power can also be used to generate electricity in an urban environment. This trend has mainly been seen in Europe, where the integration of SWTs in the built environment is being actively discussed. New wind turbines are under development for this application, which is looking mainly for quiet and efficient devices under turbulent and skewed wind flow. As well as the installation of wind turbines around and on buildings, there is also interest in ‘building-integrated’ wind turbines, where the turbine is part of the building structure or façade. For these applications, due attention should be paid to the quality of available wind resource onsite prior to installation.
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